-
Synthesis, Characterization and Catalyst Testing of Metal
Modified Zeolites for
Isomerization of n-butane and Application in Fine Chemicals
Masters Thesis
SEELAM PREM KUMAR
Laboratory of Industrial Chemistry
Process Chemistry Centre
Faculty of Technology
Department of Chemical Engineering
bo Akademi University
Turku, Finland.
2007.
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Preface
The present masters thesis was carried out at the Laboratory of
Industrial
Chemistry, Process Chemistry Centre, Faculty of Technology,
Department of
Chemical Engineering, bo Akademi University, Turku, Finland
during the
academic year 2005-2006 as part of my graduation studies as an
international
student from India. I would like to thank to bo Akademi
University for giving me
the opportunity to study in Finland.
I wish to express my sincere thanks and gratitude to Prof. Tapio
Salmi and Prof.
Dmitry Yu. Murzin, for giving me the opportunity to work in the
top research
center in catalysis i.e. Laboratory of Industrial Chemistry. My
special thanks to
my supervisor Docent Dr. Narendra Kumar who has been so helpful
to me and
whose technical and moral guidance remained a source of research
knowledge
in my life and I learnt so many things during interactions with
N.Kumar. I would
like to express my gratitude to Laboratory of Inorganic
Chemistry and Heat
Engineering Laboratory for giving me the opportunity to work
during my studies.
I would like to thank the persons at the Laboratory of
Industrial Chemistry, in
particular to Laboratory manager Dr. Kari Eranen for his help
with the reactor
system. Special thanks go to Aton Tokarev for helping me in
DCP
measurements, explanation of lactose oxidation results and also
to Elena
Murzina for testing my catalysts in oxidation of lactose
reaction. I am thankful to
Thomas Finnas for helping me in FTIR measurements and also to
Jose Villegas
in catalyst testing. Finally, I am grateful to Ping Ping Lee for
DCP and ICP
measurements at Laboratory of Analytical Chemistry.
I am very kind to my parents who made me a person with good
character and
hard working; especially my elder brothers always encouraged me
in the studies.
I wish to thank my friends for their valuable moral support in
my life in
Hyderabad, India. And also special thanks to my friends in
Denmark and Finland
for their valuable support in my life. Financial support from bo
Akademi,
Laboratory of Industrial Chemistry is gratefully
acknowledged.
i
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ABSTRACT
Seelam Prem Kumar Synthesis, Characterization and Catalytic
Testing of Metal Modified Zeolite Catalysts
For Application in Fine Chemicals and
Isomerization of n-Butane.
Masters Thesis Carried out under the supervision of Prof. Tapio
Salmi, Prof. Dmitry Yu. Murzin and
Docent Narendra Kumar, Laboratory of
Industrial Chemistry, Process Chemistry
Centre, Department of Chemical
Engineering, Faculty of Technology,
bo Akademi University, 2005-2006.
Keywords Synthesis of zeolites, MCM-22, MCM-36, MCM-48, metal
modification, Pd, Au,
ultrasound, n-butane isomerization,
lactose oxidation, deposition precipitation
method.
Iso-butane formed by isomerization of n-butane is an essential
ingredient in
refinery production of alkylates and oxygenated compounds such
as tertiary butyl
alcohol (TBA), methyl tertiary butyl ether (MTBE), isooctene,
and polyisobutene
rubber. In recent years, the technology for the isomerization of
normal butane (n-
C4) to isobutane (i-C4) has become increasingly important for
motor fuel
applications. Isobutane is a primary feedstock for motor fuel
alkylation processes,
which produces an excellent and environmentally superior
gasoline-blending
component.
The modification of zeolites is of very great importance both
from an industrial
and academic point of view because of the potential applications
of metal and
ii
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proton modified zeolite catalysts in the process of petroleum
and in the chemical
industry.
The scope of the thesis was to investigate different catalysts
preparation
methods. Proton forms of MCM-22 with varying acidities and
palladium modified
MCM-22 catalysts with 28, 30, 50, and 70 (silica to alumina)
ratios are
synthesized, characterized and tested at the laboratory. To
investigate the
influence of acidity over isomerization of n-butane and lactose
oxidation were
selected as test reactions. The effect of synthesis time of
MCM-22-30 using
ultrasound irradiation method, and effect of palladium metal
form of zeolites with
different catalyst preparation methods for applications in fine
chemicals i.e.
oxidation of lactose had been investigated. Synthesis of MCM-36
with different
MCM-22 precursors and as well as preparation of MCM-48, which is
difficult to
synthesize, and also its reproducibility are also investigated
in this thesis work.
Acidity of MCM-22 catalytic materials which can be adjusted by
silica to alumina
ratio was found to influence activity and yield of isobutane in
n-butane
isomerization. H-MCM-22-30-R was found the most promising
catalyst and more
active catalyst compared to Pd form catalysts in n-butane
isomerization and in
the case of lactose oxidation, the Pd-impregnated catalysts i.e.
MCM-22 exhibits
similar catalytic activity and Pd-H-MCM-22-30-IMP exhibits high
active compared
to other preparation methods of Pd-H-MCM-22-30 in lactose
oxidation.
The stability of the catalysts in the conversion of n-butane,
yield and selectivity of
iso-butane were investigated with Pd-H-MCM-22-28-I.E catalyst
with different
weight hourly space velocities and also effect of
temperature.
iii
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REFERAT
Seelam Prem Kumar Syntes, karakterisering och testning av
metall- modifierade zeolitkatalysatorer fr
finkemika-lie applikation och isomerisering av
n-butan.
Diplomarbete Arbetet utfrdes under handledning av Prof. Tapio
Salmi, Prof. Dmitry Yu. Murzin och
Docent Narendra Kumar, vid Laboratoriet fr
Teknisk Kemi, Processkemiska forskargrupp-
en, Kemisk-tekniska fakulteten, bo Akademi
2007.
Nyckelord Zeolitkatalysatorer, syntes, MCM-22, MCM-36, MCM-48,
metallmodifiering, Pd, Au,
ultraljud, n-butan isomerisering, laktos
oxidering.
Isobutan, en isomeriseringsprodukt av n-butan, r en viktig
frening fr
produktion av alkylat och syrehaltiga komponenter, s som
tertirbutylalkohol
(TBA) och metyltertir butyleter (MTBE), samt iso-okten och
polyisobutengummi.
Under de senaste ren har teknologin fr isomerisering av n-butan
till isobutan
blivit alltmer viktig. Isobutan r en viktig rvara fr
alkyleringsprocesser som
producerar utmrkta och miljvnliga bensinkomponenter. Modifiering
av zeoliter
genom protonering eller metalltillsats r viktig ur srl ur
industriell som
akademisk synvinkel.
Avsikten med diplomarbetet var att underska olika metoder fr
syntes av
katalysatorer. Protonerade MCM-22 med olika surhets grad, kisel
till aluminium
frhllande 28, 30, 50 och 70 syntetiserades och modifierades med
palladium.
iv
-
Drefter karaketeriserades och testades katalysatorerena i
laboratoriet. Till
testreaktioner fr att studera effekten av surhet valdes
isomerisering av n-butan
och oxidering av laktos. Syntestidens effekt studerades med hjlp
av
ultraljudsbehandling av MCM-22-30. Olika
palladiumtillsatsmetoder p zeoliter
studerades med hjlp av laktosoxidation som exempel p
finkemikalieapplikation.
Syntesen av MCM-36 utfrdes frn olika MCM-22 material. Den
ytterst krvande
syntesen av MCM-48 samt reproducerbarheten av denna, ingr ven i
detta
diplomarbete.
Olika surheter av MCM-22 katalysatormaterial pverkade
aktiviteten och utbytet
av isobutan och isomeriseringen av n-butan. Den protonerade
formen av H-
MCM-22-30-R hade hgre aktivitet n Pd-formen i isomeriseringen av
n-butan.
Bda formerna uppvisade liknande aktiviteter i oxidationen av
laktos.
Katalysatorstabiliteten, konversionen av n-butan, utbytet av och
selektiviteten till
isobutan studerades med Pd-H-MCM-22-28-I.E under olika
fldeshastigheter och
olika temperaturer.
v
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PUBLICATION RELATED TO THE TOPIC 1. A.V.Tokarev, E.V. Murzina,
Prem k. Seelam, A.J. Plomp, J.H.Bitter, N.Kumar, D.Yu.
Murzin, Influence of support nature on the catalytic activity of
Pd catalysts in lactose
oxidation 2006 (submitted).
2. N. Kumar, O. Russu, P. Seelam, T. Heikkil, V.-P. Lehto, H.
Karhu, T. Salmi, D.Yu.
