Homogeneous catalysis

Contribution of homogeneous catalytic process in chemical industry is significantly smaller compared to heterogeneous catalytic process, it is only about 17-20 %. But importance of homogeneous catalysis is increasing significantly. The significance of homogeneous catalysis is growing rapidly particularly in the area of pharmaceutical and polymer industry. Some of the important industrial processes include:

1. Oxidations of Alkenes such as production of acetaldehyde, propylene oxide etc.
2. Polymerization such as production of polyethylene, polypropylene or polyester production.

A new major development in homogeneous catalysis is the application of organometallic complexes as catalysts. The use of organometallic catalysts has revolutionized the homogeneous processes increasing economic viability. Another new area is bio-catalysis involving enzymes catalysts. Enzyme catalysts are highly selective and active for production of fine chemicals, pharmaceuticals etc. Enzyme catalysts are discussed in a separate section.

In homogeneous catalysis, all the reactants and catalysts are present in a single fluid phase and usually in the liquid phase. Homogeneous catalysts are the simple molecules or ions such as HF, H₂SO₄, Mn⁺² as well as complex molecules such as organometallic complexes, macrocyclic compounds and large enzyme molecules.

Advantages

Advantages of homogeneous processes can be summarized as follows:

• In many reactions homogeneous catalysts are more active and/or selective compared to heterogeneous catalysts.
  
  
• In homogeneous catalysis, the catalysts are molecularly dispersed within the fluid. Hence, pore diffusion limitations are absent. However, bulk phase mass transfer limitation may occurs.
  
  
• Catalytic chemistry and mechanism for homogeneous catalysis are better studied and understood. Therefore it is easier to control and manipulate the process parameters.

Disadvantages

However, homogeneous processes are also associated with some major disadvantages which result in limited use of these processes. These disadvantages are summarized below:

• Homogeneous catalysts are stable only in relatively mild conditions which limit their applicability.
  
  
• Since the catalysts are molecularly dispersed in the phase as the reactant, products and solvents, the separation at end of the process is difficult and expensive. In many cases, it is not possible to recover the catalyst.

Types of reactions

Several homogeneous catalytic systems are:

1. Acid base catalysis
2. Catalysis by metal ions
3. Catalysis by organometallic complexes
4. Catalysis by Lewis acids
5. Catalysis by porphyrin complexes
6. Catalysis by enzymes

 

1. Catalysis by acids or bases

Acid –base catalysts are used in the following types of reactions:

1. Condensation
2. Dehydration
3. Hydrolysis
4. Halogenations