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 1/6 th 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 chemicalRankCatalytic Process Sulfuric acid (H 2 SO 4 )1SO 2 oxidation to SO 3 ; heterogeneous Ethylene (CH 2 CH 2 )2 Lime (CaO)3 Ammonia (NH 3 )4N 2 + H 2 ; heterogeneous Sodium hydroxide (NaOH)5 Chlorine (Cl 2 )6Electrocatalysis; heterogeneous Phosphoric acid (H 3 PO 4 )7 Propylene (CH 3 CHCH 2 )8 Sodium carbonate (Na 2 CO 3 )9 1,2-Dichloroethane (ClCH 2 CH 2 Cl)10C 2 H 4 + Cl 2 ; homogeneous Nitric acid (HNO 3 )11NH 3 + O 2 ; heterogeneous Urea (CO(NH 2 ) 2 ) 12 Ammonium nitrate (NH 4 NO 3 ) 13 Benzene (C 6 H 6 )14Petroleum refining; heterogeneous Ethylbenzene (C 6 H 5 CH 2 CH 3 )15Alkylation of benzene; homogeneous Carbon dioxide (CO 2 )16 Vinyl chloride (CH 2 CHCl)17Chlorination of C 2 H 4 ; heterogeneous Styrene (C 6 H 5 CHCH 2 )18Dehydrogenation of ethylbenzene; heterog. Terephthalic acid (1,4-(COOH)C 6 H 4 )19Oxidation of p-xylene; homogeneous Methanol (CH 3 OH)20CO + H 2 ; 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 Catalysis Homogeneous 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 selectivity 2. Ease of heat dissipation from exothermic reactions Disadvantages of homogeneous catalysis on an industrial scale: 1. Scale-up can be costly, difficult, and dangerous 2. Separation is required

Important Homogeneous Catalytic Processes 1. Oxo process: Hydroformylation of alkenes

Important Homogeneous Catalytic Processes 2. Wacker-Hoechst process: Oxidation of alkenes

Important Homogeneous Catalytic Processes 3. Monsanto process: Carbonylation of methanol to acetic acid 4. Hydrocyanation of butadiene to adiponitrile 5. Oligomerization of ethylene

Important Homogeneous Catalytic Processes 6. Olefin metathesis: Alkene dismutation 7. Asymmetric hydrogenation of prochiral alkenes 8. Cyclotrimerization of acetylene

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, ML 4 complexes of Pd(II), Pt(II) and Rh(I) are commonly used as catalysts. E.g., Wilkinson’s catalyst

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

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.

4. Oxidation and Reduction During a catalytic cycle, metal atoms frequently alternate between two oxidation states: Cu 2+ /Cu + Co 3+ /Co 2+ Mn 3+ /Mn 2+ Pd 2+ /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

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 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-5 Na 3 (AlSi 31 O 64 ) 3 16H 2 O Typical inert supports Silica gel, SiO 2 microcrystalline γ-alumina, Al 2 O 3

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 Al 3+ acts as a Lewis acid site. Exposed O 2- acts as a Lewis base site.

Heterogeneous Catalysis: Considerations C. 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 H 2 O, reduction of M with H 2(g), etc.

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