Catalysis

Question 1

Define a catalyst.

Answer 1

A substance that increases a rate of reaction without changing the overall standard Gibbs free energy change. The position of equlibrium is unchanged but the activation barriers are lowered. The catalyst is not used up or permanently changed.

Question 2

Give key benefits of catalysis.

Answer 2

• Faster reactions
• Milder conditions
• Greater selectivity with stereochemical control
• Reduce overall cost and enviromental impact

Question 3

Describe the greeness of catalysts.

Answer 3

Catalysts reduce waste and generally allow reactions at lower temperatures. However they often rely on rare metals and may be hazardous themselves to health or the environment.

Question 4

Describe the Gibbs free energy diagram of a reaction, catalysed and not, giving key definitions and how the catalyst affects the steps.

Answer 4

The catalyst tends to increase the number of steps in the reaction with much lower activation parameters between steps. There tends to be no high peaks or troughs, no highly unstable or stable intermediates are formed.

• Intermediate: A molecular entitiy with a lifetime > molecular vibration
• Transition state: High energy arrangement of atoms which must be passed through to form an intermediate/product
• Exergenic: Negative Gibbs energy
• Endogenic: Positive Gibbs energy

Question 5

How can the Gibbs free energy of a reaction be calculated from the experimental rate constant?

Answer 5

Question 6

Give four ways a catalyst may increase the rate of reaction.

Answer 6

1. Increasing electrophilicity
2. Increasing nucleophilicity
3. Stabilising a transition state
4. Bringing reagents together in the reaction mixture

Question 7

Define catalyst activity and selectivity. Suggest factors that may affect a catalyst lifetime.

Answer 7

Catalytic activity: The rate of consumption of a reactant

Selectivity: Fraction of a specific product in the the whole of the products.

Catalyst lifetime may be affected by denaturisation, poisons, mechanical wear, or sooting (active sites/pores are blocked by carbon material)

Question 8

What are the metrics for comparing catalyst efficiency and how are they used?

Answer 8

Turn over number (TON): The number of molecules reacting per active site. This may be low due to inactivity, the site being easy to poison and potential side reactions.

Turn over frequency (TOF): The number of molecules reacting per active site per second.

Question 9

Give some examples of green chemistry metrics used to compare reactions.

Answer 9

• Yield,
• Selectivity
• Atom economy
• E factor (E_factor = Waste (kg)/Product (kg))
• Reaction mas efficiency ((M_w products x yield) / M_w reactants)
• Renewables intensity (mass of all renewably derived materials/mass of products)
• Solvents can be classified on how hazardous they are

Question 10

Define homogeneous catalysis and briefly describe it.

Answer 10

• Catalyst and reagents are in the same phase
• Most commonly dissolved in liquids
• Ligands affect the steric and electronic environment of the active site
• Often a single and define active site

Question 11

Give key points about the catalysis of the Monsanto process.

Answer 11

• The catalyst is a Rh complex (iodine and carbonyl ligands) in the I and III oxidation states
• Rh can adopt multiple stable oxidation states and coordination numbers
• Reaction produces acetic acid using HI which undergos oxidative addition on the catalyst (RDS)
• The carbonyl CO bond is weakened by synergic bonding to react with the neighbouring ligand
• High activity and selectivity (over 99%)
• The Cativia process improved the reaction using a iridium catalyst, with the RDS being 150 times faster

Question 12

Describe the general mode of action, model of action and limitations of enzymes.

Answer 12

Reacting groups are brought together through VdW and electrostatic interactions and hydrogen bondings using nearby amino acid residues.

The selectivity is controlled by a ‘lock and key’ model where the substrate perfectly fits the active site. The induce fit model suggests that the enzyme changes shape to fit the substrate after it binds, inducing a transitions state.

The limitations of enzymes are pH dependancy, solvent and denaturation even at low temperatures.

Question 13

Define and describe heterogeneous catalysts and how they work. Why is the solid surface so reactive?

Answer 13

The catalyst in a different phase to the reagents, typically a solid catalyst with a liquid reagent. The solid may be the catalyst itself, or be the support for a catalytic species such as nanoparticles or a tethered complex. The surface is so reactive as the surface storms are exposed and have a low coordination number.

Question 14

What the key points and benefits of heterogenous catalysts?

Answer 14

• The easy separation of the catalyst is beneficial to the purification, recovery and greenness of the process.
• The structure may be highly ordered or may be amorphous, this will affect the properties. The reaction can be described as structure sensitive or insensitive.