Murzin, Pt modified MCM-22, ZSM-5 and beta zeolite catalysts for
n-butane
isomerization: influence of structure, acidity and Pt
modification, Studies in Surface
Science and Catalysis (TOCAT-5) (accepted).
vi
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CONTENTS Preface ........ i Abstract .......... ii
Referat.........................................................................................................................
iv Publication related to the
topic.................................................................................
vi Contents .................... vii 1. Introduction ...... 1
1.1 Isomerization of n-butane........1
1.2 Fine chemicals ........ 4
1.3 Oxidation of lactose ........... 5
2. Background and theory ........ 6 2.1. Introduction to
zeolites ........... 6
2.1.1 Definition ........ 6
2.1.2 Framework and structure..... ....... 6
2.1.3 History ........... 8
2.1.4 Classification of zeolites ...... 9
2.1.5 Shape selectivity ............. 11
2.1.6 Acidity .... 14
2.1.7 Synthesis of zeolites: pre and post synthesis
methods...... 15
2.1.8 Mesoporous molecular sieves .... 19
2.1.9 Zeolite pore structure and active sites... 22
2.1.10 Zeolites in industrial applications ....... 24
3. Catalyst preparation methods............. 26 3.1 Ion exchange
method ...... 26
3.2 Impregnation method ....... 28
3.3 In-situ method ....... 30
4. Characterization methods ............ 31 4.1 Surface area
measurement method...................................... 31
4.2 X-ray powder diffraction method .............. 32
4.3 Scanning electron microscopy.......... 33
4.4 Fourier transform infra red spectroscopy..........34
vii
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4.5 Direct current plasma....... 36
5. Overview on MCM-22 and MCM-36 ............. 36 6. Overview on
MCM-48 ............. 41 7. Overview on Ultra sound irradiation
method .............. 42
7.1 Introduction ....... 42
7.2 Applications ...... 43
8. Experimental Procedure ........... 44 8.1 Catalysts .......
44
8.2 Synthesis of catalysts ..... 47
8.3 Proton form catalysts........... 50
8.4 Metal modification .......... 51
8.5 Catalysts characterization . 54
8.5.1 Surface area measurement .... 54
8.5.2 X-ray powder diffraction... 55
8.5.3 Scanning electro micrograph...... 55
8.5.4 Direct current plasma... 55
8.5.5 Fourier transform infrared spectroscopy ...... 55
9. Catalytic Testing ........... 56 9.1 Isomerization of
n-Butane ..... 56
9.2 Oxidation of lactose ........ 59
10. Results and Discussions ............ 60 10.1 Catalyst
synthesis and characterization results ....... 60
10.2 Catalyst testing in n-butane isomerization and lactose
oxidation....... 79
10.2.1 Influence of acidity in isomerization of n-butane
.......... 79
10.2.2 Influence of palladium form of H-MCM-22
in isomerization of n-butane ....... 80
10.2.3 Effect of
temperature.........................................................................
84
10.2.4 Effect of space velocity on isomerization of n-butane ...
87
10.2.5 Influence of catalysts preparation methods on lactose
oxidation.. 90
10.2.6 Influence of SiO2/Al2O3 ratio on lactose
oxidation.....93
viii
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11. Conclusion ............... 96 12. References ............
98
APPENDIX-I--------------------------------------------------------------------------------------------102
ix
-
REFERAT
Seelam Prem Kumar Syntes, karakterisering och testning av
metallmodifierade zeolitkatalysatorer fr
finkemikalieapplikation och isomerisering av
n-butan
Diplomarbete Arbetet utfrdes under handledning av Prof.
Tapio Salmi, Prof. Dmitry Yu. Murzin och
Docent Narendra Kumar, vid Laboratoriet fr
Teknisk Kemi, Processkemiska forskargruppen,
Kemisk-tekniska fakulteten, bo Akademi
2007.
Nyckelord Zeolitkatalysatorer, syntes, MCM-22, MCM-36,
MCM-48, metallmodifiering, Pd, Au, ultraljud,
n-butan isomerisering, laktos oxidering
Isobutan, en isomeriseringsprodukt av n-butan, r en viktig
frening fr produktion av
alkylat och syrehaltiga komponenter, s som tertirbutylalkohol
(TBA) och
metyltertirbutyleter (not sure) (MTBE), samt iso-okten (not
sure) och
polyisobutengummi. Under de senaste ren har teknologin fr
isomerisering av n-butan
till isobutan blivit alltmer viktig. Isobutan r en viktig rvara
fr alkyleringsprocesser,
som producerar utmrkta och miljvnliga bensinkomponenter.
Modifiering av zeoliter
genom protonering eller metalltillsats r viktig ur srl
industriell som akademisk
synvinkel.
Avsikten med diplomarbetet var att underska olika metoder fr
syntes av katalysatorer.
Protonerade MCM-22 med olika surhets grad, Si/Al (is Si/Al
correct?) frhllande 28,
30, 50 och 70 syntetiserades och modifierades med palladium.
Drefter karaketeriserades
och testades katalysatorerena i laboratoriet. Till
testreaktioner fr att studera effekten av
-
surhet valdes isomerisering av n-butan och oxidering av laktos.
Syntestidens effekt
studerades med hjlp av ultraljudsbehandling av MCM-22-30.
Olika
palladiumtillsatsmetoder p zeoliter studerades med hjlp av
laktosoxidation som
exempel p finkemikalieapplikation.
Syntesen av MCM-36 utfrdes utgende frn olika MCM-22 material.
Den ytterst
krvande syntesen av MCM-48 samt reproducerbarheten av denna,
ingr ven i detta
diplomarbete.
Olika surheter p MCM-22 katalysatormaterial pverkade aktiviteten
och utbytet av
isobutan och isomeriseringen av n-butan. Den protonerade formen
av H-MCM-22-30-R
hade hgre aktivitet n Pd-formen i isomeriseringen av n-butan.
Bda formerna
uppvisade liknande aktiviteter i oxidationen av laktos.
Katalysatorstabiliteten, konversionen av n-butan, utbytet och
selektiviteten till isobutan
studerades med Pd-H-MCM-22-28-I under olika fldeshastigheter och
olika
temperaturer.
-
1
1. Introduction 1.1. Isomerization of n-Butane
Transformation of hydrocarbons is of prime economic importance
in the
petrochemical industry. Generally, the transformation occurs
under strong acidic
conditions. For example, liquid super acids such as SbF5/HF or
SbF5/HSO3F can activate alkanes at temperatures below 273 K.
However, the drawback of liquid
acids is that they are corrosive and difficult to recover and
reuse. As a
replacement of liquid acids, solid acids exhibit a promising
alternative because of
their environmental friendly characteristics (non-corrosiveness,
ease of handling,
and easy to recover and reuse) [1]. Butane can be obtained from
catalytic and
steam crackers of oil refineries and petrochemical plants. The
isomerization of n-
butane to isobutane is an important reaction for the production
of alkylates and
oxygenated compounds such as tertiary butyl alcohol (TBA),
methyl tertiary butyl
ether (MTBE), isooctene, polyisobutene (PIB) and polyisobutene
rubber.
In the petrochemical industry, halogenated alumina and
comparable catalysts
containing precious metals, as well as organic chlorides have
been used for this
reaction in order to reach the necessary acidic strength.
Unfortunately, these are
non-environmental-friendly catalysts and, therefore, there is a
great interest
among academic and industrial researchers to find and develop
new active,
selective and resistant-to-deactivation catalysts. The main
problem associated
with the use of strongly acid zeolites, e.g. H-Mordenite, as
isomerization catalysts
is the fast deactivation [5, 6]. Zeolites such as ZSM-5, beta,
and Mesoporous
molecular sieves as well a sulfated zirconia have been reported
to be active
catalysts in the isomerization of n-butane. The stability and
activity of the
catalysts have been improved by the introduction of metals
combined with the
usage of hydrogen as a carrier [2]. Much discussion has been
going on in the
literature whether the apparently simple isomerization of
n-butane to isobutane
goes via a bi- or monomolecular mechanisms. The relative
importance of these
-
2
mechanisms depends on the reaction temperature and the surface
concentration
of reactants determined by temperature, reactant pressure and
concentration of
acid sites. Skeletal isomerization of n-butane over zeolite
catalysts is proposed
(see Figure 1) to proceed via a bimolecular
dimerization-cracking route. The first step is the formation of the
butylcarbenium ion from butane by: (i) protonation of
butane by a Brnsted acid site to form a penta-coordinated
butylcarbocation eq.
(1) and subsequent abstraction of H2, eq. (2), (3) (ii) hydride
abstraction by a
Lewis acid site eq. (3) and (iii) protonation of trace olefins
formed eq, (4). The
butyl carbenium ion reacts with a olefin (butene) forming an
octylcarbenium ion,
which yields n- and isobutane after isomerization and
beta-scission. Byproducts,
mainly propane and pentanes, are obtained from the C8
intermediates after
disproportionation. The formation of olefin species is important
for the skeletal
isomerization of n-butane, since they are needed in the
dimerization step [3].
However, the possibility of the n-butane isomerization also via
a monomolecular
mechanism has been proposed by Tran et al. [4]. The bimolecular
mechanism
shown below is a simplified version of that used to describe
n-butane
isomerization.
n-C4H10 C4H9+ + H .. (1)
C4H9+ C4H8 + H+. (2)
H+ + H H2 .. (3)
C4H8 + H+ C4H9+ (4)
C4H9+ + C4H8 C8H17+ i-C4H10 ..... (5)
Figure 1. Reaction mechanism of isomerization of n-butane
(68).
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3
The primary reactions (1.1-1.4) in butane conversion may involve
cracking
of butane into propylene and methane or ethylene and ethane,
disproportionation
of butane into pentane and propane, and isomerization of
n-butane into iso-
butane. Butane disproportionation is more dependent on strong
acid site density
of a catalyst than butane cracking (mono-molecular reaction)
since two adjacent
acidic sites are needed to start disproportionation
(bi-molecular reaction) [81].
Ethylene, propylene, propane and pentanes formed from the
primary reactions
may undergo further reactions. The secondary reactions
(1.5-1.12) may involve
oligomerization, isomerization, cracking, hydrogen transfer,
dehydrocyclization,
aromatization, and coking (which leads to catalyst
deactivation), etc. At low
conversions, the primary reactions will be predominant.
Primary reactions: n-C4H10 C3H6+CH4 (cracking)
................................. 1.1a
n-C4H10 C2H4+ C2H6 (cracking) .................................
1.1b
2 n-C4H10C5H12+C3H8 (disproportionation) ...............
