Homogeneous catalysis

In chemistry, homogeneous catalysis is a sequence of reactions that involve a catalyst in the same phase as the reactants. Most commonly, a homogeneous catalyst is codissolved in a solvent with the reactants.

Examples

The proton is the most pervasive homogeneous catalyst.^[1] Illustrative of acid catalysis is the hydrolysis of esters:

CH₃CO₂CH₃ + H₂O CH₃CO₂H + CH₃OH - with H⁺ catalyst

In the absence of protons provided by the acid, most esters do not hydrolyze at practical rates.

Often, "homogeneous catalysis" is jargon for processes that utilize organometallic compounds or metal phosphine complexes as catalysts. Some well-known examples of such homogeneous catalysis include hydroformylation and transfer hydrogenation, as well as certain kinds of Ziegler-Natta polymerization and hydrogenation.^[2] Many non-organometallic complexes are also widely used in catalysis, e.g. for the production of terephthalic acid from xylene.

Enzymes are homogeneous catalysts that are essential for life but are also harnessed for industrial processes.

Contrast with heterogeneous catalysis

Homogeneous catalysis is the opposite of heterogeneous catalysis, where the catalyst is in a different phase, usually a solid catalyst interacting with gaseous or solution of substrate. Because the liquid reactants and the solution of the catalyst are immiscible, the petrochemical alkylation process features heterogeneous catalysis. Heterogeneous catalysis offers the advantage that products are readily separated from the catalyst.

References

1. ^ R.P. Bell "The Proton in Chemistry", Chapman and Hall, London, 1973. doi: 10.1016/0022-2860(76)80186-X
2. ^ Elschenbroich, C. ”Organometallics” (2006) Wiley-VCH: Weinheim. ISBN 978-3-29390-6