2.3 Porous materials Flashcards
(30 cards)
What are the main 3 types of porous materials?
- Micro pore width under 2nm
- Meso pore width in between 2 and 50 nm
- Macro por width over 50nm
What are microporous materials mainly used for?
small organic molecules
What are zeolites?
Microporous material (2-10A) Huge SA - up to 1000m2g-1 Crystalline aluminosilicates occur naturally (from igneous rock)and synthetically v. hydrophilic-v exothermic reaction zeolites+water=steam
Chemical framework
Built from [SiO4]4- and [Alo4]5-
units can be called sites and can be represented by a tetrahedron
these sites are linked through bridging oxygens to form chains are rings
aka tetrahedrons joined at point
What is a SBU
secondary building unit -rings and cages that are repeated throughout the structure to give its shape
nMR= n member ring made out of n units
DnR= double n member ring
v popular repeating unit of zeolites
sodalite cage or beta cage
8x 6MR and 6x 4MR
can be joined up in different ways to build different frameworks
3 common zeolites made from sodalite cage
SOD- sodalite- share 4MR’s cubic structure almost
LTA- zeolite A
FAU- faujasite
Rough pore sizes
4MR too small 6MR-2A 8MR-4.A 10MR-6A 12MR-8A all rough =n-4A
CAn topology vary?
yes- 1D topology straight 1d pores the whole way through
3D pores cross and diagonal (FAU) and can get v zig zaggy and complex
What are the two properties of zeolites that make them a useful material?
Pore size and topology- used as size seperation
charge compensating ions- many application
What is a charge compensation ion?
si - 4- charge al -5 charge
everytime a al is switched into the si framework- a charge compensation cation must be introduced to preserve the neutrality
typically small K, Na, Ca, H
not bound to framework- but are loosely bound in pores
can move and are exchangable
ions are often hydrated w/ water
general formula for aluminosilicates
M(m+)(x/m)[Si1-xAlxO2]x-.nH2O
Ratio of si and al sites
zeolites can be synthesised with a variety of ratios
not possible to easily synthesise pure Si
Rule that no al-o-al bridges may be present in a structure due to the columbic repulsion of the 2 Al
Lowest Si/Al ratio=1 -strict alternation
as si:al increases random al sitribution
Uses of zeolites- natural and synthetic
Natural- cement and cat litter- dig up easy and cheap
Synthetic -LTA- zeolite A- water softner- removes hard ions such as Ca from water
FAU-Faujasite-Gasoline from crude oil and general organic synthesis seperation
What are the two types of sorption
Absorption- sorption into bulk
Adsorption- sorption onto surface
With zeolites- line blurs- some external and some internal - just use sorption
What is sorption controlled by?
soption into an empty void- entropically disfavoured
Zeolites have a negatively charged framework which sorbates can interact with
1. Accesibilty- can guest entre the pore
2.Diffusion-ease of guest moving through zeolite
3.Interactions- strength of binding energies
how can zeolites be used for seperation? 3 methods
- via size exclusion
- through ΔHsorbtion differences
- via diffusion control
Seperation via size exclusion
-Use size of pores
-LTA- used to seperate n-alkanes from branched
-can modify pore size by cation choice
Sentinal effect
increase ion size ‘block the pore up/line it slightly’
only for same charge +1
Ca2+ smaller- half as many due to 2+charge and hence are rarely stored around the pores
Separation due to differences in ΔHsorption
eg. N2 and O2 in ca zeolite X - n2 interacts with framwork more- higher ΔHsorption - more n2 will be sorped than o2 meaning gas leaving is oxygen rich
large hydration enthalpy– drys gases
use in facemasks self heating
separation via diffusion control
molecules interact with zeolites to a different extent and therefore diffuse at difference rates
eg separations of xylenes (benzene with 2 methyl groups
p»o and m
3 major uses of zeolites
- seperation
- nuclear waste
- heterogeneous catalysis
Use: ion exchange
Extra framework cations are weakly bound and are exchangeable
controlled by TD, diffusion rates, ion charge and size, and hydration effects
main use: laundry detergents - ca replaces 2 Na from hard warder
ca binds more strongly to framework - fav
radioisotopes can also bind stronglyto zeolites and partake in ion exchange - useful in waste water and nuclear containment
heterogeneous catalysis
- acid catalysis (expanded on later)
- shape selectivity - of reactant, TS or products
- cracking catalysis
acid catalysis
when the ccc=H+ the zeolite is a solid acid
-extremely acidic but non toxic or corrosive because to access acidity- have to access pores
-strength of acid dependant of local environment and si/al ratio
high acidity - isolated sites better and small sites better
high si/al ratio-more isolated sites
low si/al ratio high activity- more ccc
