Precious Metal Catalysts for Homogeneous Catalysis
Homogeneous catalysis with precious metals is used in the chemical industry to carry out reactions carefully and selectively. For industrial applications, the platinum group metals rhodium, iridium, ruthenium, palladium and platinum are particularly important. In contrast to heterogeneous catalysis, the reacting substances, the products and the catalyst form one phase during the whole duration of the reaction. Certain chemical reactions can only be carried out by means of homogeneous catalysis. This is the case, for example, for hydroformylation and metathesis.
Because of the high costs of catalysts containing precious metals, the catalyst is normally separated out after the reaction and then re-used or recycled. A number of methods have been developed to separate the catalyst from the reaction mixture (e.g. distillation, adsorption).
Most homogeneous precious metal catalysts are clearly defined compounds. Frequently, these are complex precious metal chemicals.
- Wilkinson’s catalyst
A homogeneous catalyst with great commercial significance is Wilkinson's catalyst, [RhCl(PPh3)3]. This complex was discovered in 1965 by Geoffrey Wilkinson. Wilkinson was a pioneer of organometallic chemistry and was awarded the Nobel Prize in 1973 for his ground-breaking work on the chemistry of the so-called sandwich compounds.
Wilkinson’s catalyst is a rhodium complex. It is isolated in the form of red crystals (Abb. 1) and is stable in air in its solid form.
The catalyst was one of the first homogeneous precious metal catalysts used in technical applications. In industry it plays an important role in the hydrogenation of olefine double-bonds. An example of this type is the synthesis of ivermectin, an anti-parasitic agent used in veterinary medicine. The catalyst is also used on a large technical scale for the hydrogenation of caoutchoucs, thus rendering them particularly resistant to aggressive chemicals and extreme temperatures.
The great significance of homogeneous catalysis can be recognized from the fact that in the recent years three Nobel Prizes for chemistry (2001, 2005 und 2010) have already been awarded to nine laureates who have worked in the field of homogeneous catalysis.
In the following sections a selection of outstanding examples for homogeneously catalyzed processes which are used on a large industrial scale will be described.1)