1.1c
n- C4H10i-C4H10 (isomerization)
................................. 1.1d
Secondary reactions: i- C5H12 C3H8 + C2H4 (cracking)
................................ 1.1e
C3H8 C2H4+CH4 (cracking)
........................................ 1.1f
2 C2H4n-C4H8 (oligomerization) .................................
1.1g
C2H4+C3H6i-C5H10 (oligomerization)
...........................1.1h
i-C4H8 +i- C5H12i- C4H10+i-C5H12 (hydrogen transfer)...1.1i
n- C4H8i-C4H8
(isomerization).......................................1.1j
C6H12 C2H4+ C4H8 (cracking)
.......................................1.1k
Olefins Aromatics + H2 (dehydrocyclization)
................1.1l
In the temperature range, except for cracking reactions
(Reactions 1.1a,
1.1b, 1.1e, 1.1f, 1.1k) that are very endothermic; the other
reactions are
exothermic or have small heats of reaction. Oligomerizations
(Reactions 1.1g
-
4
and 1.1h) have large exothermic heats of reaction. The effect of
temperature on
the equilibria of the reactions is different. For cracking
reactions, a high
temperature is favored; while for oligomerization, a low
temperature is favored.
For hydrogen transfer reactions and isomerization reactions, the
effect of
temperature on reaction equilibrium is not as significant as on
the other reactions
[81].
The zeolites to be used in skeletal isomerization of n-butane
should have at least
10-membered-ring channels (0.45-0.6 nm), if the molecular sizes
of reactant n-
butane (critical diameter 0.49 nm) and the product isobutane
(critical diameter
0.56 nm) are taken into consideration. The channels of
8-membered-ring
zeolites (0.35-0.45 nm) are too small to allow branched
molecules to diffuse with
any considerable rate, which results in a rapid deactivation of
the catalyst.
1.2. Fine Chemicals Zeolites are important catalysts in a
multitude of contemporary chemical
production processes. A continually increasing number of
applications is
emerging in the production of organic intermediates and fine
chemicals. In drug
manufacture, fine chemicals are pure, single chemical substances
that are
produced by chemical reactions. Examples of fine chemicals are
intermediates
for drug production and bulk active pharmaceutical ingredients
ready to be
compounded with inert pigments, solvents and excipients and made
into dosage
forms [7]. Most of the fine chemicals are used in the
manufacturing of life saving
drugs; in order to prepare the fine chemicals we need catalysts
with high activity
and selectivity towards desire product.
-
5
1.3. Oxidation of Lactose
Lactose oxidation is a consecutive reaction, resulting first in
lactobionic acid, and
then in 2-keto-lactobionic acid (Figure 2). From industrial
viewpoint this reaction is interesting because lactobionic acid
possesses useful properties and can,
thus, be used as acidulant, complexing agent or antioxidant in
food, pharmacy
and medicine. Lactose, an abundant disaccharide, is a by-product
of dairy
industry available in big amounts [26-28].
Figure 2. Reaction of lactose oxidation.
Oxidation of sugars is very sensitive to several factors such as
pH of reaction
media, since the formation of corresponding carbohydrate acids
results into big pH
OO
O
OH
OH
OH
OH
OH
OH
OH
H
OH
OOCOONa
OH
OH
OH
OH
OH
OH
OH
OOCOONa
OH
OH
OH
OH
OH
OH
OH
OH
OOOH
OH
OH
OH
O
O
HO
OH
OH
Lactose Lactobionic acid (LBA), sodium salt
Lactulose 2-keto-Na-Lactobionate
IsomerizationOH-
O2
O2
OH
-
6
change. Another important parameter is the oxygen feed rate,
since the reaction
rate can be retarded either due to the lack or due to oversupply
of oxygen (i.e.
oxygen poisoning) at high oxygen feed rate.
2. Background and Theory
2.1 Introduction to zeolites
2.1.1. Definition
Zeolites are crystalline aluminosilicates characterized by a
structure that
comprises a three-dimensional and regular framework formed by
linked TO4
tetrahedral (T= Si, Al, etc), each oxygen being between two
T-elements. This
tetrahedron is the fundamental building unit of all zeolites.
These simple
tetrahedra are combined in a complex way to form secondary
building units
(SBU), forming the building block of the different framework
structures of zeolite
crystals [27].
Classical zeolite structures are composed from primary units of
AlO4 and SiO4
and polyhedra with secondary units to form frame network with
Si-O-Al atoms. At
the movement there are over 170 types of zeolites in which 50
are naturally
occurring zeolites. The aluminum atoms, particularly in high
silica zeolites, may
not be uniformity distributed, often zoning effect is
observed.
2.1.2. Framework and Structure
A defining feature of zeolites is that their frameworks are made
up of 4-
connected networks of atoms (see Figure 3). One way of thinking
about this is in terms of tetrahedra, with a silicon atom in the
middle and oxygen atoms at the
-
7
corners. These tetrahedra can then link together by their
corners (see illustration)
to from a rich variety of beautiful structures. The framework
structure may contain
linked cages, cavities or channels, which are of the right size
to allow small
molecules to enter - i.e. the limiting pore sizes are roughly
between 3 and 10 in
diameter.
In all, over 130 different framework structures are now known.
In addition to
having silicon or aluminum as the tetrahedral atom, other
compositions have also
been synthesized, including the growing category of
microporous
aluminophosphates, known as ALPOs [34].
Figure 3. Frame work of aluminosilicates in zeolites (78).
Figure 4. Zeolite structure (77) (ZSM-5). Figure 5. Zeolite cage
(77).
The structure has channels and cavities with molecular sizes
(Figures 4 and 5) that can host the charge-compensating cations,
water or other molecules and
salts. A schematic representation of a chain of tetrahedral is
shown in Fig. 3. In a real crystal, however, all four oxygen atoms
are bridging atoms except where the
macromolecule terminates at the crystal faces, in which case a
proton
-
8
coordinates to the non-bridging oxygen atom. Each tetrahedron
containing
aluminum formally has one unit of negative charge because the
aluminum atom
has a formal charge of +3 and each oxygen atom has a formal
charge of -2.
There are enough metal cations, such as Na+, K+, Ca+2, Mg+2 or
Sr+2, present in
the interstices of the aluminosilicate framework to make the
crystal electrically
neutral. These cations are usually mobile and are responsible
for the ion
exchange with another metal.
2.1.3. History of zeolites
The discovery of zeolite catalysts in the late 1950s stimulated
the interest of
chemists, physicists, and engineers in the study of zeolite
catalysts and reactions
catalyzed by them and before the discovery of zeolites in
catalysis, there is much
information in the history (see Table 1) which tells about the
application of zeolite materials.
Table 1. History of Zeolites (38).
Date Facts of zeolite History
1756 Zeolite mineral identified
1850 Ion-exchange properties
1858 Reversible adsorption/Desorption of water
1862 Synthetic zeolite
1926 Molecular sieves defined
1929 Modern concepts of zeolite structure identification
1948 Synthetic zeolite without natural occurrence
1953 Synthetic zeolite commercialized
1967 First International Conference on Zeolites, foundation of
IZA
1972 High silica zeolites
1979 Pillared clays
1982 Aluminophosphates
1991 Mesoporous molecular sieves
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9
2.1.4. Classification of zeolites
The natural or synthetic zeolites are hydrated micro porous
molecular sieves
(
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10
several purposes, such as oil dewaxing and ethylbenzene
production. In 1980s
and 1990s, zeolites and molecular sieves were used for several
petroleum-
refining applications particularly for hydrocarbon cracking and
production of
octane-enhancement additives. The zeolites are also classified
into different
groups depending upon the silica to alumina ratio as mentioned
in Table 2.
Table 2. Zeolites classification (74) (depends on silica to
alumina ratio).
Si/Al ratio Zeolite Properties
Low(1-1.5) A,X Relatively low stability of
framework, low stability in acids,
High stability in bases ,
High concentration of acid groups
with moderate acid strength
Hydrophilic
Intermediate(2-5) Erionite,
Chabazite,
Clinoptilolite,
Mordenite
High(10-infinite) ZSM-5, Erionite,
Mordenite,MCM-
41,MCM-22,MCM-
36, etc.
Relatively high stability of frame
work, high stability in acids and
low stability in bases.
The effect of silica to alumina ratio on physicochemical
properties of the zeolites
as explained:
a. Increasing SiO2/Al2O3 ratio affects the following physical
properties of the
zeolite:
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11
Increases acid resistance Increases thermal stability Increases
hydrophobicity Decreases affinity for polar adsorbents Decreases
cation content.
b. Decreasing SiO2/Al2O3 ratio affects following physical
properties of the
zeolite:
Increases hydrophilicity Increases cation exchange properties
Decreases the pore size for same numbering ring, as Al has
lower atomic radius than Si.
2.1.5. Shape selectivity
The shape selectivity provided by the microporous crystalline
structures of
zeolites is of crucial importance in hydrocarbon transformation.
The rate of
deactivation by coke largely depends on the shape selectivity.
The crystalline
structures of zeolites can be altered to a large extent
achieving desired shape
selective properties. Three different types of shape selectivity
are observed over
zeolites (shown in Figure 6) and summarized shortly here
[32].
1. Reactant selectivity occurs when some of the molecules in the
reactant
mixture are too large to diffuse through the catalyst pores.
2. Product selectivity occurs when some of the products formed
within the
pores are too bulky to diffuse out. The bulky molecules are
either converted to
less bulky molecules or coked, that eventually deactivates the
catalyst.
3. Restricted transition-state selectivity occurs when certain
reactions are
prevented because the corresponding transition-state would
require more space
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12
than the available in the cavities or in the pores. Neither the
reactant nor the
products are prevented from diffusing through the pores.
