Selective oxidation of methane in sulfuric acid

Selective Oxidation of Methane in

Sulfuric Acid: Understanding and

Improving Catalyst Activity, Stability,

and Selectivity in the Periana System

Dissertation

zur Erlangung des Grades eines Doktors der Naturwissenschaften

Vorgelegt der Fakultät der Chemie und Biochemie der

Ruhr-Universität Bochum

von

Tobias Zimmermann

geboren am 01.02.1986 in Pforzheim

Bochum, 2015

Die vorliegende Arbeit wurde von August 2011 bis Oktober 2015 am

Max-Planck-Institut für Kohlenforschung in Mülheim an der Ruhr unter der Leitung

von Prof. Dr. Ferdi Schüth angefertigt.

Referent: Prof. Dr. Ferdi Schüth

Koreferent: Prof. Dr. Roland A. Fischer

Das ist leider geil – muss ich sagen!

Strietzel, 1990

I

ACKNOWLEDGEMENTS

Up to now I have been very lucky and always been supported, developed and

promoted by senior persons who were wiser and more experienced than I. These

acknowledgements are for these mentors, friends and persons who influenced me and

this thesis.

Vielen Dank an meine Eltern Bernd und Friedlinde sowie meine Geschwister Conny

und Matze für die Unterstützung von klein auf. Danke Mama und Papa, dass ihr mich

auch unterstützt, wenn ihr selbst eine andere Entscheidung treffen würdet, wie zum

Beispiel mit der Chemie, die aber nicht so gefährlich ist wie ihr denkt.

Vielen Dank an meine Freunde aus Schule und Sport, insbesondere an Cle, Lolle, Juli,

Schimpanze Jonas und eure Familien, für die gemeinsame Zeit und die damit

verbundene Erholung von den Strapazen der Wissenschaft (wenn sie mal nicht so

will/wollte wie ich ).

Thanks to the TU München and its teaching staff for the excellent education.

Especially thanks to the groups of Ueli Heiz and Matthias Arenz. Thanks Ueli for

your versatile support during my studies, the master thesis, and the time after.

Thanks a lot Sebastian for getting me into AK Heiz and your scientific influence on

me during my master studies. Also, it has always been a pleasure to spent leisure with

you and to learn from you. The same goes to Tschurli, one of the persons why you

should love Austria.

Great influence for the choice of my PhD topic had Mirza Cokoja, as well as

Professor Herrmann and Professor Kühn. In their lectures I heard for the first time

from the Periana system which was also the topic of the oral examination. Also thanks

for your support in getting a stipend.

Thanks to my friend from university, Lennart, for the legendary days in Munic and

Mülheim, the good team work in our undergraduate lab courses, and the knowledge

that I can rely on you.

Danke Wolfgang und Uschi, meine garchinger “Adoptiveltern”, dass ihr euch immer

liebevoll um mich gekümmert habt und ich immer wusste, dass jemand da ist, den ich

um Hilfe fragen kann.

Coming closer to people who were directly involved in this thesis, I would like to

thank a long list of persons: Danke an alle, die in irgendeiner Form Verbindung zum

Max Planck Institut für Kohlenforschung haben. Insbesondere danke an diejenigen,

denen ich während meiner Zeit hier begegnet bin und die hier arbeiten.

I am especially grateful to my Doktorvater (supervisor) Ferdi for the scientific

freedom I had, the trust in me, and all the opportunities I was offered. I am glad that I

was allowed to do mistakes, and I always enjoyed your stories. To make a long story

short: I always had the feeling that I could come with any problem to you.

Thanks to the examination board, especially to Professor Roland Fischer, for agreeing

on being my second examinator.

Another important person of my methane oxidation adventure is Mario: I think Team

Methan is just awesome! Also thanks for the friendship and the good evenings we

had. In this context I also have to mention Kommune MH and Caro Columbina: If I

will ever share a flat it will be with you (or Schimpanze).

Thanks Teo that I could learn from you in the acetylene project. It is good to know

that I have friends like you, I can rely on.

Thank you, Wolfgang and Claudia that you are there in case something goes terribly

wrong, your reassuring advice, and the nice evenings.

II

Danke an meine Mädels Kirsten, Annette und Sarah: Mütterliche Fürsorge und etwas

Gegacker ist durch nichts zu ersetzen!

Thanks to all group members for the pleasant time. Also thanks to the former group

members who gave me a warm welcome, wherever I met them (Joker you are the

kindest person on earth). It is nice to be part of the big Schüth family. I always felt at

home and the right place!

Thanks to JP, Tim, Stefano, Felix, and Andre for going climbing, sailing, running, and

playing squash with me, and to Niki for the hard party experiences.

Thanks Gonzalo that you are never tired of answering my questions! It was a pleasure

to meet such an excellent scientist.

Thanks to my office mates Jorro aka JoJo der Reifendealer, ManoGangsta, Ivy, Dr.

Pille, Julia, Hequing, boss of the entire agent Wang clan, and Nico, an expert on

pseudo plastic extrusion.

Vielen Dank allen Technikern und Mitarbeitern der analytischen Abteilungen für alle

Hilfestellungen: Danke Laila, Udo, Klaus, Marjan, Uli, Sebastian! Danke Heike,

Alfred und Marie! Danke Christophé! Danke Hans-Josef Bongard und Bernd

Spliethoff!

Danke an das Drucktechnikum und seine komplette Crew, insbesondere Lars!

Vielen Dank an IT und Verwaltung und die gute Seele des Instituts Madalena.

Ein ganz spezielles Dankeschön geht an alle aus Elektrik, Feinmechanik, Schreinerei,

Schlosserei, Hausdienst,…! Nicht nur weil ich ohne Wolfgang, Knut, Dirk, Jürgen,

Sebastian und Ralf mein Thema wohl nach einem Monat wegen Korrosionsproblemen

aufgeben hätte müssen, sondern auch weil es ohne euch nur halb so lustig gewesen

wäre. Danke für die super Zeit beim Mopped fahren, Paintball spielen, Fußball

schauen, Feiern oder auch einfach nur für ein paar aufmunternde Worte zwischen

durch.

Rallye- und Rennlegende Sebastian P. muss ich an dieser Stelle nochmal separat

erwähnen: Es war mir eine Freude mit dir zu fahren!

I would also like to thank some institutions: Thanks to the Fonds der Chemischen

Industrie for granting me a Kekulé stipend and supporting my scientific curiosity.

Thanks to the Ruhr Universität Bochum and all chairs of the Lehrverbund Katalyse

for making the Ruhr area a good place for catalysis.

Many thanks to the Max Planck Gesellschaft and the Max Planck Institut für

Kohlenforschung, not only for funding but also for all the great experiences, like

participation in the Lindau Nobel Laureate Meeting 2013, I could make during my

PhD. Thanks to all the persons who contributed to the development of these

institutions, I personally owe a lot.

I also have to thank A. E. Shilov, who unfortunately died last year, for his research:

Spasibo dlya vashey sistemy! In the same way I have to thank Roy A. Periana for his

contributions to the field: Thanks for all the acid! Thanks to all other researchers in

the field, on their results I could build my thesis. Special thanks to Jay Labinger and

John Bercaw whose excellent reviews helped me a lot in improving my understanding

of this chemistry.

I hope that I did not forget anybody and finally would like to thank Porsche for

building the 911 and Eugene Houdry for his special inspiration.

III

ZUSAMMENFASSUNG

Diese Arbeit beschäftigt sich mit der selektiven Oxidation von Methan zu

Methylbisulfat, einem Methanolderivat, in konzentrierter und rauchender

Schwefelsäure mittels verschiedener Katalysatoren, größtenteils Platin-basierten.

Dieses System – gewöhnlich mit dem Namen Roy A. Periana und Catalytica Inc. in

Verbindung gebracht – ist aufgrund seiner hohen Selektivität ein vielversprechender

Kandidat für die direkte Umsetzung von Methan.

In vorhergehenden Arbeiten waren feste Katalysatoren für diese Reaktion entwickelt

worden, die auf Polyacrylnitril-basierten stickstoffhaltigen Kohlenstoffmaterialien

beruhten, die mit Kaliumtetrachloroplatinat modifiziert waren. Diese wurden

makroskopisch verändert, um in einem kontinuierlichen Reaktor verwendet werden zu

können. Darauf folgende Stabilitätstests zeigten Probleme in Bezug auf

Kohlenstoffkorrosion. Des Weiteren ließ die Verwendung von

Kaliumtetrachloroplatinat als Katalysator zur Ermittlung der Beiträge ausgelaugter

Spezies zur katalytischen Aktivität Fragen in Bezug auf das fundamentale Verständnis

des Systems aufkommen.

Daraufhin wurden die Grundlagen des Systems umfassend erforscht und der Einfluss

von Schwefeltrioxid, Katalysator und Additivkonzentration auf Aktivität, Stabilität

und Selektivität verschiedener molekularer Katalysatoren untersucht.

Weitere vorläufige Experimente zielten auf ein mechanistisches Verständnis ab und

deuteten darauf hin, dass Pt(IV) eine aktive Rolle in diesem System spielt. Zusätzlich

ergab ein Vergleich verschiedener katalytisch aktiver Elemente, dass platinbasierte

Katalysatoren zu den Besten gehören, Jod-basierte allerdings die

Vielversprechendsten sind.

Bemerkenswerterweise übertreffen optimierte Reaktionsbedingungen Vorgaben für

den Katalysator in Hinblick auf industrielle Anwendung. Die Aktivität, die bisher als

größte Hürde angesehen wurde, liegt im Bereich großer industrieller Prozesse, und

damit bleibt Produktabtrennung das letzte große Problem auf dem Weg zur

Aufskalierung.

Die Aktivitäten, die in dieser Thesis gezeigt werden, sind sehr wahrscheinlich die

höchsten, die jemals für die direkte Oxidation von Methan berichtet wurden, und

übertreffen die in bisherigen Veröffentlichungen beschriebenen um mehrere

Größenordnungen.

IV

ABSTRACT

This thesis deals with the selective oxidation of methane to methyl bisulfate, a

methanol derivative, in concentrated and fuming sulfuric acid with different catalysts,

mostly platinum-based. This system – usually related with the name of Roy A.

Periana and Catalytica Inc. – is, due to its high selectivity, a very promising candidate

in the field of direct conversion of methane.

Previously invented solid catalysts for this reaction, i.e., Polyacrylonitrile-based

nitrogen containing carbon materials, modified with potassium tetrachloroplatinate,

were macroscopically customized to be used in a continuous reactor. Subsequent

stability tests revealed general problems with carbon corrosion. Furthermore, use of

potassium tetrachloroplatinate as catalyst for evaluation of contributions from leached

species put question marks on the general understanding of the system.

Following these observations, a comprehensive study towards the fundamentals of the

system was undertaken, and the influence of sulfur trioxide, catalyst, and additive

concentration on activity, stability, and selectivity of various molecular catalysts was

investigated.

Further preliminary studies aimed at a mechanistic understanding. The results point at

Pt(IV) to have an active role in this system. Additionally, comparison of several

catalytically active elements identifies platinum-based catalysts amongst the best, but

iodine-based ones as most promising.

Notably, improved reaction conditions resulted in activities higher than described in

catalyst guidelines for viable industrial application. Activity, considered as the biggest

issue so far, was shown to be in the range of large scale industrial processes, leaving

product separation as the last big problem towards upscaling.

The activities presented in this thesis are, very likely, the highest ever reported for

direct oxidation of methane, and several orders of magnitude higher compared to

those described in previous reports.

V

ABBREVIATIONS

1 η2-(2,2´-bipyrimidyl)dichloroplatinum(II)

AMBN 2,2′-azobis(2-methylbutyronitrile)

AN acrylonitrile

ASU air separation unit

ATR autothermal reforming

bpym 2,2´-bipyrimidyl

(NH3)2PtCl2 cis-diamminedichloroplatinum(II)

C conversion

CTAB hexadecyltrimethylammonium bromide

CTF covalent triazine framework

DMSO-d6 deuterated dimethyl sulfoxide

EA elemental analysis

EDX energy dispersive X-ray analysis

e.u. Entropic units

HVC high value chemical

HPLC high performance liquid chromatography

IEA International Energy Agency

IR infrared spectroscopy

K quasi steady state constant of platinum methane σ-complex

kMBS rate constant of methyl bisulfate formation

kox rate constant of methyl bisulfate oxidation

Kp equilibrium constant for hydrolysis of methyl bisulfate to methanol

MBS methyl bisulfate

MSA methanesulfonic acid

MTA methanol to aromatics process

MTO methanol to olefins process

MTP methanol to propylene process

NGL natural gas liquid

NMR nuclear magnetic resonance spectroscopy

OCM oxidative coupling of methane

PAN polyacrylonitrile

VI

PAN-C polyacrylonitrile derived nitrogen containing carbon

PEG poly(ethylene glycol)

POX partial oxidation

Pt(acac)2 bis(acetylacetonato)platinum

(PtCH4) platinum methane σ-complex

Pt-CTF platinum(II) modified covalent triazine framework

Pt-PAN-C platinum(II) modified polyacrylonnitrile based nitrogen containing

carbon

rMBS rate of methyl bisulfate formation

S selectivity

SEM scanning electron microscopy

SMR steam reforming

STEM scanning transmission electron microscopy

STY space time yield

Syngas synthesis gas

TEM transmission electron microscopy

TEOS tetraethyl orthosilicate

TOF turnover frequency

TON turnover number

WSA water steam activation

Y yield

VII

CONTENT

1 Introduction ..................................................................................... 1

1.1 A changing raw material situation in organic chemical industry ........................1

1.2 Scope of this thesis...............................................................................................4

2 Conversion processes for methane ................................................ 6

2.1 Syngas production and methanol synthesis .........................................................6

2.1.1 Syngas production ....................................................................................... 6

2.1.2 Industrial methanol production ................................................................... 7

2.2 Direct conversion processes .................................................................................8

2.3 Selective oxidation based on C-H activation .....................................................12

2.3.1 Shilov system ............................................................................................ 13

2.3.2 Periana/Catalytica system ......................................................................... 14

2.3.2.1 General features .....................................................................................14

2.3.2.2 Early catalysts – Hg(II) ..........................................................................15

2.3.2.3 Pt(bpym)Cl2 - the Periana catalyst .........................................................16

2.3.2.4 Further catalysts and developments .......................................................18

2.3.2.5 Critical remarks on methane oxidation in sulfuric acid .........................19

2.4 Perspective for methane conversion ..................................................................22

3 Results and discussion .................................................................. 23

3.1 Reading advice ...................................................................................................23

3.2 Experimental issues and approach .....................................................................24

3.3 Thesis of Mario Soorholtz .................................................................................28

3.4 Solid catalysts ....................................................................................................31

3.4.1 Macroscopic modification ........................................................................ 31

3.4.1.1 Water steam activation ...........................................................................32

VIII

3.4.1.2 Hard templating .....................................................................................35

3.4.2 Continuous experiments and hot filtration ................................................ 37

3.4.3 Carbon corrosion ....................................................................................... 41

3.5 Molecular catalysts ............................................................................................46

3.5.1 Influence of sulfur trioxide concentration: 100-fold activity increase in

oleum......................................................................................................... 46

3.5.1.1 Periana catalyst ......................................................................................46

3.5.1.2 Potassium tetrachloroplatinate ...............................................................48

3.5.1.3 Pressure dependency ..............................................................................49

3.5.1.4 Catalyst stability.....................................................................................51

3.5.1.5 Methane oxidation with pure sulfur trioxide .........................................53

3.5.1.6 Discussion of sulfur trioxide concentration series .................................56

3.5.1.6.1 Differences between sulfuric acid and oleum – oxidation

potential 56

3.5.1.6.2 Decrease in activity above 40% oleum - solubility of methane . 58

3.5.1.6.3 Comparison of different catalyst precursors .............................. 61

3.5.1.6.4 Oxidant, oxidation state of Pt, and over-oxidation dilemma...... 64

3.5.1.6.5 NMR studies ............................................................................... 66

3.5.1.6.6 Mechanistic considerations ........................................................ 69

3.5.1.6.7 Associated issues ........................................................................ 72

3.5.2 Influence of catalyst concentration ........................................................... 74

3.5.2.1 Catalyst concentration series in 20% oleum ..........................................77

3.5.2.2 Convolution of catalyst concentration and methane

concentration – pre-equilibrium model .................................................79

3.5.2.3 Transformation and speciation of potassium

tetrachloroplatinate ................................................................................86

3.5.2.4 Reactions of Cl- in sulfuric acid and oleum ...........................................89

3.5.2.5 Additional observations with respect to solubility.................................89

3.5.3 Further catalysts and aspects ..................................................................... 90

3.5.3.1 Bis(acetylacetonato)platinum(II) ...........................................................90

3.5.3.2 Other catalytically active elements and poisoning .................................95

IX

3.5.4 Selectivity ................................................................................................. 98

3.5.4.1 Methyl bisulfate: Intermediate in a consecutive reaction ......................99

3.5.4.2 High activity leads to high selectivity ..................................................101

3.5.4.2.1 Selectivity of the catalyst concentration series ........................ 103

3.5.4.2.2 Selectivity of the sulfur trioxide concentration series .............. 108

3.5.4.3 Methyl bisulfate decomposition ...........................................................110

3.5.4.3.1 “Uncatalyzed” methyl bisulfate decomposition and other issues

112

4 Conclusion and Outlook ............................................................. 115

5 Experimental ............................................................................... 118

5.1 Catalytic testing ...............................................................................................118

5.1.1 Continuous methane oxidation ............................................................... 118

5.1.2 Autoclaves, materials, and equipment .................................................... 118

5.1.3 Hot filtration............................................................................................ 120

5.1.4 Standard experiment ............................................................................... 120

5.1.5 Methane oxidation with sulfur trioxide ................................................... 123

5.2 Error assessment ..............................................................................................123

5.3 Calculations......................................................................................................124

5.3.1 Determination of methane and carbon dioxide ....................................... 124

5.3.2 Turnover number, turnover frequency, conversion, and selectivity ....... 125

5.3.3 Extrapolation of the sulfur trioxide concentration series ........................ 125

5.3.4 Equations used in section 3.4.3 (selectivity) ........................................... 126

5.4 Chemicals .........................................................................................................127

5.5 Synthesis and handling ....................................................................................128

5.5.1 η2-(2,2´-bipyrimidyl)dichloroplatinum(II) [(bpym)PtCl2, 1] .................. 128

5.5.2 Synthesis of Polyacrylonitrile based carbons ......................................... 129

5.5.2.1 Water steam activation route ...............................................................129

5.5.2.2 Hard templating route ..........................................................................129

5.5.3 Sulfur trioxide ......................................................................................... 130

X

5.6 Instruments and analysis ..................................................................................130

5.6.1 Nitrogen physisorption............................................................................ 130

5.6.2 Elemental analysis .................................................................................. 130

5.6.3 Powder X-ray diffraction ........................................................................ 131

5.6.4 Transmission electron microscopy ......................................................... 131

5.6.5 Scanning electron microscopy ................................................................ 131

5.6.6 Liquid phase nuclear magnetic resonance .............................................. 131

5.6.7 High performance liquid chromatography .............................................. 132

5.6.8 Titration................................................................................................... 133

5.6.9 Infrared spectroscopy .............................................................................. 133

6 Curriculum vitae ......................................................................... 135

7 Appendix ...................................................................................... 138

8 References .................................................................................... 158

Introduction

1

1 Introduction

1.1 A changing raw material situation in organic

chemical industry

Currently, petrochemistry uses predominantly oil and natural gas as raw materials to

produce aromatics, lower olefins, and synthesis gas (Syngas), i.e., a mixture of

hydrogen, H2, and carbon oxides, COx, important building blocks for many base

chemicals, e.g., methanol. An important route for the production of olefins is steam

cracking where a feed of longer chain saturated hydrocarbons is thermally cracked in

the presence of steam into a stream, containing shorter chain unsaturated

hydrocarbons. Steam reforming (SMR) on the other hand is one method for the

production of Syngas where steam reacts with hydrocarbons on a solid catalyst to

form a mixture of H2 and CO. Although it is possible to run the same type of process

with different raw materials, natural gas, respectively its main component methane, is

mainly used for producing Syngas, whereas natural gas liquids (NGLs), i.e., a mixture

of ethane, propane, butane and pentane, and naphtha, a fraction of crude oil, is used

for steam cracking.1

However, the raw material basis for industrial organic chemistry has not always been

like that and this basis changed several times in history. At the beginning of organic

chemical industry, coal and tar distillation were the entry into organic synthesis until

acetylene chemistry, still based on coal, took over, after the turn of the last century.

With the end of the Second World War petroleum resources became cheap and

abundant, which paved the way for modern chemical industry, and olefins to replace

acetylene.1,2

Syngas based production of chemicals, e.g., methanol, was available

already at an earlier time in history, but usually based on coal gasification, and SMR

was introduced in 1930 in the US, and became important after 1962 with the

introduction of ICI´s tubular reforming under pressure.3

Selective oxidation of methane in sulfuric acid

2

Oil and gas majors4 and the International Energy Agency (IEA)

5 expect that even

though the share of renewable energy will continue to increase, fossil fuels will

remain the main source of energy until 2035 (figure 1a). More importantly, the share

of natural gas as primary energy source is anticipated to grow, in contrast to oil and

coal (figure 1b). Consumption in absolute numbers will increase, due to economic

growth and increasing population on earth.

Since the chemical industry is tied to the energy sector, which is by volume

significantly larger, and both use the same raw materials, organic chemical industry

will likely undergo another, albeit slower and smoother, transition of the basic

resources used for chemical production and follow the energy sector. As a result,

growing importance of natural gas and methane for production of chemicals can be

foreseen.

Figure 1: a): Energy production by source. b): Share of primary energy sources of total energy

consumption. Based on reference 4.

Indeed, exploitation of shale gas can be seen as one driver for the increasing

importance of natural gas, and first consequences of the shale gas boom in North

America on the chemical industry are already visible.6-8

Major chemical companies

announced or started to install new steam cracking capacities in the US to make use of

cheap NGLs produced by hydraulic fracturing of shale deposits.9,10

The increasing

share of NGLs as feed for steam crackers and the different product spectrum

compared to naphtha feeds will, as a consequence, lead to less production of

propylene by steam crackers. Thus, additional capacity for “on-purpose” propylene

production, e.g., by the methanol-to-propylene (MTP) process, which was already

tested in a demonstration plant in the 1980´s, could be needed and also be installed.11

a b

Introduction

3

This means a change in the raw material situation can boost commercialized

processesa.

Also the availability of a cheap feedstock, as it is currently the case for natural gas in

the US, might lead to commercialization of new processes. The 2008 founded

company Siluria Technologies started up a demonstration plant for production of

ethylene by oxidative coupling of methane (OCM) earlier this year, the first plant of

its kind in history.12-15

Even if the scenario expected by the IEA and others does not come true, and a

groundbreaking discovery makes hydrogen available from renewable resources,

methane and methanol will remain important for industrial organic chemistry, as they

represent versatile C1 building blocks.

The roles of methane and methanol in a changing raw material situation are

summarized and supplemented in the following three statements11

:

- Production and importance of natural gas, respectively methane, will

continuously increase over the next decades.

- Straightforward manufacture of high value chemicals (HVCs) from methane is

a topic of great interest and importance.

- Methanol is an important platform molecule which can be used as a fuel, a

source for producing oxygenates, e.g., formaldehyde, or a source for olefins

and aromatics, via MTP/MTO and methanol-to-aromatics (MTA).

Consequently, aside from improvements in the established Syngas based multi-step

processes, alternative and ideally direct ways to convert methane into HVCs should

be established.16

A direct route with less process steps comprises one possibility to

increase efficiency and consume less energy and raw material, a major goal in

chemical industry.

Any selective oxidation of methane into oxygenates is such an alternative, and the

investigation of such a reaction, i.e., the selective oxidation of methane to methyl

bisulfate (MBS), a methanol derivative, in concentrated/fuming sulfuric acid is the

scope of this thesis.

aThe first world scale MTP complex started operation in 2010 in China. It is, however, based on coal

gasification and the drivers are different in this case, i.e., not cheap shale gas and a shift in the

ethylene/propylene production ratio but cheap coal and vital economic growth. Thus, China is building

capacity for MTP as well as methanol-to-olefin (MTO) to produce ethylene.


4

1.2 Scope of this thesis

As is evident from the previous section, the selective oxidation of methane,

respectively any saturated hydrocarbon,17,18

is an obvious dream reaction for

chemists.19

Compared to Syngas such a type of reaction has the inherent problem that

the desired HVC is an intermediate in a complex reaction network, and consecutive

reactions take place, leading to undesired total oxidation to carbon dioxide. This issue

can be overcome by several tricks, each of them having usually also a drawback. One

of these tricks is protection of the intermediate, the desired HVC, with a protecting

group. This concept has been used in the selective oxidation of methane to methanol

in solution, based on C-H activation with metal complexes.

Methane oxidation with metal complexes has originally been achieved with the so-

called Shilov system which operates in aqueous hydrochloric acid and leads directly

to methanol.20-22

It was shown that methane and methanol have approximately the

same intrinsic reactivity towards the catalyst23-25

which is for direct methane

conversion already quite good.

Subsequent improvements in selectivity were achieved by the group of Roy A.

Periana which transferred this chemistry into concentrated/fuming sulfuric acid

thereby giving not directly methanol, but methyl bisulfate which was estimated to

have - compared to methane - approximately 100 times lower reactivity to the

catalyst. This system was actually developed at a company, Catalytica Inc., and is

known as the Catalytica or Periana system.26-30

However, one drawback is the molecular catalyst used, posing problems on separation

and recycling of the same. This issue had been approached in previous studies by the

development of a solid catalyst, mimicking the molecular one, in our group at the

Max Planck Institut für Kohlenforschung in Mülheim an der Ruhr, Germany.31

This

first generation solid catalyst was established to be indeed a solid counterpart of the

molecular one.32

Second33

and third generation solid catalysts were developed mainly

by my predecessor Mario Soorholtz.32

The original proposal for my thesis was to modify the third generation solid catalyst

that it could be used in a continuous way, in contrast to its development in batch

reactors. However, first experiments on the stability of a modified third generation

Introduction

5

solid catalyst resulted in questions regarding the fundamental understanding of the

system, notably relevant for both molecular and solid catalysts.

Therefore, the focus of my thesis was changed towards the fundamental

understanding of the molecular system covering the three aspects, relevant to describe

a catalyst, i.e., activity, stability and selectivity.34

Investigation of these aspects

comprises the main part of the thesis.

For the successful implementation of a catalyst all three parameters need to be

addressed and balanced, which is not always simple, due to their interplay. As an

example, OCM has been known since 198235

but rapid deactivation of the catalysts

due to the high temperatures has been a showstopper for this reaction although

activity and selectivity were acceptable. At the heart of Siluria´s OCM technology,

now tested in a demonstration plant, is a dedicated catalyst which tackles the problem

of high temperatures and rapid deactivation. In this case a more active catalyst allows

for lower operational temperatures, and consequently longer catalyst lifetime, and

most likely also higher selectivity are achieved.12-15

It should be noted that Mario Soorholtz and I worked more than one year together in

the laboratory on this project and part of the developments are a result of this team

effort. Consequently, there are aspects in our theses which overlap. Also, part of this

thesis is in preparation for publication and word-for-word matching with the

manuscripts exists.

The thesis is structured in the following way: After a brief overview about industrial

production of methanol and comparison with possible direct conversion routes for

methane, including C-H activation and the Periana system, the main difficulties with

respect to experiments are given, as well as, a short summary of Mario Soorholtz´s

thesis, 32

. Then the macroscopic modification of the third generation solid catalyst is

discussed, together with stability tests which lead to questions on the system in

general. These questions are answered in the next section, discussing the influence of

sulfur trioxide concentration and catalyst concentration on activity, stability, and

selectivity. This part is mainly in preparation for publication, but has been

complemented by additional results. The summary and outlook section comments

critically on the achievements made, evaluates the Periana system, or in a broader

sense the selective oxidation of methane to methyl bisulfate in concentrated/fuming

sulfuric acid, and guides the work for further improvement.


6

2 Conversion processes for methane

Around 10% of the produced methane is used for production of chemicals. More than

90% of this fraction is utilized via intermediate production of Syngas, and other direct

uses, e.g., production of halogenated methanes, prussic acid, acetylene and carbon

disulfide, represent minor applications. Syngas itself is the most important way for

production of hydrogen, which is used for synthesis of ammonia (51%), hydrotreating

and hydrochracking in refineries (35%), methanol production (8%), and Oxo-

synthesis.11

2.1 Syngas production and methanol synthesis

2.1.1 Syngas production

Requirements on composition and purity of Syngas differ widely, and depend on its

use. Specifications can be met by choice of feed (natural gas, naphtha, coal, etc.), type

of process (SMR, autothermal reforming, and partial oxidation), and post treatment

(water gas shift reaction). Processes for Syngas production have distinct advantages

and disadvantages, which can serve as a framework for the requirements, that direct

conversion processes should meet.

One big advantage of Syngas is obviously that it does not suffer from consecutive or

side reactions (carbon formation can be excluded if the right conditions are chosen),

and thermodynamic equilibrium controls the reaction, resulting in high selectivity.

Many direct processes instead are rather kinetically controlled.

Nevertheless, the SMR reaction (1) is endothermic, catalyzed by nickel supported on

oxides, and requires temperatures between 800 and 950 °C at 20 bar. This is achieved

by external heat supply with a considerable amount of heat-exchange operations.11

𝐶𝐻4 + 𝐻2𝑂 → 𝐶𝑂 + 3𝐻2 𝛥𝐻 > 0 (1)

In contrast, autothermal reforming (ATR) and partial oxidation (POX), irrespective if

catalytic or non-catalytic, are exothermic and combust a part of the hydrocarbon

Conversion processes for methane

7

source inside the reactor, reaction (2), respectively oxidize the hydrocarbon partially,

reaction (3). Actually, ATR shows high selectivity towards reaction (3) and thus

represents in a sense such a dream reaction, mentioned in chapter 1.2.11

𝐶𝐻4 + 2𝑂2 → 𝐶𝑂2 + 2𝐻2𝑂 𝛥𝐻 < 0 (2)

𝐶𝐻4 +1

2𝑂2 → 𝐶𝑂 + 2𝐻2 𝛥𝐻 < 0 (3)

ATR and POX do not need external heat supply, but generally pure oxygen is

required instead, usually produced by energy intensive cryogenic distillation of air in

an air separation unit (ASU). If air were used instead of oxygen, too much inert gas

would be carried through the whole process, lowering the volumetric productivity and

overall efficiency. The choice of the process depends much on factors like available

feedstock and size of the plant, but in general big plants prefer ATR and small ones

SMR.11

Consequently, in order to improve efficiency of methane conversion any direct

conversion process should ideally be (mildly) exothermic and not require energy

intensive separation or recycling steps, i.e., utilize at best air as oxidant. SMR loses

efficiency as (1) is endothermic and part of the feedstock has to be burned externally

for heating purposes. ATR loses efficiency by energy intensive cryogenic distillation

of air.11

2.1.2 Industrial methanol production

Production of methanol is rapidly growing with an annual output of 34.5 106 tons in

2005, more than 40 106 tons in 2009 and around 60 10

6 tons in 2012. Annual growth

rates of 10 to 20% are expected, mainly due to increasing production in China.11

Usually, synthesis proceeds at 220 to 280 °C and 50 to 100 bar over a copper zinc

oxide catalyst which is structurally promoted by alumina. The reaction is exothermic

and limited by thermodynamic equilibrium under these conditions which makes

recycling of unconverted Syngas necessary. The mechanism is still under debate,

however, carbon dioxide in the feed is needed for high activity and the stoichiometric

number, i.e., the ratio of hydrogen minus carbon dioxide to the sum of carbon oxides,

is slightly above 2.11

Selectivity for methanol formation is very high, i.e., greater than 99%. However, in

the overall process efficiency is lost during Syngas formation and carbon yield for


8

methanol production from natural gas lies in a range of 70 to 80%a.36

Another

important benchmark is the space-time-yield (STY) as a measure of productivity

which lies for commercial processes around 1 t m-3

h-1

.

A less superficial overview on Syngas and methanol production can be found in the

literature.11

2.2 Direct conversion processes

Given the maturity of Syngas based indirect routes, direct ones hardly seem to be able

to compete,37

and the selective and direct conversion of methane to HVCs is one of

the most challenging problems in chemistry (with respect to both economy and

chemistry) and often designated as “a holy grail of chemistry”.16,38-40

Part of the

chemical challenge lies in the high standard enthalpy for homolytic bond dissociation

of the methane C-H bond, approximately 439 kJ mol-1

, and the fact that the enthalpies

of functionalized molecules like methanol are lower, 402 for the C-H and 385 kJ mol-

1 for the C-O bond

b.41

Accordingly, various approaches and numerous attempts from several branches of

chemical science exist, and the overview given here is neither exhaustive nor are the

examples comprehensively described.

One rare example of an endothermic reaction used for the direct conversion of

methane is the aromatization of methane with a MoC2/H-ZSM-5 catalyst to benzene

and hydrogen.42,43

The reaction proceeds at temperatures of 700 °C at moderate

pressure and equilibrium conversion of 12% can be obtained. Recently, also the

conversion of methane to ethylene, hydrogen and aromatics was reported over iron

exchanged zeolite at 1090 °C.44

These approaches have similar to SMR the drawback

that external heat supply is required which decreases the overall carbon yield, but

aExact values are difficult to find and vary. In case of MTO process with SAPO-34 an overall carbon

yield to HVCs of 62% is reported along with 10% coke formation. Thus, the carbon yield for Syngas

production and methanol formation is in the given range. bThis is oversimplified since H2 is readily activated although the bond dissociation energy of

436 kJ mol-1

is similar to methane. Also symmetry and position of the molecular orbitals contributing

to bond energy have to be considered.


9

aromatics represent interesting products, usually not formed with other direct

conversion processesa.

OCM has already been mentioned and designates the reaction of methane, at or above

atmospheric pressure, forming methyl radicals on the surface of a solid catalyst,

mostly different classes of oxide materials. The radicals desorb and couple in the gas

phase to first yield ethane, which undergoes subsequent oxidative dehydrogenation to

ethylene.35,45,46

Undesired side reactions include the reaction of methyl radicals with

dioxygen leading to carbon oxides. The reaction is exothermic and the abstracted

hydrogen atoms are oxidized to water. The high reaction temperature (above 800 °C;

claimed to be lower in the Siluria process) and the associated problem with catalyst

lifetime have already been mentioned and it remains to be answered if Siluria will

succeed.12-15,47

In any case, catalysts are known which exhibit at 30% conversion

selectivity towards C2 hydrocarbons greater than 70%, corresponding to a carbon

yield that is in the range of the benchmark.

