homogeneous catalysis

File:ConstrainedGeomCmpx.png. Such precatalysts are used for the production of polyolefins such as polyethylene and polypropylene.{{cite journal|doi=10.1021/acs.accounts.5b00065|title=Development of Group IV Molecular Catalysts for High Temperature Ethylene-α-Olefin Copolymerization Reactions|year=2015|last1=Klosin|first1=Jerzy|last2=Fontaine|first2=Philip P.|last3=Figueroa|first3=Ruth|journal=Accounts of Chemical Research|volume=48|issue=7|pages=2004–2016|pmid=26151395|doi-access=free}}]]

{{main|acid catalysis}}

The proton is a pervasive homogeneous catalyst{{Cite book|last=Bell|first=R. P.|url=https://books.google.com/books?id=oefcBwAAQBAJ|title=The Proton in Chemistry|date=11 November 2013|publisher=Springer Science & Business Media|isbn=978-1-4757-1592-7|location=New York, NY|language=en|oclc=1066192105}} 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 {{eqm}} CH₃CO₂H + CH₃OH

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

File:WilkinsonCycleJMBrown.png.]]

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.

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.{{cite book|doi=10.1002/9783527651733.ch2|chapter=Hydroformylation|title=Applied Homogeneous Catalysis with Organometallic Compounds|year=2017|last1=Cornils|first1=Boy|last2=Börner|first2=Armin|last3=Franke|first3=Robert|last4=Zhang|first4=Baoxin|last5=Wiebus|first5=Ernst|last6=Schmid|first6=Klaus|pages=23–90|isbn=9783527328970}}

A related carbonylation is the conversion of alcohols to carboxylic acids. MeOH and CO react in the presence of homogeneous catalysts to give acetic acid, as practiced in the Monsanto process and Cativa processes. Related reactions include hydrocarboxylation and hydroesterifications.

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.Elschenbroich, C. ”Organometallics” (2006) Wiley-VCH: Weinheim. {{ISBN|978-3-527-29390-2}} Olefin metathesis is usually catalyzed heterogeneously in industry, but homogeneous variants are valuable in fine chemical synthesis.{{cite book|doi=10.1002/9783527651733.ch4|chapter=Polymerization and Copolymerization|title=Applied Homogeneous Catalysis with Organometallic Compounds|year=2017|last1=Beckerle|first1=Klaus|last2=Okuda|first2=Jun|last3=Kaminsky|first3=Walter|last4=Luinstra|first4=Gerrit A.|last5=Baier|first5=Moritz C.|last6=Mecking|first6=Stefan|last7=Ricci|first7=Giovanni|last8=Leone|first8=Giuseppe|last9=Mleczko|first9=Leslaw|last10=Wolf|first10=Aurel|last11=Grosse Böwing|first11=Alexandra|pages=191–306|isbn=9783527328970}}

Homogeneous catalysts are also used in a variety of oxidations. In the Wacker process, acetaldehyde is produced from ethene and 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 and the Sharpless dihydroxylation.

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 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, which can be viewed as large coordination complexes. Acrylamide is prepared by the enzyme-catalyzed hydrolysis of acrylonitrile.{{Ullmann|doi=10.1002/14356007.a01_161.pub2|title=Acrylic Acid and Derivatives|year=2003|last1=Ohara|first1=Takashi|last2=Sato|first2=Takahisa|last3=Shimizu|first3=Noboru|last4=Prescher|first4=Günter|last5=Schwind|first5=Helmut|last6=Weiberg|first6=Otto|last7=Marten|first7=Klaus|last8=Greim|first8=Helmut|isbn=3527306730}} US demand for acrylamide was {{Convert|253000000|lbs|kg}} as of 2007.

• 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.G. O. Spessard and G. L. Miessler "Organometallic Chemistry", Prentice Hall, Upper Saddle River, NJ, 1997, pp. 249-251.

• 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, however, operate near 200 °C.{{cite journal|doi=10.1021/ar3000713|title=Alkane Metathesis by Tandem Alkane-Dehydrogenation–Olefin-Metathesis Catalysis and Related Chemistry|year=2012|last1=Haibach|first1=Michael C.|last2=Kundu|first2=Sabuj|last3=Brookhart|first3=Maurice|last4=Goldman|first4=Alan S.|journal=Accounts of Chemical Research|volume=45|issue=6|pages=947–958|pmid=22584036}}

• Concurrent tandem catalysis

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Category:Catalysis