The most important type of shape selectivity in the skeletal
isomerization of n-
butane is the restricted transition-state selectivity (Figure
6). Concerning the restricted transition state-type selectivity,
the lower transition state molecule is
easier to accommodate in the cavities than the upper one.
Figure 6. Schematic representation of the three types of
shape-selectivity (70).
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13
The Figure 7 gives the idea of the shape selectivity of the
molecules entering into the zeolite pores which depends upon the
size and shape selectivity of the
molecules. There are in principle as many different shapes, as
shown in Figure 7,
and dimensions of intracrystalline cavity or channels, as there
are zeolite
topologies. All molecular sieves are classified according to
their dimensions (pore
diameter) into microporous (pore mouth less than 2 nm) and
macroporous (pore
mouth more than 2 nm) materials. Zeolites are microporous
molecular sieves and
are also divided in small, medium or large pores.
Figure 7. Shape-selective environments in different zeolite
structure types (71): (a) large molecules have access to
interrupted cavities and channel intersections
for pore mouth catalysis; (b) molecules are plugged into the
pore aperture; (c)
molecules are converted in multiple pore mouths according to
key-lock catalysis;
(d) molecules are converted in the intra-crystalline
shape-selective environment.
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14
2.1.6. Acidity
Zeolites and zeolite-type catalysts are usually acidic according
to both definitions
of acidity e.g. Brnsted (proton donor) and Lewis (electron pair
acceptor) acidity.
In hydrocarbon transformation, Brnsted acid sites (Figure 8) are
considered to be more important. The proton form of a molecular
sieve containing Brnsted
acid sites can easily be obtained from the ammonium form by
calcination.
However, Brnsted acid sites can be present already in the
as-synthesized
molecular sieves, as determined by the choice of the template
[1]. The number of
acid sites is proportional to the concentration of the framework
trivalent T-atoms.
There are, however, factors that affect the acid strength, like
the nature of
trivalent T-atom and the Si/T-atom ratio in the framework. The
acidity decreases
in the order: Al>Ga>Fe>>B. Brnsted acid sites in
zeolites account for their
catalytic properties and can be investigated by NMR, FTIR or
TPD
measurements.
Figure 8. Brnsted acid site as present in a zeolite (72).
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15
2.1.7. Synthesis of zeolites
Zeolites are crystalline aluminosilicates with the general
formula Mn/2.O.Al2O3
.ySiO2. By varying the "template" molecule added to an aqueous
solution of
mineralized silica (and co metal) nearly 100 different zeolites
have been
synthesized. Other synthetic variables including the source of
the inorganic
precursors, the mineralizing agent (OH-, or F-) and the reactant
concentrations
have also resulted in new crystalline materials. Various
zeolites are synthesized
with varying the Si/Al ratio with minute value 1 to
infinite.
a. Pre-synthesis Method
Aluminosilicates zeolites are formed by hydrothermal synthesis,
typically under
mild conditions. Zeolites are synthesized by mixing a silica
source (SiO2), an
alumina source (Al2O3), organic template, mineralizing agent
(OH- anion, NaOH)
and water as solvent to form an aluminosilicate gel.
The nucleation and crystallization of the gel takes place in an
autoclave (shown
in Figure 11). The zeolite crystals that are formed are
filtered, washed and dried. Washing is required to reduce the
alkalinity because of the addition of NaOH.
In order to remove the organic template, zeolites are typically
calcined after
drying. Different calcinations techniques have an effect on the
properties of the
synthesized zeolite. After calcinations the zeolite is in the so
called sodium form,
denoted by Na at the beginning of the e.g. Na-ZSM-5 because it
contains sodium
ions to balance the framework charge induced by Al as
T-atoms.
Zeolite crystallization mainly depends on the sources of silica
and alumina,
temperature, pressure, synthesis time, solution pH, composition
of the gel and
the nature of the organic templates, pretreatment of the
reactants, inclusion of
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16
special additives, homogeneity or heterogeneity of the reactant
mixture as well
as seeding effects.
The pH of the solution is an important parameter in the zeolite
synthesis.
Increasing the alkalinity at constant temperature influences the
kinetics of zeolite
crystallization in the same way as increasing the temperature at
constant
alkalinity does. The induction time decreases strongly and the
crystal growth is
accelerated with an increase in pH [14]. The decrease in the
nucleation time and
enhanced rate of crystal growth with rising pH can be attributed
to the much
greater concentrations of dissolved species. The induction time
in zeolite
crystallization decreases rapidly with increasing temperature,
up to a certain
point. Alkalinity also has effect on the Si:Al ratio of the
zeolite [16]. Since OH-
ions serve as a mineralizing agent, the pH of the gel solution
is of utmost
important. The OH ions bring the Si and Al oxides or hydroxides
into the solution
at a particular rate. Crystallinity of zeolites increases with
time and usually the as-
synthesized form of zeolite is not the most suitable for the
catalytic purpose.
Template makes the stability, control the formation of a
microporous framework
structure (see Figure 9) of the zeolite and then the template
can be removed. Thereafter, the microporous voids channels and
cavities are created by
calcination. A large number of organic molecules can be used as
templates for
zeolites synthesis. Typical templates are alkyl ammonium cations
R4N+, alkyl
phosphates (R4P+) and organic complexes, which act as structure
directing
agents and help in the formation of the zeolite lattice. The
templates contribute to
the stability by forming new bonds such as hydrogen and
electrostatic bonds,
and assist in the formation of the particular structure through
their form and size.
While choosing a template for the synthesis of a zeolite,
important properties of
the template such as its solubility, stability during the
synthesis, steric
compatibility, framework stabilization and removal of the
template without
destroying the frame work structure of the zeolite must be taken
into account.
The structure directing agents can be organic or inorganic
compounds. They are
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17
cations, ion pairs and neutral molecules. Of the three species,
cations are the
most important because they not only acts as structure directing
agents but also
affect the rate of zeolite crystallization. The mechanism of the
zeolite synthesis is
illustrated in the Figure 11.
Figure 9. Zeolite pore structure formation (72).
Generally used silica, alumina and template sources for the
synthesis of zeolites
as mentioned below.
1. Silica Sources: Tetra methyl orthosilicate (Si(OCH3)4), Tetra
ethyl orthosilicate (Si(OC2H5)4), waterglass (Na2SiO3.9H2O), Na2O
11%, SiO2
29%, Ludox-AS-40 (colloidal silica), Fumed silica, Aerosil-200,
etc.
2. Alumina Sources: sodium aluminate (NaAlO2) 54% Na2O,
Al2O3,
Al (OH)3 aluminum hydroxide Gibbsite, etc.
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18
3. Templates: Alkyl ammonium cations R4N+, alkyl phosphates
(R4P+) and
organic complexes, and also inorganic cations like Na+, Li+, K+,
etc.
b. Post synthesis method
The sodium form of zeolite is made to acidic form by
ion-exchanging sodium
cations with NH4Cl or NH4NO3 to achieve an ammonium form (Figure
10). The ammonium form of zeolite is dried and calcined to obtain
the proton form of
zeolite i.e. acidic form of zeolite.
Figure 10. Zeolite synthesis flow sheet (i.e. proton form).
Sodium form of zeolite formed (Na-zeolite form, parent
zeolite)
Drying in order to remove water molecules at 100 oC and then
calcination process to remove template or surfactant.
Nucleation and crystal growth takes place in autoclave at 150oC,
1-7 days
Sodium form to proton form with 1M NH4Cl sol. by ion exchange
and then dried and calcined to remove ammonia molecules to get
proton form.
Silica source + alumina source + NaOH (mineralizing agent) +
Organic template ----------- leads to gel formation
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19
Figure 11. Synthesis mechanism of zeolite (73).
c. Present day research on zeolites
Synthetic goals in research include the optimization of reported
syntheses,
variation of the composition of zeolites with known topologies
and understanding
the influence of synthetic conditions like synthesis time,
silica to alumina ratio,
etc. that favor the formation of these low-symmetry crystalline
phases and
moreover the high quality zeolites should be more feasible, high
porosity, eco-
friendly, easy handling, easy recovery, less expensive, high
thermal stable, high
activity, longer life and high selectivity.
2.1.8. Mesoporous molecular sieves
Mesoporous materials are those with pores in the range 2-50 nm
in diameter.
They have huge surface areas, providing a vast number of sites
where sorption
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20
processes can occur. These materials have numerous applications
in catalysis,
separation and many other fields. The synthesis of these
materials is of
considerable interest and is constantly being developed to
introduce different
properties. Molecular sieves are porous solids with pores of the
size of molecular
dimensions, 0.3-2.0 nm in diameter or more. Examples include
zeolites, carbons,
glasses and oxides. Some are crystalline with a uniform pore
size delineated by
their crystal structure, e.g., zeolites, while others are
amorphous carbon
molecular sieves. The most common commercial molecular sieves
are zeolites. Mobile composite of matter (MCM) materials are
mesoporous templated
molecular sieves (Figure 12). M41S are typically semi
crystalline mesoporous silicates and aluminosilicates with pores
2-10 nm related to phyllosilicate
minerals, imogolite and alophane [19]. MCM-22, MCM-36, MCM-48
and MCM-50
are the members of mesoporous with high surface areas.
Figure 12. Ordered mesoporous materials
Amongst the current developments in the field of hierarchical
pore structures, the
creation of mesopores in zeolite crystals is the most frequently
employed way to
combine micropores with mesopores in one material. There are
different
approaches to generate and characterize mesopores in zeolite
crystals and
establish their impact on the catalytic action with better mass
transport.
