Lecture 7 Silica and Alusilicate-based Nanostructures

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Silica and Aluminosilicate-based Nanostructures and Metal-Organic Framework

Dr Montree Sawangphruk (DPhil)

Chemical Engineering, Kasetsart University, Room #1248, email:[email protected]

Porous Materials

Protein

Microporous Mesoporous

2 nm

Macroporous

Zeolites MCMs (silica) Bio-foams

50 nm

Molecule

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Silicon Dioxide or Silica

Silicon dioxide is the main component of the crust of the earth.

Combined with the oxides of magnesium, aluminum, calcium,

and iron, it forms the silicate minerals in our rocks and soil.

Functional Groups of a Silica Particle

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Mesoporous Silica

Mesoporous silica is a form of silica and a recent

development in nanotechnology. The most common

types of mesoporous nanoparticles are MCM-41 and

SBA-15, 16. Research continues on the particles, which

have applications in catalysis, drug delivery and imaging.

Synthesis of Mesoporous Silica (SBA-16)

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Santa Barbara Amorphous (SBA) type

material SBA-16

Mobil Composition of Matter (MCM)

Mobil Composition of Matter (MCM) is the initial name given for a series of mesoporous materials that were first synthesized by Mobil's researchers in 1992.

MCM-41 (Mobil Composition of Matter No. 41) and MCM-48 (Mobil Composition of Matter No. 48) are two of the most popular mesoporous molecular sieves that are keenly studied by researchers.

The most striking fact about the MCM-41 and MCM-48 is that, although composed of amorphous silica wall, they possess long range ordered framework with uniform mesopores.

These materials also possess large surface area, which can up to more than 1000 m2g-1.

Moreover, the pore diameter of these materials can be nicely controlled within mesoporous range between 1.5 to 20 nm by adjusting the synthesis conditions and/or by employing surfactants with different chain lengths in their preparation.

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MCM-41 (Mobile Crystalline Material-41)

hexagonal pore arrangement

Applications of MCM

MCM-41 and MCM-48 have been applied as

catalysts for various chemical reactions

a support for drug delivery system

adsorbent in waste water treatment

etc

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An MCM-41 type mesoporous silica nanosphere-

based (MSN) controlled-release delivery system

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Carbon Nanotube Synthesis Using

Mesoporous Silica Templates

What are zeolites?

Natural or synthetic crystalline, microporous,

aluminosilicate materials with well-defined

structures and unique characteristics

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Zeolite types

Natural zeolites form where volcanic rocks and ash layers react with alkaline groundwater.

Synthetic zeolites are created through a slow crystallization process using a combination of silica and alumina and using a foundation of alkali and organic templates.

Synthetic Zeolite

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Porous Materials

Protein

Microporous Mesoporous

2 nm

Macroporous

Zeolites MCMs Bio-foams

50 nm

Molecule

Structure

SiO44- and AlO4

5- tetrahedra linked in several ways, resulting

in over 130 different 3D framework structures

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Structure

The presence of Al in the framework induces a negative

charge that is balanced by an extraframework cation.

Classification

Depending on the pores dimension and on the framework structure, zeolites can

be A, X, Y or L type

Zeolites A Zeolite Y

A. H. Roy, R. R. Broudy, S. M. Auerbach and W. J. Vining, The Chemical Educator, Vol. 4, No. 3, S1430-4171(99)03300-2,

10.1007/s00897990300a, © 1999 Springer-Verlag New York, Inc.

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Schematic

structure of natural

zeolite -

TETRAHEDRON

The cells: POLYHEDRONS inside of which there are voids of different

sizes dependent on the zeolite type A- 0.4 nm i type X- 0.9 nm

sodalite faujasite

Zeolite are aluminosilicates composed of [SiO4]4− and [AlO4]

4− tetrahedra.

For each Al, a negative charge is created.

The negative charge is compensated by cation.

The Faujasite Zeolite

Cations occupy different sites.

Cations are exchangeable.

The zeolite has cages.

2 types of Faujasite, X and Y.

Si/Al = 1.2 (NaX), 2.4 (NaY).

Salient chemical properties:

NaY unreactive, no supercage Na+

Basicity of supercage O atoms

Cations attract anions formed

Sodalite cage (0.67 nm)

Supercage (1.3 nm)

Faujasite structure [J. Phys. Chem. B. 109,

4738 (2005)]

Al

O

OO

SiO

O

OO

M+

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Classification

Zeolite A is a small-pore zeolite in which the Si:Al ratio is 1 and thecages are linked octahedrically.

The pore diameter varies from about 3 to 5 Å.

