Homogeneous catalyst

In chemistry, homogeneous catalysis is 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. 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.

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:

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

Processes that utilize soluble organometallic compounds 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. Homogeneous catalysts have also been used in a variety of industrial processes, such as the Wacker process Acetaldehyde (conversion of ethylene to acetaldehyde) as well as the Monsanto process and the Cativa process for the conversion of MeOH and CO to acetic acid.

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