Question 15

Describe the two main types of heterogeneous catalysts.

Answer 15

1. Finely divided solids with the active site on the particle surface. These can often be on a support stucture
2. Where the active sites are located at internal surfaces such as within pores or cavities such as zeolites.

Question 16

Describe the role of the catalyst support in heterogeneous systems and the size classifications of pores.

Answer 16

A good catalyst has a high surface area so there will be a thermodynamic driving force to reduce this energy. One method is by sintering, where small particles fuse and reduce in energy. The solid support prevents this. A solid catalyst can have various pore sizes such as macropores (>50 nm), mesopores (2 - 50 nm) and micropores (<2 nm). These have different roles to allow liquid to flow and to increase surface area and hence activity.

Question 17

Describe the structure of a catalytic converter, the reactions it operates, the advantages and limitations.

Answer 17

A honeycomb stainless steel structure will silica or alumina deposited onto it with embedded nanoparticles of Pt, Rh and Pd. The structure prevents sintering and allows gas flow.

The main reactions are oxidising CO, soot and NO.

Advantages are reduction of hazardous emission gases and high activity.

Limitations are high temperature dependence of performance, deactivation at high temperatures and use of precious metals.

Question 18

Describe the structure of zeolites and how it links to the activity of the catalyst.

Answer 18

Zeolites are mostly crystalline aluminosilicates based on tetrahedral structures of SiO₄ and AlO₄. They form fine crystals with large pores producing a large surface area. The Al³⁺ in the structure distorts the lattice to form different structures and pore shapes and sizes. It also causes a negative charge so must be balanced by another cation (can be an acidic proton). The ions and water in the pores can move quite freely allowing for ion exchange.

Question 19

Describe the selectivity and uses of zeolites.

Answer 19

The size of the channels can preferentially absorb products that can fit into its channels better, such as certain isomers, this can then cause further reactions to occur to the slower reacting isomers. As a result, the main use is adsorptions, separations and ion exchange. They are often used in the catalytic cracking of hydrocarbons or acid catalysis. They are quite expensive however.

Question 20

Describe the 5 general steps for a reaction on a solid surface.

Answer 20

1. Substrate diffusion to near the solid.
2. Adsorption to the surface - thermodynamically favourable, cannot be too strong or weak.
3. Surface diffusion to the species it will react with - low energy barrier.
4. Surface reaction to form absorped product.
5. Product desorption and diffusion.

Question 21

Describe the type of adsorption onto solid surfaces. How is this studied?

Answer 21

Physisorption - involves weak VdW interactions and electrostatic polarisation effects. No new chemical bonds are formed. Chemisorption - chemical bonds formed to the surface. May cause a bond breaking within the substrate. About 10x stronger than physisorption.

The adsorption is studied using a probe molecule.

Question 22

Describe the factors that affect the selectivity of a heterogeneous catalyst.

Answer 22

The selectivity is often lower than for homogeneous catalysts as multiple active sites are present which may have different reactivity. The strength of adsorption onto the surface can affect the selectivity.

Question 23

List the factors affecting the catalytic activity of solid catalysts and describe the types of active sites.

Answer 23

• Lifetime
• Surface area and number of pores
• Particle size
• Strength of surface adsorption

One type of active site is an acidic or basic site, and the other is metal particles which can be studied with IR of the binding of CO.

Question 24

Define and describe promotors and poisons.

Answer 24

Promotors are substances that enhance the activity of a catalyst when added in a small amount, typically an electropositive element. They may also increase selectivity by enhancing a specific reactions rate or increase the catalyst lifetime.

Poisons tend to be electronegative elements which block the active sites of a catalyst, but may preferentially block an undesirable pathway.

Question 25

Compare homogeneous and heterogeneous catalysts over a number of factors.

Answer 25

1. Recoverabiliy and reuseability - Homo are hard to seperate and remove and unlikely to be recovered and reused directly. Hetero are easy to seperate and often suitible for reuse.
2. Selectivity - Homo can be highly selective whereas Hetero can be variable.
3. Activity - Homo tend to have a higher activity.
4. Tunability - Homo can be highly tuned whereas Hetero depends on the reaction.
5. Ease to study - Homo are often easy to study, Hetero are often very difficult.
6. Sensitivity - Homo is often sensitive to oxygen, water and temperature, Hetero tend to be more robust
7. Suitable for exothermic reactions - Homo is suitable as the heat can be easily dissapated whereas Hetero are not suitable.

