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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  • 1Zeolitic Materials: Ion Exchange and Shape Selective Catalysis

    Angus P. WilkinsonSchool of Chemistry and Biochemistry

    Georgia Institute of TechnologyAtlanta, GA 30332-0400


    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


    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

  • 10


    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


    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

  • 11

    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

  • 12

    Typical zeolite products Usually, zeolites are polycrystalline


    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

  • 13

    Zeolites and powder diffraction

    Electron microscopy and zeolites

  • 14

    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

  • 15

    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

  • 16

    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

  • 17

    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

  • 18

    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+


    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

  • 19

    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

  • 20

    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


  • 21

    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


  • 22

    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. .