Methane was also shown to be oxidized with oxide materials to methanol and

formaldehyde by dioxygen at temperatures ranging from 400 to 800 °C. However, at

pressures higher than 10 bar, radical side reactions prevail, and the reactor material is

catalytically active. At atmospheric pressure systematic study of catalytic materials

was possible. However, in both cases selectivity towards methanol or formaldehyde is

rarely above 50%, even though conversion is usually kept below 10%.48

Thus, the low

yields make this approach seemingly unattractive. Improvements were made by using

oxidants other than O2, especially nitrous oxide,49

but the use of any expensive

oxidant, i.e., other than air or pure oxygen, even if recycled, contributes to process

costs and allows their use only for the production of intermediates, e.g.,

cyclododecanone,50

and not base chemicals. It still shows another issue for oxidative

approaches, i.e., activation of dioxygen. Dioxygen is a diradical in a triplet state and

direct insertion into C-H bonds is spin-forbidden. Instead radical pathways with

hydrogen abstraction prevail.22,51

Consequently, several approaches in the field of direct methane conversion make use

of transfer oxidants.

aBased on thermodynamic considerations aromatization could be more carbon efficient than SMR.

Based on standard heat of formation SMR could reach a theoretical carbon yield of roughly 80%

whereas aromatization could yield 90%.


10

Catalytic chlorination of methane with subsequent hydrolysis of the methyl chloride

to give methanol and HCl was suggested by George A. Olah.52,53

Amongst others,

platinum nanoparticles on alumina can be used as catalyst at temperatures from 180 to

250 °C and atmospheric pressure with 99% selectivity to methyl chloride at 36%

conversion, and in contrast to the commercial production of chloromethanes this

process does not involve radicals. Hydrolysis requires higher temperatures, and

besides methanol also dimethylether is formed. These values look promising, but the

massive corrosion encountered with Cl2/HCl and little experience with HCl oxidation

on industrial scale (Deacon process) questions the viability of such a technology.54

Current research focuses on Br2 as transfer oxidant55

and Gas Reaction Technologies

Inc., founded in 1999, tried to commercialize a methane to gasoline process based on

this chemistry together with Marathon Oil Corporation.56

No brand-new

announcements appeared recently, and the technology was sold to Reaction35 LLC

where the focus of this technology lies on converting NGLs to lower olefins and

aromatics.57

It is thus questionable if this technology will be used for converting

methane, but the technology uses a closed cycle, has a claimed carbon efficiency of

70%, and has been tested in a demonstration plant.56-60

Other examples with alternative oxidants utilize H2O2 and often precious metal based

catalysts.61-65

Reaction temperatures are usually below 100 °C, pressure ranges from

30 to 50 bar, and aqueous media are utilized. H2O2 is either supplied directly or

generated in situ by direct synthesis from H2 (which might be generated by water gas

shift reaction from CO and H2O) and O2. These systems are obviously quite selective

towards C1-oxygenates but due to the expensive oxidant (and non-productive

decomposition of it) not expected to reach commercialization. Higher temperatures

would also be desired in order to make use of the heat released by the exothermic

reaction.

In this context also carboxylation under oxidative conditions and oxidative

carbonylation of methane to acetic acid should be briefly mentioneda. This chemistry

was strongly influenced by the group of Fujiwara and is often catalyzed by

Pd(OAc)2/Cu(OAc)2 or VO(acac)2 with K2S2O8 or O2 as oxidant in trifluoro acetic

acid.66-69

Depending on the catalysts the reaction might proceed via electrophilic or


11

radical mechanisms.68

In case of carbon dioxide as a carbon source70

this chemistry is

for good reasons debated since the reaction is thermodynamically restricted.71,72

Thus

oxidative conditions are necessary, and several other pathways for formation of acetic

acid have been proposed.73

Some of the examples in aqueous phase are related, or can actually be seen as

biomimetic approaches, mimicking enzymes, called methane monooxygenases.

Structural elucidation of these enzymes, and mimicking of their properties is a wide

field where either only the reaction cascades are mimicked, as just exemplified for the

H2O2 mediated reaction, or the structure as well, either with metal complexes, often

porphyrine complexes,74

or solid catalysts, mostly ion exchanged zeolites.75-77

However, a big problem of truly biomimetic systems is to generate closed cycles, and

if this is realized, to achieve reasonable activity. Thus, such approaches might be seen

as least developed and far from practical application.

With the exception of aromatization and catalytic halogenation, radicals, or at least

peroxo species which readily form radicals, are involved in the direct conversion

reactions discussed so far. Since radicals are reactive intermediates, they can lead to

undesired side reactions, as in case of OCM where radicals do not only react to ethane

but also carbon oxides, or to formic acid instead of methanol in H2O2 mediated

oxidations of methane. Consequently, non-radical conversion should have an intrinsic

advantage.78,79

C-H activation with metal complexes is another way of activating methane without

formation of radicals and part of the catalytic transformation investigated in this

thesis. Some basics of C-H activation and subsequent functionalization of alkanes are

given in the following. It should be stated, however, that a clear distinction can be

difficult, and activation of alkanes via metalloradical pathways is sometimes included

in this field.

aThe authors named the formation of acetic acid from methane carboxylation, irrespective if CO or CO2

is the source of the second carbon atom. For the sake of clarity I prefer the nomenclature oxidative

carbonylation and carboxylation under oxidative conditions.


12

2.3 Selective oxidation based on C-H activation

C-H activation, as it is used here, describes the insertion of a metal complex into a

carbon hydrogen bond of an alkane, forming a metal carbon bond. One of the early

discoveries of C-H activation at saturated hydrocarbons is Shilov chemistry, and this

is also one of the few examples which leads to an actual functionalization of the C-H

bond. Over the course of time, more and more examples of C-H activation have been

found, and understanding of the underlying molecular mechanisms continuously

improved. In general, C-H activation is facile, and can proceed even below room

temperature, but conditions required for actual functionalization, e.g., the presence of

oxidants or water, are incompatible with many organometallic complexes, and

examples of functionalization are relatively rare.80

The two prominent examples, where functionalization occurs, are the Shilov20

and the

Periana system.29

Both are believed to operate by electrophilic C-H activation, a term

which has mechanistic implications, i.e., release of a proton during C-H activation

which is accepted by a base, and usually observed with late transition metals in protic

media. Scheme 1 shows a generally accepted mechanism, which is a sequence of four

steps, i.e., coordination of methane leading to a σ-complex, breaking of the C-H bond

(sometimes σ-complex formation and breaking of the C-H bond are taken together as

C-H activation), oxidation and functionalization.23

Further mechanistic details are

comprehensively reviewed and not further discussed here.80-82

Scheme 1: Mechanism of Pt(II) catalyzed methane functionalization by electrophilic C-H activation.


13

2.3.1 Shilov system

The Shilov system was reported in 1972 and uses a mixture of Pt(II) and Pt(IV)

chloro complexes in aqueous hydrochloric acid at around 120 °C and up to 100 bar

methane, to oxidize amongst other substrates methane to methanol and methyl

chloride. Pt(II) serves thereby as catalyst whereas Pt(IV) is a stoichiometric oxidant

and not capable of C-H activation. The reaction is autocatalytic since Pt(II) is

generated by reduction of Pt(IV). Recycling of the, if stoichiometrically used,

unsuitable oxidant Pt(IV) in a Wacker oxidation like fashion showed limited success

and subsequent work aimed at fundamental understanding of the reaction.20-23

It was found that the platinum chloro species undergo speciation in solution with

changing population of different chloro complexes, depending on chloride

concentration. Most active in terms of both, H/D exchange and oxidation, is

PtCl2(H2O)2. Furthermore, the substrate scope was extended and chemoselectivity

found to favor primary over secondary and secondary over tertiary C-H bonds.23

Since features of H/D exchange by Pt(II) and functionalization in the Shilov system

are similar with respect to rate and selectivity, C-H activation (actually methane

coordination) is accepted to be the rate determining step. Only under special

conditions, i.e., low oxidant concentration and temperatures above 100 °C in acetic

acid, oxidation was found limiting.23

With respect to practical application, the use of Pt(IV) as oxidant and only few

turnovers of the catalyst due to deactivation by formation of metallic platinum,

presumably as a result of Pt(II)-CH3 oxidation by inorganic Pt(II) are clear

showstoppers. Intrinsic selectivity of the catalyst towards unfunctionalized and

functionalized C-H bonds seems to be in an acceptable range of around 3:2. Clearly,

this would not allow full conversion in one pass, but operation at 30% conversion

would still result in high selectivitya.21

Productivity in the range of 0.2 mol L-1

h-1

with a starting concentration of 15 mM

potassium tetrachloroplatinate and 50% conversion of Pt(IV) is too low, but this rate

was measured at 140 °C and rates at higher temperatures have not been reported to

my knowledge. Extrapolation to a useful rate of 20 mol L-1

h-1

would require a

aBased on figure 2 in reference 21, and assuming that overoxidation to CO2 would also require Pt(IV),

it can be concluded that at 50% oxidant conversion selectivity to methanol and methyl chloride is close

to 100% and still around 80% at full oxidant conversion.


14

temperature of approximately 220 °C (with the given EA,app of 108.9 kJ mol-1

and

0.02 mol L-1

h-1

at 120 °C).21

A comprehensive overview and mechanistic details are given elsewhere.23

2.3.2 Periana/Catalytica system

In the 80´s and 90´s modifications of Shilov chemistry with different solvents and

catalysts appeared. One important modification is the work of Sen´s group which used

Pd(CH3COO)2 for stoichiometric oxidation of methane in trifluoroacetic acid to the

corresponding methyl ester.83

Subsequent work realized a catalytic transformation

with Pd(CF3COO)2 as catalyst and H2O2 as oxidant, and mentions that methanol is

more easily oxidized than the ester.84

Around the same time Moiseev´s group reported

the catalytic oxidation of methane with Co(CF3COO)3 and air in trifluoroacetic acid

and showed methyl trifluoroacetate to be stable under typical reaction conditions.85

Although it is not clear if these examples proceed by electrophilic C-H activation

(involvement of peroxo compounds!) and inefficient catalysis appears to be a bigger

issue than selectivity in the Shilov system, the concept of esterification was picked up

and extended to methane oxidation in concentrated sulfuric acid/oleum and coined

product protection by Roy A. Periana and Henry Taube at Catalytica Inc. A truly

noteworthy retro perspective of the considerations and activities leading to the

development of methane oxidation in sulfuric acid, respectively the Periana system, is

available.86

2.3.2.1 General features

The concept of the Periana system consists of three reactions, (4) to (6).27,29

Reaction

(4) is the oxidation of methane to methyl bisulfate with simultaneous reduction of

sulfur trioxide to sulfur dioxide and formation of water. Reaction (5) is the hydrolysis

of the ester, leading to methanol and regeneration of sulfuric acid. Reaction (6) is

reoxidation of sulfur dioxide which is on paper the same reaction used for large scale

production of sulfuric acid. Combination of equations (4) to (6) basically result in

equation (7): Oxidation of methane to methanol with oxygen, the desired dream

reaction. Since reactions (5) and (6) are necessary to obtain methanol it can be

questioned if this is a direct conversion at all, but increase in the overall carbon yield

could still be possible.


15

In any case, only reaction (4) is investigated in this thesis!

𝐶𝐻4 + 𝑆𝑂3 + 𝐻2𝑆𝑂4 → 𝐶𝐻3𝑂𝑆𝑂3𝐻 + 𝑆𝑂2 + 𝐻2𝑂 (4)

𝐶𝐻3𝑂𝑆𝑂3𝐻 + 𝐻2𝑂 → 𝐶𝐻3𝑂𝐻 + 𝐻2𝑆𝑂4 (5)

𝑆𝑂2 +1

2𝑂2 → 𝑆𝑂3 (6)

𝐶𝐻4 +1

2𝑂2 → 𝐶𝐻3𝑂𝐻 (7)

Sulfuric acid has the following four functions:

- Solvent, thus the catalyst should show good solubility in the medium

- Protective group, as mentioned in the historical review

- Reactant, i.e., oxidant, thus should be considered as such

- Cocatalyst, as discussed below for Pt(bpym)Cl2

2.3.2.2 Early catalysts – Hg(II)

The first academic report and patent about this system mention several cations which

can be used to oxidize methane to methyl bisulfate, sometimes catalytically. Usually

these cations are soft electrophiles and comprise, e.g., Tl(III), Pd(II), Pt(II), Au(III)

and Hg(II). Initial focus was on Hg(II). At 180 °C 50% methane (at 34.5 bar) is

converted within 3 h and 85% selectivity (43% yield) in 100% sulfuric acid to methyl

bisulfate with 0.1 M Hg(OSO3H)2 as catalyst (gas/liquid ratio 2.33:1). This

corresponds to volumetric productivities of 0.4 mol L-1

h-1

and a turnover frequency

(TOF) of 3.6 h-1

.26,27

There had already been indications that mercury salts in combination with sulfur

trioxide as oxidant might catalyze the selective transformation of methane, but the

previous example was a rather undefined process with pure, gaseous sulfur trioxide,

and the subsequent work of Periana´s group was clearly a milestone.87,88

Remarkably, the intermediate CH3HgOSO3H was identifieda, and based on

independent synthesis and treatment of this compound a mechanism was proposed,

consisting of electrophilic, meaning heterolytic cleavage of the C-H bond, methane

activation by mercuric bisulfate and formation of methyl mercuric bisulfate,

aTo my knowledge this is the only intermediate in the field of methane conversion in sulfuric acid

which has been undisputed spectroscopically identified!


16

functionalization with sulfuric acid to give methyl bisulfate and mercurous bisulfate,

which is finally reoxidized to mercuric bisulfate.

Different mercury compounds were subsequently studied as catalyst by other groups,

but without significant improvement towards a viable process.25,89,90

2.3.2.3 Pt(bpym)Cl2 - the Periana catalyst

Although platinum salts were found to be active they were described as “poor (…)

because of the irreversible bulk-metal formation”a, and in the search for more efficient

catalysts focus was put on development of ligands that stabilize cationic platinum

species in sulfuric acid.29

This search resulted in the complex η2-(2,2´-

bipyrimidyl)dichloroplatinum(II) ((bpym)PtCl2, 1, scheme 2) which is often named

Periana/Catalytica catalyst, respectively the

combination with concentrated sulfuric acid or oleum

Periana/Catalytica system.

At 220 °C 90% of methane (at 34 bar) is converted

within 2.5 h and 81% selectivity (73% yield) in 102%

sulfuric acid to methyl bisulfate with 50 mM 1 as

catalyst (gas/liquid ratio 2.75:1). This corresponds to

volumetric productivities of 0.5 mol L-1

h-1

and a TOF

of 10 h-1

. In separate experiments volumetric

productivities of 3.6 mol L-1

h-1

and a TOF of 36 h-1

were found, and the final methyl bisulfate

concentration was 1 mol L-1

. Based on these values 1

was claimed to be more effective than mercuric salts.29

It is important to note that 1 is often designated as a highly active catalyst. However,

too low activity is one of the biggest issues for this catalyst, respectively, the system.

For industrial application, activity should be two to three orders of magnitude

higher.23,91,92

Indeed, Periana clearly points out that cis-diamminedichloroplatinum(II)

shows higher initial activity, but is transformed and precipitates with a half-life of

aIt has been found in this thesis that in case of K2PtCl4 precipitate is not bulk metal, but PtCl2.


17

15 min at 180 °Ca. The 2,2´-bipyrimidyl ligand leads to reduced activity, but keeps the

cationic platinum species solubilized.

Based on the fact that H/D exchange is observed at 150 °C in D2SO4 - not oleum -,

but no oxidation, it is accepted that the oxidation step is limiting above an acid

concentration of 85%.29

Subsequent reports concerning 1 and related complexes with nitrogen containing

ligands were often computational studies and aimed at mechanistic understanding.

Presumably due to the special solvent, these studies often lead to conflicting

conclusions.93-100

Without being exhaustive, the cocatalytic effect of the acid is briefly surveyed as an

example. This effect was conceptually developed for C-H activation, based on the

finding that Pt(II) catalyzed H/D exchange below acid concentrations of 80% sulfuric

acid essentially does not proceed. However, it increases continuously, if the

concentration of the acid is increased. In 96% sulfuric acid H/D exchange is around

1000 times faster compared to 80% sulfuric acid.

In analogy to alkane dehydrogenation systems, ground state stabilization was given as

explanation.91,92

Ground state stabilization describes the coordination of ligands,

stronger than the substrate, to the metal center, which inhibits the reaction. If acidity

drops in sulfuric acid, nucleophiles, e.g., methanol or water, are less and less

protonated and might coordinate to the metal center. This was substantiated by

computational methods and sounds generally reasonable, but is without clear proof,

and it has to be considered that the Shilov system operates in aqueous solution and

does obviously not face serious problems with ground state stabilization (methane

coordination is limiting under common conditions but proceeds at measurable rates).

A later computational96

study finds methane to readily replace water from the

platinum center and suggests instead acid catalyzed ligand exchange as reason, i.e.,

protonation of the leaving bisulfate ligand accelerates coordination of methane.

Unambiguous proof for this hypothesis is still missing.

Recently a new mechanistic study appeared,101

refining the original mechanistic

proposal, and addressing another important issue in Pt(II) catalyzed methane

functionalization. Pt(IV) catalyzed C-H activation is rarely observed, and, if so, under

aInitial TOF of 3.6 h

-1 at 180 °C with cis-(NH3)2PtCl2 is reported without further information about the

used conditions.


18

special conditions, e.g., photochemical activation.80,100,102-104

Consequently, strongly

oxidizing conditions could lead to depletion of active Pt(II) and deactivation of the

whole system. The recent study concludes that this so-called over-oxidation

dilemma23,105

could be unproblematic, if a self-repair mechanism was in operation,

where Pt(IV) is the actual oxidant and regenerates Pt(II). Again, clear proof for that is

missing.

One relatively young direction in this field is the development of solid catalysts which

mimic 1.31

The development of solid catalysts for large volume chemicals is crucial,

since catalyst separation is more difficult with molecular ones. A proof of concept

was given with Pt-CTF, a Pt(II) modified covalent triazine framework (CTF), and

subsequent work revealed close similarities between 1 and Pt-CTF on the molecular

level.32

More solid analogues have been developed,33,106

and reference 33 has led to

important consequences for this thesis.

2.3.2.4 Further catalysts and developments

1 is perhaps the most investigated catalyst for reaction (4), and although platinum

salts were described as not promising, studies exist which make use of platinum salts,

sometimes even Pt(IV) compounds which should be even less promising in terms of

C-H activation.107,108

This is surprising, because PtCl4, which was used as catalyst, is

insoluble in sulfuric acid or oleum, but TON of approximately 800 was obtained. The

difference between studies undertaken by Periana and the just mentioned ones lies

mainly in the use of sulfuric acid or oleum.

The importance of this difference becomes evident in a study about a modified

system, i.e., the platinum catalyzed oxidative carbonylation of methane with carbon

monoxide to acetic acid.109

One series of experiments in this study was conducted

without carbon monoxide, i.e., under standard conditions to give methyl bisulfate, and

clearly shows that the amount of methyl bisulfate formed in oleum is around two

orders of magnitude higher. Additionally, this is one of the few studies where a direct

comparison between 1 and other platinum compounds was undertaken, clearly

showing 1 to be inferior to other platinum compounds.

Actually, oxidative condensation or carbonylation of methane in sulfuric acid to give

acetic acid is usually done with palladium catalysts and was first reported by Periana

with PdSO4.110,111

Additional carbon monoxide is not needed with palladium catalysts,


19

most likely due to formation of CO from methyl bisulfate. Again a clear influence of

sulfuric acid and oleum with respect to the formed product (methyl bisulfate or acetic

acid) as well as the amount of formed product is observed.112,113

The catalyst scope was further extended to Au(I) and Au(III) with Se(VI) as oxidant

in sulfuric acid,114

and iodine as well as iodine compounds in oleum with low sulfur

trioxide115

concentration, or high sulfur trioxide116,117

concentration. It was reported

that I2 in sulfuric acid is not active and oleum is required for the reaction.115

Furthermore, it is commented that differences in these publications with respect to

yield, reaction order, etc. are likely a result of the differences in the sulfur trioxide

concentration.116

Especially the activity in the study with 65% oleum is promising,

and the TOFs exceed for the first time in methane oxidation in sulfuric acid/oleum

clearly a value of 100 h-1

.

Summarizing selective oxidation of methane in sulfuric acid/oleum it is clear that

Periana not only invented this transformation but also contributed a lot to subsequent

developments. However, Periana usually used concentrated sulfuric acid as medium

whereas other groups more often used oleum.

Further developments targeted at ligand design to increase the rate of C-H activation,

sometimes in non-oxidizing strong acids, with other substrates, but limited

success.118,119

Two noteworthy directions are the use of ionic liquids to increase the solubility of

platinum compounds in sulfuric acid120

and methane oxidation based on C-H

activation with air as oxidant. Incorporation of air to obtain a closed cycle is

paramount for further progress in this field. Examples are the use of redox

couples,121,122

Wacker type systems,111

silica supported hybrid compounds consisting

of an analogue of 1 and polyoxometalate for activation of oxygen123

and a high-

throughput approach testing combinations of catalyst/ligand/cocatalyst

compositions.124

In some cases, methane oxidation might proceed by other pathways,

involving radicals or peroxo species.

2.3.2.5 Critical remarks on methane oxidation in sulfuric acid

Actually, a huge number of compounds has been used for oxyfunctionalization of

methane in sulfuric acid, more commonly in oleum, including V2O5,125

TiO2, Cr2O72-

,126

but also metal free compounds like K2S2O8127,128

and other peroxides129

or


20

combinations like urea/H2O2/RhCl3.130

This is only a small selection and especially

patent literature has many more versions of oxyfunctionalization in sulfuric

acid/oleum.26,28,30,131-133

The feature by which these systems can be categorized is if functionalization occurs

via pathways involving radicals or not. Systems based on a free radical mechanism

usually operate below 100 °C and form methanesulfonic acid. Although formation of

carbon dioxide is not considered in reports on these systems, selectivity seems to be

comparable to the systems described beforea. Nevertheless, the actual problem in

categorizing is that methane sulfonic acid is converted in oleum to methyl bisulfate

above 150 °C, even without catalyst.134

Since the systems, claimed to operate by C-H

activation, usually work above this temperature, it is not a priori clear, if activation

proceeds by a non-radical mechanism. It could be argued that this is not important, as

also the radical systems are selective, but if we want to understand the chemistry and

rationally design a system, this is an important question.

Pt(II), Pd(II), and iodine based systems are indeed very likely to operate by a non-

radical mechanism since methane sulfonic acid has never been observed as main

product with these catalysts.130

Other halogens and mercury, however, were found to

form methanesulfonic acid, depending on the conditions, as main product. It might

even be possible that Hg(II) initiates radical reactions as well as catalyzes methane

conversion by C-H activation.25,90,127

Aside from mechanistic ambiguity another problem is evident in the field of methane

oxidation via C-H activation in sulfuric acid/oleum. Comparison of the performance

of catalysts is sometimes based on results from different publications, obtained under

different conditions. 1, for example, has been claimed to be more effective than

Hg(II).29

This might indeed hold for selectivity if methyl bisulfate is the desired

product and not methane sulfonic acid, but not necessarily for activity. Comparing

TOFs reveals that Hg(II) exhibits a TOF of 3.6 h-1

, whereas 1 shows a value of 10 to

36 h-1

. The latter value is higher, but it was obtained at 220 °C, whereas the former at

180 °C. The same applies to conversion: 90% clearly looks better than 50%, but the

gas/liquid ratios, and the sulfur trioxide concentration are different. In case of Hg(II)

100% sulfuric acid was reported, while 102% sulfuric acid was used with 1.

aProductivity lies in a comparable range to Periana´s work but does not seem to be easily increased,

since decomposition of initiator is difficult to control at higher temperature.


21

This might look like almost identical but in fact the nomenclature for sulfuric acid and

oleum can be misleading by referring either to mol% or wt%. To avoid any

misunderstanding only wt% is used in this thesis and corresponds in case of oleum

(where the molar ratio of sulfur trioxide to water exceeds 1, i.e., sulfur trioxide is

present in other forms than sulfuric acid, H2SO4, e.g., as H2S2O7) to the wt% of excess

sulfur trioxide present in the sulfur trioxide/sulfuric acid mixture and in case of

sulfuric acid to the wt% of H2SO4 present in the sulfuric acid/water mixture. With

respect to 102% sulfuric acid in Periana´s original work,29

the used oleum

specification was clarified in a subsequent theoretical study coauthored by Periana,96

where it is stated that 102% sulfuric acid corresponds to 9% oleuma. Literature tends

to be not very precise on this and sometimes no clear distinction is made between

sulfuric acid and oleum. Furthermore, cited conditions can differ from the conditions

in the original report.31,39,95,106

Table 1 Comparison of conditions used for testing Hg(II)27

and 129

in the oxidation of methane

parameter Hg(II) 1

solvent 300 mL 100% sulfuric acid 80 mL 9% oleum

temperature /°C 180 220

methane /mmol 640 115a

“free” SO3 /mmol 0 90

“accessible” H2SO4 /mmolb 990 260

maximum possible CCH4 /%c 77 100

experimental CCH4 /% 50 90 aIf the volume of the gas phase given in the experimental description is used for the calculation the amount of methane would be

185 mmol. bbased on the difference between 100 and 85% sulfuric acid cassuming a 2:1 stoichiometry of accessible oxidant and

methane; The exact stoichiometry with respect to the oxidant is not clear, yet.

Finally, 100% sulfuric acid corresponds to 18.7 mol L-1

H2SO4 and 9% oleum to a

solution formally containing 2.1 mol L-1

sulfur trioxide and 17.6 mol L-1

H2SO4.

Considering that methane oxidation slows down upon dilution and does not proceed

below 85% sulfuric acid (15.4 mol L-1

H2SO4) it is clear that these differences are

significant and cannot be neglected. Table 1 gives a rough estimate of the amount of

a102% sulfuric acid is obviously used to describe a molar ratio, most likely the molar ratio of SO3:H2O

which would be then 1.02:1. The conversion to wt% is likely achieved by correcting the 2 mol% excess

SO3 by the molar weights of SO3 and H2O (factor of 4.44). This procedure can be found in the internet,

but is questionable, since the reference should be the total mass of the SO3/H2SO4 mixture and not only

of the excess SO3 and H2O.


22

methane and oxidant (sulfur trioxide and sulfuric acid above 85% sulfuric acid/ sulfur

trioxide alone) as well as the maximum theoretical conversion of methane for Hg(II)

and 1.

These numbers show that the conditions used in previous publications differ with

respect to the composition of the solution as well as sulfuric acid/sulfur trioxide has to

be considered as reactant and can be limiting! Consequently, use of inappropriate

amounts of oxidant and/or the particular experimental approach can obscure intrinsic

properties of the investigated catalyst.

2.4 Perspective for methane conversion

The Periana System improved several drawbacks of the Shilov system, e.g., use of a

more practical oxidant, higher selectivity, and catalyst stability. However, aside from

the inherent corrosion issues, there are two major hurdles, preventing

commercialization: Separation of methyl bisulfate from the reaction solution, which is

complicated by the large sulfuric acid to methane molar ratio,135

and, more

importantly, the catalyst activity, which seems to be two to three orders of magnitude

too low for industrial application.23,39,91

It is difficult to foresee if any of the mentioned approaches will be implemented on

commercial scale. The ones, currently tested in demonstration plants, are favored, and

the Periana system seems to lag behind. Syngas, however, can be expected to

maintain a strong position for economic reasons, e.g., depreciated plants, and the

efficiency of this mature process.37

If alternatives are implemented, it will be first for

small scale applications. Nevertheless, technologies for Syngas production and

conversion on small scale are also developed, based on once through concepts,

meaning air can be used in Syngas production and subsequent conversion does not

need recycling of unconverted Syngas.136,137

In case of methanol several concepts

exist. Liquid phase processes could be beneficial for good heat control.11,138,139

Without the need to recycle gas, it might be affordable to carry undesired ballast gas

through the reactor. These developments show that separation and processing are

important factors for production on large scale, and any possible solution has to be

analyzed comprehensively. Focus on a catalyst alone is not sufficient. Separation,

e.g., of product from the reaction mixture, is of exceptional importance.

Results and discussion

23

3 Results and discussion

3.1 Reading advice

The previous section already hints that there are still many open questions with

respect to methane oxidation in sulfuric acid. Also, comparison of the experimental

conditions, used for testing Hg(II) and 1, shows differences, and indicates that results,

reported in the literature, might be masked.

Indeed, several findings, presented in this thesis, seemingly conflict with some of the

conclusions presented in the literature survey. In order to improve readability, and to

avoid confusion, the most important findings are given here as brief notes. This

should be seen as supplement to the introduction.

- Acid and sulfur trioxide concentration have a significant impact on reactivity.

- Differences in sulfur trioxide concentration can lead to orders of magnitude

difference in catalyst activity.

- Many conclusions in literature are based on experiments in concentrated

sulfuric acid and not oleum.

- Platinum salts are not unstable catalysts, and achieve under certain conditions

high activity and stability.

- The TOF is a function of catalyst concentration, and many conclusions in

literature are based on high catalyst concentrations.

- Different catalyst concentration leads to different speciation of catalyst

precursors.

- Oxidation of methane to methyl bisulfate is part of a consecutive reaction

network which has to be kept in mind with respect to experimental approach

and selectivity.

Currently, literature generalizes from a narrow set of conditions. However, this thesis

shows that specific statements only apply to a specific set of conditions, and revisits

the whole picture, based on experiments from a broad range of conditions.


24

3.2 Experimental issues and approach

From the critical discussion in chapter 2.3.2.5 it is clear that mechanistic as well as

basic experimental issues are still present and need to be solved, before textbook

understanding of the Periana system is achieved.

Since the focus of this thesis is mainly on Pt(II) catalyzed methane oxidation, radical

mechanisms are of minor concern. All experiments yielded methyl bisulfate as the

main product with carbon dioxide being a side product. Other possible products, like

methanesulfonic acid, were not detected. Small signals in 1H NMR spectra from trace

impurities were present in some cases but hard to assign. It remains elusive, although

unlikely, if methyl bisulfate is formed directly or via methanesulfonic acid as

intermediatea.

However, one important aspect is comparability of measurements and conditions as

well as description of activity: A specific conversion value after a certain time is

clearly not sufficient to describe the system. Consequently, attention was paid at the

experimental approach and obtaining values which can be compared between

different experiments.

One challenge lies in the use of a batch setup where as a rule of thumb conversion of

reactants with a reaction order of 1 should stay below 10% in order to obtain rates

which are not obscured by consumption of reactant.140

As methane and also sulfur

trioxide/sulfuric acid - likely with a reaction order greater than one - are reactants, the

conversion of both has to be limited. In case of sulfur trioxide and sulfuric acid this is

complicated by the fact that not just the total amount has to be considered but also

which part is accessible and can participate in the reaction. The experimental setup

which was developed together with Mario Soorholtz accounts for this and limits under

standard conditions (20% oleum, reaction stopped after defined pressure drop) the

conversion of methane to a maximum of 30%, but usually 20%. The conversion of

sulfur trioxide is under assumption of a 2:1 stoichiometry for sulfur trioxide/methane

comparable. Importantly, there is no transition from oleum to sulfuric acid in most

cases, since sulfur trioxide is in slight excess. A brief description of a standard

experiment is given in the following and further details can be found in the


25

experimental part. Additional experimental issues are discussed where they are

relevant.

Scheme 3: Sketch of the experimental setup; taken and modified from reference 32.

In a typical experiment with the two autoclave setup, depicted in scheme 3, where one

autoclave serves as reactor and the other as reservoir for methane, 15 mL oleum (this

corresponds in case of 20% oleum to 70 mmol free sulfur trioxide) with catalyst

(typically around 600 µM) is heated under stirring (1000 rpm) to reaction

temperature, usually 215 °C, pressurized with methane to a total pressure of around

70 bar (30 mmol methane, pCH4 60 bar), which had been preheated to the same

temperature, and finally all reactions, except where otherwise stated, are stopped after

different reaction times, but at the same drop of the pressure-volume product,

indicating similar conversion, by quenching in a water bath. Figure 2 shows typical

p,T-t profiles: The pressure in the reactor increases during heat-up, due to evaporation

of sulfur trioxide, and after pressurizing with methane decreases (from a higher level)

due to conversion of methane.

aThe unexpected signals in

1H NMR spectra may originate from impurities in the capillary, added as

lock reference. The same signals are observed by measuring the capillary immersed in pure sulfuric

acid.


26

Figure 2: p,T-t profiles of a typical experiment.

Lower conversions would be desirable, but reaction time and time for pressurizing

and quenching have to be accounted for, too. If possible, comparisons are only made

at identical conversion. Nevertheless, it is still a simple batch setup and improvements

can be made, albeit with modifications of hardware. All in all this procedure reflects

the catalytic activity for a given set of conditions rather than the maximum possible

yield. For the understanding of the system, both types of information are important.

Also the lowest level of acidity, at which a catalyst is still working, would be

important for the description and comparison of catalysts. It might be that a less active

catalyst tolerates more dilute conditions.

Previous work of the group did partly follow integral approaches, and results should

be seen in that context: The initial work on Pt-CTF reported TONs based on the

amount of Pt(II) used in the first reaction in five recycling experiments and showed

this TON to stay constant.31

Final methyl bisulfate concentration indicates that this

does not necessarily mean that the initial amount of Pt is stably coordinated to CTF,

and also not that the activity is constant after recycling but due to the long reaction

times the reaction essentially consumes all accessible oxidant, respectively sulfur

trioxide, and the TON is predetermined, within error margin, by the sulfur trioxide

concentration. It is expected that the fresh catalyst ends up faster in the sulfuric acid

regime, and also leached platinum species contribute to activity (compare with the

thesis of Mario Soorholtz where it was shown that a typical CTF with 15 wt%


27

nitrogen content can accommodate 5 wt% platinum after recycling several times

while the platinum content of Pt-CTF reported in the initial report amounted to

approximately 40 wt%). For the recycled catalyst the reaction time is still sufficient to

reach the sulfuric acid regime, although a part of the platinum has been leached.

This problem with an integral approach was further substantiated and addressed in a

subsequent study33

where the TON of a solid catalyst in recycle experiments was

shown to increase with an integral approach if the reference for each recycle is the

actual amount of Pt used in each reaction. Essentially all reactions lead to the same

amount of methyl bisulfate along with less and less platinum used in each experiment.

