Homogeneous catalysis

In chemistry, homogeneous catalysis is catalysis in a solution by a soluble catalyst. Strictly speaking, homogeneous catalysis are catalytic reactions where the catalyst is in the same phase as the reactants, so homogeneous catalysis applies to reactions in the gas phase and even in a solid. Heterogeneous catalysis is the alternative to homogeneous catalysis, where the catalysis occurs at the interface of two phases, typically gas-solid.^[1] The term is used almost exclusively to describe solutions and it is often implies catalysis by organometallic compounds. The area is one of intense research and many practical apprehended applications, e.g., the production of acetic acid. Enzymes are examples of homogeneous catalysts.^[2]

Examples

Acid catalysis

The proton is the most pervasive homogeneous catalyst ^[3] because water is the most common solvent. Water forms protons by the process of self-ionization of water. In an illustrative case, acids accelerate (catalyze) the hydrolysis of esters:

CH₃CO₂CH₃ + H₂O $\overrightarrow{\leftarrow}$ CH₃CO₂H + CH₃OH

In the absence of acids, aqueous solutions of most esters do not hydrolyze at practical rates.

Organometallic chemistry

Processes that utilize soluble organometallic compounds as catalysts fall under the category of homogeneous catalysis, as opposed to processes that use bulk metal or metal on a solid support, which are examples of heterogeneous catalysis. Some well-known examples of homogeneous catalysis include hydroformylation and transfer hydrogenation, as well as certain kinds of Ziegler-Natta polymerization and hydrogenation.^[4] Homogeneous catalysts has also been used in a variety of industrial processes such as the Wacker process Acetaldehyde (conversion of ethylene to acetaldehyde) as well as the Monsanto process and the Cativa process for the conversion of MeOH and CO to acetic acid.

Many non-organometallic complexes are also widely used in catalysis, e.g. for the production of terephthalic acid from xylene.

Other forms of homogeneous catalysis

Enzymes are homogeneous catalysts that are essential for life but are also harnessed for industrial processes. A well studied example carbonic anhydrase, which catalyzes the release of CO₂ into the lungs from the blood stream.

Contrast with heterogeneous catalysis

Homogeneous catalysis differs from heterogeneous catalysis in that the catalyst is in a different phase than the reactants. One example of heterogeneous catalysis is the petrochemical alkylation process, where the liquid reactants are immiscible with a solution containing the catalyst. Heterogeneous catalysis offers the advantage that products are readily separated from the catalyst, and heterogeneous catalysts are often more stable and degrade much slower than homogeneous catalysts. However, heterogeneous catalysts are difficult to study, so their reaction mechanisms are often unknown.^[5]

Enzymes possess properties of both homogeneous and heterogeneous catalysts. As such, they are usually regarded as a third, separate category of catalyst.

References

1. http://goldbook.iupac.org/C00876.html
2. P. W. N. M. van Leeuwen and J. C. Chadwick "Homogeneous Catalysts" Wiley-VCH, Weinheim, 2011.
3. R.P. Bell "The Proton in Chemistry", Chapman and Hall, London, 1973. doi: 10.1016/0022-2860(76)80186-X
4. Elschenbroich, C. ”Organometallics” (2006) Wiley-VCH: Weinheim. ISBN 978-3-527-29390-2
5. G. O. Spessard and G. L. Miessler "Organometallic Chemistry", Prentice Hall, Upper Saddle River, NJ, 1997, pp. 249-251.