Catalysis Flashcards
What is a catalyst
Speeds up reaction without effecting eqm position and is not consumed in the process
However in practice catalyst degrades overtime and activity will change
What properties would an ideal catalyst have?
Stable, cheap
Accelerate reaction
Active in low quantities
Recoverable and reusable
High yield
Arrhenius equation
k = Ae^(-Ea/RT)
lnk = lnA - Ea/RT
Gibbs Free energy equation
G = H - TS
Eyring Equation
k = kbT/h e^(ΔS/R) e^(-ΔH/RT)
How to assess catalyst reactivity
Rate
Turnover frequency (TOF)
Turnover Number (TON)
Equation for rate
converted amount of substrate/ catalyst volume or mass x time
Equation of TOF
number of cycles/ number of centres x time
Equation of TON
moles of product/ moles of catalyst
Equation for selectivity
quantity of desired products x 100 / amount of reactants consumed
Equation for efficiency
m.w of desired product x100 / m.w of all products from reaction
What is syngas? and what are some sources?
Mixture of H2, CO and CO2
Natural gas, coal, biomass, liquid petroleum gas
Homogeneous
Catalyst is in the same phases as the reactants
Heterogeneous
Catalyst is in a different phase to the reactants e.g. solid catalyst in liquid reactants
Advantages of heterogeneous catalyst
easy to recover and reuse
Thermodynamically and mechanically stable
Disadvantages of heterogeneous catalyst
Mechanism not always known
often low selectivity
Can degrade over time - poisoning
Hard to control shape/ size of catalyst at molecular level
Advantages of homogeneous catalyst
Very active so high rates
Often selective and easily modifiable
Work at low temp and conc
well defined reaction sites = obtaining mechanistic info easier
Disadvantages of homogeneous catalyst
Hard to separate from reactants
Low thermal stability means temp below 200°c often used
Advantages of enzyme catalyst
Highly specific
homogeneous
Disadvantages of enzyme catalyst
Subject to regulation by inhibitors and activators
Sensitive to their environment
What are solvent effects?
When the solvent used in a chemical reaction interacts with the solute to influence outcomes
Why is proton transfer in acid-base catalysis involving water so fast?
Because protons in hydrogen bonded structures e.g. water move rapidly bc their mass is low
What is specific acid catalysis
In specific acid catalysis, protonated solvent is the catalyst. The reaction rate is proportional to the concentration of the protonated solvent molecules SH+ and therefore is proportional to the pH
What are the reaction steps in specific acid catalysis
First is fast protonation of molecule via strong dissociated acid
Then nucleophilic attack by e.g. H2O
What is general acid catalysis?
All species able to donate a proton contribute to reaction rate
Proton transfer is from a now undissociated acid HA. This is much slower than dissociated H+
What are the reaction steps for general acid catalysis
Slow protonation by a weak undissociated acid in solution at same time as nucleophilic attack
Difference between general and specific acid catalysis?
Specific catalysis is detected by taking rate measurements at constant pH but different buffer concentrations. If the rate of the reaction is independent of the concentration of the buffer, specific acid catalysis is involved.
How do enzymes stabilise reaction transition states
Amino acids on enzymes control the properties e.g. potential acid or base and can stabilise reaction transition states increasing the reactions rate (via general acid/base catalysis mechanism) Both acid and base effects simultaneously to lower E barrier
Example of acidic amino acid that can be used to stabilise T.S
Protonated His
Example of basic amino acid that can be used to stabilise T.S
Unprotonated Asp, Glu or His
What are zeolite catalysts?
Microporous crystalline aluminosilicates composed of SiO4 and AlO4 tetrahedra
SiO4 neutral
AlO4 neg charged
Charge is compensated by extra framework cations e.g. Na+
Zeolite formation is a synthetic process, this means we can control the pore diameter what effect does this have on reactivity?
Only molecules that are smaller than the pore diameter fit and undergo reaction
What occurs during catalytic cracking
Hydrocarbons mixed with zeolite (strong acid)
Mixture blown through reaction chamber at 500°c
Hydrocarbons break down randomly producing a mixture of smaller hydrocarbons some w C=C bonds
How do carbenium ions form during catalytic cracking with zeolite?
Carbenium ions form when bronsted acid sites reacts with alkene or lewis acid site abstracts H-
Which type of carbenium ion is most stable and how does this effect rate of reaction
Tertiary carbenium ion
Rate depends on the stabilities of product and reactant carbenium ions
e.g. 2 tertiary ions fast
How can carbenium ions rearrange to achieve more stable configuration?
Rearrange via H- and CH3- movement
How can zeolites be used as molecular sieves?
1) reactant selectivity - only certain size molecule will fit into zeolite pores eg. non branched alkane
2) Product selectivity - only some products can leave the cage/ pore for example only para subs benzene ring not meta or ortho
3) Restricted T.S selectivity - cage/ pore shape restricts size/shape of intermediates
What is Ziegler Natta Polymerisation
TiCl3 and AlEt3 combine to produce heterogeneous ethene polymerisation catalyst
Produces linear polymer
What is an isotactic polymer?
alternating chiral centres along back bone all have same geometry
What is syndiotactic polymer?
Alternating chiral centres along back bone have alternating geometry
What is an atactic polymer?
Alternating chiral centres along polymer backbone have random geometries
Methods of termination of a polymerisation sequence off a metal centre
1) -βH
2) +H2
3) +HX e.g. H2O
How do we control the reactivity of catalysts used in polymer formation
Modify the catalyst precursor e.g. tether the 2 Cp rings together using Me2Si which changes bond angle and orbital overlap changing reactivity leading to high isotacticity polymers
