Sam - Defects, zeolites, MOFs

Question 1

what are Schottky defects?

Answer 1

a pair of vacancies: anion and cation missing to maintain neutral charge

Question 2

Frenkel defects?

Answer 2

ion (usually cation bc smaller) moves into an interstitial site that is normally unoccupied, common for AgX

Question 3

what are extended defects?

Answer 3

defects extend beyond a small no. of sites throughout a larger volume of the crystal, may cluster or form planes

dislocations: additional plane where shouldn’t be,

grain boundaries, stacking faults, clusters

Question 4

describe zeolite structure

Answer 4

corner sharing [SiO4]4- and [AlO4]5- tetrahedra with oxygen bridges

high component of covalent bonding so more stable and more gaps (cavities) in the structure which host charge-balancing cations

Question 5

describe silicate component of zeolites

Answer 5

Si-O-Si link, made of corner sharing [SiO4]4-

Question 6

describe sodalite cage

Answer 6

known as beta cage

24 tetrahedra in 4 and 6 rings

large enough to host small molecules (absorb) - small pore zeolite

truncated octahedron shape

Question 7

what is zeolite A (LTA) ?

Answer 7

a small pore zeolite made of sodalite units and linked by oxygen bridges connecting the 4 rings

large truncated cuboctahedron linked to 6 others - forms orthogonal channels with apertures

Question 8

how are zeolites suitable as desiccants and why?

Answer 8

small pore zeolites like sodalite and zeolite A selectively absorb polar molecules (e.g. water) due to their hydrophilicity (high Si/Al ratio)

water molecules are small enough to pass through the windows to enter cavities = size selectivity

allows diffusion through the crystal structure

Question 9

how are zeolites suitable for water softening and why?

Answer 9

charge balancing cations in the structure of zeolite A can be exchanged for other cations

Na-zeolite A and Na-zeolite P are used in laundry detergent

Na+ ions exchange for cations such as Mg2+ and Ca2+ which cause hard water

avoids polyphosphates which cause environmental problems such as algal bloom

Question 10

how are zeolites suitable for environmental remediation and why?

Answer 10

zeolites can show high affinities for various metal cations, particularly larger ones

cation exchange with heavy metals causing radioactive contamination

Question 11

how are zeolites suitable for hydrocarbon separation and why?

Answer 11

small pore zeolites can accommodate linear molecular (straight chain) hydrocarbons, primary alcohols, and amines but exclude branched = shape selectivity

involved tuning window diameters by ion exchange to allow access to specific molecules

Ca-zeolite A used for fuel

Question 12

how are zeolites suitable for heterogeneous catalysis and why?

Answer 12

zeolites can act as bronsted acid catalysts with protons as counter ions forming a hydroxylated bridges

or lewis acids due to open Si/Al sites from heating bronsted acid framework

or catalytic metals such as Pd2+ or Ni2+ can be exchanged into the framework

shape and size selectivity can promote selectivity of reaction by reactant, product, or TS

Question 13

how does tuning the Al/Si ratio affect the properties of a zeolite?

Answer 13

• affects structure type
• hydrophobicity of the cavities - therefore the polarity
• the available pore volume (no. of cations)
• the chemical stability of the framework - charged Al decreases the stability

Question 14

how does tuning the counter cations affect the properties of a zeolite?

Answer 14

affects the channel and cage size
the chemical properties - acidity, polarity, catalysis

Question 15

how does tuning templating affect the properties of a zeolite?

Answer 15

affects the specific large cations present during synthesis
promotes the desired structure type
stabilises a high Al/Si ratio