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Porous materials

Materials with an open framework consisting of pores/cages/channels/windows
Have molecular sieving properties


Examples of porous materials

Zeolite-like materials e.g.
Aluminophosphates (AlPOs)
Covalent Organic Frameworks (COFs)
Metal Organic Frameworks (MOFs)
Zeolitic Imidazolate Frameworks (ZIFs)


General definition of zeolites

3D framework of tetrahedrally-coordinated T-atoms with cavities/channels
T-atoms are the atoms connected into tetrahedra through O atoms e.f. Si, Al, P, As, Ga etc


Classical definition of zeolites

Open aluminosilicate network of corner-sharing [AlO4] and [SiO4] tetrahedra
The charge of the framework is compensated for by mono- or divalent cations or protons within with cavities/channels
Also additional water molecules present in the cavities


How are zeolites different to dense silicates e.g. quartz?

Quartz has smaller channels that can only fit small cations e.g. Na+, Li+, whereas zeolites have larger pores that can fit larger cations/molecules


General formula of zeolites

(M^m+)y/m [(SiO2)x(AlO2)y]nH2O
Open framework structure


Primary building units of zeolites

Single tetrahedral units e.g. SiO4, AlO4, GeO4, GeS4


Secondary building units of zeolites

Collections of tetrahedral units, arranged in a specific way, repeated through the material
Rare for materials to have different combinations of SBUs within the zeolite framework
SBUs can contain up to 16 T-atoms


Zeolite chemistries

Bronsted acid sites
Lewis acid sites


Bronsted acid sites in zeolites

Arise from the creation of "hydroxyls" within the zeolite pore structure
Their strength and position are important in order to maximise their effect
The protons have great mobility, especially at T > 200 oC
At T > 500 oC, they are lost as water and form Lewis acid sites


Methods of synthesis of zeolite Bronsted acid sites

1. Ammonium ion exchange
NaZ(s) + NH4+(aq) NH4Z(s) + Na+(aq)
Then calcine NH4Z(s) to give NH3(g) and HZ(s)

2. Polyvalent ion exchange
NaZ(s) + M(H2O)n+(aq) M(H2O)n+Z(s) + nNa+(aq)
Then calcine M(H2O)n+Z(s) to make M(OH)^n-1 + HZ(s)


Lewis acid sites in zeolites

Unstable, especially in the presence of water vapour/steam
This produces "true" Lewis acid sites by ejecting Al from the framework


How can the 'extra-framework' Al ejected from Lewis acid sites be identified?

Because the 2 Al sites have different environments


Lowenstein's rule

Al-O-Al avoidance rule
Whenever 2 tetrahedra are linked by one oxygen bridge, the centre of only one of the tetrahedra can be occupied by Al - the other centre must be occupied by Si/another small ion of electrovalence >= 4 e.g. P
Means that a framework can never have > 50 % Al (i.e. max 1:1 Al:Si ratio)


What is believed to be the origin of Lowenstein's rule?

Charge of Al
Size of AlO4 tetrahedra (they are larger than SiO4 tetrahedra)
Position of the charge-balancing cations - i.e. each Al must be charge balanced, but there might not be enough space to house all these cations


Formula for calculating Si/Al ratio from SS NMR studies

(I4 + I3 + I2 +I1 +I0) / (I4 + 0.75I3 + 0.5I2 + 0.25I1)


Angles in zeolites

Alpha = O-T-O = 109.47 degrees. Little variability, tetrahedra quite rigid
Beta = T-O-T = more variable, depends on the species i.e.
Si-O = broad T-O-T angles of 130-179, maxima 145


Synthesis of zeolites

All zeolites today are synthesised by hydrothermal methods
RT - 250 oC (most syntheses are 80 - 150 oC)
(These temps are mild compared with those for high density silicates (~1000 oC))


Synthesis process

(see notes)


More recent method for zeolite synthesis

Ionothermal synthesis
Uses ionic liquids as both the solvent and template (structure-directing agent)
Some, but not all zeolites, are easier to grow with this method


Advantages and disadvantages of using microwaves instead of conventional synthesis

Can control temp. and pressure

Can be too fast (fast kinetics) meaning can't control kinetics of zeolite formation
No control of morphology



Organic Structure-Directing Agent


Advantages of OSDAs

Stabilise structures
Improved crystallinity - get small homogeneous crystals
Low degree of agglomeration - i.e. particles don't stick together
High external surface area
Diverse chemical composition


Disadvantages of OSDAs

Require post-synthesis treatment to "burn-out" template e.g. calcination
Release of Al
Partial collapse of structures
Increased production costs
Environmental problems


Processes for zeolite nucleation

1. Polymerisation-depolymerisation
2. Solution-precipitation
3. Nucleation-crystallisation



An equilibrium
pH dependent



Most relevant to nanozeolites
Si + Al species mixed together, then precipitate out in the shape of the crystal nanozeolite
Phase precipitates first before nucleating eventual shape of zeolite



See graph


Pathway of formation of LTA zeolite crystals

Primary building units, tetrahedra, grow
Primary building units assemble into SBUs
SBUs form the 3D framework


Zeolites are...