Classification

Zeolites X and Y are large-pore zeolites (6-8 Å) containing various

Si:Al ratios and tetrahedrically linked cages

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Framework Structure

Crystalline, well-defined nano-pore structure

Charged framework

Spatial arrangement

Extreme Thermal & Chemical Stability

Zeolites

Pores of

Zeolite Y

7.4×13Å

Classification

Zeolite L - the crystals

are very flat cylinders of

"hockeypuck" or "coin"

shape

Linear channels in which

one-dimensional diffusion

takes place

Hexagonal crystal system

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Introduction- Registered Zeolites

(ii) IR assignments

1436 cm-1: Lewis (PyNa+)

1548 cm-1: Bronsted(PyH+)

1492 cm-1: superposition of

1436 cm-1 and 1548 cm-1

(i) Titration using NaOH:

Zeolite acidity increased

dramatically after adsorption of 2-

chlorobutane.

9001100130015001700

Wavenumber (cm-1)

Tra

nsm

itta

nce

1436

14921548

a

b

c

Elimination

No HCl gas produced pointing

to acid zeolite (HX).

OAl

Si

Na OAl

Si

RCHXCH3

H RCH CH2

NaX zeolite HX zeolite

+ + + Na+X-

Character of Zeolite Elimination Reaction

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Zeolite Entrapment

Size Discrimination

Ion Exchange Encapsulation

Properties

Ion-exchange

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Properties

Molecular Sieve Effect

Properties

Acidity

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Thermodynamics says NOTHING about the

rate of a reaction.

Thermodynamics : Will a reaction occur ?

Kinetics : If so, how fast ?

Zeolite Used as Catalyst

Reaction path for conversion of A + B into AB

Zeolite Used as Catalyst

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Activation Energy

Activation Energy : The energy required to overcome the reaction

barrier. Usually given a symbol Ea or ∆G≠

The Activation Energy (Ea) determines how fast a reaction occurs, the higher

Activation barrier, the slower the reaction rate. The lower the Activation

barrier, the faster the reaction

Catalyst lowers the activation energy for both forward

and reverse reactions.

Activation Energy

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Activation Energy

This means , the catalyst changes the reaction path by

lowering its activation energy and consequently the

catalyst increases the rate of reaction.

Zeolite membranes – Recent developments

and progress (see Chapter 4)

Development of articles in open literature with (zeolite* OR molecular sieve) AND (membrane* OR coating OR film).

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Scientific publications on membrane reactors and zeolite

membrane reactors, respectively (Scifinder search)

Zeolites in the petrochemical industry

http://omusinternational.com/db3/00225/omusinternational.com/_uimages/nightrefinery.JPG

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Introduction

Zeolites contain void space that can host cations, water, or other molecules

Molecular sieves

Do not allow molecules larger than 8 to 10 nm to enter lattice

Zeolites:

40 known natural zeolites

> 140 synthetic zeolites

Introduction – Major Applications

Adsorption

Drying, purification, and separation

Powerful desiccants- able to hold 25% of their weight in water

Remove volatile organic compound (VOC) from air streams &

separate gases

Catalysis

Shape-selective catalyst- on the basis of molecular diameter

Acid catalysts – used in the petrochemical industry

Ion Exchange

Detergent formulas- replace phosphates as water softening

agents

Exchange Na in zeolite for Ca or Mg in water

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Introduction- Environment

Contribute to a cleaner, safer environment

In powder detergents, zeolitesreplaced harmful phosphate builders

Solid acids, zeolites reduce the need for corrosive liquid acids

Redox catalysts and sorbents Remove atmospheric pollutants,

such as engine exhaust gases and ozone-depleting CFCs.

Zeolites can also be used to separate harmful organics from water Heavy metals and NH4

+

Picture: http://www.cerpa.appstate.edu/images/environment.jpg

Introduction - Environment

Zeolites can be regenerated by

Heating to remove adsorbed materials

Ion exchanging with sodium to remove cations

Pressure swing to remove adsorbed gases.

