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Homogeneous catalysis using transition metal complexes is an area of research that has grown enormously in recent years. Many amazing catalytic discoveries have been reported by researchers both in industry and in academia. In chemistry, homogeneous catalysis is catalysis in a solution by a soluble catalyst. Homogeneous catalysis refers to catalytic reactions where the catalyst is in the same phase as the reactants. Homogeneous catalysis applies to reactions in the gas phase and even in solids. Heterogeneous catalysis is the alternative to homogeneous catalysis, where the catalysis occurs at the interface of two phases, typically gas-solid. The term is used almost exclusively to describe solutions and often implies catalysis by organometallic compounds. The area is one of intense research and many practical applications, e.g., the production of acetic acid. Enzymes are examples of homogeneous catalysts.

Theory
When materials react to form products, there may be a single reaction or multiple reactions occurring. A single reaction can be called an elementary. For most reactions, the sequence of elementary steps that constitute their mechanism is not known. Sometimes only a single reaction is observed. The reason is that the amount of the intermediate species formed is very small and escapes detection. The speed with which these intermediate species are created and destroyed also makes them difficult to detect. The general scheme is 'reactants --> several intermediate species --> products'.In the case of a non-elementary reaction, the test of the validity of the mechanism proposed is whether the predicted kinetics corresponds to experimental observation. Sometimes reactions may proceed by more than one mechanism, or else their kinetics can be explained by several mechanisms. There are zero-order, first-order, second-order, and higher-order processes.

Advantages
• Homogeneous catalysts are effective at being highly selective towards producing the desired product.

• If a reaction is exothermic, it will release a lot of heat. It is easier to release heat from a solution (as one would do for a homogeneous catalyst) than if one were to use a heterogeneous catalyst, which tends to be an insoluble solid in the solution that adsorbs reaction participants onto it.

• Reactants can easily access the homogeneous catalyst because it is in solution already! This promotes high catalytic activity.

• Species in solution are easier to characterize (e.g. with various spectroscopy techniques) than species adsorbed onto a solid surface.

Disadvantages
• A catalyst capable of dissolving in solution is going to have to be separated later if it is to be recycled for re-usage (as per the principles of green chemistry).

• The homogeneous catalyst may have issues at high temperatures since we don't want our solution (which contains this catalyst) and any volatile reactants to evaporate, even though high temperatures usually promotes faster reactions.

1.Acid catalysis
The proton is the most 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 of esters:
 * CH3CO2CH3 +  H2O    CH3CO2H  +  CH3OH

In the absence of acids, aqueous solutions of most esters do not hydrolyze at practical rates.

2.Transition metal-catalysis
Many transformations classically considered homogeneous catalysis utilize soluble coordination and organometallic compounds as catalysts. These catalysts differ from solid catalysts, which are traditionally associated with heterogeneous catalysis. Homogeneous catalysis can be subdivided according to their substrates or their reductive/oxidative character.

2-1.Reductions
A prominent class of reductive transformations involve hydrogenation. H2 is added to unsaturated substrates directly using hydrogen gas or indirectly by transferring hydrogen from substrates. The latter is called transfer hydrogenation. Related reactions entail "HX additions" where X = silyl (hydrosilylation) and CN (hydrocyanation).

2-2.Carbonylations
Hydroformylation is a prominent form of carbonylation, involving the addition of H and "C(O)H" across a double bond. MeOH and CO react in the presence of homogeneous catalysts to give acetic acid, as practiced in the Monsanto process and Cativa processes. Carbon monoxide is a common substrate in homogeneous catalysis and diverse reactions have been commercialized, including hydrocarboxylation and hydroesterifications.

2-3.Polymerization and metathesis of alkenes
A number of polyolefins are produced by Ziegler-Natta catalysis. Alkenes such as ethylene, propylene, and butadiene are common substrates. Olefin metathesis is usually practiced heterogeneously, but homogeneous variants are known.

Oxidations
Homogeneous catalysts are also used in a variety of oxidations. Is the Wacker process, acetaldehyde is produced from ethylene to 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.

2-4.Hydration and hydrolysis
Water is a common reagent in homogeneous catalysis. Esters and amides are slow to hydrolyze in neutral water, but the rates are sharply affected by metal complexes. The fastest and most selective catalysts for such reactions are often metalloenzymes, which can be viewed as large coordination complexes. The hydration of nitriles, alkenes, and alkynes are all catalyzed by metal complexes.

Other forms of homogeneous catalysis
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.

Industrial Applications
1. Neohexene (3,3-dimethyl-1-butene): an important intermediate in the synthesis of fine chemicals, is produced from the dimer of isobutene, which consists of a mixture of 2,4,4-trimethyl-2-pentene and 2,4,4-trimethyl-1-pentene. Cross-metathesis of the former with ethene yields the desired product. The catalyst is a mixture of WO3/SiO2 for metathesis and MgO for isomerization at 370 °C and 30 bar. The isobutene is recycled to the isobutene dimerization unit.

2. Vestenamer 8012: a predominantly trans polymer of cyclooctene made with a WCl6 based catalyst (Degussa-Hüls), used in blends and poly-norbornene made by a catalyst generated from RuCl3 in t-BuOH in the presence of air (Norsorex by Atofina, a polymer with a very high molecular weight).

3.Polydicyclopentadiene (DCPD): Dicyclopentadiene is the Diels-Alder adduct of cyclopentadiene, an abundant product from the cracker in the refinery. It contains a strained norbornene ring and a less strained cyclopentene ring. ROMP leads to opening of the norbornene ring, but some reaction of the cyclopentene ring also takes place, which leads to cross-linking.

4. For a recent update of industrial applications, many references exist.

Contrast with heterogeneous catalysis
Homogeneous catalysis differs from heterogeneous catalysis in that the catalyst is in a different phase than the reactants. One example of heterogeneous catalysis is the petrochemical alkylation process, where the liquid reactants are immiscible with a solution containing the catalyst. Heterogeneous catalysis offers the advantage that products are readily separated from the catalyst, and heterogeneous catalysts are often more stable and degrade much slower than homogeneous catalysts. However, heterogeneous catalysts are difficult to study, so their reaction mechanisms are often unknown.

Enzymes possess properties of both homogeneous and heterogeneous catalysts. As such, they are usually regarded as a third, separate category of catalyst.