2.3 Porous materials

Question 1

What are the main 3 types of porous materials?

Answer 1

• Micro pore width under 2nm
• Meso pore width in between 2 and 50 nm
• Macro por width over 50nm

Question 2

What are microporous materials mainly used for?

Answer 2

small organic molecules

Question 3

What are zeolites?

Answer 3

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

Question 4

Chemical framework

Answer 4

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

Question 5

What is a SBU

Answer 5

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

Question 6

v popular repeating unit of zeolites

Answer 6

sodalite cage or beta cage
8x 6MR and 6x 4MR
can be joined up in different ways to build different frameworks

Question 7

3 common zeolites made from sodalite cage

Answer 7

SOD- sodalite- share 4MR’s cubic structure almost
LTA- zeolite A
FAU- faujasite

Question 8

Rough pore sizes

Answer 8

4MR too small
6MR-2A
8MR-4.A
10MR-6A
12MR-8A
all rough =n-4A

Question 9

CAn topology vary?

Answer 9

yes- 1D topology straight 1d pores the whole way through

3D pores cross and diagonal (FAU) and can get v zig zaggy and complex

Question 10

What are the two properties of zeolites that make them a useful material?

Answer 10

Pore size and topology- used as size seperation

charge compensating ions- many application

Question 11

What is a charge compensation ion?

Answer 11

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

Question 12

general formula for aluminosilicates

Answer 12

M(m+)(x/m)[Si1-xAlxO2]x-.nH2O

Question 13

Ratio of si and al sites

Answer 13

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

Question 14

Uses of zeolites- natural and synthetic

Answer 14

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

Question 15

What are the two types of sorption

Answer 15

Absorption- sorption into bulk
Adsorption- sorption onto surface
With zeolites- line blurs- some external and some internal - just use sorption

Question 16

What is sorption controlled by?

Answer 16

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

Question 17

how can zeolites be used for seperation? 3 methods

Answer 17

1. via size exclusion
2. through ΔHsorbtion differences
3. via diffusion control

Question 18

Seperation via size exclusion

Answer 18

-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

Question 19

Separation due to differences in ΔHsorption

Answer 19

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

Question 20

separation via diffusion control

Answer 20

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

Question 21

3 major uses of zeolites

Answer 21

• seperation
• nuclear waste
• heterogeneous catalysis

Question 22

Use: ion exchange

Answer 22

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

Question 23

heterogeneous catalysis

Answer 23

• acid catalysis (expanded on later)
• shape selectivity - of reactant, TS or products
• cracking catalysis

Question 24

acid catalysis

Answer 24

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

Question 25

cracking catalysis general

Answer 25

zeolites are used to crack crude oil into petroleum via c-c scission reactions
high braching wanted- better combustion and less knocking

zeolite catalyst cokes up- and clogs with carbon particles but can be heated in air to remove

Question 26

Cracking catalysis mechanism

Answer 26

Initiation: alkane + H+ –> akane (- an h) and h2
ion produced will rearrange to most stab 3>2>1

Cracking reaction at beta C aka the one with the charge and its neighbour making branched alkene and another carbenium ion

termination h transferred back –> alkene

Question 27

Methanol to gasoline process

Answer 27

acid site in MFI zeolite can convert methanol to akanes and alkenes
ch3oh- picks up and h –> water and methoxy
(Si(OH)Al) goes to Si(OCH3)Al which can then go on to react with others etc etc

MFI also allows for product selectivity as pore width doesnt allow highly brached alkanes or long chains
]

Question 28

Mesoporous catalysts

Answer 28

can combine hetero and homo catalysts
selectivity of homo but ease of hetero
High density of silanol (siOH) species on silicate surfaces of pores that M catalysts can be grafted to
can get enantioselectivity depending on M and L’s grafted to frame

Question 29

MOF’s what are they

Answer 29

Metal Organic frameworks are a coordination network with organic ligands containing potential voids
aka organic groups can link up metal clusters
many forms and not all necessarily cubic
some MOF’s have peptides as links and tend to be particularly flexible

post syntheic functionalisation

Question 30

MOF’s properties and uses

Answer 30

ability to do sorption
catalysis potential
could conduct/be magnetic

applications
catalysis
drug delivery
storage
fuel cells