University of New York at Stony Brook

Developed a zeolite to trap radioactive strontium

Heating the material makes the holes clamp shut, sealing the radioactive waste inside

http://www.ecofriendlymag.com/wp-content/plugins/wp-o-matic/cache/3ad14_us-

import-radioactive-waste.jpg

http://jdlong.files.wordpress.com/2009/05/nuclear-plant.jpg

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Introduction- Applications

Introduction Membranes

Applications

Desalination

~90% salt rejection

Ethanol Dehydration

Replaces azeotropic

distillation

Separate CO2 from air

A method of CO2

sequestration

H2 Separation

http://www.freepatentsonline.com/20040144712-0-large.jpg

http://cdn.physorg.com/newman/gfx/news/hires/energyeffici.jpg

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Introduction - Membranes

Research Efforts

Decrease membrane thickness

Increase flux

Determine sustainability/durability

Analyze replacement time and cost for industrial applications

Potential to replace current energy consuming separation devices

Distillation Column

http://bioage.typepad.com/photos/uncategorized/2007/07/11/mitsui_2.png

Motivation

Reduce Operating Costs

Lower reaction temperature and pressure

Superior control of reaction selectivity

Reduces feed costs – less waste and treatment streams

Challenges

Energy efficiency – CO2 emissions

Product/Process specifications- heavier & dirtier crudes

Changing feedstocks (biofuel, biomass, unconventional oils)

http://gozonews.com/wp-content/uploads/2008/05/biofuels.jpg

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Motivation- Global Market

http://www.theage.com.au/ffximage/2005/03/24/oil_generic_wideweb__430x317.jpg

Metal-Organic Framework (MOF)

Metal-organic frameworks (MOFs) are materials in which metal – to-organic ligand interactions yield porous coordination networks with record-setting surface areas surpassing activated carbons and zeolites.

The applications of MOFs include storage and separations of gases, sensors, catalysis and others.

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MOF

High Surface Area (MOF-14)

a surface area of 4526 m2/g

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Synthesis of MOFs

MOFs and zeolites alike are produced almost exclusively by hydrothermal or solvothermal

techniques, where crystals are slowly grown from a hot solution of metal precursor, such as

metal nitrates, and bridging ligands.

Zeolite synthesis often makes use of a variety of templates, or structure-directing compounds,

and a few examples of templating, particularly by organic anions, are seen in the MOF

literature as well.

A particular templating approach that is useful for MOFs intended for gas storage is the use of

metal-binding solvents such as N,N-diethylformamide and water.

In these cases, metal sites are exposed when the solvent is fully evacuated, allowing hydrogen

to bind at these sites.

MOFs for hydrogen storage

Hydrogen has the potential to be an attractive option

because it has a high energy content (120 MJ/kg

compared to 44 MJ/kg for gasoline), produces clean

exhaust product (water vapour without CO2 or NOx),

and can be derived from a variety of primary energy

sources.

However, the specific energy of uncompressed hydrogen

gas is very low, and considerable attention must be given

to denser storage methods if hydrogen is to emerge as a

serious option for energy storage.

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MOFs for hydrogen storage

Metal Organic Frameworks (MOFs) attract attention as materials for adsorptive hydrogen storage because of their exceptionally high specific surface areas and chemically-tunablestructures.

MOFs can be thought of as a three-dimensional grid in which the vertices are metal ions or clusters of metal ions that are connected to each other by organic molecules called linkers.

Hydrogen molecules are stored in a MOF by adsorbing to its surface.

Compared to an empty gas cylinder, a MOF-filled gas cylinder can store more gas because of adsorption that takes place on the surface of MOFs.

MOFs for hydrogen storage

Furthermore, MOFs are free of dead-volume, so there is almost no loss of storage capacity as a result of space-blocking by non-accessible volume.

Also, MOFs have a fully reversible uptake-and-release behavior since the storage mechanism is based primarily on physisorption, there are no large activation barriers to be overcome when liberating the adsorbed hydrogen.

The storage capacity of a MOF is limited by the liquid-phase density of hydrogen because the benefits provided by MOFs can be realized only if the hydrogen is in its gaseous state.

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Metal organic frameworks for Hydrogen

Storage

1 – Hydrogen tank

2 – Radiator

3 – Stack Module (Hydrogen Fuel Cell)

4 – System Module (Hydrogen Fuel Cell)

5 – Power Distribution Unit

6 – LiPoly Battery to start the fuel cell system

7 –Total Rescue System

Why store Hydrogen?

We need Clean energy.

Hydrogen if combined with Oxygen releases energy (an energy carrier).

Water is the byproduct (Completely harmless, clean). This reaction can

replace another popular but polluting source of energy-gasoline.

Hydrogen generated from diverse domestic resources can reduce demand

for oil by more than 11 million barrels per day by the year 2040.

For this reaction we need a source of hydrogen. (Attaching a hydrogen tank

with the mobile vehicle.)

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How much Hydrogen?

DOE Targets (for 2010) : Future hydrogen cars should have :

Hydrogen storage tank carrying approximately 5 kg of H2 (a range of

300 miles (480 km)).

Maximum allowed pressure of 100 bar for a storage device.

