Lecture 7 Silica and Alusilicate-based Nanostructures
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Transcript of Lecture 7 Silica and Alusilicate-based Nanostructures
17/01/2011
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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)