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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. Enzymes are examples of homogeneous catalysts. [2]
Homogeneous catalysts function in the same phase as the reactants. Typically homogeneous catalysts are dissolved in a solvent with the substrates. One example of homogeneous catalysis involves the influence of H + on the esterification of carboxylic acids, such as the formation of methyl acetate from acetic acid and methanol. [27]
Homogeneous catalysts usually based on complexes of the group 4 metals titanium, zirconium or hafnium. They are usually used in combination with a different organoaluminum cocatalyst, methylaluminoxane (or methylalumoxane, MAO). These catalysts traditionally contain metallocenes but also feature multidentate oxygen- and nitrogen-based ligands. [2]
The concept of cone angle is of practical importance in homogeneous catalysis because the size of the ligand affects the reactivity of the attached metal center. In an example, [ 10 ] the selectivity of hydroformylation catalysts is strongly influenced by the size of the coligands.
One of the first applications of phosphine ligands in catalysis was the use of triphenylphosphine in "Reppe" chemistry (1948), which included reactions of alkynes, carbon monoxide, and alcohols. [16] In his studies, Reppe discovered that this reaction more efficiently produced acrylic esters using NiBr 2 (PPh 3) 2 as a catalyst instead of NiBr 2.
Oxidation catalysis is conducted by both heterogeneous catalysis and homogeneous catalysis. In the heterogeneous processes, gaseous substrate and oxygen (or air) are passed over solid catalysts. Typical catalysts are platinum, and redox-active oxides of iron, vanadium, and molybdenum.
The dominant technology for abiological nitrogen fixation is the Haber process, which uses iron-based heterogeneous catalysts and H 2 to convert N 2 to NH 3. This article focuses on homogeneous (soluble) catalysts for the same or similar conversions. [1]
Catalysts are often based on palladium, which is frequently selected due to high functional group tolerance. Organopalladium compounds are generally stable towards water and air. Palladium catalysts can be problematic for the pharmaceutical industry, which faces extensive regulation regarding heavy metals.