# Homogeneous catalysis > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Homogeneous_catalysis > Markdown URL: https://mediated.wiki/source/Homogeneous_catalysis.md > Source: https://en.wikipedia.org/wiki/Homogeneous_catalysis > Source revision: 1351573288 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) In chemistry, **homogeneous catalysis** is [catalysis](/source/Catalysis) where the catalyst is in same phase as reactants, principally by a soluble catalyst in a solution. In contrast, [heterogeneous catalysis](/source/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 by [organometallic compounds](/source/Organometallic_compound). Homogeneous catalysis is an established technology that continues to evolve. An illustrative major application is the production of [acetic acid](/source/Acetic_acid). Enzymes are examples of homogeneous catalysts.[2] ## Examples ### Acid catalyst Main article: [acid catalysis](/source/Acid_catalysis) The [proton](/source/Proton) is a pervasive homogeneous catalyst[3] because water is the most common solvent. Water forms protons by the process of [self-ionization of water](/source/Self-ionization_of_water). In an illustrative case, acids accelerate (catalyze) the [hydrolysis](/source/Hydrolysis) of [esters](/source/Ester): - CH3CO2CH3 + H2O CH3CO2H + CH3OH At neutral pH, aqueous solutions of most esters do not hydrolyze at practical rates. ### Transition metal-catalysis #### Hydrogenation and related reactions A prominent class of reductive transformations are [hydrogenations](/source/Hydrogenation). In this process, H2not added to unsaturated substrates. A related methodology, [transfer hydrogenation](/source/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](/source/Hydrosilylation)) and CN ([hydrocyanation](/source/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](/source/Hydroformylation), a prominent form of [carbonylation](/source/Carbonylation), involves the addition of H and "C(O)H" across a double bond. This process is almost exclusively conducted with soluble [rhodium](/source/Rhodium)- and [cobalt](/source/Cobalt)-containing complexes.[4] A related carbonylation is the conversion of alcohols to carboxylic acids. [MeOH](/source/Methanol) and [CO](/source/Carbon_monoxide) react in the presence of homogeneous catalysts to give [acetic acid](/source/Acetic_acid), as practiced in the [Monsanto process](/source/Monsanto_process) and [Cativa processes](/source/Cativa_process). Related reactions include [hydrocarboxylation](/source/Hydrocarboxylation) and [hydroesterifications](/source/Hydroesterification). #### Polymerization and metathesis of alkenes A number of polyolefins, e.g. polyethylene and polypropylene, are produced from ethylene and propylene by [Ziegler-Natta catalysis](/source/Ziegler-Natta_catalysis). Heterogeneous catalysts dominate, but many soluble catalysts are employed especially for stereospecific polymers.[5] [Olefin metathesis](/source/Olefin_metathesis) is usually catalyzed heterogeneously in industry, but homogeneous variants are valuable in fine chemical synthesis.[6] #### Oxidations Homogeneous catalysts are also used in a variety of oxidations. In the [Wacker process](/source/Wacker_process), acetaldehyde is produced from [ethene](/source/Ethene) and [oxygen](/source/Oxygen). Many non-organometallic complexes are also widely used in catalysis, e.g. for the production of [terephthalic acid](/source/Terephthalic_acid) from [xylene](/source/Xylene). Alkenes are epoxidized and dihydroxylated by metal complexes, as illustrated by the [Halcon process](/source/Halcon_process) and the [Sharpless dihydroxylation](/source/Sharpless_dihydroxylation). ### Enzymes (including metalloenzymes) [Enzymes](/source/Enzyme) are homogeneous catalysts that are essential for life but are also harnessed for industrial processes. A well-studied example is [carbonic anhydrase](/source/Carbonic_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 by [metalloenzymes](/source/Metalloenzyme), which can be viewed as large coordination complexes. Acrylamide is prepared by the enzyme-catalyzed hydrolysis of [acrylonitrile](/source/Acrylonitrile).[7] US demand for [acrylamide](/source/Acrylamide) was 253,000,000 pounds (115,000,000 kg) as of 2007. ## Advantages and disadvantages ### Advantages - Homogeneous catalysts are often more selective than heterogeneous catalysts. - For exothermic processes, homogeneous catalysts dump heat into the solvent. - Homogeneous catalysts are easier to characterize, making their reaction mechanisms amenable to rational manipulation.[8] ### 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, distillation can extract volatile organic products. - Homogeneous catalysts have limited thermal stability compared to heterogeneous catalysts. Many organometallic complexes degrade below 100 °C. Some [pincer-based catalysts](/source/Pincer_complex), however, operate near 200 °C.[9] ## See also - [Concurrent tandem catalysis](/source/Concurrent_tandem_catalysis) ## References 1. van Leeuwen & Chadwick, J. C. (2011). [*Homogeneous Catalysts: Activity - Stability - Deactivation*](https://books.google.com/books?id=bvKkcQAACAAJ). Wiley-VCH, Weinheim. ISBN 9783527635993. [OCLC 739118524](https://www.worldcat.org/oclc/739118524). 1. Bell, R. P. (11 November 2013). [*The Proton in Chemistry*](https://books.google.com/books?id=oefcBwAAQBAJ). New York, NY: Springer Science & Business Media. ISBN 978-1-4757-1592-7. [OCLC 1066192105](https://www.worldcat.org/oclc/1066192105) 1. Cornils, Boy; Börner, Armin; Franke, Robert; Zhang, Baoxin; Wiebus, Ernst; Schmid, Klaus (2017). "Hydroformylation". *Applied Homogeneous Catalysis with Organometallic Compounds*. pp. 23–90. [doi:10.1002/9783527651733.ch2](https://doi.org/10.1002/9783527651733.ch2). ISBN 9783527328970. 1. Elschenbroich, C. ”Organometallics” (2006) Wiley-VCH: Weinheim. ISBN 978-3-527-29390-2 1. Beckerle, Klaus; Okuda, Jun; Kaminsky, Walter; Luinstra, Gerrit A.; Baier, Moritz C.; Mecking, Stefan; Ricci, Giovanni; Leone, Giuseppe; Mleczko, Leslaw; Wolf, Aurel; Grosse Böwing, Alexandra (2017). "Polymerization and Copolymerization". *Applied Homogeneous Catalysis with Organometallic Compounds*. pp. 191–306. [doi:10.1002/9783527651733.ch4](https://doi.org/10.1002/9783527651733.ch4). ISBN 9783527328970. 1. G. O. Spessard and G. L. Miessler "Organometallic Chemistry", Prentice Hall, Upper Saddle River, NJ, 1997, pp. 249-251. 1. Haibach, Michael C.; Kundu, Sabuj; Brookhart, Maurice; Goldman, Alan S. (2012). "Alkane Metathesis by Tandem Alkane-Dehydrogenation–Olefin-Metathesis Catalysis and Related Chemistry". *Accounts of Chemical Research*. **45** (6): 947–958. [doi:10.1021/ar3000713](https://doi.org/10.1021/ar3000713). [PMID 22584036](https://pubmed.ncbi.nlm.nih.gov/22584036) --- Adapted from the Wikipedia article [Homogeneous catalysis](https://en.wikipedia.org/wiki/Homogeneous_catalysis) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Homogeneous_catalysis?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.