Nevertheless, although the Pt content is constant at ~6 wt% for the Pt(II) modified

nitrogen containing carbon investigated, the TOF measured with the differential

approach decreases in contrast to the TON from the integral approach which shows

that deactivation other than leaching of platinum exists. The same behavior has been

observed for polyacrylonitrile derived N-containing carbon materials modified with

Pt(II) (Pt-PAN-C).32,141

This underlines the importance of the experimental approach.

However, also as a result of the dependency of the TOF on catalyst concentration,

vide infra, the new “differential” approach in batch reactors is not necessarily

sufficient to understand the complex deactivation behavior and continuous

experiments seem to be the only way for unravelling these intricate mechanisms.

Attempts to better understand the deactivation behavior of the solid catalysts are still

ongoing.

In general, previous publications should be critically revised, considering the

particular experimental approach chosen.

The stated TOFs and rates are based on the product methyl bisulfate in the reaction

mixture, determined by 1H NMR, although the TOFs calculated either by the pressure

drop or the amount of methanol determined by HPLC after hydrolysis show the same

trends and both parameters describe the catalytic activity reasonably well. The

pressure drop was used for the determination of rates in previous work33,89,116,117

but

the exact composition of the gas phase during the experiment is not determined and

remains undefineda.

aAlthough it is reasonable to use the pressure drop as measure of activity, the actual composition of the

gas phase is not known and changes in the amount of CH4, CO2, SO2 and SO3 contribute to the pressure

drop. Under the mentioned standard conditions with 30% conversion based on the amount of methyl


28

In any case, this experimental approach differs from most previous reports. Until now

most reports follow the integral approach of Periana´s 1998 Science paper,29

reporting

a yield of oxidation product after a certain reaction time, usually a few hours, and not

at the same conversion.107,112

In some studies, the integral approach seems to be

appropriate because the used conditions (low sulfur trioxide concentration or low

temperature) usually lead to rates which can still be satisfactorily resolved, even if the

reaction is conducted for several hours. However, most of the experiments presented

in this thesis show rates that would not be resolved by an integral approach.

Furthermore, it is important to keep in mind that the reported TOFs, respectively

rates, in this work as well as in all previous work (including H/D exchange) are

apparent. They are significantly influenced by the changing properties of sulfuric

acid/oleum with changing sulfur trioxide concentration and different catalyst

concentrations.109

Consequently, care has to be taken in comparing results from

different publications.

3.3 Thesis of Mario Soorholtz

Since parts of Mario Soorholtz´s thesis32

were the starting point for this thesis it seems

convenient to wrap up the main conclusions from that work, which had two main

directions. The first direction was the characterization of the first generation solid

catalyst Pt-CTF, and the elucidation of its structure at the atomic level. It was

essentially found that it is adequate to call Pt-CTF a solid analogue of 1 although

subtle differences between both exist.

The second direction was the development of solids that can act as ligand for Pt(II)

and after modification with Pt(II) catalyze the oxidation of methane in oleum/sulfuric

acid. Initially the CTF material was modified by applying milder synthesis conditions,

i.e., lower synthesis temperature. This resulted in materials which were chemically

different, albeit unstable in oleum/sulfuric acid at 215 °C. Later experiments with

other nitrogen containing carbons as potential solid ligands for methane oxidation

showed that the parent material has to be treated at last at 600 °C, preferentially at

higher temperature, to avoid complete decomposition after 2.5 h in oleum at 215 °C.

bisulfate, determined by 1H NMR, the pressure drop would indicate 15% conversion if a change in the

partial pressure of methane was the only contribution.


29

The higher stability might be induced by mild carbonization at or above 600 °C and

formation of graphitic domains. Nevertheless, by no means should it be concluded

that materials surviving 2.5 h at 215 °C in oleum/sulfuric acid are infinitely stable

towards the harsh conditions.

Different precursors were used to vary the atomic level (different coordination

environment) as well as the structural properties (porosity) of CTFs. Micro- and

mesoporous CTFs with different nitrogen content as well as different ratios of surface

nitrogen species were obtained and catalytic tests were conducted in batch

experiments similar to Periana´s integral approach, which was shown to be possibly

inappropriate in the previous sections. A comparison between most of these tests

seems to be reasonable because the majority shows an effective rate of reaction too

low to reach the maximum possible conversion (both, intrinsic activity and amount of

catalyst were rather low). In recycling experiments only the original Pt-CTF showed a

stable platinum loading between 4 and 5 wt%. Higher TONs, comparable to values

obtained with 1, but unstable platinum coordination were obtained if the CTF

exhibited a higher C/N ratio. Actually, comparable values were obtained in these two

cases, as conversion of oxidant was close to maximum conversion.

However, improvement of activity with unchanged stability was desired and a variety

of nitrogen containing carbons was screened for their ability as solid ligand in

methane oxidation. Interestingly, materials were identified which exhibited greater

TONs than 1 and showed increasing TONs in recycling experiments. This at first

sight peculiar behavior has just been mentioned and is a result of the experimental

approach, together with the relatively high activity of the catalysts. In subsequent

experiments less platinum was used, as part of it had been leached. In addition,

recycling resulted in additional loss of material, but the activity of the catalyst and the

rate of reaction were still sufficiently high to get close to the maximum possible

conversion. In case of 20% oleum care has to be taken if the methyl bisulfate

concentration gets higher than 1 mol L-1

(this concentration had previously been

reached with 1, and also the most active, but unstable, Pt-CTF at a platinum

concentration of 1 mM.).

This made modification of the setup mandatory. At the same time another material,

derived from lobster shells, did not only show increased activity, but also stable

platinum loading in recycling experiments. This was the first solid catalyst


30

investigated with the setup described in the previous section (at this stage reactions

lasted 30 min which still does not allow comparison at identical conversion but the

TOFs were determined from a defined pressure drop at the beginning.). Remarkably,

the material was around twice as active as 1 and showed a stable platinum

coordination over five recycling experiments.33

Figure 3: Platinum loading after four recycling steps as function of the specific surface area for

different Pt-PAN-C carbonized at 1100 °C. Taken and modified from reference 32.

Unfortunately, this material, as it is derived from lobster shells, is not well suited for a

systematic investigation. Luckily, at the same time Pt-PAN-Cs were also found to be

promising candidates. These materials can chemically be modified by different

thermal treatments,142

resulting in different nitrogen content as well as different ratios

of the surface nitrogen species. Furthermore, also the pore structure can be varied, and

two synthesis routes are available: The first one is based on hard templating, where

the pore structure can be influenced by use of different silica templates. The second

one is based on post synthesis treatment of a non-porous bulk carbon by means of

water steam activation. These two routes allow for a wide variety of hierarchical

structures, including micro and mesoporous carbons, but also core shell materials.

Investigation of the Pt-PAN-C revealed that, depending on the nitrogen content and

specific surface area, a certain amount of platinum is probably stably coordinated.


31

This is shown in figure 3 with a modified graph from the thesis of Mario Soorholtz,

where “stable” platinum loading corresponds to the value of apparently constant wt%

of platinum during the course of four recycling experiments in 20% oleum for 2.5 h at

215 °C.32

Furthermore, these studies corroborated what had already been evident:

- Higher nitrogen content, as a result of lower carbonization temperature, gives

materials which exhibit more sites for stable platinum coordination.

- Higher nitrogen content leads to less active materials.

- Deactivation not attributable to platinum leaching occurs, but is very difficult

to investigate due to interaction of various factors.

3.4 Solid catalysts

The PAN based catalysts looked quite promising and were the starting point of this

thesis with the original proposal to use them in a continuous manner. In order to

evaluate if they are suitable for application in a continuous setup, deactivation needs

to be better understood. Aside from deactivation mechanisms different from simple

platinum leaching, it is also important to clarify if platinum is released under reaction

conditions and formation of methyl bisulfate is catalyzed by dissolved species which

are coordinated again upon cooling the reaction mixture. To answer this question hot

filtration experiments are necessary which means that the operating catalyst is

removed from the hot reaction mixture. As oleum at 215 °C is challenging to filter,

the catalyst needs to be modified macroscopically that filtration can be done by means

of a frit or capillary. Consequently, the size of the carbon particles has to be increased

from around 10 - 100 µm to several mm.

3.4.1 Macroscopic modification

Due to the physical properties of the carbon, pelletizing by simply pressing is not

possible, and also the use of binders did not result in useful pellets. Thus, the

approach was changed from post carbonization shaping to pre-carbonization shaping.


32

Two ways were pursued: Post synthesis generation of porosity by water steam

activation (WSA)a, and hard templating with a pre-shaped template.

3.4.1.1 Water steam activation

In contrast to the resulting carbon, powdered PAN can be readily pelletized or already

polymerized in a mold which allows basically shapes of any kind, obviously ideally

suited for introduction of porosity by WSA. In comparison to the synthesis protocol of

the powdered Pt-PAN-C obtained by WSA there is no big difference: Radical

polymerization slightly above room temperature, stabilization at 200 °C in air,

subsequent carbonization at 1100 °C, and finally introduction of micropores at 850 °C

by WSA. Some examples of shaped bodies (pellets, crushed material, monoliths) and

different stages during synthesis of a monolith are shown in figure 4.

Figure 4: Photograph of porous PAN-C pellets, crushed material and monoliths (lower part; from left to

right PAN monolith, stabilized PAN monolith, PAN monolith after carbonization and WSA) obtained

by WSA route.

As shown by nitrogen physisorption isotherms in figure 5 of crushed PAN-C before

and after WSA, porosity is introduced by WSA, and the resulting material is

microporous, exhibiting a type I isotherm.

aWater steam activation is oxidative treatment of carbons with steam to create micropores.


33

Figure 5: Nitrogen physisorption isotherms of PAN-C before and after WSA.

BET equivalent surface area increased from 1.6 to 556 m² g-1

(ZIR-ZA-035-04;

corresponds to upper right material in figure 4). This material was modified with

potassium tetrachloroplatinate to give a formal platinum loading of 3.2 wt%. At the

time of these experiments the correlation seen in figure 4 was not known yet which

means that part of the platinum is expected to leach during methane oxidation in

concentrated sulfuric acid in the retro perspective.

Block face analysis with scanning electron microscopy/energy dispersive X-ray

analysis (SEM/EDX) mapping indicated that the bodies have a core shell structure

with a (highly) porous shell and a solid core. Figure 6 shows the electron image (left)

of a block face cut from embedded crushed material and the corresponding EDX

mapping of platinum (right). Platinum seems to be enriched at the edges of the

material.


34

Figure 6: SE micrograph of crushed Pt-PAN-C-WSA (left) and corresponding Pt EDX mapping (right)

Also simply cutting some of the monoliths showed that there is a solid core. Core

shell structure has already been shown for the powdered PAN-C, obtained by WSA.32

Analysis of some carbon particles, which had fallen off during cutting the embedded

material, with SEM and scanning transmission electron microscopy (STEM) revealed

the presence of platinum nanoparticles smaller than 3 nm (figure 7).

Figure 7: Micrographs of carbon particles from cutting embedded Pt-PAN-C-WSA; from left to right:

SEM, STEM dark field, STEM bright field

Pt-PAN-C-WSA was tested for methane oxidation in a trickle bed reactor at

atmospheric pressure as well as in hot filtration under standard conditions and the

results are described and discussed in the following sections.

Nevertheless, the synthesis of the shaped materials needs to be better understood and

improved as treatment of bulky material is likely to result in different properties

compared to the powdered materials. For example, monolithic PAN-C after

stabilization and carbonization showed inhomogeneous optical appearance and the

solid core was sometimes not carbon at all but hard, glassy polymer. These

differences may result from limited access to the surface. Furthermore, thermal

treatment of the shaped PAN led to mechanical stress and often resulted in cracks.

Also the formation of platinum nanoparticles is not desired at all and also here

synthesis protocols need to be adapted.


35

3.4.1.2 Hard templating

Since without changing the synthesis protocol the materials scope of PAN-C by WSA

is limited to materials which only stabilize a low amount of platinum, and mechanical

stability proved to be an issue in catalytic testing, another route was followed: Hard

templating with preshaped silica templates which offers more variability with respect

to pore structures as well as carbonization temperatures. Based on literature reports

silica monoliths were synthesized by soft templating.143

After calcination of the

resulting material, a PAN-C replica of the silica monolith was obtained, following the

standard synthesis protocol for powdered PAN-C: Impregnation of the silica with a

monomer/radical initiator mixture, polymerization slightly above room temperature,

stabilization at 200 °C under air, carbonization at the desired temperature, and finally

leaching of the silica template. Since the silica can be molded in various ways, also

here a wide variety of bodies can be synthesized. Figure 8 shows the photograph of a

silica monolith and its carbon replica after silica leaching.

Figure 8: Photograph of a porous silica monolith and a PAN-C replica after silica leaching

The silica showed ordered mesoporosity as evidenced by low angle X-ray diffraction

(XRD) shown in figure 9 and exhibited a bimodal pore system as measured by

nitrogen physisoprtion, shown in figure 10. This was expected, since two different

structural directing agents were used for synthesis. Adsorption properties are given in

table 2.


36

Figure 9: X-ray diffractogram of a crushed silica monolith

Figure 10: Nitrogen physisorption isotherms of SiO2 monoliths, the composite material after

polymerization and at various stages towards Pt-PAN-C monoliths, as well as the PAN-C monoliths.


37

Table 2: Results of analysis of N2 physisorption for SiO2 monolith and PAN-C replica

parameter SiO2 PAN-C

BET /m² g-1

864 612

total pore volume at p/p0 0.97 /cm3 g

-1 1.01 0.58

mesopore range /nm (3-4), 20 (3) - 6

In qualitative agreement with the powdered PAN-C elemental analysis gave a

nitrogen content of 3.9%, if 1100 °C had been used during carbonization, and 16.4%

in case of carbonization at 650 °C.

Elemental analysis also revealed that modifying PAN-C with Pt(II) might be more

difficult than in case of powdered material. In case of Pt-PAN-C with a formal

loading of 3.8% this value varied with sampling and parts from the shell showed

higher platinum content.

Since in the meanwhile the focus of the thesis was changed towards understanding of

the molecular system, these materials were neither further analyzed nor tested for

methane oxidation.

Still, future studies should use hard templating rather than WSA, since templating has

a broader scope with respect to carbonization temperature, and PAN-Cs are less bulky

and likely closer to the powdered materials. Similar to the monoliths obtained by

WSA, magnetic stirring led to abrasion of small particles. Elaborate synthesis of the

silica template does not seem to be necessary and pelletizing of silica particles used as

template in the synthesis of powdered PAN-C seems sufficient.

3.4.2 Continuous experiments and hot filtration

Continuous experiments and hot filtration were only performed with catalysts

obtained by the WSA route.

Continuous methane oxidation was done in a trickle bed reactor at atmospheric

pressure of methane. The reactor was made of glass and the Pt-PAN-C was supported

on a glass frit. Method development and knowledge about the system was small at the

time of the experiments, and thus results were only of limited value. Anyway,

conceptually it is possible to carry out continuous methane oxidation in this way.

Mostly sulfuric acid (not oleum) was used as reactant, and traces of methyl bisulfate


38

/methanol were found by NMRa. The signal was slightly bigger, if 20% oleum was

used, but not present if PAN-C was in the reactor instead of Pt-PAN-C. Qualitatively,

after around 13 h of methane oxidation at 215 °C and 200 mL of sulfuric acid had

passed over the catalyst still platinum could be found by elemental analysis in the

used catalyst (100 mg of catalyst were initially used and in the middle of the

experiment the amount was reduced to 33 mg). Consequently, these experiments

prove that methane oxidation with Pt-PAN-C is possible in a continuous fixed bed

reactor, although the use of atmospheric pressure and sulfuric acid, instead of oleum,

vide infra, led to low rates with limited practical use.

Hot filtration was carried out with a modified one autoclave batch setup: The

autoclave was equipped with two valves. One valve was used for supplying methane;

the other valve was connected to a capillary in the inside of the autoclave. The

capillary reached to the bottom which means that upon opening the valve a

sufficiently pressurized autoclave will release more than 90% of the liquid.

Preconditioning, by first immersing the catalysts in sulfuric acid, and only afterwards

in oleum, was necessary, since immediate contact of the catalyst with 20% oleum

often led to disintegration of the catalyst.

In one series, the catalyst was first used in 96% sulfuric acid for 2.5 h at 215 °C and a

total pressure of approximately 95 bar. The catalyst was recycled after cooling to

room temperature and stored in 20% oleum without washing and drying. This way of

recycling the catalyst, i.e., recovering from the reaction mixture at room temperature,

was repeated five times in 20% oleum (1st 15 mL oleum, subsequent runs with

20 mL). Subsequently, the catalyst was recycled five times by hot filtration at 215 °C,

in 20% oleum after 2.5 h of reaction. Figure 11 shows the obtained amount of methyl

bisulfate for this series in comparison with the amount of methyl bisulfate formed

under identical conditions with a similar amount of PAN-C material not modified

with Pt(II) (“blind”). Since the amount of methyl bisulfate formed in these

experiments is rather low, limitations by oxidant or methane are not a concern at all

and conducting the reaction for 2.5 h is justified. It is obvious that the amount of

methyl bisulfate formed in the 2nd

run after the first cold filtration (first experiment

aFor comparison, if 1 (2.5 – 5 mM) was used continuously in a 2:1 mixture of 96% sulfuric acid/ 20%

oleum MBS concentration was in the range of 20 mM which is around 1 order of magnitude more than

with the solid catalysts. This experiment was conducted over 5 weeks and the concentration of MBS

went through a maximum. If this was due to deactivation of the catalyst or drop in acidity is not clear.


39

with oleum) is around three times higher compared to all other experiments. This is

likely due to unstably coordinated platinum (the initial platinum loading was 2.6 wt%

at BET equivalent surface area of 556 m² g-1

; figure 3 suggests that the use of 1.5%

would be more appropriate). However, subsequent cold and hot filtrations showed a

stable methyl bisulfate production, which is twice as big as the amount formed with

PAN-C in blind experiments under identical conditions. Only the last hot filtration

shows a decrease in the amount of formed methyl bisulfate, but this could already be a

result of loss of catalyst particles split off from the shaped body. This series is

promising and indicates that there seems to be no difference between hot and cold

filtration and more important that not all of the platinum is released at 215 °C and

reccordinated at room temperature. Another indication for this hypothesis is that

platinum was still detected by elemental analysis in the catalyst (0.5 wt% after three

more cold filtrations; activity in these experiments was not different from blind

experiments, but the catalyst had been washed and dried in between which could lead

to other types of deactivation). However, the activity in recycling tests could be

blurred by mass transfer limitations as the shaped catalyst has only a low external

surface area.

Figure 11: Amount of methyl bisulfate formed in 2.5 h at 215 °C in recycling experiments.


40

In order to get more indication about the stability of the platinum coordination, the

activity of the filtrate, obtained by hot and cold filtration was tested with Pt-PAN-C

pellets from the same batch. In both cases 150 mg of shaped catalyst (pellets, 1 wt%

platinum, 370 m² g-1

; pellets seem to be less resistant to attrition compared to

monoliths or crushed material.) was heated in 20 mL 20% oleum with methane to

215 °C. Then, either hot filtration was performed, or the autoclave was rapidly cooled

in a water bath to room temperature, where the solution was filtered off “cold” by

opening the filtration valve.

In case of hot filtration, the filtrate activity was 0.0200 mol h-1

, compared to

0.0175 mol h-1

for cold filtration (methane was used for filtration and the filtrate

activity was corrected for the already formed amount of methyl bisulfate during heat

up in the filtration). Activity in hot filtration is higher but not significantly and it is

clear that differences during filtration, e.g., as a result of viscosity or the pressure

difference, might contribute to the difference. Actually, the sulfur trioxide partial

pressure, which is a good measure of sulfur trioxide concentration, indicated that in

case of testing the filtrate of the hot filtration experiment, the sulfur trioxide

concentration was higher. BET equivalent surface area decreased in both experiments

(to 184 m² g-1

in case of cold filtration, 198 m² g-1

in case of hot filtration), indicating

that substantial loss of catalytically active particles had happened. However,

meaningful comparison would also require testing the activity of the parent Pt-PAN-C

pellets. Such experiments were not undertaken, since these materials did generally not

show the required support stability for a meaningful complete dataset. For

comparison, if half of the platinum was leached out, one could expect a rate of

0.0713 mol h-1

(based on a measurement with 30 µM potassium tetrachloroplatinate).

100 mg of powdered Pt-PAN-C (carbonized at 1100 °C, 1wt% platinum) showed a

rate of 0.0025 mol h-1

. These values indicate that a substantial part of the platinum

might be leached under reaction conditions, since the rate of both filtrates lies in the

range of potassium tetrachloroplatinate. However, the comparison between the

powdered Pt-PAN-C and the Pt-PAN-C pellets, used for hot filtration should be taken

with caution, since they are different materials, with respect to porosity (micro vs.

meso porous and hierarchy) and surface chemistry (pellets are synthesized with an

additional WSA step).


41

Hot filtration with 430 mg of monolith (330 m² g-1

, 0.8wt% platinum) and N2 showed

filtrate activity (0.062 mol h-1

), but the solid catalyst was still active afterwards

(0.025 mol h-1

), and platinum was found in the solid by elemental analysis. This

suggests, in agreement with the previous experiments, an intermediate situation,

where only part of the platinum is leached, albeit the leached species seem to be more

active.

In general, continuous methane oxidation and hot filtration experiments do not

satisfactorily answer the question, if catalytic action originates from truly

heterogeneous catalysis or if platinum is, under reaction conditions, released to the

solution and recoordinated upon cooling. The filtrate showed substantial activity,

which suggests, either leaching of platinum under reaction conditions, or support

instability and loss of catalyst particles. Based on comparison with potassium

tetrachloroplatinate and powdered Pt-PAN-C it seems reasonable that a substantial

part of the filtrate activity originates from leached platinum. Nevertheless, continuous

experiments and recycling of Pt-PAN-C after hot filtration also shows that a fraction -

although a minor one - of the activity and platinum is retained on the material. Future

experiments should improve the experimental setup and the stability of the pellets,

because mass transfer limitations and attrition/support corrosion might mask results.

3.4.3 Carbon corrosion

Deactivation of a solid catalyst can occur by means of several processes, e.g.,

poisoning of active sites, sintering, or formation of deposits. In liquid phase processes

leaching is a big concern. However, also corrosion and abrasion of support, especially

with carbon supports under oxidizing conditions, should not be neglected.144

Abrasion and/or carbon corrosion was already obvious in hot filtration experiments.

To further test these deactivation pathways, 200 mg of powdered mesoporous PAN-

C-1100 was heated to 215 °C in a 5/1 mixture of concentrated sulfuric acid and 20%

oleum. After two weeks 135 mg of carbon was recovered and the solution had

significantly darkened. However, this result is ambiguous, because the flask was

obviously leaking, and air could have got in the flask. Nevertheless, a strong smell of

sulfur dioxide was observed, indicating sulfuric acid as oxidant and not aira. In order

aThermogravimetry revealed PAN-C to be kinetically stable in air until around 400 °C.


42

to remove sulfuric acid from the micropores, part of the material was subsequently

carbonized for a second time at 1100 °C and around half of the material was lost

during this step. Nitrogen physisorption and elemental analysis was conducted at all

three stages. Isotherms are shown in figure 12 and table 3 summarizes adsorption

properties before and after sulfuric acid treatment.

Figure 12: Nitrogen physisorption isotherms of PAN-C before and after sulfuric acid treatment.

Table 3: Results of analysis of N2 physisorption for PAN-C before and after sulfuric acid treatment

parameter before

treatment

after

treatment

after 2nd

carbonization

BET /m² g-1

453 323 334

total pore volume at p/p0 0.97 /cm3 g

-1 0.954 0.238 0.253

mesopore range /nm 6-8 (3 – 4) (3 – 4)

Besides oxidation of carbon as evidenced by weight loss, especially the mesoporous

structure suffers from the sulfuric acid treatment, and mesoporosity is essentially lost.

This is unlikely due to blockage of pores by sulfuric acid, because the isotherm is

hardly influenced by a second carbonization at 1100 °C, which should lead to


43

evaporation of any remaining sulfuric acid. However, additional material is lost

during this second carbonization step, maybe due to loss of surface functionalities like

SO3H groups.

The structural changes are corroborated by transmission electron microscopy (TEM)

which also shows loss of mesoporous structures. Figure 13 shows typical TEM

micrographs of mesoporous PAN-C (left column) and of PAN-C after sulfuric acid

treatment (right column). Clearly, the mesoporous structures are lost upon prolonged

heating in sulfuric acid.

Figure 13: TEM micrographs of mesoporous PAN-C before (left column) and after sulfuric acid

treatment (right column) at 215 °C for two weeks.

Elemental analysis supports the hypothesis of surface functionalization and loss of

these functionalities after a second carbonization. Table 4 lists the C, N and S content


44

of the materials. Upon sulfuric acid treatment a considerable amount of oxygen, the

balance of the weight, is found in the material, and the sulfur content is lower after the

second carbonization.

Table 4: C, N, S content and weight of PAN-C at the various stages of the stability test

sample C /% N /% S /% weight /mg

before treatment 79.4 3.5 n.a. 200

after treatment 63 3.1 3.7 135

after 2nd

carbonization 67 2.2 0.7 68

The stability of different materials is apparently different. PAN-C carbonized at

1100 °C is faster oxidized than CTF carbonized at 600 °C in 20% oleum, as shown by

the pressure time curve in figure 14, where 200 mg of material was heated to 215 °C.

Carbon dioxide was the majority of infrared active component in the gas phase after

completion of these experiments.

Figure 14: Pressure time curves of stability tests in 20% oleum.

Rough estimation with the ideal gas law results in around 2 mmol of carbon dioxide

in case of PAN-C. Elemental analysis shows substantial oxygen and sulfur

modification of the materials after oleum treatment.


45

Another sign of support corrosion is found in liquid phase 1H NMR spectra of all

reaction solutions obtained with nitrogen containing carbons. Exemplarily the 1H

NMR spectrum of the solution from the stability test with PAN-C in 20% oleum is

shown in figure 15. The signal at 10 ppm corresponds to the acid protons, the signal at

2.9 ppm corresponds to methanesulfonic acid added as standard, and the signal at

2.5 ppm originates from the lock standard DMSO-d6. The other signals were not

assigned, except the triplet at around 5 ppm with a 1:1:1 intensity which originates

from 14

NH4+, a clear sign of nitrogen release from the support.

Figure 15: 1H NMR spectrum of the solution from the stability test with PAN-C in 20% oleum.

Consequently, there is strong indication that the carbon support is slowly (but very

likely still too fast for application in a continuous process) oxidized under reaction

conditions, and it can be questioned if a carbon support can be successful under these

conditionsa.

In conclusion nitrogen containing carbons can be used as solid ligands for methane

oxidation. However, there are several drawbacks:

aIt is also known that PAN-C can be used for the oxidative removal of SO2 from flue gases. Oxygen is

present in these cases but it is clear that the surface is not inert towards the components present under

reaction conditions.145


46

- High nitrogen contents are needed to prevent platinum from leaching which is

detrimental for activity

- Activity of presumably stably coordinated platinum is low

- Stability of the support against oxidation and attrition needs to be improved

3.5 Molecular catalysts

Already during the thesis of Mario Soorholtz as well as in the hot filtration

experiments with catalysts of high platinum loading and low nitrogen content it

became evident that leached species might actually be very active and contribute to

formation of methyl bisulfate. In order to quantify how much dissolved species

contribute to catalysis in the filtrate activity tests described in the previous section, a

small amount of potassium tetrachloroplatinate was used as catalyst in 20% oleum.

This combination was not only very active, but also stable with several hundred

turnovers. This result was unexpected and different from literature where platinum

salts are described as unstable. The unexpected stability, the high activity, and the

consequences for solid catalysts (small amounts of leached species might be

responsible for the majority of the activity) led to the decision to focus on the

understanding of the molecular system before further development of the more

complex system with solid catalysts should continue.

Since the catalytic activity observed was much higher compared to literature, and one

obvious difference to the work of Periana is the sulfur trioxide content of oleum, the

starting point was a systematic investigation of this parameter.

3.5.1 Influence of sulfur trioxide concentration: 100-fold

activity increase in oleum

The main content of this section will be published in “Revisiting the Periana System”.

3.5.1.1 Periana catalyst

The dependency of the TOF of 1 on the sulfur trioxide concentration is shown in

figure 15 (TOF is given as molMBS molPt-1

h-1

; molMBS determined by means of

1H NMR; most of the values are still underestimated due to the not strictly differential


47

approach and oxidation of methyl bisulfate to carbon dioxide). Compared to 96%

sulfuric acid (18 mol L-1

H2SO4), the use of ~40% oleum (9.5 mol L-1

SO3,

11.6 mol L-1

H2SO4) leads to an increase of the TOF by a factor of 100. Further

increase of the sulfur trioxide concentration decreases the TOF by a factor of 2. These

measurements were done with a catalyst concentration of 600 µM which is around

two orders of magnitude lower compared to the 50 mM of Periana´s original paper.29

The highest observed TOF50bar is ~1250 h-1

, 35 times higher than in Periana´s original

report.29

The methyl bisulfate concentration in these experiments usually reached

0.5 mol L-1

, only a factor of two lower. Consequently, with two orders of magnitude

lower catalyst loading, but the right sulfur trioxide concentration, a similar volumetric

productivity compared to Periana´s original report is reached.29

The TOFs in figures

16 and 17, vide infra, are extrapolated to a partial pressure of 50 bar methane, the

reaction order of which is assumed to be 1 (the raw data are given in figures 18 and 19

and a detailed description of the extrapolation is described in the experimental part).

Figure 16: Dependency of TOF for methane oxidation with 1 on sulfur trioxide concentration at

215 °C. TOF based on methyl bisulfate formed, 15 mL solution, [1] 600 µM, CCH4 < 30%, TOFs

extrapolated to methane partial pressure of 50 bar (see experimental part for details, actual partial

pressures at 215 °C 35-75 bar).


48

The partial pressure of methane is calculated based on the amount of methane

introduced to the reactor. Extrapolation does not change the general picture of the raw

data but seems to be necessary as the observed partial pressure of sulfur trioxide at

215 °C differs drastically for different sulfur trioxide concentrations (~1 bar for 100%

sulfuric acid, up to 60 bar for 65% oleum), and it is thus difficult to precisely control

the partial pressure of methane introduced to start the reaction.

3.5.1.2 Potassium tetrachloroplatinate

Due to the strong influence of the sulfur trioxide concentration, also potassium

tetrachloroplatinate was investigated as catalyst. In general, simple Pt compounds are

known to be even more active than 1a, but believed to deactivate after few turnovers,

respectively after a short time, upon formation of polymeric Pt compounds, e.g.,

platinum(II) chloride, or “Pt black” (based on standard electrode potentials Pt(0) does

not seem to be a pathway for deactivation in the Periana system and formation of

Pt(0) seems only possible under less oxidizing conditions, e.g., Shilov conditions.).29

However, closer inspection of the patent,30

which served as the basis for Periana´s

original report,29

reveals that e.g., (NH3)2PtCl2 shows a better performance than 1 in

some instances (c.f. table 2 ibidem). Indeed, at low catalyst concentrations

deactivation of potassium tetrachloroplatinate after few turnovers is not observed,

and, as illustrated in figure 17, a similar behavior with respect to sulfur trioxide

concentration compared to 1 is found, albeit more pronounced, overall with higher

activity over the whole sulfur trioxide concentration investigated. At the optimum

sulfur trioxide concentration the TOF50bar is more than 20 times higher compared to 1,

i.e., ~27000 h-1

. Such a value is to my knowledge far bigger than anything else

reported for the direct conversion of methane75

and clearly in the range of large scale

industrial homogenously catalyzed processes.

aSimilar to the comparison between Hg(II) and 1, a TOF of 3.6 h

-1 at 180 °C is given for (NH3)2PtCl2

without further specification of the used conditions.29


49

Figure 17: Dependency of TOF for methane oxidation with K2PtCl4 (1 shown for comparison) on

sulfur trioxide concentration at 215 °C. TOF based on methyl bisulfate, 15 mL solution, [K2PtCl4]

600 µM, CCH4 < 30%, TOFs extrapolated to methane partial pressure of 50 bar (see experimental part

for details, actual partial pressures at 215 °C 35-75 bar).

3.5.1.3 Pressure dependency

Figure 18 shows the raw data of both sulfur trioxide concentration series. As

mentioned, the general trend does not depend on extrapolation.

Figure 18: Raw data of the sulfur trioxide dependency series for a): 1 and b): K2PtCl4 (1 shown for

comparison). The pressure of methane in the preheating autoclave was kept constant at approximately

53 bar at room temperature and the partial pressure in the reactor thus changes.

a b


50

To exclude that methane has a substantially different reaction order than one, pressure

dependency series were measured in 96% sulfuric acid, 20% oleum and 65% oleum

for both catalysts. The results are shown in figure 19. The partial pressure of methane

at reaction temperature (x-axes) is again calculated based on the amount of methane

introduced into the reactora.

Most of the series show within error margin a linear correlation between the partial

pressure and the TOF. In 96% sulfuric acid with 1 as catalyst (figure 19a) the TOF

seems to become saturated with increasing pressure. However, the TOF is based on

methyl bisulfate found in the crude reaction mixture, and the carbon dioxide formed

in the experiment with the highest pressure is higher compared to the other

experiments of this series. This illustrates the problem of underestimation of TOF if

rMBS is low in general. It might be in this case that the rate of methyl bisulfate

decomposition already equals rMBS.

In case of the pressure dependent experiments in 96% sulfuric acid the conversion of

methane was below 5%. In 20% oleum conversion ranged from 15 to 40% and in 65%

oleum it was ~10% in case of 1 and ~30% for potassium tetrachloroplatinate. Due to

the differences in conversion these results should be seen qualitatively and a true

order of reaction should not be extracted. Due to the interdependencies of the

parameters it is difficult to perform a clean and distinct pressure series. To solve this

problem, the experimental setup has to be further modified, so that operation under

constant pressure is possible, i.e., methane should be continuously fed into the reactor

and the conversion of sulfur trioxide limited. It seems still acceptable to use a reaction

order of one for extrapolation in the sulfur trioxide concentration series.

aA table with the values of figure 19 is in the appendix and shows that the calculated partial pressure

might have a considerable error. The values in oleum cannot be compared because the partial pressure

of SO3 contributes significantly. In case of sulfuric acid, however, the calculated partial pressure is

higher compared to the measured total pressure. This is unexpected since the volume of the gas phase

should be, due to expansion of the liquid phase, smaller than assumed for the calculations.