Mesopores can be created (see Figure 13) via several routes from
which steaming and acid leaching are the most frequently applied.
Novel approaches
using secondary carbon templates that are removed after
syntheses have
recently been launched. For the characterization of mesopores,
nitrogen
physisorption and electron microscopy are commonly used. More
recently, it was
shown that electron tomography, a form of three-dimensional
transmission
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21
electron microscopy, is able to reveal the three-dimensional
shape, size and
connectivity of the mesopores. The effect of the presence of
mesopores for
catalysis is demonstrated for several industrially applied
processes that make
use of zeolite catalysts in the cracking of heavy oil fractions
and synthesis of fine
chemicals over zeolite Y, and the production of cumene and
hydroisomerization
of alkanes over mordenite. For these processes, the mesopores
ensure an
optimal accessibility and transport of reactants and products,
while the zeolite
micropores induce the preferred shape-selective properties
[38-39].
Figure 13. Schematic representation of the assembly of zeolite
nanocrystals to a mesoporous structure (31).
The fundamental reagents used in the synthesis of mesoporous
templated
molecular sieves are
1. The long chain quaternary ammonium ions used as surfactants
form micelles or
liquid crystals in aqueous solutions.
2. The silicate or aluminosilicate species, reflecting the
micellar array condense and
polymerize around the hydrophilic parts of the surfactant.
3. Condensed inorganic species aggregates to form the walls of
the porous solid.
4. Calcination of the materials removes the organic templates.
This procedure
improves the stability of the structure.
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22
2.1.9. Zeolite pore structure and active sites
The adsorption and catalytic process is over zeolites involve
diffusion of
molecules in the zeolites pores, only those with a minimum of 8
tetrahedral
atoms apertures allowing this diffusion are generally considered
for the particular
reactions [12], for e.g.:
i. Small pore zeolite with eight membered ring pore aperture
having free
diameters 0.3-0.45 nm.
ii. Medium pore zeolites with ten membered ring apertures
0.45-0.60 nm in free
diameter.
iii. Large pore zeolites with 12 membered ring apertures 0.6-0.8
nm as
represented in Figure 14.
Figure 14. Pore openings of common zeolites with 12, 10 and 8
member ring structures (74).
Active sites in zeolite pores act as an acid, acid base, or
bifunctional catalysis.
For example in fluid catalytic cracking, a catalyst containing
acid sites e.g., FAU
zeolite is applicable. Also methanol conversion to olefins
reactions, acetylation
reactions etc, need acid sites. The hydrocarbon reactions as
well as many
transformations functionalized compounds are catalyzed by
protonic sites only.
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23
The maximum number of protonic sites is equal to the number of
framework
aluminum atoms. The number of protonic sites can be adjusted
either during the
synthesis or during post synthesis pretreatment of the zeolite:
dealumination, ion-
exchange, etc. The parameters determining the acid strength of
the zeolite
protonic sites are important in catalytic applications. The
first feature of zeolites is
their stronger acidity compared to amorphous aluminosilicates. A
relation exists
between the T-O-T bond angles and the acid strength of the
associated proton in
the zeolites. Hence, the greater the angle, the stronger the
active sites in
zeolites. The protonic sites (bond angles) of the zeolites are
influenced by the
basicity of the reactants and the temperature will also play a
role. Some of the
zeolites catalysts acidity measurements using pyridine
adsorption are presented
in Table 3. For example, an acid site giving surface hydroxyl
groups (-OH groups) i.e. the Brnsted acid site, which are more
selective property for a zeolite
in acid catalysis. The higher Al atoms content in zeolite
framework is proportional
to higher Brnsted acid sites (maximum acidity at Si/Al =
9-12).
Table 3. Brnsted and Lewis acid sites of the different zeolite
catalysts (74).
S.No Catalyst Brnsted acid
sites (mol/g.cat)
Lewis acid sites
(mol/g.cat)
1 H-Beta-11 183 128
2 Mordenite 294 109
3 H -Y 291 165
4 H-MCM-41 89 168
5 H-MCM-22 187 175
6 Silica 0 7
7 Alumina 7 156
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24
2.1.10. Zeolites in Industrial Applications
Applications of zeolites are in three main areas [13]:
i. Ion exchange: It is one of the important properties of the
zeolites. The most
important end application of synthetic zeolites is in
detergents. Zeolites replaced
phosphates builders that are caused algae overgrowth and
consequently oxygen
deficit in water resources. Zeolites in sewage waters accumulate
and immobilize
phosphates etc.
ii. Adsorption: zeolites are important industrial adsorbents for
the separation of
both gases and liquids. They possess excellent capacity to
remove volatile
organic chemicals from air streams and to separate isomers and
mixtures of
gases.
iii. Catalysis: with the use of X and Y zeolites in
isomerization and cracking,
hydrocarbon transformation, etc. Many industrial applications
are presented in
the Table 4.
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25
Table 4. Application of zeolites in various industries (10).
Zeolite/microporous material
Process or application technology
LTA (A-type zeolites) Detergent builder, separation,
desiccation
FAU(X-and Y-type
zeolites)
Catalytic cracking, hydrocracking, separation,
Purification and desiccation, aromatic alkylation.
BEA (Beta zeolite) FCC additive, cumene and ethylbenzene
production.
MOR (Mordenite) Hydrocracking, hydroisomerisation, dewaxing,
NOx
reduction, adsorption, cumene synthesis, transalkylation of
aromatics.
MWW (MCM-22) Ethyl benzene and cumene production, Isomerization
etc.
MFI (ZSM-5) Dewaxing, hydrocracking, ethyl benzene
(Mobil-Badger) and
styrene production, xylene isomerisation, methanol to
gasoline (MTG), benzene alkylation, adsorption, catalytic
aromatization, FCC additive, toluene disproportionation
ERI (Erionite) Selectoforming, hydrocracking
LTL (KL-type zeolites) Catalytic aromatization
CHA (SAPO-34) Methanol to olefins (MTO)
FER (Ferrierite) n-Butene skeletal isomerisation
TON (Theta-1, ZSM-22) Long-chain paraffin isomerisation
AEL (SAPO-11) Long-chain paraffin isomerisation
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26
3. Catalyst preparation methods Methods of metal modification of
zeolites and mesoporous sieves are ion
exchange, impregnation, and in-situ. New methods of metal
modification are
direct metal introduction (in-situ), isomorphous substitution of
metal in framework
of zeolite and chemical anchoring.
3.1. Ion-exchange method
After post synthesis method, an acidic form i.e. proton form
zeolite catalysts is
obtained. To this metal precursor solution is added in order to
get metal modified
form under continuous stirring for 24 h and measure the pH value
then filtration,
drying, and step calcination are performed.
Ion-exchange method consists of replacing an ion in an
electrostatic interaction
with the surface of a support by another ion species. The
support containing ions
A is plunged into an excess volume (compared to the pore volume)
of a solution
containing ions B. Ions B gradually penetrate into the pore
space of the support,
while ions A pass into the solution, until equilibrium is
established corresponding
to a given distribution of the two ions between the solid and
the solution. For
example, using a proper salt solution at ca. 100 C (to increase
the exchange
rate), it is possible to prepare the acid form of zeolite by
exchanging NH4+ for Na+
and successive calcination.
Natural exchangers are composed of a framework bearing electric
charges
neutral by ions of opposite sign. For zeolites, for example,
these charges are
negative and are due to the particular environment of aluminum.
Aluminum, just
like silicon, is effectively situated in the center of a
tetrahedron of four oxygen
atoms, which provides it with four negative charges, whereas the
aluminum itself
has only three positive charges. The tetrahedron (AlO4) is thus
an overall bearer
of a negative charge distributed over the oxygen atoms, and this
charge is
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27
neutralized by the presence of various cations Na+, K+ etc.
These cations are not
definitely linked to the framework but may be replaced by the
other cations during
an ion exchange operation. Whatever the exchanger conditions,
and in particular
the pH, zeolites are cation exchangers and have a constant
number of exchange
sites, which is equal to the number of aluminum atoms in their
framework.
There are natural ion exchangers other than zeolites. Clays and
silicates are
cation exchangers, whereas hydrotalcites are anion exchangers.
As in the case
of zeolites, the number of exchange sites is not pH
dependent.
Oxide surfaces contacted with water are generally covered with
hydroxyl groups
which can be schematically represented as S-OH, where S stands
for Al, Si, Ti,
Fe, etc. Some of these groups may behave as Brnsted acids,
whereas other
hydroxy groups may behave as Brnsted bases, giving rise to the
following
equation:
S-OH = S-O- + H+ . (1)
S-OH + H+ = S-OH+2 (2)
The resulting surface charge which arises from an excess of one
type of charged
site over the other, is a function of the solution pH. A given
value of pH exists for
which the particle is not charged overall. This value is
characteristic of the oxide
and is called the pristine point of zero charge (PPZC or ZPC).
Alumina is
amphoteric and may adsorb cations as well as anions and it
ranges between 7
and 9. Silica ZPC values range between 1.5 and 3 and silica may
adsorb cations.
The general procedure to introduce cations into the zeolite
framework consists of
exchanging the Na+ cations, which balance the negative charge
born by AlO4
tetrahedra with a solution of metal salt of the ion-exchange
method. It has been
found that the equilibrium of the exchange reaction depends on
temperature and
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28
the concentration of the exchanging metal salt solution [12].
Cation exchange
into the hydrogen form of the zeolite is more difficult than
into the Na-form
because of the strong bonding of the protons with the lattice
oxygen. The proton
exchange limitation can be overcome by transforming the H-form
into the NH4-
form, which has an ion-exchange property similar to that of
Na+-form zeolite.
