Homogeneous catalysis

Source: Wikipedia, the free encyclopedia.

In chemistry, homogeneous catalysis is

organometallic compounds. Homogeneous catalysis is an established technology that continues to evolve. An illustrative major application is the production of acetic acid. Enzymes are examples of homogeneous catalysts.[2]

Examples

A constrained geometry complex. Such precatalysts are used for the production of polyolefins such as polyethylene and polypropylene.[3]

Acid catalysis

Main article: acid catalysis

The proton is a pervasive homogeneous catalyst[4] 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

At neutral pH, aqueous solutions of most esters do not hydrolyze at practical rates.

Transition metal-catalysis

Mechanism for the hydrogenation of an alkene catalyzed by the homogeneous catalyst Wilkinson's catalyst.

Hydrogenation and related reactions

A prominent class of reductive transformations are hydrogenations. In this process, H2 added to unsaturated substrates. A related methodology, transfer hydrogenation, involves by transfer of hydrogen from one substrate (the hydrogen donor) to another (the hydrogen acceptor). Related reactions entail "HX additions" where X = silyl (hydrosilylation) and CN (hydrocyanation). Most large-scale industrial hydrogenations – margarine, ammonia, benzene-to-cyclohexane – are conducted with heterogeneous catalysts. Fine chemical syntheses, however, often rely on homogeneous catalysts.

Carbonylations

Hydroformylation, a prominent form of carbonylation, involves the addition of H and "C(O)H" across a double bond. This process is almost exclusively conducted with soluble rhodium- and cobalt-containing complexes.[5]

A related carbonylation is the conversion of alcohols to carboxylic acids.

hydroesterifications
.

Polymerization and metathesis of alkenes

A number of polyolefins, e.g. polyethylene and polypropylene, are produced from ethylene and propylene by

Ziegler-Natta catalysis. Heterogeneous catalysts dominate, but many soluble catalysts are employed especially for stereospecific polymers.[6] Olefin metathesis is usually catalyzed heterogeneously in industry, but homogeneous variants are valuable in fine chemical synthesis.[7]

Oxidations

Homogeneous catalysts are also used in a variety of oxidations. In the

Sharpless dihydroxylation
.

Enzymes (including metalloenzymes)

metalloenzymes, which can be viewed as large coordination complexes. Acrylamide is prepared by the enzyme-catalyzed hydrolysis of acrylonitrile.[8] US demand for acrylamide
was 253,000,000 pounds (115,000,000 kg) as of 2007.

Advantages and disadvantages

Advantages

Disadvantages

  • The separation of homogeneous catalysts from products can be challenging. In some cases involving high activity catalysts, the catalyst is not removed from the product. In other cases, organic products are sufficiently volatile that they can be separated by distillation.
  • Homogeneous catalyst have limited thermal stability compared to heterogeneous catalysts. Many organometallic complexes degrade <100 °C. Some
    pincer-based catalysts, however, operate near 200 °C.[10]

See also

References

  1. doi:10.1351/goldbook.C00876
  2. OCLC 739118524
    ..
  3. PMID 26151395
    .
  4. OCLC 1066192105
    .
  5. ISBN 9783527328970
    .
  6. ISBN 978-3-527-29390-2
  7. ISBN 9783527328970
    .
  8. ISBN 978-3527306732
    .
  9. ^ G. O. Spessard and G. L. Miessler "Organometallic Chemistry", Prentice Hall, Upper Saddle River, NJ, 1997, pp. 249-251.
  10. PMID 22584036
    .
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