51

Figure 19: Pressure dependency of the TOF for a): 1 in 96% sulfuric acid, b): K2PtCl4 in 96% sulfuric

acid, c): 1 in 20% and 65% oleum, and d): K2PtCl4 in 20% and 65% oleum at 215 °C with a catalyst

concentration of 600 µM in 15 mL sulfuric acid/oleum. The ranges of methane partial pressure are

different due to the increasingly higher sulfur trioxide partial pressures limiting the experimentally

accessible window. The total pressure, the calculated methane partial pressure, TOF and temperature of

the respective experiments can be found in the appendix.

3.5.1.4 Catalyst stability

With simple platinum salts, deactivation has always been described as a problem in

literature. However, at a catalyst concentration of 600 µM the reaction solution

remains clear, and, more importantly, the TONs usually exhibit values greater than

500 (both for 1 and potassium tetrachloroplatinate). Also in 96% sulfuric acid a TON

greater than 150 was observed for potassium tetrachloroplatinate (The TON of 1 is

only 18 at identical reaction time due to the lower activity. Although TON is usually

related to stability it should be noted that a low TON can also result from a stable

catalyst with low activity combined with a “short” reaction time.) As for cis-

(NH3)2PtCl2 a half-life of 15 min at 180 °C was reported,29

and some of the reaction

times presented for potassium tetrachloroplatinate are below this value, additional

experiments were performed with potassium tetrachloroplatinate where TON > 16000

was observed (40 µM, 20% oleum, 2 h). Titration of the solution after reaction yields

a b

c d


52

a concentration of sulfur trioxide equivalents of 19.1 mol L-1

. Although this value

corresponds to oleum, the contributions from methyl bisulfate and sulfur dioxide to

the titrated acidity are not known, and an unambiguous conclusion is not possible.

Also, the pressure vs. time curve indicated activity over the whole reaction time. The

pressure drop significantly decreases over the course of the reaction which, however,

can be expected due to the consumption of sulfur trioxide and the shown dependency

of activity on sulfur trioxide concentration. Consequently, the quoted TON can be

considered as lower limit. It can be expected that with a constant sulfur trioxide

concentration the TON would be significantly higher.

Furthermore, keeping the reactor for 5 h at reaction temperature (this is two orders of

magnitude longer compared to the shortest reaction time, and 20 times longer

compared to the reported half-life of (NH3)2PtCl2 at 180 °C) before pressurizing with

methane shows the same activity within experimental error (320 µM, 20% oleum,

TOFwithout preheating 25540 h-1

, TOF5 h preheating 27670 h-1

).

In order to prove that deactivation is in general not a problem for simple Pt

compounds and not only particularly for potassium tetrachloroplatinate, several

platinum compounds were briefly investigated, including cis-(NH3)2PtCl2 and

platinum(II) chloride. The compounds tested in 20% oleum are listed in table 5. It is

remarkable that all of the platinum containing precursors exhibit a decent stability

with a TON greater than 500, and deactivation does not seem to be a considerable

issue. Furthermore, the activity of all tested compounds is at least one order of

magnitude higher compared to 1 and additional bpym (5 extra equivalents, entry 2)

reduces the activity of 1 by a factor of 2. This decrease of activity can perhaps also

explain the previously observed lower activity of the solid analogue Pt-CTF compared

to 1: Whereas 1 has a 4:1 molar ratio of N:Pt, addition of 5 extra equivalents of bpym

leads to a ratio of 24:1 which is much closer to a 5 wt% Pt-CTF which has

approximately a 42:1 ratioa.

The differences in selectivity are not necessarily intrinsic, and selectivity will be

discussed in detail later. It is interesting to note that the system shows high reactivity,

irrespective of whether the oxidation state of platinum in the precursor is 0, II or IV. It

is important to realize that the higher activity in oleum compensates the lower catalyst

aA sound comparison should be made based on the amount of surface nitrogen. However, this was not

attempted.


53

concentration and the volumetric productivity is even higher compared to the original

report (4 vs. 11 mol L-1

h-1

).29

Table 5: Performance of platinum compounds for methane oxidation in 20% oleum at 215 °C

entry catalyst

precursor

concentration

/µM

TOF

/h-1

TONa

SMBS

/%

rMBS

/mol h-1

1 1 600 1281 651 96 0.0115

2 1/bpym 600/3000 558 764 91 0.0050

3 cis-(NH3)2PtCl2 600 19201 651 98 0.1728

4 PtCl2 600 15214 769 97 0.1373

5 (bipy)PtCl2b

600 14399 840 97 0.1296

6 Pt black 680 20094 837 98 0.2060

7 Pt(acac)2 680 22532 883 97 0.2292

8 K2PtCl4

600 22998 945 98 0.2050

9 K2PtCl4

670 23246 768 99 0.2352

10 K2PtCl6

670 24011 774 98 0.2421

Reaction conditions: 15 mL 20% oleum, CCH4 < 30%, pCH4,215 °C ~65 bar, ptotal,215 °C ~72 bar

adifferences in TON are a result of slightly different conversion (20-30%) and catalyst concentrations bbipy stands for 2,2´-

bipyridyl and should not be confused with bpym.

3.5.1.5 Methane oxidation with pure sulfur trioxide

Before discussing the results presented so far in more detail, a brief discussion of

methane functionalization with pure sulfur trioxide seems appropriate at this point. A

patent of Dow, using amongst other materials Pd modified Nafion as catalyst,146

and

patents from the 1950´s, utilizing mercury sulfate as catalyst,87,88

have reported the

selective oxidation of methane with pure sulfur trioxide. In order to test potassium

tetrachloroplatinate as possible catalyst, it was loaded together with liquid sulfur

trioxide in an autoclave, heated to 215 °C and pressurized with preheated methane.

Some activity was observed, but it appeared to be rather low compared to methane

oxidation in oleum. The TOF based on the pressure drop is approximately 160 h-1

(figure 20). This value is obtained with the ideal gas law under the assumption of the

stoichiometry in equation (8):

𝐶𝐻4 + 2𝑆𝑂3 → 𝑀𝐵𝑆 + 𝑆𝑂2 (8)

This means three gas molecules would be transformed into a liquid and a gas

molecule. Consequently, per bar methane converted the pressure would drop by two


54

bar. The IR spectrum of the gas phase after cool down revealed the presence of

methane, carbon dioxide and sulfur dioxide. Sulfur trioxide is difficult to assign as

absorption by sulfur trioxide overlaps with absorption bands of sulfur dioxide.

Figure 20: Pressure vs. time curve for methane oxidation with pure sulfur trioxide as oxidant and

K2PtCl4 as catalyst at 215 °C.

After opening the reactor, a few drops of a strongly green liquid were seen which

could be collected by rinsing with concentrated sulfuric acid. A 1H NMR of the

resulting solution was measured (figure 21). The intense signal at 10.4 ppm originates

from the acid protons. At 3.5 ppm a broad, presumably due to leached metal cations

from the reactor, signal in the range expected for the methyl resonance of methyl

bisulfate is found. In contrast the quintet at 2.5 ppm (residual protons of DMSO-d6)

and the singlet at 3.3 ppm (H2O) are sharp, as DMSO-d6 is added in a capillary to the

NMR tube and thus has no contact to the cations.


55

Figure 21: a): NMR spectrum of the sulfuric acid rinse of methane oxidation with K2PtCl4 at 215 °C

and pure sulfur trioxide as oxidant b): extension from 2 to 4 ppm.

It is yet unclear, if the lower activity can be explained by the fact that the solid

potassium tetrachloroplatinate exhibits only a low surface area and a solid catalyst

with atomically dispersed platinum31,33

could lead to a higher TOF. Despite the

a

b


56

analytical issues it remains to be answered if a process utilizing pure sulfur trioxide

could add any advantage. Possibly separation might be easier. A pure gas phase

process seems appealing, but will not be easy to realize, as moisture will be present in

any case, e.g., as a result from over oxidation of methane, which would react to

sulfuric acid with the sulfur trioxide.

3.5.1.6 Discussion of sulfur trioxide concentration series

Two key points of these results need a thorough discussion: First why potassium

tetrachloroplatinate and other platinum compounds are significantly more active than

1 over the whole sulfur trioxide concentration range investigated, and second why

both catalysts show a strong and at first sight unusual dependency on sulfur trioxide

concentration.

Regarding the higher activity of potassium tetrachloroplatinate it has already been

mentioned that simple Pt compounds are known to be more active than 1,29

but no

direct comparison has been presented up to now showing the extreme difference in

activity. The higher activity is usually traced back to the ease of oxidation of the

different compounds and the compound which is more easily oxidized shows higher

activity. Obviously, bpym exhibits π-acceptor properties which makes oxidation

difficult, respectively stabilizes the oxidation state (II). However, the actual surprise is

the stability of these materials which disproves the current standpoint of the scientific

community. Anyway, there has been already clear evidence for the stability of simple

platinum compounds in literature.107

A reason for the misunderstanding with respect

to catalyst stability over the last years might be the lack of details regarding the

influence of process parameters like sulfur trioxide or catalyst concentration and

especially their interplay. Later the reason for this misunderstanding will become

more obvious when the influence of the catalyst concentration on the rate of methyl

bisulfate formation is discussed. A crucial point is the much lower catalyst

concentration of the present study in combination with the individual solubility of the

compounds used herein, and compensation effects caused by the tremendous

differences in activity in oleum and sulfuric acid.

3.5.1.6.1 Differences between sulfuric acid and oleum – oxidation potential

The dependency of the activity on the sulfur trioxide concentration of 1 and potassium

tetrachloroplatinate is similar. Therefore it seems reasonable to assume the same


57

factors being causal. The merit of the quasi differential approach is to resolve these

features (fuming of the solutions after reaction reveal that experiments with 20%

oleum applying the standard protocol remain in the oleum region; titrations indicate

the same but are ambiguous as mentioned).

An integral approach would likely show a monotonic increase in the yield with

increasing sulfur trioxide concentration, provided a constant gas to liquid ratio,

methane in excess, and sufficient reaction time. I.e., 65% oleum should lead to a

higher final methyl bisulfate concentration and higher yield compared to 40% oleum,

where the maximum rate is observed. The aforementioned studies on related systems

indicate such a behavior.89,107,109,112,113,116,117,125

A comprehensive overview of these

systems is given elsewhere, but the conditions used in the individual studies should be

evaluated carefully.147

Similar dependencies of activity on the composition of the reaction medium have

been observed for the oxidative carbonylation of methane in CF3COOH/H2O. They

were explained by speciation of the rhodium catalyst due to protonation of OH-

ligands depending on proton activity.148

Furthermore, bases can participate in

oxidation reactions and influence the rate.22

Aromatic nitration in sulfuric acid/oleum

is such an example where it has been found that the rate of nitration is a function of

the sulfur trioxide concentration with a maximum at 92 - 94% sulfuric acid.149

In this

context the concentration of a proton acceptor and hence the dissociation properties of

sulfuric acid were found to be important. However, another obvious explanation could

be a change of the oxidation potential with change in sulfur trioxide concentration. It

is reasonable that the rate is correlated with the oxidation potential. This is known for

the Wacker oxidation of ethylene to acetaldehyde, where the rate of oxidation in the

two step process (oxidation of ethylene and regeneration of catalyst in separate

reactors) is not stationary and changes with the potential of the solution.150

The latter seems to be an obvious explanation for the increase in activity up to around

40% oleum. Oleum is known to be a stronger oxidant and shows a higher oxidation

potential compared to sulfuric acid.151-153

As oxidation is believed to be rate limiting

an increase of the oxidation potential could lead to a higher rate. It can be expected

that the reaction pathway does not change with sulfur trioxide concentration. Thus, it

seems appropriate to assume a linear activation energy – reaction energy relationship


58

like the Brønsted-Evans-Polanyi relation. Gibbs free energy, ΔG, and potential, E, are

linked by equation (9) which suggests proportionality between rate and potential.

∆𝐺 = −𝑛𝐹𝐸 (9)

Indeed, the resemblance of TOF and oxidation potential at the transition from sulfuric

acid to oleum is intriguing. At this point the TOF as well as the oxidation potential

increase drastically, in case of the potential by 0.6 Va.152

A similar change can be

observed for the Hammett parameter in this region.153,154

This is not surprising, since

the Nernst equation (10) couples proton activity and potential if protons participate in

the redox reaction, which is often the case. It can be expected that reduction of sulfur

trioxide and concomitant formation of water during methane oxidation involves the

transfer of protons. However, in the context of oxidations the redox potential seems to

be more appropriate for a discussion instead of proton activity.

𝐸 = 𝐸0 +𝑅𝑇

𝑧𝐹𝑙𝑛

𝑎(𝑂𝑥)

𝑎(𝑅𝑒𝑑) (10)

3.5.1.6.2 Decrease in activity above 40% oleum - solubility of methane

One might be tempted, to interpret the decrease above 40% oleum as a consequence

of the over-oxidation dilemma (see explanation of the over-oxidation dilemma in the

introduction). However, this has not been proven experimentally for the Periana

system, yet. Moreover, the same holds for the oxidation state of the active species. As

both catalyst precursors, 1 and potassium tetrachloroplatinate, show the same

dependency, the properties of the solvent should also be considered here for possible

explanations of the decrease of the TOF for high sulfur trioxide concentration.

As mentioned, the availability of a base could be crucial. In the proposed mechanisms

HSO4- appears usually and the self-dissociation of sulfuric acid/oleum changes over

the whole concentration range.155

Consequently, the concentration of the bisulfate

anion continuously changes. In order to increase the concentration of the bisulfate

anion, respectively the hydrogen disulfate anion, potassium bisulfate or potassium

disulfate was added to the reaction mixture. Adding potassium salt to 65% oleum at a

concentration of 1.6 mol L-1

resulted in a completely unexpected outcome: The TOF


59

Table 6: Influence of salt addition by salting out on methane oxidation

entry salt, [K+, NH4

+] /M [SO3] /M

a TOF /h

-1 TOFwith/TOFwithout /%

65% oleum

blind testb

1 no addition, 0 23.55 144 2.1

1

2 no addition, 0 23.55 330 100

3 K2S2O7, 1.6 21.43 184 55.8

K2PtCl4

4 no addition, 0 23.55 6742 100

5 KHSO4, 0.8 21.96 1891 28.0

6 K2S2O7, 1.6 21.02 755 11.2

7 (NH4)2SO4, 1.6 21.70 813 12.1

20% oleum

blind testb

8 no addition, 0 20.20 53 0.23

K2PtCl4

9 no addition, 0 20.20 22998 100

10 KHSO4, 0.8 19.86 16886 73.4

11 KHSO4, 1.6 19.18 9300 40.4

12 K2S2O7, 0.8 19.89 18911 82.2

13 K2S2O7, 1.6 19.45 11958 52.0

14 (NH4)2SO4, 0.8 19.70 19528 84.9

15 (NH4)2SO4, 1.6 19.10 10011 43.5 aThis concentration corresponds to the titrated amount of sulfur trioxide equivalents. The sulfur trioxide partial pressure of

experiments with salt addition was slightly lower compared to the experiments without salt addition which indicates slight

dilution in agreement with the titrations. However, both traits agree with the range of sulfur trioxide concentration discussed.

bTOFs of blind tests were calculated as if 600 µM of catalyst was present. Blind test in 65% oleum was conducted for 5 min (as

for the measurements in 65% oleum with K2PtCl4) and 2.5 h in 20% oleum. The experiment with 1 in 65% oleum was done for

0.5 h and thus the blind test is not directly comparable. As the difference between blind activity and catalytic activity is more

significant for K2PtCl4 this catalyst was used for most of the investigations. In 65% oleum much higher blind activity is observed

which is not fully clear yet. Measurements in 65% oleum are more challenging due to temperature increase and material stability

issues especially for Teflon parts which might release more active contaminations in 65% oleum (not clear if due to the higher

sulfur trioxide concentration or temperature effects).

aTo assess the exact redox potential in oleum is particularly challenging. By measuring the potential of

a concentration cell I could qualitatively confirm that the oxidation potential increases with increasing

SO3 content. However, I was unable to determine and reproduce exact values.


60

of 1 in 65% oleum decreased by approximately a factor of 2. This phenomenon was

further investigated with potassium tetrachloroplatinate (600 µM) in 65% and 20%

oleum with different concentrations of K+ and NH4

+. The results are summarized in

table 6, and it is obvious that addition of substantial amounts of salt decreases the

TOF. Although the decrease of TOF is unexpected at first sight, it is known that the

solubility of nonpolar substances is decreased upon addition of salt to the solution (in

case of methane-water at similar temperature and pressure, the addition of 1 M NaCl

reduces the solubility of methane by around 20%.156,157

). This effect of salting out

decreases the anyhow low concentration of methane further, and consequently also

decreases the rate of methyl bisulfate formation.

These observations and a publication that shows that the solubility of methane in

aqueous sulfuric acid is strongly dependent on the concentration of the acid158

evidence that the decrease of TOF at high sulfur trioxide concentrations is most

probably due to a lower solubility of methane in oleum with a high sulfur trioxide

content, respectively the TOF is in general convoluted by the concentration of

methane in solution. This implies that not only added ions can lead to salting out, but

also self-dissociation or changes in speciation of the medium. Interestingly, the

maximum of the TOF occurs close to 44.9% oleum where the medium has the formal

composition of H2S2O7 and a turning point for several properties.155

It seems unlikely

that the decrease can be explained by a reaction order of methane different from one

and differences in partial pressure (see figure 20): Comparison of the data from the

pressure dependency measurements in 20 and 65% oleum supports a lower solubility

of methane in 65% oleum, where the TOF values are lower, respectively level off

towards higher pressure.

A closer look shows that addition of 1.6 mol L-1

of K+ or NH4

+ to 65% oleum (entries

6 and 7) decreases the TOF by a factor of ten compared to the TOF without salt

addition (entry 4). In 20% oleum the same amount of salt decreases the TOF only by a

factor of 2 to 3 and the TOF is still higher than in 65% oleum without salt addition

(entries 11, 13 and 15). In addition to the bigger effect of salt addition in 65% oleum

it has to be considered that addition of salt dilutes the medium and it can be expected

that the concentration of sulfur trioxide, thus also acidity and oxidation potential, are

reduced. Therefore, if salting out was not the reason for the decrease, and, e.g., the

oxidation potential instead, the dilution by salt addition to 65% oleum should increase


61

the TOF because the starting point of the reaction is moved from the very right side of

figures 16 and 17 towards the maximum of the TOF which is obviously not the case

(the titrated sulfur trioxide concentration of entries 6 and 7 is close to the optimum).

For 20% oleum a decrease is expected anyway because the reaction proceeds on the

left side of the maximum in figures 15 and 16, and dilution by salt addition moves the

starting point of the reaction further to lower sulfur trioxide concentrations. In

conclusion these experiments show that solubility of methane seems to be a bigger

issue in 65% oleum than in 20%. In any case, for the understanding of this system it is

necessary to consider all physicochemical properties of the solvent, and not only

acidity.

Summarizing, the dependency of the activity on the composition of the medium can

be explained by redox and solubility properties. Both factors have already been

mentioned in a publication by the group of Bell for the carbonylation of methane to

acetic acid with Pd2+

in sulfuric acid.112

3.5.1.6.3 Comparison of different catalyst precursors

The lower activity of 1 compared to the other platinum compounds is likely a

consequence of differences in the ease of oxidation, as has been investigated

computationally for cis-(NH3)2PtCl2 and 1.93,99

As the oxidation step is assumed to be

rate limiting in the Periana system, a compound, respectively an intermediate of the

catalytic cycle, which is more easily oxidized should show higher activitya.

The main difference between 1 and the other Pt(II) compounds is the bpym ligand.

Whereas ammine and chloro ligands function as σ-donors, bpym also functions as π-

acceptor. Donors should rather stabilize Pt(IV), easing oxidation of Pt(II) to Pt(IV),

whereas acceptors stabilize Pt(II) and oxidation to Pt(IV) is more difficult.159

The

reverse step, reduction of Pt(IV) to Pt(II) has been studied in detail with the same

conclusions.160

The stabilizing effect for Pt(II) should additionally be increased by

protonation of bpym. It might be that such a picture is oversimplified and it has been

suggested in a computational study that considering bpym as a π-acceptor might not

be entirely complete and also its ability as σ-donor should be considered.

Nevertheless, the weaker σ-donor capability of bpym would still lead to the same

aIf the oxidation step was not limiting, inferior methane coordination properties would result in lower

activity. Compare to the catalyst concentration series, the model based on quasi steady state, and that


62

trend in the ease of oxidation. Also protonation of bpym reduces donation capability,

but influences on the other steps of the catalytic cycle have to be considered too.93,94

Furthermore, a study on the trends in the ease of oxidation shows the unexpected

result that (tmeda)PtI2Me2 (a Pt(IV) compound with σ-donors only) is capable of

oxidizing Pt(II) ligated with α-diimine ligands.161

However, in a subsequent study it

was clearly shown that diamine complexes of Pt(II) are more easily oxidized than the

corresponding α-diimine complexes.105

Both studies mention the importance of steric

factors. Consequently, even though the various influences are complex and do not

seem to be fully understood, bpym seems to stabilize Pt(II) and makes oxidation to

Pt(IV) more difficult.

The detrimental effect of excess bpym (table 6, entry 2) in combination with the

significantly higher activity of e.g., platinum(II) chloride (table 6, entry 4), raises

actually the question, if 1 is active for methane oxidation or a dormant species and

unligated Pt species are the origin of catalysis (in Periana´s original report free ligand

was observed at 200 °C in 20% oleum!29

). Addition of excess ligand would shift the

equilibrium of complex formation to the side of ligated platinum and reduce the

concentration of catalytically active species. Further studies are needed for

clarification, and this experiment is not a clear proof that 1 is only a dormant species.

Importantly, as will be shown later, 1 shows under certain conditions a better

performance than platinum(II) chloride, see figure 29 and table 9; the performance of

platinum(II) chloride would mark an upper limit for performance of 1, if 1 itself was

inactive, which would be reached if 1 was completely dissociated, leaving bpym and

platinum(II) chloride. Instead, the decrease could also be a consequence of

coordination of more than one bpym moiety to platinum in the weakly coordinating

reaction medium which would block methane coordination to the metal.162

For the more active catalysts, respectively precursors, it is difficult to make a clear

statement. It seems to be obvious that the compounds used are precursors and the

working catalyst is not the same entity as put into the reactor. Especially the oxidation

state of the working catalyst is not clear. Otherwise it would not be possible to

explain, why Pt(0), Pt(II) and Pt(IV) show activity in the same range.

for Shilov chemistry methane coordination is often rate determining and strong ligands are said to be

detrimental for activity.


63

The differences between the more active compounds should not be stressed too much,

especially as potassium tetrachloroplatinate was observed to be a sensitive system and

considerable differences between single measurements can be obtained. A reason for

differences in single measurements might be the reactions potassium

tetrachloroplatinate likely undergoes in oleum/sulfuric acid. Nevertheless, the trends

are still reproducible. If the oxidation state was neglected and if solubility was not an

issue, which could be possible at these concentrations, a rough qualitative ranking

would be: K2PtCl4, K2PtCl6 ≥ Pt(acac)2, Pt black > cis-(NH3)2PtCl2, PtCl2,

(bipy)PtCl2.

If (NH3)2PtCl2 and (bipy)PtCl2 are decomposed, this group would lead to the same

active species, namely PtCl2 (sol.). Pt black and bis(acetylacetonato)platinum(II) - acac

is decomposed under reaction conditions - would give a dissolved platinum species

without chloro ligands. Potassium tetrachloroplatinate and potassium

hexachloroplatinate would lead to Pt chloro complexes with more than two chloro

ligands.

All together this might indicate different speciation under reaction conditions. This

will be further evidenced in the section about the influence of catalyst concentration,

where also the transformation of catalyst precursors will be shown. In general, the

influence of speciation is known from Shilov chemistry. It was shown experimentally

that PtCl2(H2O)2 is more active than PtCl3(H2O)- and PtCl4

2- for H/D exchange of

cyclohexane in DAc/D2O,159

and it was confirmed computationally for methane

activation in case that coordination of methane proceeds by a dissociative mechanism.

In case of an associative mechanism, PtCl3(H2O)- was found to be more active.

163 A

direct comparison is difficult, because for the examples mentioned, alkane

coordination is rate determining and the conditions of these studies are significantly

different from the conditions used here. In any case, slight differences in the

reactivity, e.g., the ease of oxidation for different chloro complexes, can also be

expected. Actually, this would also imply that a difference between potassium

tetrachloroplatinate and potassium hexachloroplatinate should be observed which is

not the case and shows that further experiments with a higher degree of experimental

control are necessary. Still valid is that all tested platinum sources, irrespective of the

oxidation state, are much more active than 1.


64

3.5.1.6.4 Oxidant, oxidation state of Pt, and over-oxidation dilemma

The virtually same reactivity of potassium tetrachloroplatinate and potassium

hexachloroplatinate goes hand in hand with the question to the actual oxidant. A

conclusion if Pt(IV)101

or sulfur trioxide98

serves as the oxidant cannot be drawn from

these results. According to Periana, Pt(IV) as oxidant is important for catalyst

stability. Whether Pt(IV) is solely responsible for product formation is not clear, and

it is likely that in oleum Pt(IV) and sulfur trioxide both contribute to product

formation. As can be seen from the Nernst equation (10), a higher oxidation potential

results in general in a higher activity of the oxidized species, i.e., higher Pt(IV)

concentration, which would also lead to an increase in the rate of oxidation if Pt(IV)

were the oxidant.

One observation which supports Pt(IV) as oxidant is the difference in the colors, if 1

is dissolved in sulfuric acid or oleum (50 mM, room temperature). The solution of 1

in sulfuric acid has an orange color (indicative for Pt(II)) whereas a yellow solution

(indicative for Pt(IV)) is obtained in 20% oleum. A similar color change has been

mentioned by Periana´s group upon heating a solution of 1 in 98% sulfuric acid to

200 °Ca.101

This means, it is likely that already before the catalyst solution is

contacted with methane a large part (or maybe all) of the platinum is in the oxidation

state (IV).

On the other hand there is a study which demonstrates oxidation of [Pt(II)CH3Cl3]2-

with several different one and two electron oxidants,164

indicating little constraints on

the oxidant, and supporting either both or rather sulfur trioxide (due to the higher

concentration) as oxidant. This does not necessarily include molecular oxygen as

possible oxidant as molecular oxygen usually occurs in a triplet state and requires

activation, too.165-167

In the same study it is also mentioned that [Pt(II)Cl4]2-

is oxidized as well under

otherwise identical conditions. If Pt(IV) is not active for C-H activation, as is

commonly assumed, this would lead to an inactive system. On the basis of (10) it can

also be expected that in the experiments presented here Pt(II) is gradually depleted

with increasing sulfur trioxide concentrationb and therefore the over oxidation

aUnfortunately this information was only mentioned and not shown in the supporting information,

although announced. bNote that the oxidation potential increases rapidly at the transition from sulfuric acid to oleum and

steadily but slowly in oleum.


65

dilemma might look like a reasonable explanation for the decrease of TOF at high

sulfur trioxide concentrations. This implies that increasing oxidation potential

accelerates the rate of the oxidation step, but at the same time slows the rate of C-H

activation due to depletion of Pt(II) until C-H activation is limiting at high oxidation

potential.23,105

However, there is evidence that the over oxidation dilemma is unlikely to be a serious

issue for methane oxidation in sulfuric acid/oleum: It was recently reported that using

a model of 1 in the oxidation state (IV) gives the same outcome as using 1 directly.101

Similarly, PtCl4 was shown to be an effective catalyst for methane oxidation.107

Also

there is no obvious difference in using potassium tetrachloroplatinate or potassium

hexachloroplatinate as catalyst (670 µM, 20% oleum, see table 5 entries 9, 10). This

does not necessarily prove Pt(IV) as active catalyst for C-H activation, as Pt(IV)

compounds can be contaminated by traces of Pt(II) and vice versa24

or a self-repair

mechanism could be in operation,101

but it shows that if over oxidation was an issue in

sulfuric acid/oleum it would only be so at very high ratios of Pt(IV) to Pt(II),

respectively complete depletion of Pt(II). Pt(II) in turn would need to be tremendously

active for C-H activation. One should therefore not exclude Pt(IV) as active for C-H

activation, especially not in an unusual solvent like sulfuric acid/oleum at 215 °C.

Indeed, Pt(IV) is much less used for C-H activation and examples either activate

aromatic C-H bonds or if aliphatic bonds are activated making use of

photochemistry.80,100,102-104

It should also be considered that 215 °C is a high and usually not investigated

temperature in the field of C-H activation with defined organometallic complexes.80

This means that the higher temperature used for methane oxidation might compensate

an indeed lower but perhaps present activity of Pt(IV) towards aliphatic C-H bonds,

which would allow the overall reaction to proceed at a decent rate.

In the context discussed here also the research of Wickleder´s group as well as others

is worth emphasizing: Their work did not only show that concentrated sulfuric acid

above 350 °C is capable of oxidizing elemental platinum (explaining why Pt black is

a working precursor and deactivation by formation of Pt black is unlikely), but also

that different precursors lead to compounds with a common motif of platinum dimers

in the oxidation state (III).168-173

Recently it was shown that 65% oleum at 160 °C


66

oxidizes platinum into the oxidation state (IV).174

The possible role of these

compounds for methane oxidation in sulfuric acid/oleum remains to be answered.

3.5.1.6.5 NMR studies

To clarify some of these questions and gain a better mechanistic understanding,

spectroscopic observation of this reaction is obviously a must. NMR studies were

undertaken to follow this direction. This is quite challenging for several reasons:

- presence of protonated species

- relatively low substance concentration

- unusual viscos solvent

- NMR properties of 195

Pt

It has recently been mentioned that “after numerous attempts (…) no 195

Pt-NMR

resonances could be clearly observed”.101

Nevertheless, Christophe Farès managed to

obtain spectra which showed, amongst others, clear 195

Pt resonances. One key was

short repetition time since the presence of 14

N leads to fast relaxation.175

These studies

can serve as starting point for interesting mechanistic studies in the future. NMR data

of 1 in DMSO-d6 is also available.176

Some of the spectra are shown here with

emphasis on the difference between oleum and sulfuric acid but not discussed in

detail as sound understanding has not been established, yet. Deuterated oleum was

prepared by dissolving liquid sulfur trioxide in D2SO4.

Both 1H and

15N HMBC (see appendix) show that the symmetry of bpym is broken

and platinum is coordinated to one side of bpym. In accordance with the different

colors also the 1H spectrum shows differences with respect to sulfuric acid or oleum.

Figure 22 shows that the spectra of 50 mM solution of 1 in both solvents exhibit three

main signals, albeit at different chemical shifts. Furthermore, each main signal has a

corresponding minor signal, probably from protonated species. Multiply protonated

species have been proposed to lead to the complex 1H NMR spectra;

101 protonation is

indeed likely and should occur at the nitrogen atoms; resonances from protonation

should be observable. In non-deuterated solvent a triplet with 1:1:1 intensity would be

expected due to coupling with 14

N. This was the case for bpym, as shown in the

appendix. In case of 1 precipitated by dilution of sulfuric acid or oleum and measured

in DMSO-d6 a triplet at around 7 ppm is observed, vide appendix. Nevertheless, in

oleum the smaller signal is upfield whereas in sulfuric acid the smaller signal is


67

downfield. In sulfuric acid also free ligand might be present, corresponding to the

signals at 9.4 and 10.3 ppm, which are also visible in precipitated samples.

Figure 22: 1H NMR of 1 in a): d-oleum and b): D2SO4.

Upon heating the spectra significantly change. 50 mM solutions of 1 were first

measured at room temperature and then step wise heated to 100 °C, measured at each

temperature step, and cooled back to room temperature. The spectra of the complete

series are in the appendix. Figure 23 shows the ones at room temperature (lower two

spectra, identical to figure 22) after heating to 100 °C (middle two spectra) and after

cooling again to 40 °C (upper two spectra). Clearly, heating changes the system

substantially, and more changes happen in oleum. Aside from minor changes in

D2SO4, the basic structure of the signals is retained, with three main signals and a

corresponding smaller downfield shift. In d-oleum it seems that the two presumably

differently protonated species get equally populated. Other species are present as well,

and after cooling to 40 °C all of these species are still there.

a

b

ZIR-ZA-251-01

ZIR-ZA-245-01


68

Figure 23: 1H NMR of heating series of 1 in D2SO4 and d-oleum: a): room temperature sulfuric acid,

b): room temperature oleum, c): 100 °C sulfuric acid, d): 100 °C oleum, e): 40 °C sulfuric acid, and f):

40 °C oleum.

Nevertheless, since these spectra show the features of many different species a clear

assignment is difficult. During the same heating series also 195

Pt NMR spectra were

measured. Figure 24 presents the 195

Pt NMR spectra of 1 in D2SO4 (bottom spectrum,

a) which does not significantly change upon heating and subsequent cooling with

three signals at -559, -2198 and -2321 ppm (relative to Na2PtCl6).

a

b

c

d

e

f


69

Figure 24: 195

Pt NMR spectra of 50 mM 1 in a): D2SO4 at room temperature, b): d-oleum at room

temperature, and c): d-oleum at 100 °C.

The spectrum in d-oleum at room temperature (middle, b) shows two signals at -1576

and -1899 ppm. Interesting, however, is the appearance of two new signals at roughly

-2800 and -3400 ppm after heating. Actually, all these signals appear in a region

where typically Pt(0), Pt(II) and Pt(IV) compounds resonate, which does not allow a

clear assignment.177

However, it seems logical to assume that the new species formed

in d-oleum play a role in the catalytic transformation of methane. Catalytic testing at

lower temperature, corresponding to the temperatures of NMR experiments would

give deeper mechanistic insight and should be a goal in the future.

Solutions should be prepared freshly since aging occurs (50 mM solution of 1 in

D2SO4 slowly turns yellow). Furthermore, heating of a 50 mM solution of 1 in

sulfuric acid leads to precipitation of a red solid.

Also free ligand was observed after external heating to 170 °C for 1.5 h. More spectra

can be found in the appendix.

