Homogeneous catalysis

In chemistry, homogeneous catalysis is catalysis in a solution by a soluble catalyst. Strictly speaking, homogeneous catalysis refers to catalytic reactions where the catalyst is in the same phase as the reactants. Homogeneous catalysis applies to reactions in the gas phase and even in solids. 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 often implies catalysis by organometallic compounds. The area is one of intense research and many practical 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 ⇌ CH3CO2H + CH3OH

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

Transition metal-catalysis

Many transformations classically considered homogeneous catalysis utilize soluble coordination and organometallic compounds as catalysts. These catalysts differ from solid catalysts, which are traditionally associated with heterogeneous catalysis. Homogeneous catalysis can be subdivided according to their substrates or their reductive/oxidative character.

Reductions

A prominent class of reductive transformations involve hydrogenation. H2 is added to unsaturated substrates directly using hydrogen gas or indirectly by transferring hydrogen from substrates. The latter is called transfer hydrogenation. Related reactions entail "HX additions" where X = silyl (hydrosilylation) and CN (hydrocyanation).

Carbonylations

Hydroformylation is a prominent form of carbonylation, involving the addition of H and "C(O)H" across a double bond. MeOH and CO react in the presence of homogeneous catalysts to give acetic acid, as practiced in the Monsanto process and Cativa processes. Carbon monoxide is a common substrate in homogeneous catalysis and diverse reactions have been commercialized, including hydrocarboxylation and hydroesterifications.

Polymerization and metathesis of alkenes

A number of polyolefins are produced by Ziegler-Natta catalysis. Alkenes such as ethylene, propylene, and butadiene are common substrates.[4] Olefin metathesis is usually practiced heterogeneously, but homogeneous variants are known.

Oxidations

Homogeneous catalysts are also used in a variety of oxidations. Is the Wacker process, acetaldehyde is produced from ethylene to oxygen. Many non-organometallic complexes are also widely used in catalysis, e.g. for the production of terephthalic acid from xylene. Alkenes are epoxidized and dihydroxylated by metal complexes, as illustrated by the Halcon Process.

Hydration and hydrolysis

Water is a common reagent in homogeneous catalysis. Esters and amides are slow to hydrolyze in neutral water, but the rates are sharply affected by metal complexes. The fastest and most selective catalysts for such reactions are often metalloenzymes, which can be viewed as large coordination complexes. The hydration of nitriles, alkenes, and alkynes are all catalyzed by metal complexes.

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 is 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.

See also

References

  1. ↑ http://goldbook.iupac.org/C00876.html
  2. ↑ P. W. N. M. van Leeuwen and J. C. Chadwick "Homogeneous Catalysts: Activity - Stability - Deactivation" Wiley-VCH, Weinheim, 2011. Online ISBN 9783527635993.
  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.
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.