Question 26

Describe how nanoparticles can act as catalysts and what must be considered when they are in a catalyst system. Describe the activity of a nanoparticle catalyst.

Answer 26

They can be made purposely or may form after a complex breaks down and the metal atoms aggregate together. The catalyst should be studied to determine if the catalyst is the actual nanoparticle, or just the free metal ions being released from the nanoparticles. This can be used to tune the activity.

The larger the particle, the lower the catalytic activity due to the surface area decrease. The shape of the nanoparticle can also affect the tpe of catalysis that will occur.

Question 27

Describe how you test for the presence and activity of nanoparticles.

Answer 27

They can be detected by light scattering or electron microscopy experiments. This can determine the size and shape distribution of the nanoparticles.

Kinetic studies show that when nanoparticles are the active catalysts, there can be a induction period while the nanoparticles form, after which there is the catalysed reaction. This is shown by a sigmoidal curve.

Mercury poisoning will deactivate heterogeneous catalysts - can be used to detect if nanoparticles are being formed from the homogeneous catalyst.

Question 28

Describe and give the benefits of hybrid catalysts.

Answer 28

Attaching a catalyst to a solid surface such as silica or alumina means that the homogeneous catalysts can be seperated like a heterogeneous catalyst. However they can be limited as during seperation the catalyst can leech of the solid support.

Question 29

Generally describe acid catalysis.

Answer 29

An inexpensive, widely used type of catalysis. The acid can be either classified as protic or non-protic. They typically have been homogeneous but modern catalysts are often heterogeneous.

Question 30

Give the mechanism of proton transfer with the relative rates and describe the differences between specific and general acid catalysis.

Answer 30

Step 1: Formation of a hydrogen bond between the base and proton - diffusion controlled.

Step 2: Bond and hydrogen bond switch between the base and conjugate base - rate depends on the atom that is being transferred to, faster for more nucleophilic atoms.

Step 3: Dissociation of the hydrogen bond to form the product - diffusion controlled.

Specifc acid catalysis is where the proton is transferred before the RDS and usually occurs under strongly acidic conditions.

General acid catalysis is where the proton is transferred during the RDS and usually occurs under weakly acidic conditions.

Question 31

Give the factors that reduce the rate of proton transfer and state the Bronsted catalysis law.

Answer 31

1. Intramolecular hydrogen bonding of the acid.
2. Requirement to electronically reorganise to stabilise the charge.

Under ideal conditions (dilute with low concentration solutions), there is good correlation between acid strength and the rate of acid-catalysed reactions.

Question 32

Describe how acidity is measured, when this becomes a problem and how it is corrected for. Define and describe how to form a super acid.

Answer 32

pH = -log[H⁺] pK_a = -logK_a

This only measures with respect to water as a base, which misrepresents strong acids. The Hammett acidity function, H₀, which measures the tendancy of a solution to protonate a neutral base. This can describe more concentrated solutions of acid and super acids.

A super acid is any acid more acidic than 18M sulfuric acid, made by dissolving a powerful lewis acid (SbF₅) in a bronsted acid such as HF.

Question 33

How can use of organic solvents affect the rate of catalysis for acid catalysts?

Answer 33

Water can stablise acids which means the acid is a less effective catalyst. Using an organic solvent means that there is a smaller activation energy as the protonated forms are less stabilised and the reaction will proceed faster.

Question 34

What must be considered when using non-protic (Lewis) acid catalysts? Why may they be used specifically?

Answer 34

Lewis acids are lone pair acceptors such as AlCl₃. They are much bulkier than protons so steric behaviour must be considered. AlCl₃ can remove water from reactions when used in large quantities by formining Al(OH)₃ and HCl. However using this in reactions will cause large amounts of waste. This can be substituted with HF.

Question 35

Give the benefits and limitations of zeolite acid catalysts.

Answer 35

Benefits: Safer to handle, less sensitive to water, can be recovered (soild).

Limitations: Less reactive, higher temperatures/longer reactions may be required, can become poisoned by large molecules.

Question 36

List the information about a catalyst that it would be useful to know and how you could gather that information.

Answer 36

• Surface area - Use a probe molecule which can form a monolayer such as N₂, measured as a function of pressure
• Number of active sites - Use probe molecule to fit into each active site that can then be detected
• Bulk and surface structures - power x-ray differaction and specific chemisorption respectively
• Particle size - Dynamic light scattering
• Nature of the active site
• The true catalyitic species
• How to increase activity
• Mechanism of catalysis
• Rate determining step
• How to improve selectivity