3.5.1.6.6 Mechanistic considerations

Even though further studies are needed to clarify especially the oxidation state of the

active species, the drastically increased activity in oleum actually raises another

question, namely if the oxidation step is still limiting in oleum. The conclusion that

a

b

c


70

oxidation is rate limiting is based on the fact, that H/D exchange happens in D2SO4 at

150 °C, but not oxidation. This question has not answered by combining H/D

exchange experiments with mass spectrometry, but approached by careful estimation

and comparison of TOF values in oleum and literature values in concentrated sulfuric

acid. Therefore expected TOFs from ΔG‡, based on the values given in reference 101,

were calculated (H/D exchange ΔH‡ 28 kcal mol

-1, ΔS

‡ -11 e.u.; methyl bisulfate

formation ΔH‡ 34.3 kcal mol

-1, ΔS

‡ -3.8 e.u.; all values determined from

measurements between 169 – 190 °C). Because these values are apparent and apply

strictly only for the set of conditions used in that study, the calculated values are

compared with experimental ones that are measured under similar conditions, i.e.,

much higher catalyst concentration compared to the experiments discussed so far.

Only values for 1 are presented, and they are shown in table 7.

Table 7: comparison of expected and observed TOF

entry reaction and conditions ΔG‡

/kJ mol-1

TOFcalc.

/h-1

TOFobserved

/h-1

215 °C

1 H/D exchange, 35 mM (1), 98% D2SO4 139.6 42.0

2 MBS formation, 35 mM (1), 98% H2SO4 151.3 2.4

3 MBS formation, 50 mM (1), 98% H2SO4a 6

4 MBS formation, 50 mM (1), 20% oleum 264

150 °C

5 H/D exchange, 35 mM (1), 98% D2SO4 136.6 0.4

6 MBS formation, 35 mM (1) 98% H2SO4 150.2 0.009

7 MBS formation, 35 mM (1) 20% oleum 26 a integral measurement

Entry 1 and 2 show that H/D exchange proceeds faster in sulfuric acid than methyl

bisulfate formation. The experiment with 50 mM 1 in 98% sulfuric acid (entry 3)

confirms former studies.29,101,120

In 20% oleum (entry 4) methyl bisulfate formation

proceeds with a significantly higher TOF compared to H/D exchange in sulfuric acid.

At 150 °C the same applies (entries 5-7): Whereas H/D exchange happens with a

higher rate in sulfuric acid, the TOF measured for methyl bisulfate formation in 20%

oleum is two orders of magnitude higher compared to H/D exchange in sulfuric acid.

This and especially the abrupt change at the transition from sulfuric acid to oleum


71

puts some question marks on the generalization of oxidation being rate determining

above 85% sulfuric acid.

Due to the known increase of the rate of H/D exchange with increase in

acidity91,92,101,120

it could be argued that in oleum also H/D exchange is faster and

oxidation should still be the bottleneck. Experimentally, this has not been checked;

the increase in the rate of H/D exchange has to my knowledge only been measured in

sulfuric acid and not in oleum. Two reasons for this increase in rate of H/D exchange

with acidity have been discussed so far: Less ground state stabilization with

increasing acidity due to protonation of methanol or water, which would coordinate to

the platinum,91,92

or protonation of the HSO4- ligand, which was found

computationally to facilitate uptake of methane.96

With respect to the former, any

nucleophile should be protonated in superacid medium and ground state stabilization

is unlikely to occur at all, no matter if the reaction is done in 5, 20 or 40% oleum, and

no difference in rate would be expected with respect to H/D exchange unless another

effect may be present.

With respect to the latter, ionic self-dissociation should play a decisive role and

determine the amount of bisulfate present. As electrical conductivity is a good

measure of the extent of ionic self-dissociation, H/D exchange would be expected to

correlate inversely with conductivity of the solution. In sulfuric acid this correlation

between conductivity and rate of H/D exchange exists from around 90 to 100% acid,

although this does not mean that there is a cause-effect relationship. In oleum,

however, the conductivity first increases and then decreases again.155

Comparison of

conductivity with figure 16 and 17 reveals no obvious correlation. Thus, for both

cases, no correlation between expected behavior of H/D exchange and measured TOF

of the overall cycle exist. This rather speaks against H/D exchange as rate determining

step.

Thus, a final conclusion is not possible, which is due to two reasons: The apparent

nature of the TOFs complicates any mechanistic study. The concentration of methane

can be expected to be different in 20% oleum which influences the rate of H/D

exchange as well as the overall cycle. Furthermore, as all four steps of the catalytic

cycle proceed in the experiment at 150 °C in 20% oleum, ambiguity remains in any

case. Therefore, further studies are necessary. It would be interesting to see if this

problem can be solved by computational methods, accounting for the changing


72

properties, especially the oxidation potential, of sulfuric acid and oleum as well as

concentrations of catalyst and methane therein. This, however, would entail

methodological developments, as so far DFT studies usually account for solvation

effects in sulfuric acid by modeling its properties only summarily with a solvent

radius of 2.205 Å and a dielectric constant of 98 F m-1

.93-96

3.5.1.6.7 Associated issues

Other reasons for the decrease of the TOF with high sulfur trioxide concentrations

cannot be fully excluded, and it should be mentioned that the non-isothermal behavior

of some experiments, e.g., with 65% oleum, is a problem; also error margins are

bigger if high sulfur trioxide concentrations are used. Under the used conditions the

partial pressure of sulfur trioxide reaches 50 to 60 bar before pressurizing with

methane. Due to non-ideality of the system, a part which is crudely estimated, based

on the difference of the theoretical partial pressure of methane and the measured total

pressure, to 5-10 bar of the sulfur trioxide dissolves back upon pressurizing with

methane. Due to the exothermicity of the dissolution of sulfur trioxide in oleum a

temperature increase of up to 15 K can be observed, also in the experiments with salt

addition and blind tests. Actually, with increasing sulfur trioxide concentration the

temperature increase upon pressurizing with methane gradually increases. It cannot be

fully excluded that these temperature effects cause, respectively contribute, to the

decrease of the TOF. However, it does not seem likely to be the major contribution to

the lower activity in 65% oleum, because in case of high absolute rates in 20% oleum

(0.4 mol h-1

, vide infra) also non-isothermal behavior has been observed leading to

temperature increase of 7 K. In this case, a blind experiment did not show any

significant change of temperature after pressurizing with methane, and hence the

increase in temperature for this experiment can be attributed to the oxidation of

methane to methyl bisulfate and not to redissolution of sulfur trioxide in oleum. It

might be, however, that in some cases both effects contribute to a temperature

increase.

All experiments presented so far reveal an obvious problem in comparing TOFs from

different publications: Not only might the ratio of sulfuric acid/oleum to methane and

the experimental approach be different in different publications. Even if this was not

the case, a comparison of TOFs would be difficult, because the TOFs are apparent and


73

depend on many factors. Consequently, one should always use benchmarks, e.g., 1

and potassium tetrachloroplatinate, for comparison.

The most obvious convolution seems to be the concentration of methane: Even if the

reactions are carried out at the same partial pressure of methane but at different sulfur

trioxide concentration, the liquid phase concentration of methane can be substantially

different. Unfortunately, it seems that only solubility values for sulfuric acid at room

temperature and low pressure (few mbar) are published, and no data for oleum are

available at all. But from these values it becomes obvious that in a relatively narrow

window of acid concentration the liquid phase concentration of methane can easily

differ by a factor of four (α decreases from 100 at 75% to 25 at 100% sulfuric acid).158

It would be extremely useful to have more experimental results on the

physicochemical properties of the system methane - oleum. Attempts to measure the

solubility of methane in oleum are under preparation.

The sulfur trioxide and pressure dependency series also indicated that determining

rates from the pressure drop might be dangerous: Although both series were

conducted with the same autoclave, had almost the same pressure at the beginning of

the reaction and were stopped after the same pressure drop, the series with potassium

tetrachloroplatinate led usually to conversion of 25%, whereas the series with 1 to

conversions of 15%. Also the carbon balance of the series with 1 was slightly lower

compared to that with potassium tetrachloroplatinate. The origins remain completely

unclear and need to be clarified in the future.

The most important facts gained from experiments with molecular catalysts up to this

point are:

- oleum with its high oxidation potential leads to drastic increase in activity

compared to sulfuric acid

- solubility of methane in the medium seems to be critical

- simple platinum compounds are much more active than 1 and stable

- many mechanistic questions remain open, including the nature of the active

species


74

3.5.2 Influence of catalyst concentration


These results, especially the high TON, respectively stability, of potassium

tetrachloroplatinate and other platinum compounds, seem to be contradicting

Periana´s original publication.29

However, as already mentioned, apart from the sulfur

trioxide concentration, the conditions used in the original report differ also

substantially in the catalyst concentration, specifically 600 µM mostly discussed so

far versus 50 mM in the original report. The patent,30

which is doubtless the basis for

Periana´s original publication,29

strengthens the view that these seemingly conflicting

results can originate from the different catalyst concentrations used: Most of the

experiments described in the patent were performed in 96% sulfuric acid and with

catalyst concentrations ranging from 50 to 100 mM. Some experiments actually

indicate that also simple Pt compounds can be used as efficient catalyst for methane

oxidation.30

Because at low catalyst concentration in 96% sulfuric acid potassium

tetrachloroplatinate still shows higher activity than 1, vide supra, either the high

catalyst concentration or the combination of high catalyst concentration and low

sulfur trioxide concentration should have a detrimental effect on the performance of

potassium tetrachloroplatinate. Consequently, methane oxidation with a catalyst

concentration of 50 mM in 98% sulfuric acid was investigated. Because

computational studies predict the transformation of cis-(NH3)2PtCl2 to platinum(II)

chloride,94

and, as will be shown later, also potassium tetrachloroplatinate is

transformed to platinum(II) chloride, these experiments were performed with 1,

potassium tetrachloroplatinate and platinum(II) chloride as catalyst. Figure 25 shows

the pressure vs. time curves of these experiments. Reaction time for these experiments

was 2.5 h, which means that in case of 1 this was by far not differential and optimal:

After approximately 30 min, the experiment with 1 reached the pressure drop after

which the reaction was stopped in the previous experiments, and shortly after the

pressure started to increase again, due to over oxidation to carbon dioxide.


75

Figure 25: Pressure time curves of methane oxidation in 98% sulfuric acid (15 mL) at 215 °C with a

catalyst concentration of 50 mM (pCH4≈ptotal≈55 bar).

From comparison of the pressure drop it is immediately evident that activity decreases

in the order 1 >> PtCl2 > K2PtCl4. The pressure increase for 1 at longer reaction times

indicates that oxidation of methyl bisulfate and formation of carbon dioxide already

becomes significant, and the reaction time does not seem appropriate for maximum

yield, illustrating the problem associated with integral approaches. A detailed

comparison of the performance of 1, platinum(II) chloride and potassium

tetrachloroplatinate in 98% sulfuric acid and 20% oleum at low (600 µM) and high

(50 mM) catalyst concentration is given in table 8. In 98% sulfuric acid the

performance is completely altered if the catalyst concentration is increased from

600 µM to 50 mM. In 20% oleum the performance of all catalysts is better by at least

one order of magnitude, and again potassium tetrachloroplatinate is the best catalyst at

600 µM whereas at 50 mM it shows the worst performance. The best catalyst at high

concentration in 20% oleum is platinum(II) chloride. In general the TOF decreases

with increasing catalyst concentration but in case of potassium tetrachloroplatinate not

only the TOF, but also the absolute rate is lower with higher catalyst concentration.


76

Table 8: Comparison of the catalytic performance of 1, PtCl2 and K2PtCl4 in 98% sulfuric acid and

20% oleum with low (600 µM) and high (50 mM) catalyst concentration at 215 °C.

entry catalyst rMBS /mol h-1

TOF /h-1

CCH4 /% SMBS /%

98% H2SO4a 600 µM

1 1 0.00044 49 2.9 79

2 PtCl2 0.00060 66 3.6 87

3 K2PtCl4 0.00146 164 8.8 86

98% H2SO4a 50 mM

b

4 1 0.00456 6 46.1 87

5 PtCl2 0.00181 2 15.3 96

6 K2PtCl4 0.00037 0.5 4.1 83

20% oleum 600 µM

7 1c

0.01148 1281 19.2 96

8 PtCl2 0.13728 15216 20.6 97

9 K2PtCl4 0.20499 22998 24.1 98

20% oleum 50 mM

10 1 0.19831 264 30.2 98

11 PtCl2 0.37413 503 29.5 98

12 K2PtCl4d

0.06540

87

21.3

93

aTitrated acidity of the experiments with PtCl2 is slightly higher but still below oleum concentration. This might be a systematic

error as experiments were done later with other stock solutions. bCare has to be taken in comparing the values in this series: rMBS

and TOF of 1 is clearly underestimated due to oxidation of methyl bisulfate. Selectivity should not be compared at all because

conversions are significantly different and the absolute amounts of CO2, too, i.e., although in the experiment with 1 a higher

absolute amount of CO2 is formed, selectivity is still higher compared to K2PtCl4. Values correspond to experiments of figure 25

caverage of four measurements daverage of four measurements with different pretreatments for K2PtCl4 (grinding and different

preheating times)

All experiments were done with 15 mL solution at a total pressure of 55 bar (98% sulfuric acid, 50 mM), 67 (98% sulfuric acid,

600 µM), respectively 72 bar (20% oleum). Experiments in 20% oleum stopped after identical pressure drop; experiments in 98%

sulfuric acid after 2.5 h (50 mM), respectively 2 h (600 µM).

Part of the explanation for this behavior is the difference in solubility of the different

Pt compounds. 1 has a reasonable solubility in sulfuric acid/oleum, whereas neither

potassium tetrachloroplatinate nor platinum(II) chloride show significant solubility.

Although platinum(II) chloride and potassium tetrachloroplatinate have intrinsically

higher activity (shown by the experiments with low catalyst concentration) it can be

expected that a dissolved catalyst shows higher activity compared to an undissolved

solid where only surface atoms are catalytically active. Consequently at sufficiently


77

high concentration the amount of intrinsically less active but dissolved catalyst

outweighs the performance of highly active but weakly soluble catalysts.

3.5.2.1 Catalyst concentration series in 20% oleum

The comparison of the three compounds at low and high concentration in oleum and

sulfuric acid showed that previous studies with high catalyst concentration in

combination with the low solubility of the investigated compounds, as well as the use

of rather sulfuric acid than oleum, suggested that simple Pt compounds are unstable

and 1 is the better catalyst. However, these new findings resulted in two new

questions: First, why the TOF decreases with increasing catalyst concentration not

only for platinum(II) chloride and potassium tetrachloroplatinate, which could be

explained based on solubility, but also for 1, and second, why the absolute rate of

methyl bisulfate formation with potassium tetrachloroplatinate is lower if more

catalyst is deployed.

Therefore a concentration series with 1 and potassium tetrachloroplatinate in 20%

oleum was conducted; for completeness and comparison also the available values of

platinum(II) chloride are presented. A concentration of 20% oleum was chosen for the

following reasons: (i) Activity and selectivity are higher in oleum (due to the shorter

reaction time and the higher rate of methane oxidation compared to methyl bisulfate

oxidation TOFs are easier to obtain and more reliable; in some cases this is still a

problem). (ii) 20% oleum is commercially available and displays considerable

advantage with respect to corrosion over concentrated sulfuric acid, respectively

handling over oleum with a high sulfur trioxide content. (iii) The most important

reason, however, is the solubility of 1: As other factors than solubility influencing

activity are of interest, any possibility of precipitation was excluded by deciding for

20% oleum as precipitation of 1 was never observed in 20% oleum, but in

concentrated sulfuric acid upon heating.

The trend seen from table 8 is confirmed with more data points: The TOF in 20%

oleum decreases with increasing catalyst concentration for 1, platinum(II) chloride

and potassium tetrachloroplatinate (see figure 26) and drops faster in the following

order 1 << PtCl2 < K2PtCl4. At low catalyst concentration, i.e., 600 µM, potassium

tetrachloroplatinate exhibits a TOF of 22998 h-1

, platinum(II) chloride still achieves a

value of 15216 h-1

whereas 1 has a TOF which is one order of magnitude lower,


78

1281 h-1

. At high catalyst concentration the difference is substantially lower and the

performance of the catalysts is altered.

Figures 26: Dependency of TOF on catalyst concentration in 20% oleum (15 mL) at 215 °C a, c):

K2PtCl4 and PtCl2, b): 1, and d): comparison between the TOF of 1, based on methyl bisulfate only and

the sum of methyl bisulfate and CO2 (CCH4 20-30%, ptotal 72 bar, pCH4 65 bar)

For better comparison, the absolute rate of methyl bisulfate formation is plotted

against the catalyst concentration in figure 27. The absolute rate of methyl bisulfate

formation, rMBS, increases monotonically, but not linearly, for 1 and platinum(II)

chloride. Toward high catalyst concentration the increase in rate is leveling out.

Interestingly, platinum(II) chloride shows a higher activity compared to 1 over the

whole concentration range which means that in 20% oleum the intrinsic higher

activity of platinum(II) chloride outperforms the better solubility of 1. Another

interesting aspect is that although at a formal concentration of 20 mM the majority of

platinum(II) chloride does not dissolve, further increase of the rate is observed with

increasing concentration. This could mean that a reaction is happening on the surface

of the undissolved material. Nevertheless, for potassium tetrachloroplatinate the rate

shows a peculiar dependence on catalyst concentration: The rate (not TOF!) shows a

maximum at a concentration of 670 µM and decreases, if the concentration is further

increased.

a b

c d


79

Figure 27: Dependency of rMBS on catalyst concentration in 20% oleum (15 mL) at 215 °C. All

reactions stopped after the same pressure drop; ptotal 72 bar pCH4 65 bar CCH4 20-30%.

3.5.2.2 Convolution of catalyst concentration and methane

concentration – pre-equilibrium model

As mentioned, effects other than catalyst solubility are of interest for the

understanding of the system as well. Thus the discussion is limited to 1 first. Later,

with additional experiments, the characteristics of the curve for potassium

tetrachloroplatinate will be explained.

Actually, at a small catalyst concentrations and/or long reaction times (up to 15 h), a

slight complication arises for the analysis of the dependency of the reaction rate on

the concentration of 1. If the rate is based on the amount of methyl bisulfate formed,

the value is underestimated due to decomposition of methyl bisulfate. The alternative,

calculation of the rate based on the sum of methyl bisulfate and carbon dioxide (figure

26 d) is also ambiguous, since carbon dioxide can be formed via different pathways.

Thus, for the following discussion especially the values taken at higher catalyst

concentration are important, since the reaction time is sufficiently short to reduce

uncertainty with respect to methyl bisulfate formed, due to little or no methyl bisulfate


80

decomposition, and the rate and TOF can confidently be based on methyl bisulfate. It

is clear in any case that the TOFs above concentrations higher than 600 µM decrease

with increasing catalyst concentration (figure 26b) which corresponds to the leveling

off in rate seen in figure 27.

This could in principle indicate gas/liquid mass transfer limitation as has been

recently proposed for H/D exchange with 1 in 98% D2SO4 at 165 °C.101

However,

other explanations for such behavior are also possible. Mass transfer does not seem to

be an issue here: The dependency of the rate of methyl bisulfate formation with 1

(600 and 300 µM) on stirring speed is shown in figure 28. Without stirring rMBS is

similar for both catalyst concentrations indicating that mass transfer is limiting. At

300 rpm the difference between both concentrations is substantial, and further

increase in stirring speed does not lead to increase in rates further in both series.

Usually experiments are performed at 1000 rpm which should ensure sufficient gas

liquid mixing. Thus, as already at a catalyst concentration of 600 µM a decrease of

TOF can clearly be observed (figure 26b) gas/liquid mass transfer does not seem to

cause this trend.

Figure 28: Dependency of the rate of methyl bisulfate formation on the stirring speed in 20% oleum at

215 °C with 1 (CCH4 20-30%, ptotal 72 bar, pCH4 65 bar).


81

Another important evidence against mass transfer as explanation is the highest

observed rMBS of 0.374 mol h-1

(PtCl2 50 mM, 20% oleum, see figure 29). This value

is more than an order of magnitude higher than the rates where already a decrease of

TOF can be observed, and still a factor of two higher than rMBS with 50 mM of 1.

0.374 mol h-1

(reduced by some safety margin) can be seen as the rate up to which the

used reactor system does not suffer from mass transfer issues (this is a minimum

value!). Due to these facts, mass transfer limitation does not look like a reasonable

explanation for a decreasing TOF with increasing catalyst concentration.

Instead this behavior could be caused by the generally low methane concentration in

the liquid phase. Some of the cited publications assume concentrations in the range of

1 -33 mM.25,29

In general estimation of the concentration is extremely difficult

because under high temperature/high pressure conditions also counterintuitive

behavior arise. E.g., if temperature is increased, the solubility might initially decrease

but later increase again as shown for methane-water.156,157,178,179

Anyhow, in water at

210 °C and a pressure of 50 bar the liquid phase concentration of methane is

0.0556 mol kg-1

.156

With a density of water at 220 °C of 0.84 g mL-1

this corresponds

to a value of approximately 50 mM. Clearly, extrapolation from water to sulfuric acid

can be doubted, and solubility is different in sulfuric acid/oleum. Nevertheless it

might be taken as indication, and shows that catalyst concentration and methane

concentration could possibly lie in the same range.

Furthermore, methane is a weak ligand and it cannot be expected that even in case of

a 1:1 ratio of methane to platinum all of the methane coordinates to the platinum,

respectively, excess of methane would be required to bring all catalytic centers into

operation at the same time. This is commonly presented in descriptions of the

catalytic cycle by including σ-complex formation before the actual C-H activation

(see scheme 1).

Since C-H activation and functionalization are facile, the system is expected to be

described by the simplified catalytic cycle depicted in scheme 4, where only σ-

complex formation, its decay, and formation of methyl bisulfate from the σ-complex

are taken into account. Based on this cycle the following equations can be derived,

with the main assumption of quasi steady state of the σ-complex, (PtCH4).


82

Scheme 4: Simplified catalytic cycle for methane oxidation.

Neglecting consecutive reactions, the rate of methyl bisulfate formation, rMBS, is

described by equation (11), where kMBS is the rate constant of methyl bisulfate

formation.

𝑟𝑀𝐵𝑆 =𝑑𝑀𝐵𝑆

𝑑𝑡= 𝑘𝑀𝐵𝑆 ∗ (𝑃𝑡𝐶𝐻4) (11)

Based on the main assumption of quasi steady state, the concentration of the σ-

complex should not change. Thus:

𝑑(𝑃𝑡𝐶𝐻4)

𝑑𝑡= 𝑘1 ∗ 𝑃𝑡 ∗ 𝐶𝐻4,𝑠𝑜𝑙 − (𝑃𝑡𝐶𝐻4) ∗ (𝑘−1 + 𝑘𝑀𝐵𝑆) = 0 (12)

Where k1 is the rate constant of σ-complex formation, k-1 the rate constant of σ-

complex decay, Pt the concentration of platinum in solution, and CH4,sol the

concentration of dissolved but unbound methane.

With the arbitrary constant 𝐾 =𝑘−1+𝑘𝑀𝐵𝑆

𝑘1,

and the mass balance of methane 𝐶𝐻4,𝑡𝑜𝑡𝑎𝑙 = 𝐶𝐻4,𝑠𝑜𝑙 + (𝑃𝑡𝐶𝐻4), (12) can be

rewritten:

𝑃𝑡 ∗ (𝐶𝐻4,𝑡𝑜𝑡𝑎𝑙 − (𝑃𝑡𝐶𝐻4)) − (𝑃𝑡𝐶𝐻4) ∗ 𝐾 = 0 (13)

Deriving an expression for (PtCH4) from (13), and putting it into (11) gives:

𝑟𝑀𝐵𝑆 = 𝑘𝑀𝐵𝑆 𝑃𝑡∗𝐶𝐻4,𝑡𝑜𝑡𝑎𝑙

𝑃𝑡+𝐾 (14)

If 𝑃𝑡 ≫ 𝐶𝐻4,𝑡𝑜𝑡𝑎𝑙, 𝐶𝐻4,𝑡𝑜𝑡𝑎𝑙 = (𝑃𝑡𝐶𝐻4), and the maximum possible rate for a given

methane concentration will be observed. Thus:

𝑟𝑚𝑎𝑥 = 𝑘𝑀𝐵𝑆 ∗ 𝐶𝐻4,𝑡𝑜𝑡𝑎𝑙 (15)

And:

𝑟𝑀𝐵𝑆 = 𝑟𝑚𝑎𝑥 𝑃𝑡

𝑃𝑡+𝐾 (16)

A similar equation, albeit with a different constant, can be derived if equilibrium is

assumed between platinum, methane and the σ-complex. In general this formalism is

closely related to the Michaelis-Menten formalism, known from enzyme kinetics.


83

(16) can be linearized by double reciprocal plot which yields a linear relationship,

corresponding to the Lineweaver-Burk plot.

1

𝑟=

1

𝑟𝑚𝑎𝑥+

𝐾

𝑟𝑚𝑎𝑥

1

𝑃𝑡 (17)

Equation (17) was applied for values of 1 with a concentration greater than 800 µM

which is shown in figure 29. As predicted by (17) a linear correlation exists. Other

possibilities to linearize (16) are shown in figure 30, and all give clear linear

relationships in the relevant region.

Figure 29: Linearization of the dependency of rMBS on catalyst concentration by double reciprocal plot

(Lineweaver Burk plot).

Figure 30: a): Eadie Hofstee plot, and b): Hanes Woolf plot for linearization of the dependency of rMBS

on catalyst concentration (1 in 20% oleum at 215 °C).

a b


84

Furthermore a hyperbolic fit was applied to all data points which is shown in figure

31. The coefficient of determination is reasonably close to 1, indicating agreement

between model and data points.

Figure 31: Hyperbolic fit of the dependency of rMBS on catalyst concentration.

Figures 32: a): plot of the residuals of the hyperbolic fit in figure 31, and b): extension.

Table 9 summarizes the parameters rmax and K obtained by the different methods. All

methods give basically the same result, albeit with a different weighting of the data

points. The residuals of the hyperbolic fit (figure 32) indicate that the model is not

a b


85

sufficient at low catalyst concentration. This might be a result of underestimation of

rMBS at low catalyst concentrationa.

Table 9: Parameters obtained for rmax and K by linearization and fit.

entry method K /mol L-1

rmax /mol h-1

1 hyperbolic fit 0.014 0.256

2 Lineweaver Burk 0.0147 0.2540

3 Eadie Hofstee 0.0149 0.2565

4 Hanes Woolf 0.0151 0.2581

This model underlines the problem of the low solubility of methane in sulfuric

acid/oleum: If it was possible to increase the liquid phase concentration of methane,

e.g., by use of solvent mixtures or additives, the rate of methyl bisulfate formation

would be further increased due to a higher concentration of the intermediate σ-

complex, until methane would be in significant excess. In general higher pressure

leads to higher rates (figure 20) but treatment of these data is difficult due to the

interdependencies of experimental parameters (conversion, molar ratios) in the current

state of the setup.

In general, this is by far not a definite proof and other possible explanations should

not be excluded. Equilibrium between Pt(II) and Pt(IV) should also not be excluded.

Also, formation of di- or oligomers which would be favored at higher catalyst

concentration could be imagined, although Pt(II) complexes with bpym as a ligand

were found reluctant to form bridged species.162

For platinum(II) chloride and potassium tetrachloroplatinate the Michalis Menten

formalism cannot be applied, as catalyst solubility and other effects play a (bigger)

role for these compounds. If solubility does not play a role, it should be possible to

compare electrophilic catalysts for methane oxidation based on K and rmax.

Also within the same solvent and with the same amount of catalyst the pressure

dependency is likely to reflect a convolution of methane partial pressure, Henry

constant and K.

aData obtained by use of 10 mL 20% oleum and only low catalyst concentration could also be fit with a

hyperbola but gave different values for the parameters. The amount of MBS was however comparable

to the experiments with 15 mL oleum and experiments thus less differential. Experiments varying the

volume of the liquid phase should continue in order to exclude other effects.


86

3.5.2.3 Transformation and speciation of potassium

tetrachloroplatinate

In order to understand why in case of potassium tetrachloroplatinate not only the TOF

drops with increasing concentration but also the rate goes through a maximum, first

the undissolved/precipitated material was analyzed directly after filtration of the

reaction mixture from methane oxidation in 20% oleum with a formal concentration

of 20 mM potassium tetrachloroplatinate by means of XRD without further treatment

(the material was still wet from the oleum and fuming on the sample holder). The

resulting pattern, shown in figure 33, clearly reveals that at least part of the potassium

tetrachloroplatinate is transformed into platinum(II) chloride (the additional

cristobalite reflexes originate from the glass sample holder which was confirmed in a

separate measurement).

Other transformations of platinum compounds in sulfuric acid/oleum were

demonstrated by the group of Wickleder but these reactions usually proceed in

concentrated sulfuric acid above 350 °C for several days.168,171-173

One exception is

the synthesis of Ba[Pt(S2O7)3](H2SO4)0.5(H2S2O7)0.5 where Pt black was treated at

160 °C in 65% oleum for 2 days.174

Implications for methane oxidation are unclear,

yet, and Pt black at high concentrations was not investigated as catalyst for methane

oxidation. Furthermore, it is also not clear how various platinum sources react with

oleum, i.e., if potassium tetrachloroplatinate or platinum(II) chloride might also be

transformed to similar compounds as Pt black. In any case, most experiments were

done without long preheating, and a 50 mM solution of potassium tetrachloroplatinate

in 20% oleum kept for 20 h at 215 °C did not show a significant difference in catalytic

behavior compared to experiments without preheating.

Thus, if potassium tetrachloroplatinate is transformed into platinum(II) chloride in

20% oleum, the remaining components of potassium tetrachloroplatinate, K+ and Cl

-,

have to be present in the solution and might be responsible for the difference between

pure platinum(II) chloride and platinum(II) chloride formed from potassium

tetrachloroplatinate. Therefore experiments with artificial mixtures simulating

K2PtCl2(HSO4)2 (simulated by a mixture of platinum(II) chloride and potassium

bisulfate) and artificial potassium tetrachloroplatinate (simulated by a mixture of

platinum(II) chloride and KCl) were conducted and these results are presented in table


87

10 and compared to those obtained with platinum(II) chloride and potassium

tetrachloroplatinate.

Figure 33: XRD pattern of sediment after methane oxidation with K2PtCl4 (20 mM) in 20% oleum at

215 °C.

Table 10: Comparison of the rates of methyl bisulfate formation for different catalyst formulations

entry catalyst concentration /mM

rMBS /mol h-1

rMBS/rMBS,PtCl2 /%

1 PtCl2 50 0.374 100

2 K2PtCl4 50 0.065 17.4

3 PtCl2 / KHSO4 50/100 0.405 108.3

4 PtCl2 / KCl 50/100 0.014 3.9

It can be seen that the addition of K+ does not have a detrimental effect on the rate and

platinum(II) chloride/potassium bisulfate has a similar performance as platinum(II)

chloride alone (the concentration of K+ is sufficiently low that salting out of methane

can be neglected). However, addition of Cl- drastically reduces the activity. The

mixture platinum(II) chloride/potassium chloride shows a rate in the range of

potassium tetrachloroplatinate. With this information the maximum of rate for

potassium tetrachloroplatinate can be explained: As potassium tetrachloroplatinate is


88

transformed into platinum(II) chloride which is to a large extent insoluble, additional

Cl- is present in the solution which can coordinate to dissolved Pt species and block

the center for methane. It seems likely that there is an optimum ratio between

dissolved Pt and Cl- ions present, because at low catalyst concentration potassium

tetrachloroplatinate shows higher activity compared to platinum(II) chloride (table 5

and 8). This supports the hypothesis that different speciation governs this system. The

situation is sketched in scheme 5 for a low and a high initial amount of potassium

tetrachloroplatinate: In case of little potassium tetrachloroplatinate not much

additional Cl- is present in the solution, and this likely results in the formation of a

very active Pt species. In case of a high initial amount of potassium

tetrachloroplatinate much of the material precipitates as platinum(II) chloride and

leaves the small amount of dissolved platinum species with a huge excess of Cl-

which is detrimental for the catalytic activity.

Scheme 5: Illustration of the transformation of K2PtCl4 in oleum and its consequences.

Different solution speciation and an optimum concentration, respectively ratio of

additive to the active metal, is well known for a lot of systems utilizing halides as

ligands, i.e., Shilov chemistry,23,164,180

Wacker oxidation,150,181

and carbonylation of

acetic acid182

to mention just a few.


89

3.5.2.4 Reactions of Cl- in sulfuric acid and oleum

It is difficult to investigate speciation in more detail, because Cl- undergoes at least

partly reactions in sulfuric acid/oleum: If KCl is poured into concentrated sulfuric

acid, heat is evolved and massive bubbling along with the smell of gaseous HCl is

observed, a well-known method for producing HCl in the lab. In 20% oleum bubbling

is less pronounced and the smell reminds rather of Cl2 (Oxidation of Cl- by sulfur

trioxide is possible).54

Furthermore, chlorosulfuric acid, ClSO3H, is produced via

reaction of HCl with sulfur trioxide.183

This shows again that there is a difference

between sulfuric acid and oleum, and it also complicates controlling the exact amount

of Cl- present in the reaction mixture.

Due to that, measurements with potassium tetrachloroplatinate might show a bigger

statistical error compared to 1 or platinum(II) chloride. The trend of the potassium

tetrachloroplatinate series was reproducible; however, absolute values differed

substantially in single measurements. This might be due to differences in closing and

purging the reactor which can lead to different amounts of Cl- present, respectively

different losses via gaseous compounds, e.g., HCl. Different ways of pretreating

potassium tetrachloroplatinate, i.e., grinding or different preheating times (5 min –

20 h) did not lead to significant differences in the catalytic activity. If differences

were observed, e.g., higher pressure of the catalyst/oleum mixture in case of longer

preheating, these did not show a clear correlation with activity. More sophisticated

experimental approaches and detailed knowledge about the reactions of Cl- in sulfuric

acid/oleum are needed for unequivocal conclusions.

3.5.2.5 Additional observations with respect to solubility

I am not aware of any solubility data in concentrated sulfuric acid/oleum for the

compounds used, but platinum(II) chloride and potassium tetrachloroplatinate can

doubtless be considered as weakly soluble, respectively insoluble. Nevertheless, for a

tremendously active solution species a reasonable rate will be observed even for

sparsely soluble compounds. Actually, this could be the situation found for the weakly

soluble Pt compounds in oleum.