Metal-modified zeolites can be obtained either by means of a
multistage process
of ion-exchange with intermediate zeolite heating or by a
single-stage process of
ion-exchange at high temperature in autoclaving conditions. The
pH of the
exchange solution is important, firstly because it is known that
acid solutions can
partially hydrolyze and dealuminate zeolite and secondly,
because metal
solutions of different pH values can readily precipitate basic
salts within a zeolite
matrix to various degrees. The theoretical ion-exchange capacity
depends on the
chemical composition of the zeolite: the lower the Si/Al ratio
in the lattice is, the
higher the ion-exchange capacity is [13].
Ion exchange is composed of a frame work bearing electric
charges neutralized
by ions of opposite sign. For example, [PtCl6]2- (hexachloro
planitinic acid-II) ion-
exchanged with H-ZSM-5 (proton form of ZSM-5).
3.2. Impregnation Method The impregnation method is used to
obtain higher loadings and to apply active
precursors that do not adsorb on support easily. The solution of
the precursor is
broken up into small discontinuous elements presented in the
pore of the support
by gradually evaporating the solvent. Impregnation is the
easiest method of
making a catalyst.
A carrier, usually porous, is contacted with a solution of one
or more suitable
metallic compounds. The carrier is then dried, and the catalyst
is activated as in
the case of precipitated catalyst. The active agent is never
introduced into a
porous support in its final form but by the intermediary
precursor, the choice of
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29
which is very important for the quality of the final deposit.
The size and the shape
of the catalyst particles are that of the carrier. The
impregnation mechanism
which is shown in the Figure 15 gives the general idea about the
process.
Two types of impregnation can be considered, depending on
i. If an interaction exits between the support and the
precursors at the moment of
wetting, and
ii. If there is no wetting.
a. Impregnation with no interaction between support and
catalyst
If the support does not have its own catalytic activity then it
gives the fine catalyst
by adding the precursor solution in order to have contact with
the support i.e.
metal-support non-bonding. This is a relatively rapid operation
because the pores
are filled with the solution and forms air bubbles and then
released after 10
minutes. The maximum amount of metal precursor that can be
introduced will be
depending on the solubility of the precursor salt in the solvent
and also the pore
volume of the support. The impregnation technique required less
equipment
since the filtering and drying steps are eliminated and washing
may not be
needed.
b. Impregnation with interaction between support and
catalyst
Impregnation with interaction occurs when the solute deposited
and establishes a
bond with the surface of the support at the time of wetting.
Such interaction
results in a near atomic dispersion of the precursor's active
phase. The
interaction can be as ion-exchange, adsorption, or a chemical
reaction.
Impregnation is the preferred process in preparing supported
noble metal
catalysts for which it is usually economically desirable to
spread out the metal in
finely divided form. The noble metal is usually present in the
order of 1 wt% or
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30
less or 5 wt% of the total catalyst. This makes maximum use of a
very expensive
ingredient, in contrast, to precipitated catalysts where some of
the active
ingredient will usually be enclosed by other material present
and thus unavailable
for reaction. This work mainly deals with incipient wetness
impregnation, an
amount of solution is added to the support just enough to fill
the pore volume.
The disadvantage of this method is broad particle size
distribution.
Figure 15. Mechanism of impregnation method (74).
3.3. In-situ Method
Metal solution is added after preparing the gel solution of
zeolites i.e. during pre-
synthesis, In-situ method is used to get higher metal loadings
and highly metal
species dispersion into the zeolite support.
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31
4. Characterization Methods For application of zeolites as shape
selective property in catalysis and in order to
correlate the effect of the synthesis parameters with the
catalytic and physico-
chemical properties, synthesized materials need to be
characterized properly.
The important parameters, which were investigated in this thesis
work, are the
specific surface area, phase purity, crystallinity, structure,
morphology, the
presence and the amount of different acid sites, the amount of
an introduced
metal in the ion exchanged and impregnation catalysts (i.e.
metal loadings or
metal content). A brief description of the each of the methods
used is given
below.
1. Surface area measurements (N2 adsorption)
2. X-ray powder diffraction (XRD)
3. Scanning electron microscopy (SEM)
4. Fourier Transform Infra Red spectroscopy (FTIR)
5. Direct Current Plasma (DCP)
4.1. Surface area measurements The surface area of zeolites
catalysts were measured by nitrogen adsorption.
First, catalysts were dried over night at 373 K and out gassed
in a burette at 473
K for three hours.
The most common method of measuring of surface area is N2
adsorption using
BET (Brunauer, Emmett and Teller) isotherm method for mesoporous
materials.
00
)1(1)( CPV
PCCVPPV
Pmm
+= (1) where
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32
V= Volume of the adsorbed gas at pressure P
Vm= Volume of the gas adsorbed in the monolayer
P0 = Saturation pressure of adsorbate gas at the experimental
temperature
P= Experimental pressure
C= A constant related exponentially to the heat of adsorption
and heat of
liquefaction of the gas
RTHH condadeC /)( = (2)
where,
Had= Heat of adsorption on the first layer
Hcond= Heat of liquefaction of adsorbed gas on all other
layers
A graph from the nitrogen adsorption between P/V (P-P0) vs. P/
P0 gives a
straight line whose slope and intercept can be used to evaluate
volume of the
gas adsorbed in the mono layer.
iSVm +=
1 Where, S is the slope and i is the intercept.
The execution of the experiment was carried out with help of
surface area
measurement instrument i.e. Carlo Erba sorptomatic 1900
instrument using BET
for mesoporous and Dubinin isotherm for microporous
materials.
4.1 . X-ray powder diffraction (XRD) The XRD or XRPD method
gives elemental composition, catalyst structure
(phase purity) and particle size of the materials. Material
should be sufficiently
crystalline to diffract X-rays (3-5 nm) and being present in
desired amounts for In-
situ measurements.
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33
For crystal size determination the following equation is
used.
T=K / cos . (1)
T= is the thickness of crystal perpendicular to diffraction
plane.
K= is a constant that depends on instrument
B = is the full width at half maximum (FWHM) of the diffraction
peak
= angle of reflection
The purpose of XRD is to determine the unit cell parameters and
thus unit cell
volume when the zeolitic structure is known, then one can
determine if an
element has been introduced into the lattice framework
position.
XRD method is also a measure of the purity of a compound,
compared with
reference spectra. If there is no evidence of crystalline or
amorphous
contaminants present, then one must compare the intensity of the
reference with
the authentic sample to check for the same composition and
crystal size.
The principle working of diffractometer is the divergence of the
primary X-rays
beam which is limited by an automatic divergence slit (ADS) and
a 15 mm mask.
The irradiated sample length was set to a fixed 12 mm length. On
the diffracted
side, a 0.2 mm receiving slit and a 1 mm anti scatter slit is
present. The diffracted
X-rays beam was filtered with a Ni Ka filter. The measured
diffractograms were
analyzed using X pert high score software and the powder
diffraction file (PDF)
database. The PDF database was used to identify the sample peaks
and the
corresponding phases. All the samples were first screened
through an angular
range 0.5-0.9(2theta) using 0.02 steps and 1s measuring time for
each step.
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34
4.3. Scanning electron microscopy (SEM analysis)
The morphology of zeolites catalysts was investigated by SEM
analysis and SEM
of the samples were obtained by back scattering electrons. The
purpose of using
back scattering electron was to observe relatively large
crystalline materials. For
small crystalline catalysts SEM is not useful but high
resolution transmission
electron microscope was used.
The pore shape, size, channels and small crystalline mesoporous
molecular
sieves were investigated by high resolution transmission
electron microscope
(TEM).
4.4 . Fourier transform infrared spectroscopy Infrared
spectroscopy reveals information about molecular vibrations that
cause
a change in the dipole moment of molecules. It offers a
fingerprint of the
chemical bonds present within materials. FTIR (see Figure 16) is
a very powerful analytical tool for examining both inorganic and
organic materials. A
beam of radiation from the source is focused on a beam splitter,
where half the
beam is reflected to a fixed mirror and the other half of the
beam is transmitted to
a moving mirror which reflects the beam back to the beam
splitter from where it
travels, recombined with the original half beam, to the
detector. The IR intensity
variation with optical path difference (interferogram) is the
Fourier transform of
the (broadband) incident radiation. The IR absorption spectrum
can be obtained
by measuring an interferogram with and without a sample in the
beam and
transforming the interferograms into spectra [21]. For example,
the formation of
OH groups on the external and internal surface of Y-type
zeolites as studied the
adsorption of small molecules such as ammonia and ethylene and
later
employed pyridine as a probe molecule to discriminate Brnsted
and Lewis acid
sites.
-
35
Figure 16. FITR spectrometer (75). Pyridine has extensively been
used to probe Bronsted and Lewis acidity in
catalytic materials. Specific IR vibrations could be identified,
depending on the
type of interaction of the molecule with surface sites. As an
example, the
vibration mode ascribed to combined C-C stretching and N-H
bending modes
was found at 1439 cm-1 for the gas phase, 1438 cm-1 for pyridine
physically
adsorbed on silica alumina, 1450 cm-1 and 1545 cm-1 for pyridine
chemically
sorbed respectively on Lewis and Brnsted acid sites. When
adsorbed in HY
zeolite pores, pyridine was shown to form pyridinium ions
(characterized by an
intense band at 1540 cm-1) by interaction with the so-called HF
hydroxyls, siting
in the supercages and vibrating at 3640 cm-1. The HY zeolite
structure presents a
second type of hydroxyls, the so-called LF hydroxyls, vibrating
at 3550 cm-1 and
located in sterically less accessible cavities (sodalite cages
and hexagonal
prisms). Although not accessible to pyridine because of too
small cage opening,
LF sites have been shown to interact with pyridine, which
suggested the mobility
of interacting LF sites toward positions accessible to pyridine,
i.e. supercages.
