Catalysis Flashcards
What is a catalyst?
A catalyst speeds up a reaction without affecting the equilibrium position
In practice, they can degrade over time
What are some expectations of catalysts?
Stable
Cheap
Not corrosive
Active in low quantities
Recoverable and reusable
Minimal toxic waste
Soften the reaction conditions
Increase yield
High purity
High selectivity
What is the Eyring equation?
k = ((boltzmann constant x T) / h) x exp(-delta G / RT)
How can we plot the Eyring equation in a line where y=mx+c?
We can plot y = ln(k/T) and x = 1/T
What does delta H dagger tell us about in a reaction?
It yields information about the bond strength
What does delta S dagger tell us about in a reaction?
It yields us information about the changes in order
What is the equation for turnover frequency (TOF)?
TOF = number of cycles (turnovers) / number of centres x time
What is the equation for turnover number (TON), and what factors does this take into account?
TON = Moles of product / moles of catalyst
This takes into account deactivation, and reflects the maximum yield attainable from a catalyst
What are the positives of heterogenous catalysts?
Easy to regenerate, recover (filter) and reuse
Thermodynamically and mechanically stable
Often metals or metal oxides
Activity is linked to surface area
What are the negatives of heterogenous catalysts?
Selectivity is often low
Reaction control problems due to diffusion and heat transport
Can degrade over time due to leaching, poisoning, swelling or sintering
Reproducible preparations can be difficult
Hard to control the shape or size of the catalyst at the molecular level
What are the positives of homogeneous catalysts?
Very active leading to high rates
Selective and easily modifiable
Mild working conditions
Often acids, bases or transition metal complexes
Ligand and solvent variations can be used positively
Mechanistic information is easier to obtain
What are the negatives of homogeneous catalysts?
Low thermal stability
Hard to separate
Recovery of catalyst can be costly
What are the positives of enzyme catalysis?
Classified by function
Highly specific
What are the negatives of enzyme catalysis?
Subject to regulation by inhibitors and activators
Catalyse biological transformations
Sensitive to the environment e.g. pH
What is specific acid catalysis?
The hydroxonium ion is the catalyst, with the H+ being fully transferred before the RDS
Rate is proportional to [RH+]
What is general acid catalysis?
Undissociated acid, HA, is the proton donor
Proton transfer from the undissociated acid is a slow step; this changes the RDS
How can we spot a general acid catalysis?
We can obtain a constant pH via a buffer, and change the buffer concentration which will change the rate
How can we catalytically crack hydrogen carbons to form ethene?
Hydrocarbons are mixed with zeolites and blown through a reaction chamber at 500 degrees
A strong acid can protonate the hydrocarbon
The hydrocarbon breaks down randomly to form a mixture of smaller hydrocarbons
What are the key steps for catalytic cracking?
Carbenium ions are formed when Bronsted acid sites react with alkene or Lewis acid sites abstracts H-
The carbenium ion reacts with another hydrocarbon chain by H- abstraction
C-C bond beta to carbenium ion breaks heterolytically to yield an alkene and a carbenium ion
Oligomerisation is possible if an alkene and a carbenium ion combine
What is beta-scission?
The breaking of a beta C-C bond
The rate depends on the stability of the reactant and product carbenium ions
What is the Ziegler Natta polymerisation reaction?
AlEt₃ + TiCl₃ + Ethene —–> linear alkane polymer
What are the molecular weights of the products from Ziegler Natta polymerisation?
10,000 to 2,000,000
What metals polymerise alkenes nowadays?
M⁴⁺ ions, d⁰ ions - these lead to a fast migration as there is no back bonding to the alkene
Ti«Hf<Zr
What are three ways TMs can activate hydrogen to break the H-H bond?
Oxidative addition
Hydrogenolysis
Heterolytic cleavage
What are three heterogeneous supported catalyst composition?
Active phase
Promoter
Support/Carrier
What is the active phase?
This is where the reaction occurs (active site)
What is the purpose of the promoter?
Makes a catalyst more active
Electronic modifier
Poison-resistant promotor, or to introduce selectivity
What is the purpose of the support/carrier?
Increases the mechanical strength
Increases the surface area
They may or may not be catalytically active
What is the mechanism of heterogeneous hydrogenation?
The molecules are absorbed to the surface where the bond is broken and then the reaction occurs on the surface
What is the adsorption energy comparable to for chemisorption?
Comparable to the energy of a chemical bond
What happens in physisorption?
We get an elastic collision (no energy transfer) when we hit the surface
Atoms and molecules can ‘stick’ to the surface however
What is the equation for the sticking probability?
Number of molecules that stick / number of molecular impacts
How do we understand if chemisorption will occur?
We need to consider the energy needed to break the bond and the energy formed when 2 new M-X bonds are formed
What is needed for the chemisorption to be exothermic?
E X-M > D X-X / 2
How can we measure adsorption?
Have two setups where we allow gas to pass into another container, one with a surface on. The differences in pressure can be measured
How can we measure the surface coverage?
Number of occupied surface sites / overall number of surface sites
What does surface coverage depend on?
Concentration (pressure)
Temperature
What are the three key assumptions of the Langmuir Adsorption Isotherm?
(1) The surface has a fixed number of identical sites (monolayer only)
(2) The adsorption enthalpy is independent of coverage
(3) Adsorbates do not interact