In order to find out if solubility might be significantly higher, perhaps as a result of

conversion to more soluble species at reaction temperature, solutions of potassium


90

tetrachloroplatinate and platinum(II) chloride in 20% oleum with a formal

concentration of 50 mM were heated to 215 °C in glass vials. Compared to room

temperature no difference was observed and black sediment was visible. However, the

same experiment with 50 mM solutions of 1 in 96% sulfuric acid and 20% oleum

showed a difference: As mentioned, in oleum the solution is yellow and the optical

appearance does not change during heating to 215 °C and subsequent cool down to

room temperature. In 96% sulfuric acid, though, the solution is orange at room

temperature and at 215 °C an orange-reddish precipitate is formed which does neither

redissolve upon cooling to room temperature nor does it dissolve in DMSO, sulfuric

acid, or 20% oleum.

There might be also some influence due to the presence of methane and/or the time at

215 °C: In case of 20% oleum, the solution after methane oxidation is also clear and

yellow, viz no clear difference to the experiment described above. If methane

oxidation with a 50 mM solution of 1 is conducted in 98% sulfuric acid, the reaction

solution has a dark blue-yellow appearance and no precipitate is observed directly

after the reaction. Only after leaving the reaction solution for two days some orange-

green-brown precipitate is observed. The solution has still the dark blue-yellow

appearance. In any case, experiments in concentrated sulfuric acid seem to be

sensitive to the presence of methane. Actually theoretical investigations suggest that

Cl- is exchanged for HSO4

- during the first cycle of catalytic methane oxidation.

93

This could be the reason for the observed differences in case of the presence of

methane but further experiments investigating the influence of the sulfur trioxide

concentration on the solubility of 1 as well as the role of methane are needed.

Additionally, precipitation of “1” from sulfuric acid or oleum by dilution with water

leads to different optical appearance and a different solubility of the precipitated

material. This might also be important with respect to deactivation of the solid

catalyst.

3.5.3 Further catalysts and aspects

3.5.3.1 Bis(acetylacetonato)platinum(II)

Another interesting platinum compound which was briefly investigated and nicely

complements some of the aspects just discussed is bis(acetylacetonato)platinum(II),


91

Pt(acac)2. It was already noticed by the group of Bell that this compound is soluble in

sulfuric acid/oleum and also shown to be a good catalyst for the oxidative

carbonylation of methane.109

At concentrations of 80 µM it exhibits the highest TOF observed for methane

oxidation throughout this thesis, which is greater than 45000 h-1

. The concentration

dependence in 20% oleum is shown in figure 34. Methyl bisulfate formation quickly

reaches the maximum rate obtained in this system and treatment of these data under

assumption of quasi steady state is possible. In accordance with the residual plot,

deviations from the model are observed, e.g., a slight decrease of the rate above

concentrations of 10 mM. The residual plot is shown in figure 35 and the Lineweaver

Burk linearization in figure 36. Deviations from the model might be a result of dimer

formation, presence of carbon dioxide and sulfur dioxide, or lower sulfur trioxide

concentration as is discussed in the following. Nevertheless

bis(acetylacetonato)platinum(II) has a higher rmax and K value compared to 1. In

analogy to the Shilov system it could be interpreted that also in the Periana system

strong ligands (bpym vs. naked cationic platinum, vide infra) are detrimental for

methane coordination and hence for overall performance.159,184

Figure 34: Concentration dependence of rMBS in 20% oleum for Pt(acac)2 and hyperbolic fit.


92

Figure 35: Residual plot of the hyperbolic fit in figure 34.

Figure 36: Lineweaver Burk linearization of the concentration dependency of Pt(acac)2 in 20% oleum.


93

Upon heating bis(acetylacetonato)platinum(II) to 215 °C in 20% oleum, the

acetylacetonato ligand is obviously decomposed. This is evidenced by the pressure

before introducing methane which scales linearly with the amount of catalyst in

solution. Pressure can reach up to 23 bar for 50 mM bis(acetylacetonato)platinum(II)

compared to 10 to 12 bar usually observed if 20% oleum is heated. This is also the

reason why specification of selectivity does not make too much sense since the

amount of carbon dioxide formed by decomposition of acac can easily exceed the

amount formed from methane over oxidation; in case of low catalyst concentrations

selectivity to methyl bisulfate was well above 95%.

It is not clear if only one ligand is decomposed, but the ratio of the amount of carbon

atoms introduced by the catalyst to the amount of carbon dioxide after reaction

approaches a value of around 2 for 50 mM bis(acetylacetonato)platinum(II). If this is

not an artefact, only one acetylacetonato ligand would be decomposed. Also two

singlets in 1H NMR which scale with the amount of bis(acetylacetonato)platinum(II)

initially introduced were observed (5.3 and 4.5 ppm if the acid protons are arbitrarily

set to 10 ppm).

The presence of carbon dioxide or sulfur dioxide might lead to the slightly lower

activity observed for high catalyst concentrations. However, a simple explanation can

also be a lower sulfur trioxide concentration. Complete decomposition of 1 mol acac

would require 11.5 moles of sulfur trioxide. If all acac was decomposed before the

reaction was started, and the catalyst concentration was 50 mM, the sulfur trioxide

concentration would already be 1.15 mol L-1

lower, which is a significant value.

Nevertheless, in a separate experiment 50 mM of bis(acetylacetonato)platinum(II) in

20% oleum was heated to 215 °C and kept there for 30 min (note that this experiment

has two main differences compared to methane oxidation: 30 min at 215 °C instead of

5 although the pressure indicated that decomposition of acac was complete already at

reaching 215 °C, and absence of methane). After cooling to room temperature and

degassing of the solution, the presence of reddish precipitate is observed, which is

strongly hygroscopic and becomes yellow upon exposure to air. Pt2(HSO4)2(SO4)2, a

Pt(III) compound with platinum dimers, was reported to be a moisture sensitive red

compound.171

Similarly K3[Pt2(SO4)4H(HSO4)2] is red, too, and decomposes under air

to a yellow material.172

The characterization of the material obtained from treatment

of bis(acetylacetonato)platinum(II) with oleum remains to be done, but this could


94

open completely new directions with respect to mechanistic studies, viz

stoichiometric oxidation of methane with such defined compounds.

Furthermore, bis(acetylacetonato)platinum(II) shows that precursors with easily

decomposable ligands could offer a simple but generally applicable route to precious

metal sulfates: So far platinum metal or chloro compounds with low solubility have

been used at temperatures above 300 °C to obtain such compounds. Decomposing

ligands at lower temperatures might allow the preparation in flasks instead of sealed

ampoules, and heating should be possible without the need of furnaces.

Additionally, this series nicely illustrates the evolvement of heat observed after

introducing methane, and the non-isothermal behavior of the set up. Figure 37 shows

the temperature time profiles of the series around the introduction of methane. The

increase in temperature obviously scales with rMBS.

Figure 37: Temperature time profiles of methane oxidation with Pt(acac)2.

Finally, upon addition of reaction mixture to water, immediate formation of Pt black

is observed as well as a platinum mirror, formed at the wall of the vessel.

Investigation by TEM after filtration reveals the presence of agglomerated colloidal

nanoparticles below 10 nm (figure 38).


95

Figure 38: TEM micrograph of colloidal nanoparticles obtained from mixing water and reaction

mixture from Pt(acac)2 catalyzed methane oxidation.

3.5.3.2 Other catalytically active elements and poisoning

It was mentioned in the introduction that many compounds catalyze the conversion of

methane to methyl bisulfate in sulfuric acid/oleum, with the most prominent examples

reported often by the group of Periana, including Hg,27

Pt,29

Pd,110

Au,114

and I2,115

and also comparisons between these catalysts are made. Unfortunately it has been

largely neglected that conditions were different, and tiny differences have significant

influence. In a review about “Similarities and Differences between the “Relativistic”

Triad Gold, Platinum and Mercury in Catalysis” this chemistry was used as an

inappropriate example, and some of the conclusions might not apply to this

chemistry.185

A brief comparison of some compounds was made in 20% oleum, and

the results are given in table 11. AgNO3, IrCl3·3H2O, Tl2SO4 and TeO2 were tested,

too, but did not show significantly higher activity compared to experiments without

catalyst.


96

Table 11: Comparison of different compounds and further aspects of methane oxidation in sulfuric

acid/oleum

compound concentration /µM TOF /h-1

rate /mol h-1

selectivity /%

I2 630a 10600 0.100 97.4

HgSO4 740 1400 0.016 95.2

K2PdCl4 670 2340 0.024 92.6

AuBr3 670 330 0.003 86.9

1 600 1300 0.012 96.5

K2PtCl4 670 23200 0.235 98.5

This shows that Hg(II) should by no means be described as inferior compared to 1.

Catalytic turnover with gold also seems to be possible with oleum and does not

necessarily require selenic acid. Iodine and platinum salts are the most active

catalysts. To make a sound comparison, further experiments and analysis are

necessary, since some of the experiments are ambiguous: E.g., also bromine

containing compounds were reported to be active for methane oxidation.186

It is thus

unclear if reactivity of AuBr3 can be ascribed to Au cations or bromine compounds.

Nevertheless, it seems that all catalyst show a similar behavior with respect to their

activity in sulfuric acid and oleum, i.e., much higher activity in oleum. A valid

comparison with respect to selectivity towards methyl bisulfate, methanesulfonic acid

and acetic acid would be very interesting. It should also be studied whether the

different catalysts have their own operational window, i.e., an individual acid

concentration below which they do not work anymore. In general, this would also

require comparison at lower temperature and/or in sulfuric acid.

I2 deserves some more words: If the active species contains one iodine atom, the

intrinsic activity is lower compared to platinum compounds (if this is due to lower

rmax or K is not clear). However, activity is still very high, and other catalyst

properties are indeed superior.

Table 12: Experiments with a high final methyl bisulfate concentration.

conditions [MBS] /M [H+] /M rate

/mol h-1

selectivity

/%

K2PtCl4 870 µM 65% oleum 215 °C 2.7 35.3 0.0045 73.8

K2PtCl4 870 µM 65% oleum 180 °C 1.8 40.4 0.0030 90.3

I2 18 mM 65% oleum 180 °C 7.1 25 0.0121 95.6

aAssuming that the active species contains one iodine atom.


97

In order to give a guideline for studies on separation of methyl bisulfate – probably

the biggest obstacle for practical application - experiments were conducted in high

excess of methane, until pressure essentially stopped to drop and the results are

summarized in table 12. Initially this was done with 65% oleum and potassium

tetrachloroplatinate as catalyst (870 µM), and a methyl bisulfate concentration of

2.7 mol L-1

was reached. Since iodine was reported to reach a value of 5 mol L-1

at

180 °C,116

and this was not reached with potassium tetrachloroplatinate at 215 °C,

potassium tetrachloroplatinate was tested at 180 °C which resulted in a methyl

bisulfate concentration of 1.8 mol L-1

. Subsequent testing of I2 at 180 °C (36 mM)

resulted in the unprecedented value of 7 mol L-1

. With respect to a process and the

issue of separation, this is remarkable. Considering that 65% oleum has a sulfur

trioxide concentration of 15.6 mol L-1

and assuming that two molecules of oxidant are

needed per molecule of methane, it could be concluded that essentially all SO3 was

consumed. Nevertheless, the solution appeared as quite dilute sulfuric acid (with all

the ambiguity, titration of this solution gave a proton concentration of 25 M, which

would correspond to approximately 75% sulfuric acid), and indicates that iodine

might catalyze methane oxidation under certain conditions in dilute sulfuric acid. One

reason for this might be the Bunsen reaction (26) where water reacts with I2 and sulfur

dioxide to form sulfuric acid and HI.

2𝐻2𝑂 + 𝐼2 + 𝑆𝑂2 → 2𝐻𝐼 + 𝐻2𝑆𝑂4 (18)

This would lead to in-situ recycling of sulfur dioxide and H2O. Interestingly,

according to Periana iodine needs oleum to start the reaction!115

Actually, there is an

example, involving radical pathways, where sulfur dioxide is used as sulfonating

agent for methane to methanesulfonic acid conversion.187

Table 13: Experiments with addition of hydrogen sulfide.

conditions TOF /h-1

rate /mol h-1

selectivity /%

1 600 µM, 6.6 vol% H2S 1420 0.013 96.3

1 600 µM, 0.0036 mol H2S 1340 0.012 97.7

Another interesting aspect of the oxidation of methane in sulfuric acid/oleum can be

seen in the fact that natural gas might not require clean up before processing. This was

tested by addition of H2S (6 vol%) to the methane supplied to the reactor which did

not result in a change of the catalytic activity. Another experiment, where H2S (H2S:1,

400:1) was supplied to the reactor before heat up, showed that pressure of H2S can


98

hardly build up. The catalytic results of both experiments are shown in table 13. It is

not clear if H2S is only absorbed by oleum at room temperature or reacts immediately

with sulfur trioxide. Anyway this will in any case happen after raising temperature.

Activity was also unchanged in this experiment, and IR spectroscopy showed

significant amounts of sulfur dioxide. Thus, the system has self-cleaning properties by

oxidizing common catalyst poisons.

The results described in this section lead to the conclusion that the observed rate is

influenced by the catalyst concentration in the following ways:

- solubility of the used precursor, respectively the active species

- convolution of catalyst and methane concentration and complex formation

constant

- reactions of the precursor and the concentration of Cl- which leads to different

speciation

3.5.4 Selectivity


So far it has only been discussed, how activity is influenced by sulfur trioxide and

catalyst concentration, and although the Periana system is famous for its high

selectivity at high conversion and the concept of protective group has proven to be

successful, there has been to my knowledge no detailed experimental study on the

factors, influencing selectivity. Some of the studies on related systems discuss

selectivity, however, either the focus is on selectivity towards different functionalized

products (e.g., acetic acid, methane disulfonic acid or sulfoacetic acid, and not to

carbon dioxide),109

or the selectivity to methyl bisulfate was found to decrease with

increasing sulfur trioxide concentration.113

A theoretical study suggests that decreasing acid concentration is not only detrimental

for activity, but also for selectivity.95

This study investigates and compares the

oxidation of methane, methyl bisulfate and free methanol with 1. By comparison of

the interaction of methyl bisulfate and methane with the platinum fragment, which is

weaker in case of methyl bisulfate, the concept of product protection is shown to work

as originally proposed. Additionally, it is also claimed that a decrease in acid

concentration leads to increasing amounts of free methanol which has comparable


99

reactivity towards the catalyst, compared to methane, finally leading to deterioration

of the selectivity due to catalytic oxidation of free methanol.

3.5.4.1 Methyl bisulfate: Intermediate in a consecutive

reaction

The reaction network proposed in reference 95 is shown in scheme 6, which, indeed,

can describe the selectivity in the Periana system correctly.

Scheme 6: Reaction network proposed in reference 95 for explaining selectivity in the Periana system.

Clearly, the oxidation of methane to methyl bisulfate is part of a consecutive reaction

sequence, and maximum yield and selectivity, in the most general description, depend

on the ratio of the rate constants for intermediate formation and intermediate

oxidation.23,78

Also, the ratio between methyl bisulfate and methanol (Kp), which is

influenced by the concentration of sulfuric acid, is expected to be an important

parameter.

However, the cause effect relationship, described in this study, seems to be flawed,

since the decrease in selectivity, upon dilution, is ascribed to a change of Kp with

change in acid concentration. This can only be partly correct, since k1 depends on acid

concentration, too. The change of k1 with acid concentration is accounted for in the

model, but not included in the discussion.95

Furthermore, both parameters – k1 and Kp

– are changed simultaneously, and it is not possible to judge which of the parameters

has a bigger influence. Therefore the corresponding graph was reassessed and the

parameters changed independently, showing that changing k1 has a bigger influence

than changing Kp (see figure 39).

Indeed, a decrease in acidity should lead to a decrease in selectivity, but not only as a

consequence of deprotection, but also as a consequence of a slowdown of the

oxidation of methane to methyl bisulfate.

In any case, sufficient product protection is important for selective methane oxidation,

as methane oxidation without protection leads only to low product concentration,


100

especially at higher conversion also owing to the low solubility of methane.24,25

For

comparison, in the Shilov type oxidation of p-toluene sulfonic acid without

protection, the selectivity towards the primary oxidation product already drops below

90% at 11% conversion (conversion of oxidant is higher).24,188

Also in industry

protection is used as concept, e.g., in the Bashkirov oxidation of paraffins with

protection of the alcohol as boric acid ester, and despite protection conversion is kept

around 15-25%.1

Figure 39: Reassessment of the dependency of selectivity on methyl bisulfate concentration changing

k1 and Kp independently using the identical model, formulas and values described and given in

reference 95 and on page 127.

However, even with protection, formation of methyl bisulfate is part of a consecutive

reaction network, and accelerating the first reaction should not only be beneficial for

productivity but also selectivity. As an illustration, yield against conversion is plotted

in figure 40a, respectively methyl bisulfate concentration against time in figure 40b,

using the same reaction model and parameters as in reference 95 (see experimental

part for formulas).

The parameters are assumed to stay constant, i.e., constant sulfur trioxide

concentration over time. The only modification is, that methane pressure is not

constant, which represents experiments in batch mode, as it is the case here. The main


101

message from figure 40 is that high conversion is usually not desirable for a

consecutive reaction, since the concentration of the intermediate goes through a

maximum, and accessible selectivities, respectively methyl bisulfate concentration,

are higher in case of higher k1. This can already be concluded from the black curve in

figure 25. In case of high rates the reaction time has to be drastically reduced for

optimum performance, thus underlining the importance of the quasi differential

experimental approach not only for obtaining meaningful data. but also with respect

to maximizing the desired product. These pre-considerations are important for the

discussion of the selectivity pattern observed in the experiments of the previous

sections.

Figure 40: a): Optimum performance envelope for methane oxidation with further oxidation of methyl

bisulfate and unprotected methanol. The thick black line represents the limit without subsequent

oxidation. b): Dependency of methyl bisulfate concentration on time for methane oxidation with further

oxidation of methyl bisulfate and unprotected methanol.

3.5.4.2 High activity leads to high selectivity

Following these preconsiderations, analysis of the selectivity pattern, observed in this

thesis, are made, based on scheme 7. The main feature is a consecutive reaction,

where oxidation of the intermediate, methyl bisulfate, is treated, irrespective of any

equilibrium it may be involved in. In some cases, also direct oxidation of methane to

carbon dioxide plays a role, and this additional pathway should be included. It is,

however, only important in special cases and can often be neglected.

b a


102

Scheme 7. Reaction network for further discussion; the font size of the rate constants reflects their

importance for selectivity in the Periana system.

In general the selectivity of most experiments, especially the ones in oleum with

limited conversion, was well above 90%. It is expected to be slightly overestimated

and has a high relative error due to the higher solubility of carbon dioxide in sulfuric

acid/oleum, compared to methane. In the initial stage of the investigations degassing

was not performed. Degassing of the reaction solution was performed by stirring the

closed reactor at 40 to 50 °C, after the gas phase was vented into the gas sampling

bag. The composition of the newly built gas phase was subsequently analyzed, too.

However, most of the experiments should not require degassing and this should not

distort the conclusions and trends discussed, as the carbon balance is usually in a

range of 90 to 100% and most experiments are performed at conversions, where

formation of carbon dioxide is small for the respective conditions and thus the partial

pressure, too (the experiment with 50 mM 1 in 98% sulfuric acid with CCH4>40% is

an exception). Even when solutions were degassed, there are still experimental issues

with the absolute precision of the determined amount of carbon dioxide: The absolute

amount of carbon dioxide formed is relatively small, and therefore also the pressure

buildup during degassing is small (around 0.5 bar), which leads to a big relative error

in the pressure measurement at a precision of +/- 0.1 bar. The pressure measurement

is the basis for the determination of the amount of carbon dioxide. In order to improve

the precision of experiments further, degassing of the solution after the reaction

should, nevertheless, be included into standard protocols. It is obvious that the

solubility follows the order methane < carbon dioxide < sulfur dioxide, which has

been found previously116

(see figure 41 for IR spectra of subsequent degassing

cycles). As a result of the above reasons small differences in selectivity should not be

over interpreted.


103

Figure 41: IR spectra of gas released in four subsequent degassing cycles after methane oxidation at

215 °C with 50 mM 1 and 100 mM KHSO4 in 98% sulfuric acid (15 mL) for 2.5 h (pCH4≈Ptotal≈50 bar,

CCH4 46.7%). a) – d): different spectral regions.

3.5.4.2.1 Selectivity of the catalyst concentration series

In order to underline the importance of high activity for selectivity not only with a

hypothetical consecutive reaction (figures 39 and 40), the selectivity as a function of

the rate of methyl bisulfate formation is shown in figure 42 for the catalyst

concentration series. The series with 1 is fitted for a consecutive reaction at 25%

conversion with equation (19) and rMBS as variable (as methane partial pressure was

identical in all experiments rMBS equals k1,apparent) and kox,apparent as fit parameter (fit

results in a value of 0.00123 mol h-1

, respectively 0.082 mol L-1

h-1

) with the

assumption that both reactions are first order in methane, respectively methyl

bisulfate.

a b

c d


104

Figure 42: Dependency of selectivity on the rate of methyl bisulfate formation in 20% oleum (15 mL)

at 215 °C. All reactions stopped after the same pressure drop; ptotal 72 bar pCH4 65 bar CCH4 20-30%.

𝑆𝑀𝐵𝑆 =𝑘1,𝑎𝑝𝑝𝑎𝑟𝑒𝑛𝑡

(𝑘𝑜𝑥,𝑎𝑝𝑝𝑎𝑟𝑒𝑛𝑡−𝑘1,𝑎𝑝𝑝𝑎𝑟𝑒𝑛𝑡)

0.75−0.75

𝑘𝑜𝑥,𝑎𝑝𝑝𝑎𝑟𝑒𝑛𝑡𝑘1,𝑎𝑝𝑝𝑎𝑟𝑒𝑛𝑡

0.25∗ 100 (19)

Equation (19) is derived from equation (31) and (32) in the experimental part, which

describe conversion and yield of intermediate in a consecutive reaction. Equations

(31) and (32) were combined by using the relationship that yield equals conversion

times selectivity.

Figure 43: a): Plot of residuals of the fit in figure 42. b): Dependency of selectivity on k1 for a first

order consecutive reaction (same formula as used for the fit in figure 42 but adjusted for conversion

and kox).

a b


105

The advantage of the catalyst concentration series is that any possible influence of Kp

can be expected to be similar for different experiments because the acidity of all

experiments is comparable. Furthermore, the same holds for the oxidation potential of

the medium as well as the conversion. Although the quality of the fit is rather poor

(adjusted R-Square 0.79634 and plot of the residuals, figure 43a) and the selectivity in

the fit is in general overestimated, it matches the general features of the measured

data. This indicates that the selectivity pattern observed is in large parts likely a

consequence of the consecutive nature of the reaction network in combination with

changes in the rate of formation of the intermediate methyl bisulfate. It is implied that

changing k1,apparent (rMBS) by changing the catalyst concentration influences kox,apparent

only moderately which is further supported, later. Furthermore, figure 43b shows how

the dependency of selectivity on k1 changes for a consecutive reaction if conversion

or kox are changed. The same pattern is observed and selectivity is favored by a high

ratio of k1/kox and lower conversion.

Deviations from the simple model can be explained by underestimation of rMBS,

especially at low catalyst concentration due to neglecting the amount of methyl

bisulfate which already reacted to carbon dioxide, neglecting direct oxidation of

methane to carbon dioxide, and slight changes of kox,apparent with changing catalyst

concentration. Another source of error is the determination of carbon dioxide, as

described above.

The amount of carbon dioxide formed as a function of reaction time and as well of

rMBS is shown in figures 44, respectively 45. For comparison, both graphs also show

the expected amount of carbon dioxide for a consecutive reaction network at 25%

conversion with an initial amount of methane of 0.03 mol and kox,apparent

0.00123 mol h-1

. The respective equations (20) and (21) are shown below the graphs.

The trends agree qualitatively but exact prediction is fairly bad.

Overall, the issue of a consecutive reaction network seems to be more obvious and

pronounced in case of methane conversion in the gas phase over solid catalysts where

also radical reactions are involved78

but still has to be considered for any consecutive

reaction network even if the non-radical pathway and protection increase the ratio of

k1 to kox.


106

Figure 44: Amount of CO2 as a function of the reaction time in 20% oleum (15 mL) at 215 °C. All

reactions stopped after the same pressure drop; ptotal 72 bar pCH4 65 bar CCH4 20-30% [1] varied.

𝑛𝐶𝑂2,25%𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛(𝑡𝑟𝑒𝑎𝑐𝑡𝑖𝑜𝑛) =

𝑛𝐶𝐻4,0 +−

𝑙𝑛0,75

𝑡𝑛𝐶𝐻4,0

𝑘𝑜𝑥−𝑙𝑛0,75

𝑡

𝑒−𝑘𝑜𝑥𝑡 −𝑘𝑜𝑥𝑛𝐶𝐻4,0

𝑘𝑜𝑥+𝑙𝑛0,75

𝑡

0,75 (20)

Figure 45: Amount of CO2 as a function of rMBS in 20% oleum (15 mL) at 215 °C. All reactions

stopped after the same pressure drop; ptotal 72 bar pCH4 65 bar CCH4 20-30% [1, K2PtCl4] varied.


107

𝑛𝐶𝑂2,25%𝑐𝑜𝑛𝑣𝑒𝑟𝑠𝑖𝑜𝑛(𝑘1) =

𝑛𝐶𝐻4,0 +𝑘1𝑛𝐶𝐻4,0

𝑘𝑜𝑥−𝑘10,75

𝑘𝑜𝑥𝑘1 −

𝑘𝑜𝑥𝑛𝐶𝐻4,0

𝑘𝑜𝑥−𝑘10,75 (21)

Thus, high activity is necessary to also obtain high selectivity. Comparing 1 and

potassium tetrachloroplatinate reveals an interesting aspect: At high rates, the

selectivity is virtually the same for both catalysts, but the rate of methyl bisulfate

formation below which selectivity drops significantly is higher in case of potassium

tetrachloroplatinate. If the data points of potassium tetrachloroplatinate are fit with the

model of a consecutive reaction, the quality of the fit is even worse. This could

perhaps be a consequence of the fact that high formal concentrations of potassium

tetrachloroplatinate lead to substantial amounts of Cl- in the solution, and all

experiments with low selectivity in the potassium tetrachloroplatinate series are

performed with high concentration. Hence it seems reasonable to assume that not only

the lower rate due to excess of Cl- leads to lower selectivity, but additional

decomposition pathways might be opened by Cl-, leading to further decrease of the

selectivity. Additional pathways could be catalyzed or initiated by Cl-/Cl2, or upon

leaching of components from the reactor material.

Similar to activity it does not seem possible to judge generally, which catalyst is more

selective, and it obviously depends on the exact conditions. However, figure 42 hints

why nonligated Pt salts are considered to be not only less stable but also less selective

in Periana´s original report (cf. values for 1 and potassium tetrachloroplatinate at high

catalyst concentrations; although this might not hold for platinum(II) chloride).

In order to determine the specific reactivity of the catalysts towards methyl bisulfate,

further experiments are needed. However, platinum chloro complexes might not only

be intrinsically more active towards methane, but also towards methyl bisulfate, as at

high absolute rates, i.e., high catalyst concentration of 1 or low catalyst concentration

of potassium tetrachloroplatinate, virtually the same selectivity is observed. If

potassium tetrachloroplatinate had the same intrinsic reactivity towards methyl

bisulfate, a higher selectivity for potassium tetrachloroplatinate would be expected

due to the lower catalyst concentration utilized. It can be doubted if this would be

resolved with the current setup and the precision of the analysis, because selectivity is

already high and differences in kox,apparent at a high rate of methyl bisulfate formation

would not change selectivity significantly.


108

3.5.4.2.2 Selectivity of the sulfur trioxide concentration series

Experiments to study the effect of catalyst concentration were carried out in 20%

oleum. Thus, no interference with deprotection is expected which might not hold for

the sulfur trioxide concentration series. Nevertheless, plotting selectivity against rMBS

for the sulfur trioxide series gives the same picture (figure 46).

Figure 46: Dependency of selectivity on the rate of methyl bisulfate formation for the sulfur trioxide

concentration series (denoted 1 SO3 and K2PtCl4 SO3). Catalyst concentration series (denoted 1 and

K2PtCl4; identical to figure 44) are shown for comparison.

Although the correlation between rate and selectivity should also hold for these series

and explain the course of the data points in figure 46, there is an obvious problem: In

the sulfur trioxide series, several opposing parameters are changed at the same time:

k1,apparent, conversion (experiments below the oleum regime are performed with

conversion below 10% due to the otherwise impractically long reaction times),

oxidation potential of the medium, and perhaps Kp. It is not clear to which extent

these parameters balance each other: E.g., the lower rate at low acidity is detrimental

for selectivity, but the conversions in these experiments are lower, which leads to

higher selectivity.


109

Due to these opposing effects the selectivity also seems to increase with conversion

(for experiments with CCH4<20%, figure 47), which is not expected for a simple

consecutive reaction network. In such a network, the selectivity of each individual set

of starting conditions has to decrease with conversion. The catalyst concentration

series actually indicates normal behavior, i.e., decrease of selectivity with increasing

conversion.

Given the drastic change of activity if oleum instead of concentrated sulfuric acid is

used might already suggest that the change of k1 (one to two orders of magnitude)

with SO3 concentration dominates the change in selectivity also for these series. Also

the dependency of selectivity on SO3 concentration resembles the one of activity on

SO3 concentration (figure 48).

Figure 47: Plot of selectivity versus conversion.


110

Figure 48: Dependency of selectivity on sulfur trioxide concentration at 215 °C. 15 mL solution,

catalyst concentration 600 µM, CCH4 < 30%, actual partial pressures of methane at 215 °C 35-75 bar.

3.5.4.3 Methyl bisulfate decomposition

In order to achieve a better understanding of the possible roles of the oxidation

potential and Kp for selectivity and to which extent a change in sulfur trioxide

concentration influences kox, the decomposition of methyl bisulfate was investigated

in concentrated sulfuric acid and 20% oleum separately, by subjecting the sodium salt

of methyl bisulfate (NaMBS), which is commercially available, to different

treatments. 1 M solutions of NaMBS were reacted in a Hastelloy autoclave at 215 °C

for 2.5 h in different media as well as with and without addition of 20 mM of 1. The

results are shown in table 14, together with the rate of carbon dioxide formation in

blind experiments. The amount of decomposed NaMBS increased in the following

order: Concentrated sulfuric acid < 20% oleum < concentrated sulfuric acid with 1

(20 mM) < 20% oleum with 1 (20 mM). In none of the experiments methane was

observed, indicating that functionalization is irreversiblea.

aTwo publications made similar experiments: The original publication where essentially the same is

observed.29

Another publication with palladium catalysts reports the backward reaction to methane.112


111

Table 14: Decomposition of NaMBS and carbon dioxide formation in blind experiments

entry conditionsa

rox, total

/mol L-1

h-1

rox, cat only

/mol L-1

h-1

rox, cat only/rox,

total

1 96% H2SO4 0.049 1

2 20% oleum 0.065 1

3 96% H2SO4, 20 mM 1 0.119 0.070 1.43

4 20% oleum, 20 mM 1 0.137 0.072 1.11

rCH4→CO2b

/mol L-1

h-1

rCH4→CO2

/rox, total,20 mM

5 blind test 96% H2SO4 0.0058 0.049

6 blind test 20% oleum 0.0072 0.053 a215 °C, 2.5 h, 15 mL, 1 M NaMBS bbased on 15 mL reaction volume excluding gas phase oxidation

These experiments show that uncatalyzed decomposition of methyl bisulfate (or

catalyzed by non Pt compounds, vide infra) significantly contributes to methyl

bisulfate oxidation, i.e., uncatalyzed decomposition proceeds with almost the same

rate as decomposition catalyzed by 20 mM of 1 in oleum (rox,cat only is obtained by

subtracting the rate of NaMBS decomposition without catalyst from the rate obtained

with catalyst addition). Furthermore, uncatalyzed decomposition is slightly faster in

oleum which might be due to the higher oxidation potential. Catalytic decomposition,

however, seems to be rather insensitive to the sulfur trioxide concentration. Finally,

entries 5 and 6 show that direct formation of carbon dioxide from methane is around

one order of magnitude slower than carbon dioxide formation via methyl bisulfate. It

cannot be excluded that carbon dioxide formed in blind experiments originates from

methyl bisulfate with an extremely low steady state concentration. Indeed, traces of

methyl bisulfate were found in oleum without addition of catalysta. Due to the various

contributions the most general description of selectivity in this system is given by

equation (22) with integral rates.

𝑆 =𝑛𝑀𝐵𝑆

𝑛𝑀𝐵𝑆+𝑛𝐶𝑂2=

𝑟𝑀𝐵𝑆−𝑟𝑜𝑥,𝑐𝑎𝑡−𝑟𝑜𝑥,𝑛𝑜𝑛𝑐𝑎𝑡

𝑟𝑀𝐵𝑆+𝑟𝐶𝐻4→𝐶𝑂2 (22)

aCatalytically active contaminations might be present. Teflon parts, e.g. the coating of stir bars, can

soak up reaction solution (with catalyst) and release it in the next experiment. Because tiny amounts of

catalytically active material show substantial activity in oleum this has to be considered and taken care

of (experiments with 30 mM I2 as catalyst changed the appearance of Teflon parts to purple. These

parts showed substantial activity without additional catalyst). Consequently, care has to be taken and

Teflon parts have to be frequently replaced. It might indeed be the case that Teflon itself shows a small

activity. If a lot of Teflon parts were inside the reactor blind activity was slightly increased.


112

Thus, fitting selectivity as a function of rMBS with constant kox,apparent is partially

justified: Due to the relatively big contribution of noncatalytic decomposition, the

ratio of k1/kox increases with increasing catalyst concentration. If catalyzed methyl

bisulfate decomposition was similarly influenced as methane oxidation this would not

be valid and k1/kox would be constant. For low catalyst concentration decomposition,

can almost exclusively be attributed to noncatalytic decomposition. Anyway,

comparing the value of 0.082 mol L-1

h-1

obtained for kox,apparent from the fit in figure

44 and correcting it with a factor of four for the lower concentration of methyl

bisulfate (final concentration of methyl bisulfate around 0.5 mol L-1

) shows that these

values are in the same order of magnitude but by far not similar, a result of the rough

model.

Actually, the values of catalytic decomposition show that the concept of protection

works much better than assumed so far: Transforming the value, under assumption of

methyl bisulfate being first order in catalytic decomposition, of 0.07 mol L-1

h-1

into

TOF gives a value of 3.5 h-1

compared to the TOF for methane oxidation with 50 mM

of 1 in 20% oleum of 264 h-1

(table 8 entry 4). This is already two orders of

magnitude faster which gives - if a two orders of magnitude lower methane

concentration is assumed (10 mM) - around 10000-fold higher intrinsic reactivity

towards methane as a lower limit.

