introduction to zeolites
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Zeolitic Materials: Ion Exchangeand Shape Selective Catalysis
Angus P. Wilkinson
School of Chemistry and Biochemistry
Georgia Institute of TechnologyAtlanta, GA 30332-0400
Overview
Introduction
Zeolite structure
Zeolite synthesis
Zeolite application
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What is a zeolitic material?
Zeolites are inorganic crystalline solidswith small pores (1-20 Å diameter) runningthroughout the solid
They are aluminosilicate framework structures made from corner sharing SiO4
and AlO4 tetrahedra – related structures can be made from AlPO
4and
other compositions
Building up zeolite structures
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Secondary building units
Pentasil zeolites
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Pore connectivity
Chiral zeolites
A chiral zeolite would allow enantioselectivesynthesis and separations – Very difficult to get optically pure chiral zeolite
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What is special about zeolites?
They have pores with molecular dimensions – leads to shape selectivity
There is a narrow range of pores sizes in thesolid because the materials are crystalline – gives better selectivity than non-crystalline
materials
Pore sizes in zeolites
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Zeolite A, ZSM-5 and Beta
Zeolite A8 rings
Zeolite Beta
12 rings
ZSM-5, 10 rings
What types of applications arezeolites used for?
Drying agents – used for drying solvents
Shape selective separations – e.g. dewaxing diesel fuel
Shape selective catalysis – predominantly acid catalysis, but also redox
Selective ion exchangers – water softeners, radioactive waste treatment
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Shape selectivity
Key structural features
You can make materials with a wide rangeof pores sizes and shapes
Composition can be varied to tailor amaterials properties – pure SiO2 zeolites tend to be hydrophobic
– high alumina zeolites have a lot of charge balancing extraframework cations and have avery high affinity for polar molecules
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Extraframework cations
Extraframework cations are under-coordinated bythe framework – like to bind molecules in pore system to increase
coordination number
Extraframework cations in Faujasites
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Undercoordinated cations
3A, 4A, 5A etc.
What are 3A, 4A, 5Aand 13X? – The number denotes
the accessible pore size
– the letter denotes theframework
– changing cations“tunes” the pore size
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ALPOs
Microporous aluminophosphates can also be made Synthesis is usually at low pH with organic additives
ALPOs have no framework charge
SAPOs have a negative charge on the framework
ALPOs limited to ring systems with alternatingaluminum and phosphorous
Titanosilicates
It is possible to make Zeolite frameworks thatinclude tetrahedral titanium
A class of materials containing octahedral titanium
has also been prepared
These titanosilicates are useful catalysts for selective partial oxidation reactions using peroxide oxidizingagents
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Zeolite synthesis
Zeolites and aluminophosphate microporousmaterials are made hydrothermally – reactants are heated in water (100 - 250 C)
– For an aluminosilicate zeolite» silica source: Cabosil, sodium silicate or Si(OEt)4
» alumina source: high surface area aluminum oxyhydroxide,Al(OEt)3, sodium aluminate, Al3+ salts
» base (pH ~12); alkali metal hydroxide, quaternary ammoniumhydroxide etc.
» template: organic cation, hydrated metal ion etc.
Templating agents
Pore size and shape can be controlled bygrowing the zeolite around templates
TMA+ in ZK-4 TPA+ in ZSM-5
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Typical zeolite products
Usually, zeolites are polycrystalline
Characterization
Most zeolite do not grow into largesingle crystals
Structural data can be obtained from:
– i) powder diffraction techniques
– ii) electron microscopy
– iii) solid state NMR spectroscopy
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Zeolites and powder diffraction
Electron microscopy and zeolites
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29Si MAS NMR of zeolites
Ion exchange with Na-A
More sodium zeolite A is produced than anyother zeolite
It is used as a water softener in powderedlaundry detergents
In countries with low waste water treatmentstandards it is more environmentally friendlythan polyphosphate
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Other ion exchange applications
Zeolites with good selectivities for Cs andSr are available – used to remove 137Cs and 90Sr from radioactive
waste streams» concentrate waste prior to disposal
– more robust than organic ion exchange resins» not susceptible to radiation damage
Separations
Gas separations such as O2/N2
Straight chain hydrocarbons from branchedchains using Ca-A – straight chains are a problem for diesel fuel
– straight chains are useful for detergents Water from organics
– extraframework cations coordinate to the water and remove it from the organic phase
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Silicon to aluminum ratio and hydrophobicity
Zeolites can be prepared with varying silicon toaluminum ratios
High silica zeolites are hydrophobic
– they are not wet
– they can select hydrocarbons from mixtures
Low silica zeolites are hydrophilic
Hydrocarbon separations
Zeolite A can be used to separate straight chainhydrocarbons from a mix.
Straight chains used to make detergents
Ca/Na - A
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O2 / N2 separations
N2 is adsorbed more strongly than O2 in zeoliteswith a low Si/Al ratio
This is a consequence of the quadrapolemoment of N2 interacting with theextraframework cations
Used for gas separation but it has the
disadvantage of being a batch process
Catalysis
Zeolites are frequently used as acidcatalysts – ion exchange zeolite so that extraframework
cations are protons
– ion exchange zeolite so that high chargeextraframework cations bind water and release protons
– Lewis acidity at defect sites
Can do carbenium ion chemistry
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Catalysis with zeolites
The majority of the applications make use of theacidic properties of zeolites
Acid sites can be introduced by ion exchange for NH4
+ followed by thermal decomposition
Alternatively, acid sites can be introduced by ionexchange for La3+ followed by cation hydrolysisreactions
– Ln3+ + H2O ----> Ln(OH)2+ + H+
Dewaxing
Unbranched hydrocarbons have high melting pointsand tend to form waxes.
Wax forming compounds in fuels are undesirable
Unbranched hydrocarbons can be selectively crackedin the presence of branched hydrocarbons usingZSM-5
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Xylene isomerization
p-xylene is needed for the production of polyesters
Xylenes can be rearranged over ZSM-5 – can selectively obtain p-xylene
The production of p-xylene
Isomerization isused as part of a
cycle that separates p-xylene fromother compounds
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Transalkylation with xylenes
Transalkylation is an unwanted sidereaction during xylene isomerization
Transalkylation using toluene
Toluene can be converted to a much morevaluable mixture of xylenes and benzeneover ZSM-5
+
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Alcohol dehydrations
Alcohols can bedehydrated to givealkenes – selectivity depends
on pore size
Gasoline and zeolites
Most gasoline is processed using Faujasitetype zeolites – high molecular weight materials are cracked
– second largest application of zeolites
Gasoline can be made by the dehydration of methanol over ZSM-5!! – Mobile MTG process only used in New
Zealand
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Synthetic gasoline
ZSM-5 is capable of transforming methanol tohigh grade gasoline by dehydrating the alcohol
Proceeds via dimethyl ether
Not economical is most western nations
Fluid Catalytic Cracking (FCC)
FCC is the most important industrial applicationof zeolites
Carried out using zeolite Y
High molecular weight hydrocarbons are broken
down into lighter fragments This cracking process is an acid catalyzed
carbenium ion rearrangement
Zeolite inhibits formation of coke etc. .