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Zeolitic Materials: Ion Exchange and Shape Selective Catalysis

Angus P. WilkinsonSchool of Chemistry and Biochemistry

Georgia Institute of TechnologyAtlanta, GA 30332-0400

Overview

IntroductionZeolite structureZeolite synthesisZeolite application

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What is a zeolitic material?

Zeolites are inorganic crystalline solids with small pores (1-20 Å diameter) running throughout the solidThey are aluminosilicate framework structures made from corner sharing SiO4and AlO4 tetrahedra– related structures can be made from AlPO4 and

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 the solid 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 are zeolites 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 range of pores sizes and shapesComposition can be varied to tailor a materials properties– pure SiO2 zeolites tend to be hydrophobic– high alumina zeolites have a lot of charge

balancing extraframework cations and have a very high affinity for polar molecules

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Extraframework cations

Extraframework cations are under-coordinated by the 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, 5A and 13X?– The number denotes

the accessible pore size– the letter denotes the

framework– changing cations

“tunes” the pore size

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ALPOs

Microporous aluminophosphates can also be madeSynthesis is usually at low pH with organic additivesALPOs have no framework chargeSAPOs have a negative charge on the frameworkALPOs limited to ring systems with alternating aluminum and phosphorous

Titanosilicates

It is possible to make Zeolite frameworks that include 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 oxidizing agents

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Zeolite synthesisZeolites and aluminophosphate microporous materials 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 ammonium hydroxide etc.

» template: organic cation, hydrated metal ion etc.

Templating agentsPore size and shape can be controlled by growing 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 large single 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 any other zeoliteIt is used as a water softener in powdered laundry detergentsIn countries with low waste water treatment standards it is more environmentally friendly than polyphosphate

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Other ion exchange applications

Zeolites with good selectivities for Cs and Sr 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

SeparationsGas separations such as O2/N2Straight chain hydrocarbons from branched chains 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 to aluminum 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 chain hydrocarbons 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

CatalysisZeolites are frequently used as acid catalysts– ion exchange zeolite so that extraframework

cations are protons– ion exchange zeolite so that high charge

extraframework cations bind water and release protons

– Lewis acidity at defect sitesCan do carbenium ion chemistry

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Catalysis with zeolites

The majority of the applications make use of the acidic properties of zeolites

Acid sites can be introduced by ion exchange for NH4

+ followed by thermal decomposition

Alternatively, acid sites can be introduced by ion exchange for La3+ followed by cation hydrolysis reactions

– Ln3+ + H2O ----> Ln(OH)2+ + H+

Dewaxing

Unbranched hydrocarbons have high melting points and tend to form waxes.

Wax forming compounds in fuels are undesirable

Unbranched hydrocarbons can be selectively cracked in the presence of branched hydrocarbons using ZSM-5

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Xylene isomerization

p-xylene is needed for the production of polyestersXylenes can be rearranged over ZSM-5– can selectively obtain p-xylene

The production of p-xylene

Isomerization is used as part of a cycle that separates p-xylene from other compounds

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Transalkylation with xylenes

Transalkylation is an unwanted side reaction during xylene isomerization

Transalkylation using toluene

Toluene can be converted to a much more valuable mixture of xylenes and benzene over ZSM-5

+

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Alcohol dehydrations

Alcohols can be dehydrated to give alkenes– selectivity depends

on pore size

Gasoline and zeolites

Most gasoline is processed using Faujasite type 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 to high 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 application of zeolitesCarried out using zeolite YHigh molecular weight hydrocarbons are broken down into lighter fragmentsThis cracking process is an acid catalyzedcarbenium ion rearrangementZeolite inhibits formation of coke etc. .