In chemistry, homogeneous catalysis is

catalysis Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycl ...

by a soluble catalyst in a solution. Homogeneous catalysis refers to reactions where the catalyst is in the same phase as the reactants, principally in solution. In contrast, heterogeneous catalysis describes 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 by organometallic compounds. Homogeneous catalysis is an established technology that continues to evolve. An illustrative major application is the production of

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main componen ...

. Enzymes are examples of homogeneous catalysts.

Examples

Acid catalysis

The proton is a pervasive homogeneous catalyst 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 Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution, elimination, and solvation reactions in which water is the nucleophile. Biological hydrolysis ...

ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ...

s: :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

hydrogenation Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate org ...

s. In this process, H₂ added to unsaturated substrates. A related methodology,

transfer hydrogenation In chemistry, transfer hydrogenation is a chemical reaction involving the addition of hydrogen to a compound from a source other than molecular . It is applied in laboratory and industrial organic synthesis to saturate organic compounds and r ...

, 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 Carbonylation refers to reactions that introduce carbon monoxide into organic and inorganic substrates. Carbon monoxide is abundantly available and conveniently reactive, so it is widely used as a reactant in industrial chemistry. The term carbo ...

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

MeOH Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a lig ...

and CO react in the presence of homogeneous catalysts to give

, as practiced in the Monsanto process and Cativa processes. Related reactions include

hydrocarboxylation Carbonylation refers to reactions that introduce carbon monoxide into organic and inorganic substrates. Carbon monoxide is abundantly available and conveniently reactive, so it is widely used as a reactant in industrial chemistry. The term carbo ...

and 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.

Olefin metathesis Olefin metathesis is an organic reaction that entails the redistribution of fragments of alkenes (olefins) by the scission and regeneration of carbon-carbon double bonds. Because of the relative simplicity of olefin metathesis, it often creat ...

is 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. Is the

Wacker process The Wacker process or the Hoechst-Wacker process (named after the chemical companies of the same name) refers to the oxidation of ethylene to acetaldehyde in the presence of palladium(II) chloride as the catalyst. This chemical reaction was one of ...

, acetaldehyde is produced from ethene and

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

. Many non-organometallic complexes are also widely used in catalysis, e.g. for the production of

terephthalic acid Terephthalic acid is an organic compound with formula C6H4(CO2H)2. This white solid is a commodity chemical, used principally as a precursor to the polyester PET, used to make clothing and plastic bottles. Several million tonnes are produced an ...

from

xylene In organic chemistry, xylene or xylol (; IUPAC name: dimethylbenzene) are any of three organic compounds with the formula . They are derived from the substitution of two hydrogen atoms with methyl groups in a benzene ring; which hydrogens are sub ...

. Alkenes are epoxidized and dihydroxylated by metal complexes, as illustrated by the

Halcon process In chemistry, the Halcon process refers to technology for the production of propylene oxide by oxidation of propylene with tert-butyl hydroperoxide. The reaction requires metal catalysts, which typically contain molybdenum: :(CH3)3COOH + CH2 ...

and the Sharpless dihydroxylation.

Enzymes (including metalloenzymes)

Enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...

s 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 Acrylonitrile is an organic compound with the formula and the structure . It is a colorless, volatile liquid although commercial samples can be yellow due to impurities. It has a pungent odor of garlic or onions. In terms of its molecular ...

. US demand for acrylamide was 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. 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

References

Catalysis