In chemistry,homogeneous catalysis iscatalysis where the catalyst is in same phase as reactants, principally by a soluble catalyst in a solution. In contrast,heterogeneous catalysis describes processes where the catalysts and substrate are in distinct phases, typically solid and gas, respectively.[1] 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.[2]
Theproton is a pervasive homogeneous catalyst[4] because water is the most common solvent. Water forms protons by the process ofself-ionization of water. In an illustrative case, acids accelerate (catalyze) thehydrolysis ofesters:
At neutral pH, aqueous solutions of most esters do not hydrolyze at practical rates.
A prominent class of reductive transformations arehydrogenations. 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.
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.[5]
A related carbonylation is the conversion of alcohols to carboxylic acids.MeOH andCO react in the presence of homogeneous catalysts to giveacetic acid, as practiced in theMonsanto process andCativa processes. Related reactions includehydrocarboxylation andhydroesterifications.
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.[6]Olefin metathesis is usually catalyzed heterogeneously in industry, but homogeneous variants are valuable in fine chemical synthesis.[7]
Homogeneous catalysts are also used in a variety of oxidations. In theWacker process, acetaldehyde is produced fromethene andoxygen. Many non-organometallic complexes are also widely used in catalysis, e.g. for the production ofterephthalic acid fromxylene. Alkenes are epoxidized and dihydroxylated by metal complexes, as illustrated by theHalcon process and theSharpless dihydroxylation.
Enzymes are 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 CO2 into 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.[8] US demand foracrylamide was 253,000,000 pounds (115,000,000 kg) as of 2007.
