Zeolites Flashcards
What is a zeolite?
They are crystalline materials consisting of an ordered framework structure containing channels with pores of molecular dimensions.
What are the main features of zeolites?
- 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
What are the representations of zeolites?
- ball & stick
- polyhedral
- MFI framework
- channel structure
- stick
Describe the Stick representation of zeolites
Sticks link T-sites and represent T-O-T
What is Lowenstein’s rule and what is the consequence of it?
- 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
What are zeolites based on?
They concern themselves with looking at extended 3D structures with periodic or regular arrangements of holes, pores or cavities of molecular size
In the synthesis of zeolites what is some sources of silicon
- alkali silicate solutions : Na2SiO3
- a high surface area solid
- colloidal SiO2
In the synthesis of zeolites what is some sources of aluminium
- sodium aluminate Na2AlO2.nH2O
- solid Al2O3 or Al(OH)3
In the synthesis of zeolites what is some sources of cation or base
- NaOH, KOH, CsOH
- could be organic cations e.g. Me4N+, amines
In the synthesis of zeolites what is an example of a solvent used
- water
- or an alcohol
Following the collection of all the materials required for zeolite synthesis what procedures follow
- 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
What are the compounds typically used to fill the void volume in zeolites
- as the framework grows molecules must fill the void volume
- usually cations e.g. Metal, NH4+ or organic ammonium ions and solvent amines
Describe the two ways in which cations fill the void volume
- 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
What are the reaction variables important in zeolite synthesis
- temperature
- time
- reaction mixture composition
What effect does temperature have on zeolite synthesis
- 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.
What effect does time have on the synthesis of zeolites
- 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
What is ostwalds rule of successive transformations
- kinetic products convert to thermodynamic products as time increases
What is the effect of the reaction composition the synthesised zeolite
- 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
What are the important features of zeolites used as ion exchangers
- selective towards cations
- ion sieving
Why does the cation selectivity of Li+ tend to be low
- 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
What is the general relationship between anionic framework charge and charge density cations in cation selectively when zeolites are used as ion-exchangers
- a high anionic framework charge prefers higher change density cations
- a low anionic framework charge prefers lower charge cations
What are some applications of zeolites being used as ion exchangers
- in detergents
- in treating radioactive wastes
Describe how zeolites are used in detergents as ion exchangers
- 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
Describe the use of zeolites as ion exchangers in the treatment of radioactive wastes
- 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
What is the basis of using zeolites to separate mixtures of molecules from gas or liquid phase
- their effective pore size impacts their molecular sieving abilities
- the affinity of the non-framework ions and the framework for the sorbed species
What are some of the factors affecting zeolite separation properties
- ‘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
What alterations does exchanging the cation in a zeolite illicit
- the effective pore size
- the number of cations
- sites occupied
- the internal volume occupied by the cations
- the affinity for the sorbed species
What are two applications to zeolites being used as molecular sieves
- in bulk separations
- as drying agents
Describe the zeolite molecular sieving action used in bulk separations
- 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
Describe the molecular sieving action that results in zeolites being used as drying agents
- 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
Describe the separation of N2 and O2
- 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
What are some of the advantages associated with using zeolites in heterogenous catalysis
- 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
Describe shape selectivity of the reactant when zeolites are used in heterogenous catalysis
- 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
Describe shape selectivity of the product when zeolites are used in heterogenous catalysis
- 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
Describe shape selectivity of the transition state when zeolites are used in heterogenous catalysis
- 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
Describe functionalisation at acidic sites in terms of the application of zeolites
- 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
Describe functionalisation in ion exchange in terms of the catalytic application of zeolites
- 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
Describe functionalisation of metal clusters in terms of the catalytic applications of zeolites
- 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
Describe catalytic cracking the role of zeolites in it
- 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
Describe the processes that involve the cracking reactions associated with oil refining
- 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
What is hydrocracking in terms of oil refining
- heavy vacuum gas and oil residue is converted to gasoline, kerosene and gas oil
- bifunctional catalysts are used
What are bifunctional catalysts
- 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
What are some of the advantages of hydrocracking over catalytic cracking
- reaction occurs at lower temp
- produces less C1 and C2 products
- ideally feedstock molecules only one cracking event
Describe hydroisomerisation an example of a shape selective process in oil refining
- converts low octane rating (RON) straight chain own tames and hexanes to higher RON branched methylbutane and 2,2-dimethylbutane
What is the paraffins total isomerisation process
- the shape selective aspect- determining the site of the product formed in the zeolite
- molecular sieving aspect - the separation of iso/normal products
In terms of shape selective processes associated with oil refining and petrochemicals what is dewaxing
- removal of long chain n-paraffins, from lubricating and diesel oils
- n-paraffins crystallise at