It is remarkable that decomposition in 96% sulfuric acid proceeds slower, although

methanol can be observed in NMR and a part of the ester was obviously saponified.

Methanol itself might be protonated and still sufficiently protected. Anyhow, as

decomposition of NaMBS is slower in concentrated sulfuric acid, it does not seem

likely that Kp already dropped to an alarming value in 96% sulfuric acid. Thus, if

oxidation potential and solubility of methane could be increased in concentrated

sulfuric acid it might be possible to achieve reasonable activity still with high

selectivity. The determination of Kp as well as kox as a function of sulfur trioxide

concentration, especially in more dilute acid (50 – 100%), could be an important goal

in order to define a region where methane oxidation could be viable in terms of

product protection.

3.5.4.3.1 “Uncatalyzed” methyl bisulfate decomposition and other issues

In order to test the precision and generality of the values obtained in the

decomposition experiments, the amount of carbon dioxide expected to be evolved is


113

calculated based on these values and compared with the actual amount of carbon

dioxide determined experimentally. This did not prove to be very precise.

As an example: For methane oxidation in 20% oleum with 142 µM of 1 0.00188 mol

carbon dioxide were determined experimentally. The predicted amount of carbon

dioxide amounted 0.00099 mol (uncatalyzed decomposition assuming first order of

methyl bisulfate and half the final concentration as concentration is included, as well

as the amount of carbon dioxide formed in blind experiments representing direct

oxidation of methane to carbon dioxide is added; catalytic decomposition was

neglected due to the low catalyst concentration). In general this discrepancy might

partially be caused by the relatively high error of the determination of carbon dioxide

as already discussed which should rather lead to overestimation. The general picture

is retained however.

Nevertheless in order to exclude other effects, investigation of decomposition was

started with higher throughput in glass vials. Actually, the first series of experiments

(change of NaMBS concentration without catalyst in 96% sulfuric acid) raised several

new questions. Decomposition in the glass vial is roughly by a factor of 1.7 slower

compared to the Hastelloy autoclave (comparison of experiments with 1 mol L-1

NaMBS). This could possibly be explained by decomposition catalyzed by metal

cations which were leached from the autoclave material, most likely Ni the major

component of Hastelloy G35. This notion was supported by the color of solutions

after reaction (note that experiments in Hastelloy autoclaves were considered as

“uncatalyzed decomposition” up to this point). If leached cations cause

decomposition, also corrosion properties of the medium have to be considered, which

shows again that not only the dominant property of sulfuric acid/oleum, i.e., acidity,

but also other properties have to be considered. Furthermore, it was observed that the

relationship between rate and concentration of NaMBS is not linear, implying that

decomposition is not first order in NaMBS, but increases progressively with

increasing concentration (figure 49). This might be caused by decomposition via

several pathways one of which might be bimolecular via intermediate formation of

dimethyl ether. These observations reveal decomposition, respectively selectivity, to

be much more complex and less understood than expected, especially under real

conditions. Further experiments in that direction are ongoing at the moment.


114

Figure 49: Concentration dependency of NaMBS decomposition in glass vials: 1 mL 96% sulfuric acid,

glass vials, no catalyst, 2.5 h, 215 °C, CNaMBS <10%. Double logarithmic plot does not give a clear

linear correlation.

The main facts, learned about the selectivity of methane oxidation in the Periana

system, in this thesis are:

- selectivity pattern can be explained by a consecutive reaction network

- methyl bisulfate decomposition is less influenced by sulfur trioxide

concentration compared to methane oxidation

- methyl bisulfate oxidation is partly catalyzed by the catalyst for methane

oxidation, but also uncatalyzed, and/or catalyzed by contaminations

- deprotection does not seem to be an issue in 96% sulfuric acid

- quantification of carbon dioxide needs to be improved

Conclusion and Outlook

115

4 Conclusion and Outlook

In conclusion, it has been shown in this thesis that macroscopically shaped platinum

modified nitrogen containing carbon materials can be used as catalysts for continuous

methane oxidation with sulfuric acid/oleum. Activity of materials with a stable

platinum loading is rather low, and carbon corrosion occurs, posing question marks on

the long term applicability of these materials under the harsh conditions. Hot filtration

experiments were not conclusive with respect to stability of the platinum

coordination, but indicated that dissolved platinum species can be tremendously

active.

Observation of the high activity depends much on the experimental approach and is

massively influenced by the sulfur trioxide concentration: In short, oleum with its

high oxidation potential is needed for high rates. It is important to consider all

properties of sulfuric acid/oleum, and not only acidity.

Simple platinum compounds, which were regarded as pretty active but unstable

catalysts so far, were found to be unprecedentedly active with TOFs greater than

20000 h-1

, likely the highest TOF ever observed for methane oxidation, and

unexpectedly stable with TONs reaching values of 16000. Comparison with the

Periana catalyst showed superior performance of simple platinum compounds under

most conditions. Deactivation by dilution of the medium, i.e., decrease of the sulfur

trioxide concentration, is seemingly the bigger issue than deactivation of the catalyst

itself. Nevertheless, solubility of most platinum compounds is low. This is part of the

reason why these compounds are often regarded as unstable. Catalyst solubility is an

issue and needs to be approached rigorously since catalyst precursors can be

transformed under reaction conditions.

Not only solubility of the catalyst is a problem but also the low solubility of methane.

This leads to a dependency of the TOF on catalyst concentration which can be

explained with a model assuming quasi steady state of a platinum methane sigma

complex. Consequently, TOFs are always apparent and need to be compared with a

reference measured under identical conditions.


116

Selectivity was found to be determined by the laws of a consecutive reaction network,

and uncatalyzed decomposition of methyl bisulfate is a major pathway for undesired

side reactions. Deprotection in 96% sulfuric acid has not been found to be a serious

problem.

To underline the main improvements that have been achieved in this thesis a

comparison of the current performance with the one of Periana´s original report,29

recommended guidelines,91,92

as well as a large scale industrial process, i.e., the

Cativa process for methanol carbonylation to acetic acid189

and conventional methanol

synthesis11

is made in table 15.

Table 15: Comparison of several parameters of the original publication, guidelines, this thesis, the

Cativa process and conventional methanol synthesis.

entry parameter Periana

199829

guide-

line 91,92

this

thesis

Cativa

process189

MeOH

synthesis11

1 rvol /mol L-1

h-1 a

3.6 3.6 >15 40 20 - 30

2 [catalyst] /mM 50 1 0.6 18

3 TOF /h-1

10 - 100 3600 >25000 >2000

4 TON 1000 105-10

6 >16000

b

5 S /% 81 >90 >95 99 >99c

6 CCH4 /%

Coxidant /%

90

<10d

20

20

20 – 30

<10d

15 - 50

7 separation difficult easy difficult distillation distillation abased on liquid phase for liquid phase process bbased on separate experiment cselectivity of MeOH synthesis only; generation of

SynGas decreases overall selectivity dnot only based on free sulfur trioxide but sulfur trioxide and sulfuric acid to illustrate the

problem of recycling the oxidant, respectively the high sulfur trioxide/sulfuric acid to CH4 ratio

Finally, even though activities relevant for large scale production of bulk chemicals –

seen as the biggest issue so far - have been demonstrated, it is not expected that a

process relying on this reaction can enter the phase of commercialization soon,

because the conditions needed for high activity lead to sulfuric acid as a byproduct,

and ways to separate methyl bisulfate from concentrated sulfuric acid or oleum need

to be developed. Dilution of the reaction solution does not look attractive since

reconcentration of these streams would decrease the overall carbon yield.190

Knowledge about alkylation of isoparaffin/olefin mixtures in sulfuric acid, especially

about economics and plants in operation, might be helpful for the assessment of how

much methanol per sulfuric acid has to be produced before regeneration of spent acid

can be afforded.191

Conclusion and Outlook

117

First attempts with respect to separation were undertaken with limited success during

this thesisa, and some approaches can also be found in literature, e.g., low pressure

membrane distillation192

or transesterification.132

With respect to further improvements on the catalytic reaction, mechanistic

understanding should be further developed and clarified if the knowledge from this

thesis can be transferred to other conditions and solvents, which would allow easier

separation. It would be very desirable to increase the solubility of methane which

would be beneficial for activity, selectivity and methyl bisulfate concentration.

A pressing mechanistic question is the role of Pt(IV) in this chemistry. Clearly, this

thesis shows that the over oxidation dilemma does not seem to be a problem in this

system. If this can be attributed to a self-repair mechanism, as recently suggested by

Periana´s group, remains to be answered.101

(In-situ) spectroscopic studies are needed

to corroborate such mechanistic proposals. Due to the reasonable solubility in sulfuric

acid/oleum, 1 is a good compound for such studies. Identifying the actual oxidant,

respectively clarifying, if a high oxidation potential is sufficient, goes hand in hand

with closing the cycle, i.e., incorporation of air or oxygen. It is indeed obvious from

literature that redox couples can be used, little restrictions exist with respect to the

actual oxidant, and oxygen can be incorporated.63-65,111,121-124,180,193,194

Nevertheless, it

remains to be shown how these approaches can be combined with the high activity

conditions presented in this thesis.

The difference between sulfuric acid and oleum might be universal and can at least

partially be transferred to the manifold examples of non-radical methane oxidation in

sulfuric acid/oleum with electrophilic cations, e.g., Hg2+

,27,89

Pd2+

and precursors

thereof,26,110-113

cationic iodine115-117

and gold,114

vanadiumoxo compounds,125

and

many others.28,126

Besides platinum catalysts, iodine is a very interesting candidate for

this reaction, because it is not only very active, but also seems to operate under more

dilute conditions, at least in the presence of sulfur dioxide.

Nevertheless, understanding of the Periana system is still in its infancy and further

investigations are necessary to achieve the same level of understanding, developed for

other reactions. Thus, more sophisticated experiments are necessary to circumvent the

many interdependencies present in the system and overcome ambiguity.

aTransesterification was not successful but should be further pursued. Stripping of reaction solution

with steam indeed leads to hydrolysis of MBS as evidenced by NMR but sufficient amounts of liquid

could not be condensed in the outlet.


118

5 Experimental

5.1 Catalytic testing

5.1.1 Continuous methane oxidation

For continuous methane oxidation at ambient pressure glass reactors were used. The

catalyst was supported on a glass frit and the reactor had an inner diameter of

approximately 5 mm. The oven was controlled by a thermocouple inside the catalyst

bed. sulfuric acid or oleum was supplied without preheating by a syringe pump at a

rate of 15 mL h-1

, and methane with an uncalibrated mass flow controller at 0.05

standard liters per minute. The liquid was collected at the outlet and analyzed.

For continuous experiments with 1, methane at ambient pressure was bubbled through

a mixture of 96% sulfuric acid/20% oleum (2:1) at 225 °C. Samples were taken

periodically from the solution.

In both cases the effluent gas was vented and not further analyzed.

5.1.2 Autoclaves, materials, and equipment

All autoclaves were designed in house (see appendix for technical drawing), and a

schematic drawing is shown in scheme 8.

Scheme 8: Autoclave

additional capillary for hot filtration

Experimental

119

Autoclaves for methane oxidation consist of Hastelloy G-35 (balance Ni, 33% Cr, 8%

Mo, ≤ 2% Fe, ≤ 0.6% Si, ≤ 0.5% Mn, ≤ 0.4% Al, ≤ 0.05% C) and were, depending on

corrosion, remachined from time to time. The autoclave for preheating methane, vide

infra, consists of stainless steel (1.4571).

All autoclaves are equipped with pressure meters (JUMO dTrans p30; the corrosive

conditions lead to drift, and pressure meter have to be controlled and recalibrated

from time to time) and thermocouples (type K; the metal housing was additionally

protected by a Teflon hose). Autoclaves are heated separately via external heat

supply, controlled by a previously optimized PID controller, and stirring is achieved

by magnetic stirring with a Teflon coated stir bar. Pressure and temperature are

monitored and recorded by a LabView program designed by Marjan Thomas, a

screenshot of which is shown in figure 50.

Figure 50: Screenshot of the user interface of the Lab View program taken and modified from 32.

In order to reduce contact of the reaction mixture with the reactor wall, a glass inlet

was used in all reactions.

All volumes were analyzed with a physisorption analyzer by measurement of the

pressure difference between the calibrated known volume manifold and the sum of

manifold and autoclave after opening the in house made connection to the autoclave.

Helium was used as gas, and the measurement was done at room temperature and a

pressure ranging from 0.5 to 1 bar. The ideal gas law was used for calculation.

Sealing materials consist of gold or Teflon. Teflon is used at less demanding

connections and has to be replaced more frequently. In case of 65% oleum, Teflon


120

sealing proved to be inappropriate. Polychlortrifluorethylene proved to be less

suitable compared to Teflon.

Valves and connections are made of stainless steel, and were replaced when needed.

The use of Hastelloy valves resulted in prolonged valve lifetime but does not

sufficiently prevent corrosion.

5.1.3 Hot filtration

Hot filtrations were done using a modified autoclave with an additional capillary as

indicated by the arrow in scheme 5. For safety reasons the hot filtration valve

(stainless steel) could be equipped with a modified drilling machine which allowed

opening from a safe place outside the pressure proof cubicle. This is not optimal with

respect to valve lifetime since not only the hot sulfuric acid/oleum leads to increased

chemical stress, but also opening in the described way increases mechanical stress.

Opening with the drilling machine is not gentle and usually turns the valve many

times towards its endpoint. Ball valves which are easier to open have considerable

shorter life time. The released liquid was collected in a glass vessel with an opening to

the exhaust and the liquid was further analyzed and used.

5.1.4 Standard experiment

Catalytic methane oxidation was carried out in a two autoclave setup described in

section 3.1, in reference 33 and is shown in scheme 3. The two autoclaves are

connected via two valves and a short capillary (dead volume of the capillary and

valves 0.9 respectively 1.3 mL). One autoclave (54.6 mL) was exclusively filled with

methane and served as preheating reservoir. The other autoclave, named reactor, was

used for the actual reaction. Two different autoclaves were used for this purpose (the

volumes, reduced by their glass inlet and a magnetic stir bar amounted 36.2, 37.3 mL

for reactor 1 and 30.4, 32.9, 34.8 mL for reactor 2; several values are given for each

reactor corresponding to volumes after remachining).

Usually, the preheating autoclave was filled with methane to a pressure of 53 bar at

room temperature. The reactor was filled with, unless otherwise stated, 15 mL of

sulfuric acid or oleum and catalyst, respectively other substances if indicated, and

purged with Argon before closure. Both autoclaves were connected via the capillary,

Experimental

121

but still separated by valves, and placed in their heating blocks. Heating was started,

in case of the reactor together with stirring. It usually took 30 min until reaction

temperature was reached. Before pressurizing, the reactor temperature was allowed to

stabilize for ~5 min. The reaction was started by opening the valves to the preheating

autoclave for ~10 s which led to a total pressure of ~70 bar in the reactora. The

reaction was run until a certain pressure drop was reached, or, in case of slow rates,

after a certain time, mostly 2 h. The pressure drop was 8.3 bar in case of reactor 1 and

10 bar in case of reactor 2, and most experiments were performed with a drop of the

pressure volume product of approximately 180 bar mL at reaction temperature. The

reaction was stopped by removing the reactor from the heating block and quenching it

in a water bath under stirring.

The temperature and pressure time profiles of the reactor and preheating autoclave for

a typical experiment are shown in figure 3 with the heat up period (until ~1600 s)

leading to autogenous pressure increase of ~10 bar in the reactor due to evaporation of

sulfur trioxide from the solution. Upon pressurizing, the pressure of reactor and

preheating autoclave are identical, i.e., ~70 bar. The reaction, however, leads to a

pressure drop in the reactor. At the beginning of the last period (~4000 s), pressure

and temperature of both autoclaves drop rapidly due to quenching in a water bath.

After room temperature was reached, the gas phase was vented into a gas sampling

bag and part of it transferred with a gas tight syringe into the gas cell of the IR

spectrometer and the relative composition analyzed.

An aliquot (~1 mL) of the liquid phase was mixed with a weighted amount of

methane sulfonic acid (MSA) as standard and analyzed by 1H NMR (It was separately

verified that methanesulfonic acid is not formed under the used conditions.). The

amount of methyl bisulfate formed during the reaction was determined based on the

integrals of the methyl resonances of methyl bisulfate (3.6 ppm) and methanesulfonic

acid (2.9 ppm). The exact positions of the resonances depend on the acidity (lower

acidity leads to up field shifts).

Another aliquot (10 mL) of the reaction solution was added slowly to 20 mL of

chilled distilled water under stirring and heated to 90 °C for 3 h. This mixture was

afterwards analyzed by HPLC for its methanol content. In most cases hydrolysis was

aAt the beginning of this thesis the exact times were not adhered to strictly, but strong influence is not

expected.


122

not complete, and only 80% of methyl bisulfate was converted to methanol. NMR of

the hydrolysate showed that the remainder stays unaffected.

In some cases an aliquot (1 mL) of the sulfuric acid/oleum was titrated after and/or

prior to reaction. To reduce the concentration of protons, 1 mL of the solution was

mixed with distilled water to give a volume of 100 mL. 3 mL of this solution were

mixed with distilled water to give a solution of 50 mL and half of this solution was

finally titrated. Due to the amount of dilution steps, unknown contributions from

methyl bisulfate and sulfur dioxide and the vigorous reaction in the first dilution step

(especially with oleum of high sulfur trioxide content), titrated sulfur trioxide

concentrations should be taken with care (in case of salt or NaMBS addition, further

ambiguity arises): E.g., the stock solution of 96% sulfuric acid (17.97 mol L-1

H2SO4)

was titrated to contain 17.83 mol L-1

H2SO4 (~95%). After addition of NaMBS to

obtain a 1 M solution titration resulted in a value of 16.54 mol L-1

(this would

correspond to ~90% sulfuric acid). Correcting for the Na+ and the fact that the methyl

bisulfate anion does not have acidic protons this would still be 17.54 mol L-1

“SO42-

“

(corresponding to ~94% sulfuric acid). This only holds, if methyl bisulfate is not

hydrolyzed during the dilution steps, and a clear statement is difficult. Anyway, the

results of the titrations correlate with the calculated sulfur trioxide concentration as

well as with the sulfur trioxide partial pressure.

The sulfur trioxide concentration of mixtures was calculated based on the volumes of

the stock solutions, disregarding density changes upon mixing with densities of the

stock solutions of 1.8355 (96% sulfuric acid), 1.9 (20% oleum) and 1.925 g mL-1

(65% oleum). Titration and fuming of the reaction solutions indicate that experiments

with 20% oleum in quasi differential mode stay in the oleum region. Rough

estimation based on an average titration value of 20.2 mol L-1

for the combined

concentration of sulfur trioxide and H2SO4 (calculated value of 20.23 mol L-1

), a

methyl bisulfate concentration of 0.5 mol L-1

and a 2:1 stoichiometry for the reaction,

indicates the same (100% sulfuric acid has a concentration of 18.67 mol L-1

).

Finally, the reactor was rinsed with acetone and water, dried overnight in a drying

oven and checked for its tightness before the next experiment.

Experimental

123

5.1.5 Methane oxidation with sulfur trioxide

After melting sulfur trioxide, 1 mL of liquid sulfur trioxide (0.025 mol) was charged

in reactor 2 together with 3.3 mg potassium tetrachloroplatinate as catalyst and heated

to 215 °C. After pressurizing with methane to a total pressure of 66 bar a pressure

drop of 11 bar within 3.5 h was noticed (figure 23). Already at the beginning of the

experiment the pressure was above 20 bar which was due to the use of a partially

damaged pressure meter. This pressure meter could not measure pressure below

20 bar any more, but the values between 20 and 100 bar agreed well with a reference

pressure meter.

A pressure drop of 11 bar corresponds to approximately 0.0045 mol methane

respectively methyl bisulfate and a TOF of 160 h-1

.

5.2 Error assessment

Due to the amount of sources contributing to experimental error, fully quantitative

assessment is difficult and error bars are not shown in the graphs. However, trends are

reproducible, although errors under different conditions are different. As a general

measure of precision, the values of six experiments under “standard conditions”

(600 µM 1, 20% oleum) are shown in table 16.

Table 16: Reproducibility of experiments with 600 µM of 1 in 20% oleum

operator TOF1H-NMR /h-1

TOFHPLC /h-1

Tobias Zimmermann 1228 987




Mario Soorholtz n.a. 1060

Mario Soorholtz n.a. 1044

average value 1281 1011

standard deviation 39 31

These experiments were conducted over the course of one year, during which pressure

meters had to be recalibrated, autoclaves remachined, and different batches of

chemicals were used. Additionally, two operators performed the experiments, both


124

reactors, and different batches of 1 were used. As mentioned before, hydrolysis is not

complete and TOFs based on HPLC are usually only 80% of the value based on

NMR. This shows a generally high precision and reproducibility. The majority of the

other experiments are done under similar conditions (20% oleum, low catalyst

concentration).

Higher errors were observed for measurements in 65% oleum and experiments with

potassium tetrachloroplatinate, especially at high catalyst concentration. Four

measurements with 50 mM of potassium tetrachloroplatinate as precursor, albeit with

different pretreatments (grinding and waiting time at 215 °C before the reactor was

pressurized) resulted in an average rMBS of 0.0654 mol h-1

(0.0414, 0.0687, 0.0905,

and 0.0609 mol h-1

, respectively) with a standard deviation of 0.0176 which

corresponds to a relative standard deviation of 27%.

5.3 Calculations

5.3.1 Determination of methane and carbon dioxide

The amount of methane introduced in the reactor was calculated based on the

measured volume of the preheating autoclave and temperatures and pressures before

heat up and after quenching. The Benedict-Webb-Rubin (BWR, equation (23), see

table 17 for substance specific constants) equation of state was used for the

calculations.195,196

The solver add in of Excel was used for numerical solution of the

BWR equation. Using the ideal gas law leads to differences of up to 10%. The amount

of methane and carbon dioxide after the reaction was calculated with the BWR

equation, too, under assumption of Dalton´s law, i.e. the sum of the partial pressures

of methane and carbon dioxide equals the total pressure. The partial pressures are

obtained from integration of IR spectra (2800-3200 cm-1

for methane and 2150-

2550 cm-1

for carbon dioxide). Apparent integral extinction coefficients were

separately determined and resulted in a 10.55 times higher integral absorption of

carbon dioxide in the indicated spectral range which was used accordingly in the

calculation of the partial pressures.

Experimental

125

𝑝 = 𝑅𝑇𝑛

𝑉+ (𝐵0𝑅𝑇 − 𝐴0 −

𝐶0

𝑇2)

𝑛2

𝑉2+ (𝑏𝑅𝑇 − 𝑎)

𝑛3

𝑉3+ 𝑎𝛼

𝑛6

𝑉6

+𝑐

𝑛2

𝑉2

𝑇2(1 + 𝛾

𝑛2

𝑉2exp (−𝛾

𝑛2

𝑉2)) (23)

Table 17: BWR constants for methane and carbon dioxide (Units in atm, L, mol and K; R 0.08207

atm L mol-1

K-1

)

a A0 b B0 c C0 α 105 γ 103

CH4 0.0494 1.855 0.00338004 0.0426 2545 22570 12.4359 6

CO2 0.24204855 1.8367101 0.00625361 0.03201493 19008.12 176028.05 4.878407 4.280822

5.3.2 Turnover number, turnover frequency, conversion,

and selectivity

Reported TONs are calculated as the ratio of the amount of methyl bisulfate in the

crude reaction mixture, determined by 1H NMR spectroscopy, and amount of

platinum used as catalyst for the reaction.

TOFs are calculated by division of the respective TONs by the reaction time.

The observed rate of methyl bisulfate formation, rMBS, is calculated as amount of

methyl bisulfate divided by the reaction time. Volumetric productivities are based on

the liquid phase volume, i.e., 15 mL.

Conversion is defined as: 𝐶𝐶𝐻4 =𝑛𝐶𝑂2+𝑛𝑀𝐵𝑆

𝑛𝐶𝐻4 𝑖𝑛𝑖𝑡𝑖𝑎𝑙∙ 100% (24)

Selectivity is defined as: 𝑆𝑀𝐵𝑆 =𝑛𝑀𝐵𝑆

𝑛𝑀𝐵𝑆+𝑛𝐶𝑂2∙ 100% (25)

These definitions of conversion and selectivity are not biased by deviations in the

carbon balance which was usually between 90 and 100%. In case of low conversion,

influences by a not fully closed carbon balance can become significant.

5.3.3 Extrapolation of the sulfur trioxide concentration

series

Due to the difference in the sulfur trioxide partial pressure (1 to 60 bar) with changing

sulfur trioxide concentration, the amount of methane introduced into the reactor was

different throughout the series. Adaption of the amount of methane in the preheating

autoclave to keep the partial pressure in the reactor constant proved to be difficult.

Data were thus extrapolated based on the calculated partial pressure of methane. Note


126

that only the sulfur trioxide concentration series has been extrapolated. All other

comparisons were made between experiments where the difference of the partial

pressures was within experimental error. The calculated partial pressure of methane in

the reactor was obtained by calculating the pressure of methane at the measured

temperature with the BWR equation, disregarding non-ideal behavior due to the

presence of sulfur trioxide/sulfuric acid in the gas phase and dissolution of methane.

The amount of methane in the reactor was obtained by the difference in the preheating

autoclave before and after pressurizing the reactor. The extrapolation was done to

50 bar methane at reaction temperature under the assumption of a reaction order for

methane of one. Pressure dependent measurements (figure 20) show that although this

might not be entirely correct, larger deviations from an order of 1, e.g. 0.5 or 2 do in

fact not exist. As the partial pressures within the series deviate generally by less than a

factor of two, extrapolation to an intermediate value of 50 bar seems appropriate. The

raw data are shown in figures 18 and 19 and reveal the same trends as in the

extrapolated figures. The difference between the highest TOF and the TOF at high

sulfur trioxide concentration is bigger in the raw data, because the lower partial

pressure of methane additionally lowers the TOF at high sulfur trioxide concentration.

5.3.4 Equations used in section 3.4.3 (selectivity)

Equations used for figure 41:

𝐾𝑝 =𝑀𝐵𝑆

𝑀𝑒𝑂𝐻 (26)

𝑘𝑜𝑥 =𝑘2

1 + 𝐾𝑝+

𝑘3𝐾𝑝

1 + 𝐾𝑝 (27)

k2 60 s-1

, k3 0.00003 s-1

𝑆 =1 − 𝑒−𝑘𝑜𝑥𝑡

𝑘𝑜𝑥𝑡 (28)

[𝑀𝐵𝑆] =𝑘1𝑝𝐶𝐻4(1 − 𝑒−𝑘𝑜𝑥𝑡)

𝑘𝑜𝑥 (29)

Equations used for figure 42 (basic model of a consecutive first order reaction in a

batch reactor):

𝐶𝐶𝐻4 = (1 − 𝑒−𝑘1𝑡) ∗ 100 (30)

𝑌𝑀𝐵𝑆 =𝑘1

𝑘𝑜𝑥 − 𝑘1∗

𝑒−𝑘1𝑡 − 𝑒−𝑘𝑜𝑥𝑡

1∗ 100 (31)

Experimental

127

[𝑀𝐵𝑆] =𝑘1 𝑐𝐶𝐻4,0

𝑘𝑜𝑥 − 𝑘1

(𝑒−𝑘1𝑡 − 𝑒−𝑘𝑜𝑥𝑡) (32)

[CH4]0=0.115 mol/0.08 L=1.4375 M; k1pCH4 and k1 are interconverted by this factor

5.4 Chemicals

All chemicals were either purchased from Sigma Aldrich Corp. or received from

“Lager” and used without further purification. (2,2′-bipyridine)dichloroplatinum(II)

was synthesized by Mario Soorholtz. Methane was purchased from Air Liquide.

carbon dioxide was obtained from the high pressure laboratory. In case of in-house

facilities the purity of the chemical might have been not available. All chemicals are

given in table 18.

Table 18: List of used chemicals and their purity

chemical formula purity CAS-number

acrylonitrile C3H3N > 99% 107-13-1

ammonia (25%) NH4OH normapur 1336-21-6

ammonium sulfate (NH4)2SO4 >99% puriss 7783-20-2

2,2′-azobis(2-

methylbutyronitrile)

C10H16N4 >98% 13472-08-7

(2,2′-

bipyridine)dichloroplatinum(II)

C10H8N2Cl2Pt n.a. 13965-31-6

2,2′-bipyrimidine C8H6N4 95% 34671-83-5

bis(acetylacetonato)platinum(II) Pt(C5H7O2)2 97% 15170-57-7

carbon dioxide CO2 n.a. 124-38-9

cis-

diamminedichloroplatinum(II)

(NH3)2PtCl2 >99.9% 15663-27-1

deuterated sulfuric acid (96%) D2SO4 99.5% D 13813-19-9

dimethyl sulfoxide-d6 (CD3)2SO 99.96% D 2206-27-1

gold(III) bromide AuBr3 99.9%, but old 10294-28-7

hexadecyltrimethylammonium

bromide

C16H33N(CH3)3Br >95% 57-09-0

hydrogen sulfide (2.5) H2S 99.5% 7783-06-4

iodine I2 99.5% 7553-56-2

mercury(II) sulfate HgSO4 n.a. 7783-35-9

potassium bisulfate KHSO4 n.a. 7646-93-7

potassium disulfate K2S2O7 >97.5% 7790-62-7

potassium tetrachloropalladate

(II)

K2PdCl4 98% 10025-98-6

potassium

tetrachloroplatinate(II)

K2PtCl4 98% 10025-99-7

potassium

hexachloroplatinate(IV)

K2PtCl6 98% 16921-30-5

methane (N25) CH4 99.5% 74-82-8


128

methanesulfonic acid CH3SO3H 99.5% 75-75-2

nitric acid (65%) HNO3 puriss, p.a. 7697-37-2

platinum black Pt 99.9% 7440-06-4

platinum(II) chloride PtCl2 98% 10025-65-7

poly(ethylene glycol) H(OCH2CH2)nOH average Mw

35000

25322-68-3

sodium hydroxide NaOH 97% 1310-73-2

sodium methyl sulfate CH3OSO3Na n.a.a 512-42-5

sulfur trioxide SO3 99% 7446-11-9

oleum (20%, 65%) H2SO4·(SO3)x puriss, p.a. 8014-95-7

sulfuric acid (96%) H2SO4 ACS reagent 7664-93-9

silica (Davisil 150) (SiO2)n 99%, pore size

150 Å, 100-

200 mesh

112926-00-8

tetraethyl orthosilicate Si(OC2H5)4 98% 78-10-4

5.5 Synthesis and handling

5.5.1 η2-(2,2´-bipyrimidyl)dichloroplatinum(II)

[(bpym)PtCl2, 1]

1 was synthesized in a similar way as reported previously.101

In brief 0.4453 g of

potassium tetrachloroplatinate was dissolved in 50 mL distilled water at room

temperature under air. 0.170 g bpym, dissolved in 4 mL water, was added in less than

1 min under stirring. An orange precipitate formed quickly, and the solution was

filtered after 30 min. The mother liquor was stirred overnight and filtered for a second

time. Both times the precipitate was washed with water and acetone and dried at

90 °C over night, and afterwards in a vacuum drying oven at 50 °C. The yield

amounted for 93% (The yield of the first filtration was 80%). Characterization was

done by elemental analysis, 1H NMR and XRD. Spectra and diffractograms after

synthesis and after precipitation from dilution of oleum solution can be found in the

appendix.

1H NMR (300 MHz, DMSO-d6): δ 8.0 (dd, 2H, bpym H5/5´), 9.4 (dd, 2H, bpym H4/4´),

9.7 (dd, 2H, bpym H6/6´)

aSigma-Aldrich claims less than 3% methanol and less than 5% water as impurities. However, their

analysis is obviously only based on qualitative IR and NMR. One package contained considerable

amounts of chloride salt (>>10%).

Experimental

129

Analytical data for 1: Calculated: 22.7% C, 1.4% H, 13.2% N, 16.7% Cl, 46.0% Pt;

Found: 22.3% C, 13.5% N, 16.4% Cl, 45.0% Pt.

5.5.2 Synthesis of Polyacrylonitrile based carbons

Synthesis of powdered PAN material and CTF was conducted as described in 32

.

5.5.2.1 Water steam activation route

A mixture of acrylonitrile (AN) and 2,2′-azobis(2-methylbutyronitrile) (AMBN;

molar ratio approximately 300:1) was filled in the desired mold and polymerized for 2

days at 35 °C. The resulting PAN block was stabilized for 15 h under air in a

calcination oven at 200 °C (heating ramp approximately 1.17 K min-1

). The resulting

material was carbonized for 4 h at 1100 °C under N2 flow (heating ramp 10 K min-1

).

WSA was done at 850 °C (heating ramp 5 K min-1

) for 1 to 2.5 h with a N2 stream

saturated with steam (approximately 1.5 LSTP N2 min-1

, water bath refluxed at

120 °C).

Pt(II) coordination was done at room temperature in 1 L water for 2 days, and the

resulting material was thoroughly washed with water and acetone and dried at 45 °C.

5.5.2.2 Hard templating route

Silica monoliths were prepared by mixing 6 g of water, 0.5438 g nitric acid (65%) and

0.347 g poly(ethylene glycol) (PEG, average molecular weight 35000 g mol-1

). After

dissolution of PEG, 4.665 g of tetraethyl orthosilicate (TEOS) was added. As soon as

the solution was homogeneous, 1.126 g hexadecyltrimethylammoniumbromide

(CTAB) was added (molar ratio TEOS:PEG:HNO3:H2O:CTAB

1:0.000433:0.25:14.69:0.137). After complete dissolution the solution was filled in

the desired mold and gelled at 40 °C for 61.5 h. The resulting monoliths were treated

with 1 M NH4OH for 8 h at 90 °C, neutralized with 0.1 M HNO3, washed with

acetone, dried and finally calcined under air for 5 h at 550 °C (heating ramp 1 K min-

1)

After determination of the pore volume by nitrogen sorption the monoliths were

impregnated with an AN/AMBN mixture (molar ratio 800:1), and the composite was

polymerized at 35 °C for 15 h and 8 h at 60 °C. Subsequently the composite was

stabilized under air at 200 °C and carbonized at the desired temperature under N2


130

flow. The silica template was removed by NaOH leaching (1 M) for 2 days at 60 °C

and the carbon monoliths were washed with water and acetone and dried at 45 °C.

5.5.3 Sulfur trioxide

All reagents should be handled with care due to their strongly oxidizing power or

other hazards.

