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Contemporary Themes > Zeolites > Flashcards

Zeolites Flashcards

1
Q

What is a zeolite?

A

They are crystalline materials consisting of an ordered framework structure containing channels with pores of molecular dimensions.

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2
Q

What are the main features of zeolites?

A
  • channels with pores 3-20 Å
  • reversible adsorption and desorption
  • sharp pore size distribution
  • large mostly internal surface area and void volumes
  • framework can carry a negative or neutral charge
  • voids occuied by cations if framework is charged and other neutral mobile molecules i.e. Water
  • framework geometry affects chemical and physical properties
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3
Q

What are the representations of zeolites?

A
  • ball & stick
  • polyhedral
  • MFI framework
  • channel structure
  • stick
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4
Q

Describe the Stick representation of zeolites

A

Sticks link T-sites and represent T-O-T

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5
Q

What is Lowenstein’s rule and what is the consequence of it?

A
  • Al atoms do not exist in adjacent tetrahedra in zeolites
    e. g. There are no Al-O-Al bonds
  • it’s consequence is that Si/Al is never 0.5
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6
Q

What are zeolites based on?

A

They concern themselves with looking at extended 3D structures with periodic or regular arrangements of holes, pores or cavities of molecular size

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7
Q

In the synthesis of zeolites what is some sources of silicon

A
  • alkali silicate solutions : Na2SiO3
  • a high surface area solid
  • colloidal SiO2
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8
Q

In the synthesis of zeolites what is some sources of aluminium

A
  • sodium aluminate Na2AlO2.nH2O

- solid Al2O3 or Al(OH)3

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9
Q

In the synthesis of zeolites what is some sources of cation or base

A
  • NaOH, KOH, CsOH

- could be organic cations e.g. Me4N+, amines

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10
Q

In the synthesis of zeolites what is an example of a solvent used

A
  • water

- or an alcohol

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11
Q

Following the collection of all the materials required for zeolite synthesis what procedures follow

A
  • all the materials are placed in a gel of high pH
  • for 100 degrees reaction vessel if steel autoclaves
  • typical reaction temps are usually 60-225 degrees
  • pressure 30-910 bar
  • time: hours, days, weeks
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12
Q

What are the compounds typically used to fill the void volume in zeolites

A
  • as the framework grows molecules must fill the void volume
  • usually cations e.g. Metal, NH4+ or organic ammonium ions and solvent amines
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13
Q

Describe the two ways in which cations fill the void volume

A
  • unspecific space filling- the pores, cages or channels form around these species. They stabilise the resulting framework structure against dissolution
  • templating effects- where the shape of space-filling species specifies the shape of the cage or channels of the zeolite structure
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14
Q

What are the reaction variables important in zeolite synthesis

A
  • temperature
  • time
  • reaction mixture composition
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15
Q

What effect does temperature have on zeolite synthesis

A
  • reaction temp. determines:
    • product structure
    • framework density and openness- more dense at higher temps.
  • rate of reaction increases with temp. but there might be an optimum temp for the open structure desired.
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16
Q

What effect does time have on the synthesis of zeolites

A
  • time determines the structure or material formed
  • zeolites are not typically thermodynamically stable in their synthesis mixtures at reaction temps
  • initially from the disordered synthesis mixture appears more open higher entropy products
  • with increasing time there is a conversion to denser more stable phases
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17
Q

What is ostwalds rule of successive transformations

A
  • kinetic products convert to thermodynamic products as time increases
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18
Q

What is the effect of the reaction composition the synthesised zeolite

A
  • Si/Al, Mn+/Si, Rn+/Si ratios determine structure or the material formed
  • Si/Al, Mn2+/Si or Rn+/ Si ratio- the use of Rn+ or Mn+ in those species determine the framework Si/Al ratio
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19
Q

What are the important features of zeolites used as ion exchangers

A
  • selective towards cations

- ion sieving

20
Q

Why does the cation selectivity of Li+ tend to be low

A
  • Li+ has a very high charge density
  • it is a strongly hydrated ion making it hard to remove
  • it’s solvent shell consequently has a large size
21
Q

What is the general relationship between anionic framework charge and charge density cations in cation selectively when zeolites are used as ion-exchangers

A
  • a high anionic framework charge prefers higher change density cations
  • a low anionic framework charge prefers lower charge cations
22
Q

What are some applications of zeolites being used as ion exchangers

A
  • in detergents

- in treating radioactive wastes

23
Q

Describe how zeolites are used in detergents as ion exchangers

A
  • washing powders contain Na-zeolites
  • removing Ca2+ and Mg2+ improves efficiency of the detergents
  • removing these cations from solution is achieved by in exchange with Na+ ions
  • Na-zeolite-A exchanges Ca2+ and Na-Zeolite X exchangers Mg2+ ions
24
Q

Describe the use of zeolites as ion exchangers in the treatment of radioactive wastes

A
  • zeolites are used for the selective ion exchange of radionuclides
  • natural clinoptilolite and chabazite was used be used of their high selectivity and high abundance
  • they are cheap
25
Q

What is the basis of using zeolites to separate mixtures of molecules from gas or liquid phase

A
  • their effective pore size impacts their molecular sieving abilities
  • the affinity of the non-framework ions and the framework for the sorbed species
26
Q

What are some of the factors affecting zeolite separation properties

A
  • ‘the size of the window has the greatest effect on molecular sieving properties
  • 8, 10 and 12 sized ringed windows are the most influential for sieving
27
Q

What alterations does exchanging the cation in a zeolite illicit

A
  • the effective pore size
  • the number of cations
  • sites occupied
  • the internal volume occupied by the cations
  • the affinity for the sorbed species
28
Q

