Zeolites Flashcards
(55 cards)
Porous materials
Materials with an open framework consisting of pores/cages/channels/windows
Have molecular sieving properties
Examples of porous materials
Zeolites 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
- Ammonium ion exchange
NaZ(s) + NH4+(aq) NH4Z(s) + Na+(aq)
Then calcine NH4Z(s) to give NH3(g) and HZ(s) - 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?
By SS MAS NMR
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
Adv:
Faster
Can control temp. and pressure
Disadv:
Can be too fast (fast kinetics) meaning can’t control kinetics of zeolite formation
No control of morphology
OSDA
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