The nature of the bonding between pyridine and LF hydroxyls was
recently
clarified by Parker et al. a partial protonation of pyridine was
suggested [17].
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36
4.5 . Direct Current Plasma Direct Current Plasma Spectrometer
(DCP) is used for efficient determination of
major and minor elements in sample solutions. A direct-current
plasma (DCP) is
created by an electrical discharge between two electrodes. A
plasma support gas
is necessary, and Ar is most common one. Samples can be
deposited on one of
the electrodes, or conducting can make up one electrode.
Insulating solid
samples are placed near the discharge in a way that ionized gas
atoms sputter
the sample into the gas phase where the analyte atoms are
excited. This
sputtering process is often referred to as glow-discharge
excitation [23].
5. Overview on MCM-22 and MCM-36 zeolites
MCM-22 possesses a unique crystal structure and belongs to MWW
group of
zeolites (Figure 17) containing two independent
non-interconnected pore systems [51-53]. One of the channel systems
contains two-dimensional
sinusoidal 10-MR (member ring) channels 0.55 nm 0.4 nm, while
the other
system consists of large supercages (12-member ring) with
dimensions 0.71
nm 0.71 nm 1.81 nm. The super cages stack one above another
through
double prismatic six-member rings and are accessed by slightly
distorted
elliptical 10-MR connecting channels. In general, the
synthesized MCM-22
zeolites crystallized as very thin plates with an extremely
large external surface
area, on which distributed the 12-member ring pockets. The
protonic form of
MCM-22 is an active catalyst for many reactions requiring acidic
sites such as
catalytic cracking, olefin isomerization, and conversion of
paraffins to olefins and
aromatics, and alkylation of paraffins with light olefins.
Indubitably, the acidity
(number, location, and strength of the acid sites) plays an
important role in the
catalysis [54-56]. Many studies have revealed that MCM-22
behaves like both
10- and 12-member ring zeolites [15].
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37
6 10-ring viewed normal to [001] 10-ring viewed normal to
[001]
between layers within layers
7H 8H
10-ring viewed normal to [001] 10-ring viewed normal to [001]
Within layers between layers
MCM-36 is a unique porous zeolitic material comprising
microporosity inside its crystalline layers and slit-like
mesoporosity in the interlayer space. The
mesopores are created by intercalation of the zeolitic layers
with polymeric silica
species mentioned in [24] and formation of a structure similar
to that of pillared
clays. MCM-36 can be prepared from an as-synthesized layered
precursor of
zeolite MCM-22 by applying first a swelling treatment to expand
the interlayer
distances and then a pillaring (intercalation) procedure to
stabilize the expanded
structure with flat-shaped mesopores between the layers [57].
Note that the
consideration of the mesopores as flat-shaped slits can easily
be an improper
simplification as we do not know the distribution or density of
the pillars and also
their thickness. Besides the dual porosity, the material shows
another dual
nature: the crystalline zeolitic layers form together an
amorphous structure
resulting from irregular arrangement of the pillars, i.e., the
Figure 18 gives the idea of distribution, dimensions, and internal
ordering. Since the zeolitic layers
can be intercalated with various metal oxides, not only SiO2,
MCM-36 has
-
38
become a family of materials with adjustable properties. The
introduction of
various metal oxides as single or mixed composites is an
effective tool for
adjusting the MCM-36 properties, being important for potential
catalytic
applications, by: (a) tailoring mesoporosity, i.e., distance
between the layers
mentioned in ref [24], (b) creating acidic and basic centres of
various strength
due to the choice of proper metal and its amount and; (c)
influencing the acid
base character of the materials via ion-exchange (d) changing
mechanical and/or
thermal stability via formation of more stable oxide species (e)
affecting
adsorption properties in a broad sense [24].
Figure 17. MWW group of materials and its structures (76).
Pillared layered structures which are shown in the Figure 18
(MCM-36) are built
of inorganic layers with inorganic or organic pillars appended
on both sides of the
sheets. These materials are potentially most attractive for
catalysis, because they
combine high specific surface areas and good accessibility for
larger molecules
to a large number of catalytic sites. Traditionally the focus
was on pillared clays,
but pillaring of other layered phases such as zirconium
phosphates, silicas and
metal oxides has also been explored. The combination of their
specific pore
structure and catalytic properties has been exploited in many
commercial
applications. Most notably, however, these materials contain
moderately strong
to weak acid sites, much weaker than the strong Brnsted acid
sites (bridging
hydroxyl groups) in zeolites [25].
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39
Figure 18. A schematic representation of the MCM-36
structure.
In the MCM-36 phase, the polymeric silica as pillars is formed
during the
hydrolysis and condensation of silicates from
tetraethylorthosilicate. The
hydrolysis reaction replaces an alkoxy group (OR) with a
hydroxyl (OH) group.
Subsequent condensation reactions involving the silanol groups
produce
siloxane bonds (SiOSi) and alcohol or water as byproducts,
leading initially to
oligomeric and polymeric structures. Depending on the
conditions, the final
structures of the polymeric SiO2 can be formed as nearly linear
polymeric
structures or three-dimensional branched structures. Based on
the MCM-22
precursor with different Si/Al ratio, MCM-36 was produced by
swelling and
pillaring techniques. The resulting MCM-36 contains a mesoporous
region
between the microporous layers and has the properties of a
medium-pore zeolite.
With silica as a pillaring material, the surface area of MCM-36
is about 2.5 times
higher than that of MCM-22. Silica pillaring increased the
concentration of
terminal SiOH groups compared with MCM-22, the number of Brnsted
acid
-
40
sites, however, was lower in MCM-36 than in MCM-22. This is
attributed to the
dealumination induced by the swelling and pillaring processes.
Alkane sorption
reveals that the 10-membered ring channel system of MCM-22
persists
unperturbed in MCM-36. The major fraction of the acid sites, and
thus the
favored sorption sites, is located in the 10-membered ring
channel of these
layers. Only about 10% acid sites exist on the outer surface and
are accessible
through the mesopores. Consequently, alkane sorption in MCM-22
and MCM-36
at low equilibrium pressures is dominated by the 10-membered
ring channels in
the layers. Catalytic tests indicate that MCM-36 is an active,
selective and stable
solid acid catalyst for alkylation of isobutane with 2-butene,
demonstrating that
the open mesoporous structure is utilized successfully for
sorbing and converting
larger molecules [25].
MCM-36 contains more mesoporous region and less microporous
region
compared to MCM-22 and MCM-36 exhibits higher BET surface area
than MCM-
22. The MCM-36 has more terminal Si-OH groups compared to MCM-22
due to
silica pillars. The Brnsted acid sites are less in MCM-36
compared to MCM-22.
The typical XRD patterns of the MWW group materials are shown in
the Figure 19.
-
41
Figure 19. XRD patterns of MCM-22(P) and its derivatives MCM-36
[57].
6. Overview on MCM-48 (mesoporous molecular sieve)
MCM-48 is a member of M41S Mesoporous silica's. It has recently
attracted
much attention for its three-dimensional mesoporous channel
systems, which
can be used as a prospective catalyst, an adsorbent, and even a
template for the
synthesis of nanostructures. A lot of work has been done in the
synthesis of pure
silica MCM-48 [29] and the incorporation of its framework with
various
heteroatoms. MCM-48 was usually synthesized using cationic
surfactants as
supramolecular template materials. Cationic-neutral or
cationic-anionic
surfactants were also used as its structure-directing agent.
With almost no
exception, MCM-48 was prepared in basic solution, and its yields
were usually
low. The product yield was only ca. 50% using
cetyltrimethylammonium bromide
(CTAB) as a single surfactant. One can gain a yield of ca. 80%
using cationic-
neutral surfactant mixture by adjusting the pH of the reaction
mixture to 10 during
-
42
synthesis. It is important to identify procedures suitable for a
cost-efficient
synthesis of MCM-48 with high yields. Herein, by adjusting the
pH=5 of the
solution in a synthesis process, a high product yield of 98% was
gained using
CTAB as a surfactant. Some of the comparison between MCM- 48 and
MCM-41
[30] is given below.
1. Porosity of MCM-48 is similar to that of MCM-41.
2. Particles of MCM-48 are much better organized than
MCM-41.
3. Three dimensional channel system in MCM-48 as opposed to a
one
dimensional channel system in MCM-41.
4. Both have similar thermal stability.
5. MCM-48 is more difficult to synthesize than MCM-41.
6. MCM-48 is cubic and MCM-41 hexagonal structure.
7. Ultra Sound Irradiation Method for Catalyst Preparation 7.1.
Introduction
Ultrasound is simply sound pitched above the frequency bond of
human hearing.
It is a part of sonic spectrum that ranges from 20 KHz to 10 MHz
and
corresponds to the wavelengths from 10 to 103 cm. The
application of
ultrasound, in connection to chemical reactions, is called
sonochemistry. The
range from 20 KHz to around 1 MHz is used in sonochemistry,
since acoustic
cavitation in liquids can be efficiently generated within this
frequency range [45].
The origin of sonochemical effects in liquids is the phenomenon
of acoustic
cavitation. Ultrasound is transmitted through a medium via
pressure waves by
inducing vibrational motions of the molecules, which alternately
compress and
stretch the molecular structure of the medium due to a
time-varying pressure [41,
42, and 43]. Molecules start to oscillate around their mean
position and if the
strength of the acoustic field is sufficiently intense, cavities
are created in liquids.
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43
This will happen if the negative pressure exceeds the local
tensile strength of the
liquid.
