Catalysis

Question 1

Why use catalysts?

Answer 1

· Use of catalyst should reduce 
	- energy demand
	- by product formation
	- need for stoichiometric proportions
Waste

Question 2

Disadvantages of catalysts

Answer 2

• Many catalysts use heavy metals
• May be toxic
• Need to separate catalyst from product
• Recycle catalyst
• Catalyst may degrade - products may also be toxic
• Generally more economic and environmentally friendly to use catalyst than not

Question 3

Describe the role of a catalyst in a chemical reaction

Answer 3

• Increase in rate of chemical reaction due to participation of catalyst

• Not consumed by reaction itself
• May participate in several chemical transformations
• Effect dependent on presence of other substances => reduced by inhibitors, increased by promoters

Question 4

Types of catalyst

Answer 4

· Homogenous - reagents and catalyst in same phase (usually solution) e.g. acid catalysis
· Heterogeneous - industrially important - reactant and catalyst in different phases (reagent usually liquid or gas, catalyst usually solid e.g. metal alloy or metal oxides) e.g. catalytic converters => diffusion of reagents to surface and diffusion of products from surface
·Biocatalysis - use of enzyme for reaction catalysis

Question 5

Catalyst properties: design/selection

Answer 5

· SA and porosity - heterogeneous catalysts => high SA materials formed as either finely divided substrate or crystallites with accessible internal pores => dense solid is unsuitable, examples include MgO, porosity types (lecture before)
· Surface acidic and basic sites - highly active for catalytic reactions e.g. alcohol dehydration and alkene isomerization
· Surface (active) metal sites - small metal particles on ceramic oxide substrates are very active catalysts e.g. catalytic converters
· Chemisorption and desorption - adsorption essential for heterogeneous catalysis to occur but must not be so strong that it blocks catalytic sites and prevents further reaction => adsorption of reactant on surfaces often activates molecules => physiorption - used to assess SA and porosity => chemisorption - used to assess accessible reactive sites
Surface migration - adsorbed atoms and molecules migrate over metal surfaces => mobility important in catalytic reactions as it allows atoms/molecules to find and approach one another

Question 6

Types of advanced material catalysts

Answer 6

·Uniform catalyst - bulk of high SA material serves as catalyst e.g. zeolite
·Multiphasic catalysts - high SA material that serves as a support on to which an active catalyst is deposited

Question 7

Define turnover frequency

Answer 7

Measure of no. of molecules produced per unit time

Question 8

Define selectivity

Answer 8

Ratio of desired product formed to undesired product formed

Question 9

Define catalyst

Answer 9

Alters chemical reaction by changing Ea and denotes/promotes routes taken whilst being unconsumed in the reaction

Question 10

Define catalytic cycle

Answer 10

A diagram illustrating how catalysts and reactants change throughout a reaction in a closed loop cycle

Question 11

Define catalyst support

Answer 11

A material that can be used as a host to an expensive catalyst to reduce cost

Question 12

Define enantioselective

Answer 12

two enantiomers formed, attack occurs at chiral centre

Question 13

Define regioselective

Answer 13

two isomers produced (e.g. tut 10 Q7, Br and OH form in similar regions but different bonds)
=> groups add at specific ends of double bond, minimal product results from steric hindrance

Question 14

Region of turnover number which confirms catalysis

Answer 14

turnover number higher than 100 is considered to be catalytic

Question 15

Define chemoselective

Answer 15

reaction occurs with limited no of functional groups