Capacity targets for a fueling system (including the tank and it's

accessories) set at 6 wt% and 45g/l of unstable H2.

System should show not decay for 1000 consecutive fueling cycles and

should allow filling to full capacity in 3 minutes.

for 2015 - 9 wt%, 60g/l, 1500 cycles. and 2.5 min.

Storing Hydrogen (Conventional methods)

Compressed gas method requires huge amount of initial pressures and safety issues arise.

Cryogenic storage requires large amount of energy input for initial condensation of hydrogen.

In complex hydrides (eg. Mg2NiH4 ) desorption usually occurs at higher temperatures than targeted conditions.

Other drawbacks are high cost, susceptibility to impurities and low reversible gravimetric capacity.

One way to improve the kinetics of storage is to maintain the Molecular

identity of H2 during the process.

Physisorbtion of molecular hydrogen in to highly porous materials.

Metal Organic Frameworks

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Storing Hydrogen in MOFs

MOFs have large apparent surface

areas.

The dinitrogen isotherm measured

for MOF-177 at 77 K exhibits the

highest uptake of N2 for any

material to date, and gives rise to a

monolayer-equivalent surface area

of 4500 m2 /g.

This framework has cavities in the

range of 11-12 A0.

MOF-177

IRMOF-8

MIL-53

Zn2-(bdc)2(dabco)

(C: black, N : green,O : red,

Zn : blue polyhedra, M: green octahedra).

O. M. Yaghi and J. L. C. Rowsell, Angew. Chem. Int. Ed. 2005, 44, 4670 –4679

Storing Hydrogen in MOFs

For an ideal adsorbate:

1. Pore size should be same as it’s own diameter.

2. Walls of the pore should be made of light elements (should be as thin as possible)

3. Walls should be highly segmented (achieved in MOFs by reticular synthesis).

4. Smaller pores in MOFs are needed to surpass the storage density of liquid hydrogen.

MOF-5: Pore diameter 15.2 A0 (Yellow sphere)

An smaller pore analogue of MOF-5 can be

stabilized by using a rigid linear dicarboxylate

C: black,

H : white,

O : red

Zn : blue

tetrahedra

Kinetic diameter of H2 molecule = 2.98 A0

A compromise between gravimetric and volumetric density of storage must be found out.

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Storing Hydrogen in MOFs

To bind the hydrogen in a better way, another adsorbatesurface can be inserted inside the pore (approach is called impregnation)

These surfaces also reduce the pore diameter.

In this case also a compromise between gravimetric and volumetric capacity is reached.

MOF-177 molecule with C60

molecule inside it’s pore.

O. M. Yaghi and J. L. C. Rowsell, Angew. Chem. Int. Ed. 2005, 44, 4670 –4679

Storing Hydrogen in MOFs

Storing hydrogen by framework catenation:

Catenation of two identical frameworks can be used to restrict the dimensions of

the pore considerably by interpenetration.

Though interpenetrated framework is more capacitive than interwoven but it has

comparatively less stability.

Repeat unit Interpenetration Modified InterweavingInterweaving

O. M. Yaghi and J. L. C. Rowsell, Angew. Chem. Int. Ed. 2005, 44, 4670 –4679

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Storing Hydrogen in MOFs

Using coordinatively unsaturated

metal sites:

An unsaturated metal site can attach to

H2 directly (a substantial increase in

H2 binding affinity)

Some strategies for synthesizing such

materials are:

1. Metal building units with

coordinatevely unsaturated centers

through solvent removal.

2. Incorporating Coordinatively

Unsaturated Metal Centers within

the Organic Linkers

3. Impregnation of Metal-Organic

Frameworks with Metal Ions

Unsaturated

metal centre

J. R. Long and M. Dinca Angew. Chem. Int. Ed. 2008, 47, 6766 – 6779

O. M. Yaghi and J. L. C. Rowsell, Angew. Chem. Int. Ed. 2005, 44, 4670 –4679

Storing Hydrogen in MOFs

Other methods

Modifying organic linkers to increase the H2 affinity

Introducing additional adsorptive sites on the SBUs.

Using light metals to reduce the framework density.

J. R. Long and M. Dinca Angew. Chem. Int. Ed. 2008, 47, 6766 – 6779

O. M. Yaghi and J. L. C. Rowsell, Angew. Chem. Int. Ed. 2005, 44, 4670 –4679

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Homework

A 2-page report, which can show how to synthesise, how

to characterise, and how important it is, on one of porous

materials below;

a. Mesoporous materials

b. Zeolites

c. Metal-organic framework (MOF)