Lecture Topic 5: Catalytic Chemistry in Industry

Premise: Catalysts are extremely important in industrial organic synthesis.

Goal: Students should be able to

1) describe the nature of a catalyst

2) describe the important reaction types involved in homogeneous catalysis

3) describe the important considerations involved in heterogeneous catalyst design

What is a Catalyst?

A catalyst is a substance that increases the rate at which a chemical reaction approaches equilibrium, but is not consumed in the process.

Thus a catalyst affects the ,

but does not affect the of a reaction.

Most catalysts used in industrial and research laboratories are inorganic (often organometallic) compounds.

Biochemical catalysts are known as

Why are Catalysts Important?

It is estimated that catalysts contribute 1/6th of the value of ALL manufactured goods in industrialized countries!

Catalysts can:• make a reaction possible under achievable conditions

• reduce the necessity of expensive & dangerous conditions

• generate high yields and high product purity

• reduce the amount of side-product and waste created

• generate non-racemic mixtures of enantiomers

• make a chemical process “greener”

Synthetic chemical Rank Catalytic Process

Sulfuric acid (H2SO4) 1 SO2 oxidation to SO3 ; heterogeneousEthylene (CH2CH2) 2Lime (CaO) 3Ammonia (NH3) 4 N2 + H2 ; heterogeneousSodium hydroxide (NaOH) 5Chlorine (Cl2) 6 Electrocatalysis; heterogeneousPhosphoric acid (H3PO4) 7Propylene (CH3CHCH2) 8Sodium carbonate (Na2CO3) 91,2-Dichloroethane (ClCH2CH2Cl) 10 C2H4 + Cl2 ; homogeneousNitric acid (HNO3) 11 NH3 + O2 ; heterogeneousUrea (CO(NH2)2) 12Ammonium nitrate (NH4NO3) 13Benzene (C6H6) 14 Petroleum refining; heterogeneousEthylbenzene (C6H5CH2CH3) 15 Alkylation of benzene; homogeneousCarbon dioxide (CO2) 16Vinyl chloride (CH2CHCl) 17 Chlorination of C2H4; heterogeneousStyrene (C6H5CHCH2) 18 Dehydrogenation of ethylbenzene; heterog.Terephthalic acid (1,4-(COOH)C6H4) 19 Oxidation of p-xylene; homogeneousMethanol (CH3OH) 20 CO + H2; heterogeneous

How Does a Catalyst Work?

A catalyst provides an alternative mechanism for the chemical reaction, with a lower activation energy.

General Principles of Catalysis

a) Homogeneous vs. Heterogeneous

• catalysts: in the same physical phase as reagents• catalysts: present in a different phase (solid state)

b) Catalytic efficiency

• Homogeneous catalysts: efficiency expressed as turnover freq., N• Heterogeneous catalysts: rate of change of reagents is important

c) Selectivity

• a “selective” catalyst yields a high proportion of desired product with a minimal amount of side-products.

Homogeneous CatalysisHomogeneous catalysts are more attractive for study in research labs because:

a) the mechanism is accessible to detailed study (i.e., the rate data is easier to interpret)

b) the species in solution are more easily characterized

Advantages of homogeneous catalysis on an industrial scale:

1. High selectivity2. Ease of heat dissipation from exothermic reactions

Disadvantages of homogeneous catalysis on an industrial scale:

1. Scale-up can be costly, difficult, and dangerous2. Separation is required

Important Homogeneous Catalytic Processes

1. Oxo process: Hydroformylation of alkenes

RHC CH2 + CO + H2Co(I) or Rh(I)

O

HR

Important Homogeneous Catalytic Processes

2. Wacker-Hoechst process: Oxidation of alkenes

RHC CH2 + O2Pd(II) + Cu(II)

O

H3C RR = H, aldehydeR = CnHn+2, ketone

Important Homogeneous Catalytic Processes

3. Monsanto process: Carbonylation of methanol to acetic acid

4. Hydrocyanation of butadiene to adiponitrile

CH3OH + CO[RhI2(CO)2]

O

H3C OH

5. Oligomerization of ethylene

+ 2 HCNNi(P(OR)3)4 N

N

H2C CH2nNiHL

H2C CH(CH2CH2)n-2CH2CH3

Important Homogeneous Catalytic Processes

6. Olefin metathesis: Alkene dismutation

7. Asymmetric hydrogenation of prochiral alkenes

8. Cyclotrimerization of acetylene

H2C CHCH32WOCl4/AlCl2Et

H2C CH2 + H3CHC CHCH3

HC CH3Ni(acac)2

R

H

NHCOR

COOR+ H2

[Rh(DiPAMP)2]RH2CC* H

NHCOR

COOR

Homogeneous Catalysis:5 Important Reaction Steps

There are 5 types of reactions (and their reverse) which, in combination, account for most homogeneous catalytic cycles involving hydrocarbons.

