Homogeneous catalysis

In chemistry,homogeneous catalysisiscatalysiswhere the catalyst is in same phase as reactants, principally by a soluble catalyst a in solution. In contrast,heterogeneous catalysisdescribes processes where the catalysts and substrate are in distinct phases, typically solid-gas, respectively.The term is used almost exclusively to describe solutions and implies catalysis byorganometallic compounds.Homogeneous catalysis is an established technology that continues to evolve. An illustrative major application is the production ofacetic acid.Enzymes are examples of homogeneous catalysts.

Examples

Aconstrained geometry complex.Such precatalysts are used for the production ofpolyolefinssuch aspolyethyleneandpolypropylene.

Acid catalysis

Main article:acid catalysis

Theprotonis a pervasive homogeneous catalystbecause water is the most common solvent. Water forms protons by the process ofself-ionization of water.In an illustrative case, acids accelerate (catalyze) thehydrolysisofesters:

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 catalystWilkinson's catalyst.

Hydrogenation and related reactions

A prominent class of reductive transformations arehydrogenations.In this process, H₂added 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 ofcarbonylation,involves the addition of H and "C(O)H" across a double bond. This process is almost exclusively conducted with solublerhodium- andcobalt-containing complexes.

A related carbonylation is the conversion of alcohols to carboxylic acids.MeOHandCOreact in the presence of homogeneous catalysts to giveacetic acid,as practiced in theMonsanto processandCativa processes.Related reactions includehydrocarboxylationandhydroesterifications.

Polymerization and metathesis of alkenes

A number of polyolefins, e.g. polyethylene and polypropylene, are produced from ethylene and propylene byZiegler-Natta catalysis.Heterogeneous catalysts dominate, but many soluble catalysts are employed especially for stereospecific polymers.Olefin metathesisis usually catalyzed heterogeneously in industry, but homogeneous variants are valuable in fine chemical synthesis.

Oxidations

Homogeneous catalysts are also used in a variety of oxidations. In theWacker process,acetaldehyde is produced frometheneandoxygen.Many non-organometallic complexes are also widely used in catalysis, e.g. for the production ofterephthalic acidfromxylene.Alkenes are epoxidized and dihydroxylated by metal complexes, as illustrated by theHalcon processand theSharpless dihydroxylation.

Enzymes (including metalloenzymes)

Enzymesare homogeneous catalysts that are essential for life but are also harnessed for industrial processes. A well-studied example iscarbonic anhydrase,which catalyzes the release of CO₂into the lungs from the bloodstream. Enzymes possess properties of both homogeneous and heterogeneous catalysts. As such, they are usually regarded as a third, separate category of catalyst. Water is a common reagent in enzymatic catalysis. Esters and amides are slow to hydrolyze in neutral water, but the rates are sharply affected bymetalloenzymes,which can be viewed as large coordination complexes. Acrylamide is prepared by the enzyme-catalyzed hydrolysis ofacrylonitrile.US demand foracrylamidewas 253,000,000 pounds (115,000,000 kg) as of 2007.

Advantages and disadvantages

Advantages

• Homogeneous catalysts are generally more selective than heterogeneous catalysts.
• For exothermic processes, homogeneous catalysts dump heat into the solvent.
• Homogeneous catalysts are easier to characterize precisely, so their reaction mechanisms are amenable to rational manipulation.

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. Somepincer-based catalysts,however, operate near 200 °C.

See also

• Concurrent tandem catalysis