The cavities are also called cavitation bubbles and the process
itself is referred to
as cavitation. Two types of cavitation are known: stable and
transient. Stable
cavities are bubbles, which form and oscillate around their
equilibrium position
over several rarefaction/compression cycles, before collapsing
or never
collapsing at all.
7.2 . Applications
Ultrasound has proved extremely useful in the synthesis of a
wide range of
nanostructured materials, including high surface area transition
metals, alloys,
carbides, oxides and colloids [41, 42]. Sonochemical
decomposition of volatile
organometallic precursors in high boiling solvents produces
nanostructured
materials in various forms with high catalytic activities.
Nanometer colloids,
nanoporous high surface area aggregates, and nanostructured
oxide supported
catalysts can all be prepared by the general route. The
mechanism of the rate
enhancements in reactions of metals has been unveiled by
monitoring the effect
of ultrasonic irradiation on the kinetics of the chemical
reactivity of the solids,
examining the effects of irradiation on surface structure and
size distributions of
powders and solids, and, determining depth profiles of the
surface elemental
composition.
Ultrasonic irradiation of liquid-powder suspensions produces
another effect: high
velocity inter-particle collisions [41-44]. Cavitation and the
shockwaves it creates
in slurry can accelerate solid particles to high velocities. The
resultant collisions
are capable of inducing dramatic changes in surface morphology,
composition,
and reactivity. Heterogeneous catalysts often require rare and
expensive metals.
The use of ultrasound offers some hope of activating less
reactive, but also less
costly, metals. For example, the effects of ultrasound on
hydrogenated catalyst
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44
i.e. Ni powder, with the chemical consequence of enormously
increasing the
catalytic rates of hydrogenation by Ni powder (>105-fold)
[44].
8. Experimental Procedure 8.1. Catalysts
This work mainly deals with three mesoporous materials i.e.
MCM-22, MCM-36,
and MCM-48 which were prepared using different methods as
explained in the
sections 2.1.8, 3.1, 3.2 and 3.3. The main focus was on MCM-22
materials with
different silica to alumina ratios which were prepared as
explained below,
characterized and tested in catalytic reactions i.e.
isomerization of n-butane and
oxidation of lactose. Synthesis of MCM-36 with MCM-22 (with
different silica to
alumina ratios) as precursors and synthesis of MCM-48 mesoporous
materials
were also prepared in the lab.
All sodium forms MCM-22, MCM-36 and MCM-48 are characterized by
nitrogen
adsorption method, X-ray powder diffraction method, and SEM
which is
explained in detail in the theory part 4.
Catalysts nomenclature which was used during the work and the
list of prepared
catalysts are given below.
Na-MCM-22-30-R means sodium form of MCM-22 with silica to
alumina ratio equal to 30 prepared by the rotation mode. The
rotation mode was used for all
MCM-22 based catalysts.
Na-MCM-36-22-30-R means sodium form of MCM-36, as MCM-22
precursor (no-
calcined) with silica to alumina ratio equal to 30 prepared by
the rotation mode.
H-MCM-22-30-R means a proton form of MCM-22 with silica to
alumina ratio equal to 30.
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45
Pd-H-MCM-22-30-R-IE stands for palladium modified proton form of
MCM-22
with silica to alumina ratio equal to 30 prepared by
ion-exchange method.
Pd-H-MCM-22-30-R-IMP stands for palladium modified proton form
of MCM-22
with silica to alumina ratio equal to 30 prepared by
impregnation method.
Pd-H-MCM-22-30-R-IS stands for palladium modified proton form of
MCM-22 with silica to alumina ratio equal to 30 prepared by in-situ
method.
Pd-H-MCM-22-30-R-SSIE stands for palladium modified proton form
of MCM-22 with silica to alumina ratio equal to 30 prepared by
solid state ion-exchange
method.
Au-H-MCM-22-30-R-DP stands for gold modified proton form of
MCM-22 with silica to alumina ratio equal to 30 prepared by
Deposition Precipitation Method.
The catalysts which are synthesized, characterized and tested
are listed below.
Proton forms
1. H-MCM-22-30-R
2. H-MCM-22-50-R
3. H-MCM-22-70-R
4. H-MCM-22-28-R
Ion-exchange catalysts
5. Pd-H-MCM-22-30-R-IE
6. Pd-H-MCM-22-28-R-IE
7. Pd-H-MCM-22-50-R-IE
8. Pd-H-MCM-22-70-R-IE
Impregnation catalysts
9. Pd-H-MCM-22-30-R-IMP
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46
10. Pd-H-MCM-22-50-R-IMP
11. Pd-H-MCM-22-70-R-IMP
12. Pd-H-MCM-22-28-R-IMP
In-situ catalyst
13. Pd-H-MCM-22-30-R-IS
Solid state ion-exchange catalyst
14. Pd-H-MCM-22-30-R-SSIE
Deposition precipitation catalyst
15. Au-H-MCM-22-30-R-DP
The Na-form of catalysts which are synthesized and characterized
in
laboratory is mentioned in the list below.
1. Na-MCM-22-28-static
2. Na-MCM-22-28-96 h-R
3. Na-MCM-22-30-R
4. Na-MCM-22-28-R
5. Na-MCM-22-50-R
6. Na-MCM-22-70-R
7. Na-MCM-36-22-30-R
8. Na-MCM-36-22-28-R
9. Na-MCM-36-22-50-R
10. Na-MCM-22-30-30 h-USI
11. Na-MCM-22-30-40 h-USI
12. Na-MCM-22-30-48 h-USI
13. Na-MCM-22-30-30 h-without USI
14. Na-MCM-22-30-40 h-without USI
15. Na-MCM-48-A and Na-MCM-48-B
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47
Figure 20. Catalysts in powder form (sieved 150 m-250 m).
Proton forms of MCM-22 with different silica to alumina ratios
i.e. 30, 50, 70 are
characterized by infrared spectroscopy for acidity measurement
which is an
important method in order to determine active sites in the
sample.
Palladium forms of catalysts were characterized by Direct
Current Plasma for
metal loading i.e. Pd wt% in the zeolites and four different
preparation methods
were employed during this thesis work i.e. ion-exchange method,
in-situ and
SSIE method with nominal loadings 2 to 3 wt% Pd (using palladium
nitrate
solution) and 5 wt% Pd as nominal loading by impregnation method
(finally the
catalysts are crushed and sieved to 150 m-250 m pellets, see
Figure 20).
8.2. Synthesis of Catalysts
Na-MCM-22-30-R (Si/Al = 30)
The Na-MCM-22-30-R was synthesized as mentioned in article [30]
with some modifications. 3.02 g of sodium aluminate was added to
345.73 g of water and
thereafter 4.16 g of sodium hydroxide was added. The mixture was
stirred for 10
minutes and pH was measured. To this mixture 24.07 g of
hexamethyleneimine
(HMI) and subsequently 28.82 g of fumed silica was added. The
mixture was
stirred for 20 minutes and the pH measured. The gel was
transferred to teflon
cups and then inserted to autoclaves. The synthesis is carried
out in rotation
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48
mode for seven days at 150oC. After the completion of synthesis,
the material is
filtered, washed (pH neutralization) with distilled water, dried
at 100oC for 24 h
and calcined at 550oC for 8 hours.
Na-MCM-2250-R (Si/Al = 50)
The Na-MCM-22-50-R was synthesized as mentioned in article [30]
with some modifications. Sodium aluminate (1.64 g) was added to
351.60 g of water and
then 2.26 g of sodium hydroxide was added. The mixture was
stirred for 10
minutes and pH measured. To this mixture 21.77 g of
hexamethyleneimine (HMI)
and then 26.12 g of fumed silica was added. The mixture was
stirred for 20
minutes and pH measured. The gel was transferred to teflon cups
and then
inserted to autoclaves. The synthesis is carried in rotation
motion at 150oC for 7
days.
After completion of synthesis material is filtered, washed (pH
neutralization) with
distilled water, dried at 100oC and calcined at 550oC for 8
hours.
Na-MCM-2270-R (Si/Al = 70)
The Na-MCM-22-70-R was synthesized as mentioned in article [30]
with some
modifications. Sodium aluminate (1.14 g) was added to 345.73 g
of water and
then 4.16 g of sodium hydroxide was added. The mixture was
stirred for 10
minutes and pH was measured. To this mixture 24.07 g of
hexamethyleneimine
(HMI) was added and then 28.82 g of fumed silica. The mixture
was stirred for 20
minutes and pH was measured. The gel was transferred to Teflon
cups and then
inserted to autoclaves. The synthesis is carried in rotation
motion and was
carried out at 150oC for 7 days.
After completion of synthesis material is filtered, washed (pH
neutralization) with
distilled water, dried at 100oC and calcined at 550oC for 8
hours.
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49
Na-MCM-36-22-30-R with MCM-22-30-R as precursor
The Na-MCM-36-22-30-R was synthesized as mentioned in article
[28] with some modifications. First, CTMACL (25% sol) and TPAOH
(20% sol) was added
to MCM-22 precursor (non-calcined MCM-22 dried) in the ratio
1:4:1.2 as
mentioned in [28] and pH was measured. The reaction mixture is
kept under
heating up to 98oC to 102 oC for 68 h under stirring. This
process is known as
swelling process. The swollen MCM-22 is added to tetraethyl
orthosilicate
(TEOS) in the ratio 1:5 heated up to 78 oC under nitrogen
pressure for 25 h. This
process is known as pillaring process. Finally, the swollen and
pillared MCM-36
is hydrolyzed in the ratio 1:10 and the pH is adjusted to 8.
Then the material is
heated up to 40 oC, filtered,