1. Ligand Coordination and Dissociation

2. Insertion and Elimination

3. Nucleophilic attack on coordinated ligands

4. Oxidation and Reduction

5. Oxidative addition and Reductive elimination

1. Ligand Coordination and Dissociation

Homogeneous catalysis requires that:• reagents be easily coordinated to the metal center• products be easily lost from the coordination sphere

THEREFORE, a metal catalyst must be:• a highly labile metal complex• coordinatively unsaturated

Many square-planar 16e- complexes meet these criteria.

Thus, ML4 complexes of Pd(II), Pt(II) and Rh(I) are commonly used as catalysts.

E.g., Wilkinson’s catalyst

RhPh3P

Cl

PPh3

Ph3P

2. Insertion and Elimination

Insertion : the migration of alkyl (R) or hydride (H) ligands from the metal center to an unsaturated ligand

Elimination: the migration of alkyl (R) or hydride (H) ligands from a ligand to the metal center

e.g., β-hydride elimination

M C

R

OL + M C

L

R

O

M

H

CH2

CH2M CH2CH3

M CH2 CH3 M CH2

H CH2

M

H

CH2

CH2M Sol

H-C2H4

+Sol

3. Nucleophilic Attack on Coordinated Ligands

A (+)ve charge on a metal-ligand complex tends to activate the coordinated C atom toward attack by a nucleophile.

Pd

L

LCL CPd

L

LL

C

C

HH

H R

2+OH2 H

H

H

R

OH

+

+ H+

4. Oxidation and Reduction

During a catalytic cycle, metal atoms frequently alternate between two oxidation states:

Cu2+/Cu+ Co3+/Co2+ Mn3+/Mn2+ Pd2+/Pd

Catalytic Oxidation: generating alcohols and carboxylic acids

The metal atom 1) initiates the formation of the radical R• 2) contributes to the formation of R-O-O• radical

R H + Co(III) R + H + Co(II)

R + O2 R O O R HR O O H + R

+ Co(II)R O O H + Co(III)OHR O + Co(III)R O O H R O O + H + Co(II)AND

5. Oxidative Addition & Reductive Elimination

Oxidative Addition of a molecule AX to a complex:

• dissociation of the A—X bond• coordination of the two fragments to the metal center

M

L

LLL + AX M

L

LX

A

L

L

Reductive Elimination is the reverse process:

• formation of a A—X bond• dissociation of the AX molecule from the coordination sphere

Heterogeneous Catalysis

1) Solid catalysts are robust at high T, P.

2) No solvents are required.

3) No separation of the product from the catalyst is required.

Heterogeneous catalysts are more widely used in industry because:

Classes of Heterogeneous Catalyst

There are 2 classes of Heterogeneous catalyst:

1. Uniform catalyst is a high surface area material.

2. Multiphasic catalyst is deposited on a high surface area material.

e.g., ZSM-5Na3(AlSi31O64)3•16H2O

Typical inert supports

Silica gel, SiO2

microcrystalline γ-alumina, Al2O3

Heterogeneous Catalysis: Considerations

A. Surface area and porosity

Special measures must be taken to ensure that reagent molecules achieve contact with catalytic sites.

B. Surface acidic and basic sites

Surface OH groups act as weak Brønsted acids.Exposed Al3+ acts as a Lewis acid site.Exposed O2- acts as a Lewis base site.

Heterogeneous Catalysis: ConsiderationsC. Surface metal sites

A supported metal particle (25 diameter) has ~40% of its atoms exposed on the surface.

The metal atoms are capable of forming a variety of bonds.

Question: Why is a support used instead of just finely divided metal powder?

D. Chemisorption and desorption

“Activation”: desorption of H2O, reduction of M with H2(g), etc.

Al

NH H

H

Pt

C

O

Pt

C

Pt

O

Pt

H

Pt

H

Pt

H2C

Pt

H

CH3

Fe

N

Fe

N

FeFe Fe

Heterogeneous Catalysts: Active Sites

The active sites of heterogeneous catalysts are NOT uniform.

Many types of sites are exposed on the surface of a material.Each type of site may serve a unique function in the catalysis.

e.g., crystalline solids

Irregularities (e.g., steps) Many exposed planes