1Zeolitic Materials: Ion Exchange and Shape Selective Catalysis

Angus P. WilkinsonSchool of Chemistry and Biochemistry

Georgia Institute of TechnologyAtlanta, GA 30332-0400

Overview

Introduction Zeolite structure Zeolite synthesis Zeolite application

2What is a zeolitic material? Zeolites are inorganic crystalline solids

with small pores (1-20 diameter) running throughout the solid

They 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

3Secondary building units

Pentasil zeolites

4Pore connectivity

Chiral zeolites

A chiral zeolite would allow enantioselectivesynthesis and separations Very difficult to get optically pure chiral zeolite

5What 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

6Zeolite 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

7Shape selectivity

Key structural features You can make materials with a wide range

of pores sizes and shapes Composition 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

8Extraframework cations

Extraframework cations are under-coordinated by the framework like to bind molecules in pore system to increase

coordination number

Extraframework cations in Faujasites

9Undercoordinated 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 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 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 synthesis Zeolites 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 agents Pore 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 zeolite It is used as a water softener in powdered

laundry detergents In 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

Separations Gas separations such as O2/N2 Straight 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

Catalysis Zeolites 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 sites Can 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

polyesters Xylenes 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 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. .