...metastable materials
Higher density frameworks ('denser phases') are formed with longer times/higher temperatures


Zeolite polymorphism

= zeolite phase transformation with increasing synthesis temperature and/or time
i.e. polymorphic zeolites are synthesised under exactly the same conditions apart from temperature
High temperatures generally means smaller pores (more dense) and vice versa for low temps


Ostwald ripening

A phenomenon that describes the change of an inhomogeneous structure over time
Small crystals/particles are "absorbed" into larger particles


Factors affecting zeolite synthesis

Batch composition (gel)
Si/Al sources (i.e. purity)
H2O content (i.e. more H2O = higher pressure at higher temps = will affect diffusion/pressure)
Inorganic cations
Stirring (or static)
Ageing (can affect nucleation and crystallisation kinetics)
Order of reagent addition
Temperature (ramp - especially important for microwave)
Reaction time
Crystallisation time and temperature


Templates can be used to...

...promote zeolite formation


True template effect

The zeolite forms around the template molecule, determining the pore topology (due to shape of template)


Pore-filling effect

The template fills the zeolite pores, providing stabilisation and preventing pore collapse


pH-stabilising effect

The template molecules have functionalities that stabilise the pH


Main chemical reagents for zeolite synthesis



Other sources of zeolite reagents

Fly ash (=by-product from burning coal)
Kaoline (= from countries with deserts/desert-like environments)


Most common methods for zeolite structure determination

Diffraction (PXD, neutron, synchrotron)
Atomistic simulations
Combined methods


Challenges associated with determining zeolite structures

Large unit cell containing 100s of atoms, so harder to build model
Si and Al cannot be differentiated between by X-rays because they have a similar no. of electrons
Single crystals not always available or too small (esp. if a powder)
Large number of sites for extra-framework cations
Possible disorder eg. disordered H2O/solvent in pores


Advantages of PXRD for zeolites

Good description of an average structure
Easy to perform
Phase identification
Possible refinement and structure determination
No need for large sample size/volume


Disadvantanges of PXRD for zeolites

Only shows average structure
Si and Al cannot be differentiated between by X rays so almost impossible to determine ordering
Large unit cells can be hard to refine
Sometimes neutrons/a bright light are required for structure determination - this can be done by combining microscopy and XRD e.g. TEM can look at separate phases


Using SS NMR for zeolite structure determination

SS NMR allows determination of the Si:Al ratio, as well as providing information on the local environment of each individual ion


Applications of zeolites

1. Ion exchange
2. Catalysis
3. Adsorption
4. Dessication


Ion exchange in zeolites

The selectivity of zeolites for cations decreases with ion radius - larger cations have more difficulty entering/leaving the zeolite
Exchange capacity halves when a monovalent ion is replaced by a divalent ion


Applications of zeolite ion exchange

In detergents
2Na+ Ca2+, in general
NaA removes Ca2+ and NaX removes Mg2+, preventing their precipitation by surfactants
Also helps to prevent eutrophication (build up of phosphates in waste water)

Removal of radionuclides from water and soil
HEU framework removes 99Sr and 137Cs

NH4+ + Na+Z NH4+Z + Na+
NH4+Z is then added to the soil, and then slowly releases nutrients - feeding the plants and preventing nutrient leaching

Ammonia removal from fish tanks/swimming pools


Applications of zeolite catalysis

Fluid Catalytic Cracking (FCC)
40 % of the petrol in the world is produced using FAU-type zeolites (X or Y)
Convert the high-boiling, high-MW hydrocarbon fractions of crude oils into more valuable gasoline, olefinic gases and other products

Methanol to hydrocarbons
Can get different products depending on the zeolite used e.g. HEU zeolite gives branched alkanes

Alkylation of ethylbenzene

Friedel-Crafts acylation
More environmentally friendly with zeolites because no solvent, no water, > 95 % yield


Effect of catalytic cycle on zeolites

Zeolites become increasingly deactivated with each catalytic cycle
C residues (soot) "choke" the zeolite so it becomes impossible to use
So need to "burn out" at high temps - zeolites are stable up to 800 oC unlike C residues


Categories of catalytic selectivity of zeolites

1. Product selectivity. Generally through introduction of acid sites/cations. Zeolite "selects" for which product is released.
2. Reactant selectivity. Zeolite "selects" for what can enter and thus react
3. Transition state selectivity
4. Site selectivity


Adsorption applications of zeolites

Used to purify CO2 from power plants
CO2 adsorbed onto zeolite surface, other gases can be removed
Then decrease pressure and increase temp to desorb CO2

Medical uses to produce pure O2


Dessicant applications of zeolites

Anhydrous zeolites readily absorb water so can be used for water removal, where the water is then stored in the zeolite framework


Current strategies for design of novel zeolites

Want to tackle "zeolite conundrum" / "zeolite bottleneck problem"

Trial and error
Combinatorial methods
Chemical substitutions e.g. Ge for Al, B for Si (but this is generally unsuccessful)
Computational methods e.g. ZEBEDDE method to predict templates
ADOR method = Assembly Disassmebly Organisation Reorganisation. Start with a zeolite containing some Ge instead of Al and selectively remove the Ge to create a 2D sheet with vacant sites, which then reorganise
(not strictly a new structure - always follows parent structure)



Where adsorbent is weakly bound by vdW and/or electrostatic forces



Where there is a covalent interaction between the absorber and adsorbate