In chemistry, homogeneous catalysis
in a solution by a soluble catalyst. Strictly speaking, 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.http://goldbook.iupac.org/C00876.html The term is used almost exclusively to describe solutions and often implies catalysis by organometallic compound
s. The area is one of intense research and many practical applications, e.g., the production of acetic acid
. Enzymes are examples of homogeneous catalysts.P. W. N. M. van Leeuwen and J. C. Chadwick "Homogeneous Catalysts: Activity - Stability - Deactivation" Wiley-VCH, Weinheim, 2011. Online .
The proton is the most pervasive homogeneous catalystR.P. Bell "The Proton in Chemistry", Chapman and Hall, London, 1973. 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
CH3CO2CH3 + H2O CH3CO2H + CH3OH
In the absence of acids, aqueous solutions of most esters do not hydrolyze at practical rates.
Processes that utilize soluble organometallic compound
s as catalysts fall under the category of homogeneous catalysis, as opposed to processes that use bulk metal or metal on a solid support, which are examples of heterogeneous catalysis
. Some well-known examples of homogeneous catalysis include hydroformylation
and transfer hydrogenation
, as well as certain kinds of Ziegler-Natta
polymerization and hydrogenation
.Elschenbroich, C. ”Organometallics” (2006) Wiley-VCH: Weinheim. Homogeneous catalysts have also been used in a variety of industrial processes, such as the Wacker process
Acetaldehyde (conversion of ethylene
) as well as the Monsanto process
and the Cativa process
for the conversion of MeOH
to acetic acid
Many non-organometallic complexes are also widely used in catalysis, e.g. for the production of terephthalic acid
Other forms of homogeneous catalysis
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 blood stream.
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.G. O. Spessard and G. L. Miessler "Organometallic Chemistry", Prentice Hall, Upper Saddle River, NJ, 1997, pp. 249-251.
s possess properties of both homogeneous and heterogeneous catalysts. As such, they are usually regarded as a third, separate category of catalyst.