What are two applications to zeolites being used as molecular sieves

A
  • in bulk separations

- as drying agents

29
Q

Describe the zeolite molecular sieving action used in bulk separations

A
  • used to separate n- and i- C10 - C18 paraffins using a Ca-A (5Å) zeolite in a gas phase process
  • the n-paraffins pass through the zeolite while the i-paraffins do not
30
Q

Describe the molecular sieving action that results in zeolites being used as drying agents

A
  • dehydrated zeolites particularly the non-framework cations have a high affinity for water
  • 3A and 4A ion exchanged zeolites are used in labs as drying agents
  • in refrigerator circuits and vehicle breaking circuits to dry organic liquids
  • in double glazing to dry Ar, SF6 gases
  • used in large scale dynamic operations to dry gases i.e. H2, Ar, O2
31
Q

Describe the separation of N2 and O2

A
  • separation is based on the strength of the interactions with the non-framework cations
  • passing air over a particular zeolite results in N2 being preferentially sorbed leaving an O2 rich gas product
  • N2 has a molecular quadrupole moment
    • therefore metal ions interact more strongly with it
32
Q

What are some of the advantages associated with using zeolites in heterogenous catalysis

A
  • crystal voidage and channels resulting in high surface area
  • ion exchange abilities
    • produces protons and other metal cation if centres as active sites
    • cotton pore size
    • enable distribution of other catalytically active centres within in zeolite
  • well defined framework with a very narrow pore size distribution enables shape selectivity
    • molecular selectivity is based on the shape, size and geometry of the framework
33
Q

Describe shape selectivity of the reactant when zeolites are used in heterogenous catalysis

A
  • arises because of the finite size of the pore entrance
  • reactants can be excluded based on their shape and size
  • shape selectivity results in only some shapes of reactants entering the zeolite
  • shape selectivity is not dependent on crystallite (particle) size
34
Q

Describe shape selectivity of the product when zeolites are used in heterogenous catalysis

A
  • arises because of finite size thus alters the relative diffusivities of the possible products out if the zeolite
  • products with high diffusivities leave the reaction area quickly
  • products with low diffusivities can not leave easily and undergo further reaction on crystallite (particle) size
35
Q

Describe shape selectivity of the transition state when zeolites are used in heterogenous catalysis

A
  • occurs when the spatial confinements of the micropore restrict the formation of a transition state for certain reactions
  • transitions state selectivity is not dependent on crystallite (particle) size
36
Q

Describe functionalisation at acidic sites in terms of the application of zeolites

A
  • there are two types of acidic site Brønsted and Lewis
  • in Brønsted acidic site formation ion-exchange in NH4Cl solution at ~100 degrees
  • Lewis acid sites are formed from Brønsted acidic sites
    • Brønsted acid site is head to ~550 degrees from framework defect the Lewis acid site forms
    • rearrangement then reforms framework and makes non-framework Lewis acid Al-species
37
Q

Describe functionalisation in ion exchange in terms of the catalytic application of zeolites

A
  • active metal cations can be introduced by ion exchange

- example is to exchange rare earth ion RE3+ into zeolite Y , X to form cracking catalyst

38
Q

Describe functionalisation of metal clusters in terms of the catalytic applications of zeolites

A
  • zeolites can act as host containers for metal clusters
    • e.g. Pt, Pd, Rh, Ni, and Fe
  • metal particles should be small and well dispersed clusters within the zeolite cages to increase catalytic activity
  • metal clusters are often formed in zeolite cages and at the surface
39
Q

Describe catalytic cracking the role of zeolites in it

A
  • the conversion of long chain hydrocarbons to a more useful mix of aromatics and small branched chain alkenes (C1-C6)
  • the catalyst is partially rare earth exchanged (La3+) protonated (H) ultra stable zeolite-Y
  • the reaction uses a fluidised bead reactor
40
Q

Describe the processes that involve the cracking reactions associated with oil refining

A
  • in this process long chain hydrocarbons are converted to a more useful mix of aromatics and small branched chain alkenes, alkenes and aromatics
  • reaction is believe to be initiated by the carbonium or carbenium ion formation
  • size of the products is limited by zeolite cages
41
Q

What is hydrocracking in terms of oil refining

A
  • heavy vacuum gas and oil residue is converted to gasoline, kerosene and gas oil
  • bifunctional catalysts are used
42
Q

What are bifunctional catalysts

A
  • they posses acidic functionality - H-zeolite
  • they posses hydrogenation or dehydrogenation functionality -
    i. e. Metal clusters such as Pd - Pt, Co - Mo, or Ni - W
  • uses high silica faujasites
  • conditions are 250 - 450 degrees
  • under H2 pressures at 15-150 bar
  • size of products limited by size of zeolite changes
43
Q

What are some of the advantages of hydrocracking over catalytic cracking

A
  • reaction occurs at lower temp
  • produces less C1 and C2 products
  • ideally feedstock molecules only one cracking event
44
Q

Describe hydroisomerisation an example of a shape selective process in oil refining

A
  • converts low octane rating (RON) straight chain own tames and hexanes to higher RON branched methylbutane and 2,2-dimethylbutane
45
Q

What is the paraffins total isomerisation process

A
  • the shape selective aspect- determining the site of the product formed in the zeolite
  • molecular sieving aspect - the separation of iso/normal products
46
Q

In terms of shape selective processes associated with oil refining and petrochemicals what is dewaxing

A
  • removal of long chain n-paraffins, from lubricating and diesel oils
  • n-paraffins crystallise at

Contemporary Themes (4 decks)

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