introduction to zeolites

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 Zeolitic Materials: Ion Exchange and Shape Selective Catalysis Angus P. Wilkinson School of Chemistry and Biochemistry Georgia Institute of Technology Atlanta, GA 30332-0400 Overview Introduction Zeolite structure Zeolite synthesis Zeolite application

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5/12/2018 Introduction to Zeolites - slidepdf.com

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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. .