Handling of sulfur trioxide is mentioned explicitly, since it posed a considerable

challenge. Sulfur trioxide is delivered as solid which is strongly hygroscopic and

cannot be handled properly. Instead the sulfur trioxide has to be slowly melted. The

solid contains stabilizers which facilitate melting. Nevertheless, melting has to be

done carefully over the course of several days in an oven at 35 °C and overheating has

to be avoided since liquid sulfur trioxide boils above 44 °C. The liquid can be

handled, and purging with inert gas is recommended.

5.6 Instruments and analysis

5.6.1 Nitrogen physisorption

Nitrogen physisorption was conducted with an ASAP 2010 sorption analyzer from

micromeritics. Samples were activated over night at 150 °C. Measurements were done

at liquid nitrogen temperature. Equilibrium condition was set to 5% relative or 5

mmHg absolute change within a time interval of 5 s. BET equivalent surface area was

determined in a range of p/p0 from 0.05 to 0.2. Pore sizes were determined according

to the Barrett-Joyner-Halenda method.

Volumes of autoclaves were determined with the same machine using an in-house

made connector to SwageLok fittings.

5.6.2 Elemental analysis

Elemental analysis was carried out by Mikroanalytisches Laboratorium Kolbe,

Mülheim, Germany, via atomic absorption spectroscopy. C, H, N, S analysis was

carried out at the same institute.

Experimental

131

5.6.3 Powder X-ray diffraction

X-ray diffraction (XRD) was measured together with Claudia Weidenthaler or André

Pommerin using a Stadi P diffractometer by Stoe with a theta/theta stage in reflection

geometry with CuKα radiation. A glass sample holder was used in case of the sample

still containing oleum and the sample was covered with a piece of household foil

(polyethylene). Diffraction pattern were measured from 10 to 50° 2θ with a step width

of 0.05° and a measurement time of 4.5 s per data point.

5.6.4 Transmission electron microscopy

TEM was measured by Bernd Spliethoff and Nicolas Duyckaerts using a Hitachi HF-

2000 microscope with a cold field-emission cathode at maximum acceleration voltage

of 200 kV or a Hitachi H-7100 with maximum acceleration voltage of 100 kV.

Samples were prepared by sprinkling dry specimen on the TEM grid consisting of a

lacy carbon film supported by a copper grid.

5.6.5 Scanning electron microscopy

SEM and STEM was measured by Hans Bongard on a Hitachi S-5500 at 30 kV

acceleration voltage.

Samples were prepared by embedding the specimen in a resin and cutting with a

diamond knife. The resulting block face was analyzed.

5.6.6 Liquid phase nuclear magnetic resonance

Liquid phase NMR of 1 (1H,

195Pt,

15N,

13C, 1D and 2D) was measured by Christophe

Farès on various instruments with different field strengths. Deuterated oleum was

prepared by dissolving liquid sulfur trioxide in D2SO4.

1H NMR spectra of reaction solutions were measured at room temperature with a

Bruker AV-300 spectrometer, using the standard Bruker pulse sequences. Average

measurement time was 2 minutes 41 seconds. Spectra were referenced to residual

protons of DMSO-d6 which was added in a coaxial capillary to the NMR tube.

Chemical shifts are stated in parts per million downfield of tetramethylsilane. In


132

general, chemical shifts are strongly influenced by the sulfur trioxide concentration,

respectively the pH. Dilution leads to upfield shifts.

Reliability was tested by determining the previously weighted amount of NaMBS in

20% oleum with this method (figure 51). The slope of the linear correlation obtained

by plotting the amount of methyl bisulfate determined as a function of the weighted

amount shows the validity of the method and is slightly below 1. This might be a

result of water impurities in methanesulfonic acid. Later titration of methanesulfonic

acid revealed 95% purity of the used solution.

Figure 51: Recovery function of NaMBS in 20% oleum by 1H NMR.

5.6.7 High performance liquid chromatography

The HPL chromatograms of the reaction mixture after hydrolysis were measured by

Heike Hinrichs and Marie Sophie Sterling at room temperature, using a Shimadzu

LC-20 chromatograph equipped with an organic acid column and a refractive index

detector. The eluent was 10 mM trifluoracetic acid.

Experimental

133

NMR analysis of hydrolyzed reaction solution revealed that saponification is not

complete under the used conditions, and only around 80% of methyl bisulfate is

hydrolyzed. The remainder stays unaffected.

A peak eluting at a retention time of around 11 min was often observed in HPLC

traces, and erroneously assigned to acetic acid which elutes after a similar retention

time. Instead this is sulfur dioxide, respectively H2SO3 as shown by addition of

NaS2O3.

5.6.8 Titration

Titrations were done with a 848 titrino plus automatic titrator from Metrohm, using

0.1 M NaOH standard solution, the exact concentration of which was determined

separately.

5.6.9 Infrared spectroscopy

IR spectra of the gas phase were collected with a Thermo Nicolet Avatar 370 FT-IR

spectrometer. For each spectrum 32 scans were accumulated with a resolution of

2 cm-1

. Background spectra were collected before each sample spectrum and

automatically subtracted. Before integration all spectra were subjected to automatic

baseline correction.

Calibration of apparent extinction coefficients of methane and carbon dioxide was

done separately with the pure gases. Figures 52 (methane) and 53 (carbon dioxide)

show the respective graphs of the integral absorbance (2800 – 3200 cm-1

for methane

and 2150 – 2550 cm-1

for carbon dioxide).


134

Figure 52: Integral absorbance from 2800 to 3200 cm-1

as function of injected volume of methane

Figure 53: Integral absorbance from 2150 to 2550 cm-1

as function of injected volume of carbon

dioxide.

Since at higher injection volumes linearity of the Beer-Lambert law is not valid any

more, 2 mL of gas were injected with a gas tight syringe for determining the relative

composition of the gas phase after catalysis. This is a compromise accounting for the

fact that carbon dioxide usually was present with less than 5 vol% and injection of

less gas would lead to errors by transferring the gas into the IR cell.

Differences in the molar extinction coefficient were accounted for by a correction

factor of 10.55: The average integral absorbance after injecting 2 mL of methane was

19.5 and 10.5 for injecting 0.1 mL carbon dioxide.

Curriculum Vitae

135

6 Curriculum vitae

Personal Information

Date of Birth February 1st 1986

Place of Birth Pforzheim, Germany

Nationality German

email [email protected]

Education

PhD chemistry

August 2011 Max-Planck-Institut für Kohlenforschung

- present Mülheim a.d. Ruhr, Germany

Kekulé stipend (Fonds der chemischen Industrie)

PhD thesis: Selective Oxidation of Methane in Sulfuric Acid:

Understanding and Improving Catalyst Activity, Stability and

Selectivity in the Periana System

Supervision: Professor Ferdi Schüth

MSc chemistry (final score 1.1)

October 2010 Technische Universität München, Chair of physical chemistry

– April 2011 Garching, Germany

Master thesis: Adsorption properties and activation of

Trichloroethene on Pt(111) and size-selected Pt clusters

Supervision: Professor Ulrich Heiz

October 2009 Technische Universität München, Germany

- September 2010 Major subject: Physical chemistry

Minor subject: Analytical chemistry

Optional subjects: Technical chemistry and catalysis, inorganic

chemistry, macromolecular chemistry

BSc chemistry (final score 1.6)

December 2008 Technische Universität München, WACKER chair of

- March 2009 macromolecular chemistry

Garching, Germany

Bachelor Thesis: Synthesis and characterization of

Polysilanebrushes

Supervision: Professor Bernhard Rieger

mailto:[email protected]


136

October 2006 Technische Universität München, Germany

- September 2009 Subjects: Organic, inorganic, physical and technical chemistry

Additional Experience

May 2009 undergraduate research assistant at the Chair of physical

- September 2010 chemistry (TUM, Prof. U. Heiz)

October 2007 student assistant teaching other undergraduates:

- February 2009 Reactivity of organic molecules

Thermodynamics and kinetics

Inorganic chemistry practical course

October 2006 Business plan, basic- and advanced seminar

- September 2007 UnternehmerTUM GmbH, Garching, Germany

Languages

German native speaker

English fluent

Membership

2005 Gesellschaft deutscher Chemiker (GdCh)

– present

Publications

(1) Schweinberger, F. F.; Crampton, A. S.; Zimmermann, T.; Kwon, G.; Ridge, C. J.; Günther, S.;

Heiz, U. Submonolayer sensitive adsorption study of trichloroethene on single crystal surfaces by

means of MIES, UPS and TPS Surface Science 2013, 609, 18.

(2) Soorholtz, M.; White, R. J.; Zimmermann, T.; Titirici, M.-M.; Antonietti, M.; Palkovits, R.;

Schüth, F. Direct methane oxidation over Pt-modified nitrogen-doped carbons Chemical

Communications 2013, 49, 240.

(3) Trotus, I.-T.†; Zimmermann, T.

†; Schüth, F. Catalytic reactions of Acetylene: A feedstock for

the chemical industry revisited Chemical Reviews 2014, 114, 1761. (†: equally contributing, co-first

authors)

(4) Trotus, I.-T.; Zimmermann, T.; Duyckaerts, N.; Geboers, J.; Schüth, F. Butadiene from

acetylene-ethylene cross-metathesis Chemical Communications 2015, 51, 7124.

(5) Soorholtz, M.; Jones, L. C.; Samuelis, D.; Weidenthaler, C.; White, R. J.; Titirici, M.-M.;

Cullen, D. A.; Zimmermann, T.; Antonietti, M.; Maier, J.; Palkovits, R.; Chmelka, B. F.; Schüth, F.

Local Platinum Environments in a Solid Analog of the Molecular Periana Catalyst submitted to ACS

Catalysis 2015.

Contributions on conferences

44. Jahrestreffen deutscher Katalytiker, Weimar, Germany; Interaction of trichlorethene on size

selected Pt Clusters (poster presentation), 2011.

Natural Gas Conversion Symposium 10, Doha, Qatar; Selective Direct Methane Oxidation over Pt-

based Solid Catalysts in Sulfuric Acid (oral presentation), 2013.

Curriculum Vitae

137

47. Jahrestreffen deutscher Katalytiker, Weimar, Germany; Selective oxidation of methane with the

Periana system: increasing the catalytic activity by orders of magnitude (poster presentation), 2014.

128th

BASF Summerschool, Ludwigshafen, Germany; New insights into the selctive oxidation of

methane with the Periana system (oral presentation), 2014.

School education and social service

August 2005 Lebenshilfe Pforzheim-Enzkreis e.V., Pforzheim, Germany

- April 2006 transport service for people with limited mobility

September 1996 Reuchlin-Gymnasium Pforzheim, Pforzheim, Germany

- June 2005 A levels (Final score 1.0)

Award for outstanding achievements in chemistry by the

Gesellschaft Deutscher Chemiker and physics by the Deutsche

physikalische Gesellschaft


138

7 Appendix

1H NMR spectrum of bpym in oleum with DMSO-d6 in a capillary (ZIR-ZA-139-03):

Signal at 10 ppm corresponds to the acid protons, resonances at 8 and 9 ppm to bpym.

The triplet at 5 ppm might originate from protonation of 14

N. The other signals have not

been assigned.

Extension of 1H NMR spectrum of bpym in oleum with DMSO-d6 in a capillary: Two

dubletts of dubletts are visible with a 2:1 intensity distribution as expected for bpym.

Appendix

139

Full range of 1H NMR spectra of 1 in D2SO4 (upper spectrum, ZIR-ZA-245-01) and d-

oleum (lower spectrum, ZIR-ZA-251-01). Significant impurities are found in d-oleum

the origin of which is not clear; the signals of the impurities have not been fully

reproducible in different samples.

13

C NMR spectra of 1 in D2SO4 (upper spectrum, ZIR-ZA-245-01) and d-oleum (lower

spectrum, ZIR-ZA-251-01). Also impurities are found in oleum, indicating carbonic

acids (in combination with 1H NMR acetic acid seems reasonable).


140

Extension of 13

C NMR spectra of 1 in D2SO4 (upper spectrum, ZIR-ZA-245-01) and d-

oleum (lower spectrum, ZIR-ZA-251-01) in the range of the ligand resonances: 13

C

agrees with 1H and shows four major signals with several smaller signals and

differences between sulfuric acid and oleum.

1H COSY of 1 in D2SO4 (ZIR-ZA-245-01): As the smaller signals resemble the pattern

of the main signals, species with different degree of protonation might be present. The

signal at 9 ppm corresponds to the protons in the 4 position, which couple with both

other protons.

Appendix

141

1H COSY of 1 in d-oleum (ZIR-ZA-251-01)

15

N HMBC via 1H of 1 in d-oleum (ZIR-ZA-177-01): Two different nitrogen are

present in accordance with coordination of platinum to bpym.


142

Extension of 15

N HMBC via 1H of 1 in d-oleum

13

C HMBC via 1H of 1 in D2SO4 (ZIR-ZA_245-01)

Appendix

143

13

C HMBC via 1H of 1 in d-oleum (ZIR-ZA-251-01)

13

C HSQC via 1H of 1 in D2SO4 (ZIR-ZA-245-01)


144

13

C HSQC via 1H of 1 in d-oleum (ZIR-ZA-251-01)

All 1H NMR of heating series with 1 in D2SO4 (ZIR-ZA-245-01): Temperature induced

reversible downfield shift with increasing temperature of the signals occurs. From

bottom to top: room temperature, 40 °C, 60 °C, 80 °C, 100 °C, 80 °C, 60 °C, 40 °C.

Appendix

145

All 1H NMR of heating series with 1 in d-oleum (ZIR-ZA-251-01): From bottom to

top: room temperature, 40 °C, 60 °C, 80 °C, 100 °C, 80 °C, 60 °C, 40 °C.

195

Pt-NMR spectra of heating series of 1 in D2SO4 (ZIR-ZA-245-01). from bottom to

top: room temperature, 60 °C, 100 °C, 40 °C


146

195

Pt-NMR spectra of heating series of 1 in d-oleum (ZIR-ZA-251-01); from bottom to

top: room temperature, 60 °C, 100 °C, 40 °C

1H NMR of 1 in DMSO-d6 directly after synthesis (lower spectrum, ZIR-ZA-075-01)

and after precipitation upon dilution with water of a 20 mM solution in 20% oleum

used in the decomposition of NaMBS (1 M, 2.5 h, 215 °C) (upper spectrum, ZIR-ZA-

223-01-R).

Appendix

147

Extension of 1H NMR of 1 in DMSO-d6 directly after synthesis (lower spectrum, ZIR-

ZA-075-01) and after precipitation upon dilution with water of a 20 mM solution in

20% oleum used in the decomposition of NaMBS (1 M, 2.5 h, 215 °C) (upper

spectrum, ZIR-ZA-223-01-R).

1H NMR of precipitated 1 in DMSO-d6: Precipitated from 20% oleum (lower spectrum,

ZIR-ZA-353-01) and sulfuric acid (upper spectrum, ZIR-ZA-352-01). At 7 ppm both

samples show signals of protonated 14

N.


148

XRD of 1 directly after synthesis (black) and after precipitation due to dilution with

water of a 20 mM solution in 20% oleum used in the decomposition of NaMBS (1 M,

2.5 h, 215 °C) (red).

Typical 1H NMR of reaction solution. methyl bisulfate resonates at 3.6 ppm and at

2.9 ppm the singlet of added methanesulfonic acid is visible.

Appendix

149

Technical drawing of an autoclave


150

TOFs, calculated partial pressure of methane (ptheo), measured total pressure and temperature for the

pressure dependency series.

1 K2PtCl4

TOF /h-1

Pcalc /bar ptot /bar T /°C TOF /h-1

pcalc /bar ptot /bar T /°C

96% sulfuric acid

9.0 46.1 39.7 215.9 32.9 41.4 38.3 215.9

17.8 70.0 66.0 216.2 60.3 66.4 67.5 215.5

18.5 102.1 92.4 215.9 89.5 97.0 90.5 215.9

20% oleum

510 32.6 41.0 215.5 7204 33.0 40.7 215.2

875 47.2 53.7 215.4 9442 33.7 40.8 215.5

1319 65.4 70.1 214.8 15165 53.3 58.3 216.0

1319 65.8 72.8 214.9 22998 65.4 69.9 215.3

65% oleum

330 27.8 83.2 216.4 6742 27.6 86.5 217.9

522 38.7 >100 225 9271 36.6 96.6 219.1

787 59.1 >100 224 10898 58.4 >100 230

Raw data of all experiments (For some datasets deviations <1% exist compared to previously given

values due to rounding errors and the solving of BWR by numerical procedure which gives in multiple

treatment of the same data slightly different values)

SO3 concentrations series K2PtCl4, liquid phase 15 mL, 215 °C, 600 µM catalyst

code ratioa

[SO3] /mol L-

1 (titrated)

CH4,in

/mol

CH4,out

/mol

MBS

/mol

CO2

/mol

pSO3

/bar

time

/h:min:s

Vgas

/mL

301-01 1:0:0 17.96 (17.6) 0.037339 0.034208 0.001074 0.000171 1.2 2:00:00 21.9

220-01 3:1:0 18.53 (18.0) 0.038381 0.031151 0.002915 0.000472 1.2 2:00:00 21.2

216-01 1:1:0 19.10 (18.6) 0.039935 0.026688 0.009235 0.000481 2.2 0:30:00 21.2

194-01 0:1:0 20.24 (20.0) 0.035688 0.025639 0.008428 0.000184 11.6 0:02:28 21.2

225-01 0:3:1 20.80 (20.5) 0.032116 0.021007 0.007652 0.000190 20.7 0:02:05 21.2

228-01 0:1:1 21.36 (21.7) 0.026327 0.017449 0.006617 0.000129 32.2 0:01:44 21.2

230-01 0:1:3 21.93 (21.8) 0.022290 0.012646 0.006520 0.000153 44 0:04:08 21.2

184-1b

0:0:1 22.49 (23.6) 0.016148 0.008358 0.005008 0.000219 57.9 0:05:00 21.2

SO3 concentrations series 1, liquid phase 15 mL, 215 °C, 600 µM catalyst

305-01 1:0:0 17.96 (17.6) 0.035879 0.030400 0.000319 0.000117 1.1 2:00:00 19.5

222-01 3:1:0 18.53 (17.8) 0.038660 0.032340 0.000884 0.000239 1.2 2:00:00 21.2

215-01 1:1:0 19.10 (18.5) 0.039905 0.028019 0.007107 0.000475 2 1:32:20 21.2

Appendix

151

159-01 0:1:0 20.24 (20.0) 0.035698 0.025013 0.005279 0.000314 12.1 0:26:48 21.2

226-01 0:3:1 20.80 (21.1) 0.031343 0.022449 0.005078 0.000621 22.2 0:23:52 21.2

229-01 0:1:1 21.36 (21.5) 0.026913 0.020036 0.003786 0.000283 33.7 0:23:40 21.2

231-01 0:1:3 21.93 (21.9) 0.022562 0.017139 0.002683 0.000257 40.5 0:28:03 21.2

186-1b

0:0:1 22.49 (23.6) 0.016267 0.013677 0.001480 0.000187 56.9 0:30:00 21.2

PtCl2 low SO3 concentration, liquid phase 15 mL, 215 °C, 600 µM catalyst

285-01 3:1:0 18.53 (18.2) 0.038286 0.035300 0.001191 0.000174 1.0 2:00:00 22.3

aratio of the volumes used for preparing the reaction solution: 96% sulfuric acid: 20% oleum: 65% oleum

btemperature increased to 230 °C (228 °C in case of 1) after pressurizing with methane

catalyst concentration series K2PtCl4, 15 mL 20% oleum, 215 °C

code conc.

/mM

CH4,in

/mol

CH4,out

/mol

MBS /mol CO2 /mol time

/h:min:s

Vgas /mL

271-01 0.135 0.034019 0.023490 0.006990 0.000178 0:05:53 19.8

131-01 0.321 0.030802 0.019409 0.009504 0.000104 0:04:38 17.9

141-01 0.321 0.028125 0.021405 0.006999 0.000232 0:03:09 17.9

194-01 0.594 0.035708 0.025629 0.008428 0.000182 0:02:28 21.2

326-01 0.594 0.036077 0.026596 0.009358 0.000131 0:02:02 21.9

123-03 0.675 0.029793 0.019321 0.007775 0.000115 0:01:59 17.9

267-01 0.675 0.036981 0.022709 0.008538 0.000232 0:02:44 19.8

111-01 3.598 0.026067 0.018385 0.006881 0.000198 0:03:29 15.4

111-09 3.630 0.029898 0.017993 0.006429 0.000152 0:04:19 15.4

111-05 6.505 0.029014 0.016878 0.009482 0.001394 0:10:12 15.4

111-07 6.505 0.030487 0.017391 0.007607 0.001025 0:07:19 15.4

111-03 10.95 0.030249 0.018467 0.006226 0.000189 0:05:21 15.4

111-11 10.95 0.030886 0.018325 0.006209 0.000206 0:07:09 15.4

270-01 20.06 0.036196 0.025823 0.006678 0.000280 0:05:01 22.3

272-01 20.08 0.035336 0.025784 0.007228 0.000211 0:03:46 22.3

217-01 50.16 0.031667 0.021300 0.006257 0.000357 0:09:04 17.9

311-01 50.13 0.034420 0.026036 0.006168 0.000236 0:05:23 21.9

317-01 50.00 0.033837 0.025502 0.006688 0.000200 0:04:26 21.9

322-01 50.24 0.033668 0.025207 0.007176 0.001371 0:07:04 21.9

catalyst concentration series 1, 15 mL 20% oleum, 215 °C

101-23 0.0159 0.027300 0.015834 0.004250 0.002849 14:40:22 15.4

101-05 0.0492 0.030167 0.016295 0.006814 0.001148 5:52:21 15.4

101-17 0.142 0.026690 0.015063 0.008387 0.001882 2:36:15 15.4


152

142-07 0.299 0.027846 0.021562 0.007114 0.000385 1:01:12 17.9

101-07 0.393 0.030047 0.018674 0.006410 0.000568 0:52:29 15.4

101-03 0.597 0.031585 0.019452 0.005309 0.000252 0:28:57 15.4

144-05 0.597 0.028838 0.018981 0.005796 0.000244 0:30:47 17.9

158-01 0.597 0.032120 0.021268 0.006943 0.000255 0:35:15 17.9

159-01 0.597 0.035743 0.025042 0.005279 0.000314 0:26:48 21.2

101-09 0.802 0.030774 0.018848 0.006367 0.000264 0:29:08 15.4

101-01 10.47 0.032159 0.019337 0.007546 0.000173 0:04:19 15.4

223-01 50.03 0.031638 0.019868 0.009365 0.000203 0:02:50 17.9

catalyst concentration series PtCl2, 15 mL 20% oleum, 215 °C

275-01 0.602 0.034839 0.026973 0.006940 0.000235 0:03:02 22.3

280-01 19.87 0.033660 0.021409 0.009722 0.000113 0:01:59 22.3

276-01 49.60 0.033113 0.021012 0.009561 0.000209 0:01:32 22.3

catalyst concentration series Pt(acac)2, 15 mL 20% oleum, 215 °C

code conc.

/µM

CH4,in

/mol

CH4,out

/mol


/h:min:s

Vgas /mL

261-01 5000 0.035820 0.025589 0.00950515 0.000585 0:01:39 22.3

262-01 20000 0.033625 0.023979 0.00843077 0.001645 0.01:40 22.3

263-01 680 0.036056 0.027137 0.0089756 0.000318 0:02:21 22.3

264-01 340 0.035407 0.027008 0.00752653 0.000226 0:02:41 22.3

265-01 78 0.035868 0.027946 0.00684881 0.000277 0:07:27 22.3

266-01 50000 0.031375 0.019569 0.00992266 0.003614 0:01:57 22.3

c5 h at 215 °C before pressurizing with methane

dK2PtCl4 ground

eK2PtCl4 ground; 5 h at 215 °C before

pressurizing with methane fK2PtCl4 ground; 20 h at 215 °C before pressurizing with methane

gtemperature

increased to 223 °C after pressurizing with methane

influence of salt addition, 215 °C, 15 mL oleum, 600 µM catalyst

code conditions CH4,in

/mol

CH4,out

/mol

MBS

/mol

CO2

/mol

time

/h:min:s

pSO3

/bar

Vgas

/mL

295-01 1, 65% oleum, 0.8 M

K2S2O7

0.015639 0.011942 0.000823

0.000088 0:30:00 59.3 21.9

297-01 K2PtCl4, 65% oleum,

0.8 M K2S2O7

0.018474 0.014610

0.000561

0.000110 0:05:00 51.5 21.9

315-01 K2PtCl4, 65% oleum,

1.6 M KHSO4

0.014733 0.011150 0.001405 0.000100 0:05:00 58.5 21.9

327-01 K2PtCl4, 65% oleum, 0.017149 0.012904 0.000604 0.000080 0:05:00 56.9 21.9

Appendix

153

0.8 M (NH4)2SO4

312-01 K2PtCl4, 20% oleum,

0.8 M KHSO4

0.034630 0.022849

0.008111 0.000112

0:03:14 9.2 19.5

319-01 K2PtCl4, 20% oleum,

1.6 M KHSO4

0.036443

0.027994

0.007530

0.000144

0:05:27 7.4 21.9

318-01 K2PtCl4, 20% oleum,

0.4 M K2S2O7

0.034480

0.024260

0.012549

0.000147

0:04:28 11.0 21.9

313-01 K2PtCl4, 20% oleum,

0.8 M K2S2O7

0.035616

0.026993

0.007254

0.000127 0:04:05 9.3 21.9

320-01 K2PtCl4, 20% oleum,

0.4 M (NH4)2SO4

0.034630

0.025761

0.008365

0.000102

0:02:53 10.4 21.9

321-01 K2PtCl4, 20% oleum,

0.8 M (NH4)2SO4

0.035821

0.027388

0.007511

0.000117

0:05:03 7.9 21.9

influence of K+ and Cl

- on PtCl2, 215 °C, 15 mL 20% oleum


/mol

CH4,out

/mol

MBS

/mol

CO2

/mol

time

/h:min:s

Vgas

/mL

309-01 50 mM PtCl2,

100 mM KHSO4

0.034710

0.024229

0.009114

0.000121

0:01:21 21.9

324-01 50 mM PtCl2,

100 mM KCl

0.033687

0.022146

0.007699

0.000294

0:31:54 19.5

high TON, high MBS concentration 215 °C


/mol

CH4,out

/mol

MBS

/mol

CO2

/mol

time

/h:min:s

Vgas

/mL

185-01 K2PtCl4 (40.6 µM),

15 mL 20% oleum

0.029698

0.016180

0.009805

0.000856

2:00:00 17.9

195-01 K2PtCl4 (870 µM),

5 mL 65% oleum

0.059021

0.041169

0.013554

0.004791

3:00:00 31.2

High MBS concentration, 180 °C, 5 mL 65% oleum

197-01 K2PtCl4 (870 µM),

5 mL 65% oleum

0.061889

0.048909

0.008926

0.000964

3:00:00 31.2

198-01 I2 (36 mM),

5 mL 65% oleum

0.084543

0.047968

0.036261

0.001677

3:00:00 31.2


154

50 mM catalyst, 15 mL 98% sulfuric acid, 215 °C

code catalyst CH4,in /mol CH4,out /mol MBS /mol CO2 /mol time /h:min:s Vgas /mL

145-03 1 0.028421 0.010040 0.011402 0.001699 2:30:00 17.9

145-01 K2PtCl4 0.026610 0.021120 0.000914 0.000185 2:30:00 17.9

290-01 PtCl2 0.030725 0.023778 0.004521 0.000188 2:30:00 19.8

pressure dependency, 600 µM catalyst, 15 mL solution, 215 °C

code condition ptotal

/bar

ptheo

/bar

CH4,in

/mol

CH4,out

/mol

MBS

/mol

CO2

/mol

time

/h:min:s

Vgas

/mL

304-01 K2PtCl4,

96%

H2SO4

38.3 41.4 0.023181

0.019302

0.000587

0.000092

0:02:00 21.9

306-01 K2PtCl4,

96%

H2SO4

67.5 66.4 0.037339

0.034208

0.001074

0.000171

0:02:00 21.9

301-01 K2PtCl4,

96%

H2SO4

90.5 97.0 0.054214 0.046712

0.001595

0.000220

0:02:00 21.9

305-01 1,

96%

H2SO4

39.7 46.1 0.023518

0.018477

0.000161

0.000072

0:02:00 19.5

307-01 1,

96%

H2SO4

66.0 70.0 0.035879

0.030400

0.000319

0.000117

0:02:00 19.5

303-01 1,

96%

H2SO4

92.4 102.1 0.052069

0.043291

0.000332

0.000164 0:02:00 19.5

190-01 K2PtCl4,

20%

oleum

40.7 33.0 0.016244

0.007815

0.006778

0.000163

0:06:20 17.9

192-01 K2PtCl4,

20%

oleum

40.8 33.7 0.018482

0.010852

0.006360

0.000157

0:04:32 21.2

193-01 K2PtCl4,

20%

oleum

58.3 53.3 0.029141

0.019277

0.007210

0.000237

0:03:12 21.2

Appendix

155

194-01 K2PtCl4,

20%

oleum

69.9 65.4 0.035727

0.025628

0.008428

0.000183

0:02:28 21.2

189-01 1,

20%

oleum

41.0 32.6 0.016101

0.007564

0.006360

0.000265

1:24:00 17.9

187-01 1,

20%

oleum

53.7 47.2 0.023065

0.013250

0.006646

0.000210

0:50:50 17.9

159-01 1,

20%

oleum

70.1 65.4 0.035698

0.025013

0.005279

0.000314

0:26:48 21.2

158-01 1,

20%

oleum

72.8 65.8 0.032056

0.021277

0.006943

0.000255

0:35:15 17.9

183-01 K2PtCl4,

65%

oleum

86.5 27.6 0.016148

0.008358

0.005008

0.000219

0:05:00 21.2

299-01 K2PtCl4,

65%

oleum

96.6 36.6 0.021775

0.011825

0.006887

0.000241

0:05:00 21.9

184-01 K2PtCl4,

65%

oleum

>100 58.4 0.032624

0.017400

0.008095

0.000538

0:05:00 21.2

188-01 1,

65%

oleum

83.2 27.8 0.016267

0.013677

0.001480

0.000187

0:30:00 21.2

300-01 1,

65%

oleum

>100 38.7 0.022900

0.017060

0.002376

0.000164

0:30:30 21.9

186-01 1,

65%

oleum

>100 59.1 0.032961

0.021512

0.003529

0.000320

0:30:00 21.2


156

dependency on stirring speed for 1, 15 mL 20% oleum, 215 °C

code conc.

/µM

stirring

speed /rpm

CH4,in

/mol

CH4,out

/mol


/h:min:s

Vgas

/mL

101-11 600 0 0.029538 0.018681 0.006952 0.000362 2:38:06 15.4

101-21 600 100 0.029243 0.016493 0.009256 0.001105 1:32:09 15.4

101-19 600 150 0.030007 0.018690 0.005700 0.000310 0:49:13 15.4

101-15 600 250 0.029111 0.018727 0.009267 0.000313 0:52:55 15.4

101-13 600 500 0.028177 0.017810 0.006708 0.000805 0:42:47 15.4

101-03 600 1000 0.031628 0.019447 0.005309 0.000252 0:28:57 15.4

144-03 600 0 0.029804 0.019543 0.006999 0.000464 1:49:12 17.9

144-01 600 100 0.026289 0.019978 0.006448 0.000281 0:53:33 17.9

144-05 600 1000 0.028864 0.018980 0.005796 0.000244 0:30:47 17.9

142-03 300 0 0.028658 0.021201 0.005901 0.000413 1:40.13 17.9

142-01 300 100 0.029029 0.021749 0.005932 0.000367 1:08:52 17.9

142-09 300 150 0.030870 0.021752 0.005840 0.000355 0:59:48 17.9

142-05 300 250 0.029230 0.021641 0.006350 0.000346 0:58:36 17.9

142-07 300 1000 0.027848 0.021571 0.007114 0.000385 1:01:12 17.9

blindtests, 215 °C, 15 mL solution

code solution CH4,in

/mol

CH4,out /mol MBS /mol CO2 /mol time /h:min:s Vgas /mL

308-01 96% H2SO4 0.037711 0.035352 0.000032 0.000174 2:00:00 21.9

294-01 20% oleum 0.033493 0.027156 0.000875 0.000271 2:32.00 19.5

281-01 20%oleum 0.034952 0.028364 0.000712 0.000346 2:30:00 19.5

242-01 65% oleum 0.015692 0.011908 0.000108 0.000131 0:05:00 21.2

Pt compounds, 215 °C, 15 mL 20% oleum

code catalyst [catalyst]

/µM

CH4,in

/mol

CH4,out

/mol


/h:min:s

Vgas

/mL

108-01 1/bpym 600/3000 0.029729 0.017490 0.006844 0.000704 1:22:09 15.4

314-01 (NH3)2PtCl2 600 0.033769 0.025407 0.005855 0.000122 0:02:02 21.9

196-01 (bipy)PtCl2 600 0.031908 0.021595 0.007560 0.000194 0:03:30 17.9

179-01 Pt black 680 0.029424 0.018552 0.008584 0.000185 0:02:30 17.9

263-01 Pt(acac)2 680 0.036137 0.027139 0.008976 0.000318 0:02:21 22.3

123-01 K2PtCl6

670 0.036939 0.018239 0.007800 0.000170 0:01:56 17.9

Appendix

157

Other elements, 215 °C, 15 mL 20% oleum

code [catalyst] /µM CH4,in /mol CH4,out

/mol


/h:min:s

Vgas

/mL

113-01 I2 1420 “I” 0.032189 0.022074 0.007691 0.000205 0:04:19 17.9

132-01 I2 630 0.032047 0.019812 0.007042 0.000213 0:04:13 17.9

124-01 K2PdCl4 670 0.030505 0.020075 0.004467 0.000408 0:11:21 17.9

125-01 AuBr3 670 0.031037 0.019466 0.005015 0.000757 1:30:00 17.9

116-01 HgSO4 740 0.031454 0.020252 0.004621 0.000234 0:17:50 17.9

H2S addition, 600 µM 1, 215 °C, 15 mL 20% oleum

code conditions CH4,in /mol CH4,out /mol MBS /mol CO2 /mol time /h:min:s Vgas /mL

156-01 6.6 vol% H2S 0.027067 0.019382 0.006840 0.000265 0:32:20 17.9

157-01 400:1 H2S:1 0.029295 0.018577 0,006768 0.000156 0:33:45 17.9


158

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Selective oxidation of methane in sulfuric acid

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