Gold Promotes Green Oxidation

Catalysis

Industrial oxidation processes that convert hydrocarbons to epoxides, ketones, aldehydes, and other oxygen-containing organic compounds could become greener with the discovery of nanocrystalline gold catalysts that work under mild conditions.

A team led by Graham J. Hutchings, professor of physical chemistry at Cardiff University, in Wales, has shown that gold nanoparticles supported on carbon activate molecular oxygen in air to convert alkenes to partial oxidation products such as epoxides at atmospheric pressure and temperatures of 60–80 C (Nature 2005, 437, 1132).

The catalyst can be fine-tuned with exceptionally high selectivity for the desired products by changing the solvent, Hutchings says. The partial oxidations can also be carried out under solvent-free conditions, so they fit with green chemistry.

In a Nature commentary, Masatake Haruta, a materials chemist at Tokyo Metropolitan University, observes that oxidation catalysts are second in usage only to polymerization catalysts and that most industrial oxidation processes tend to use chlorine or organic peroxides. The chlorine processes produce large amounts of chloride salts and significant amounts of chlorinated organic by-products. The disadvantage of organic peroxides is their expense.

The chemical industry would be transformed if selective oxidation of hydrocarbons could be achieved efficiently using cheap and clean oxygen from the air, Haruta notes. The Cardiff advance of greener methods for oxidation catalysis using gold is therefore invaluable.

Hutchings and coworkers suggest that their discovery could lead to the commercial exploitation of the high redox activity of gold nanocrystals.

We are now trying to use a continuous-flow reactor to improve yields to a commercial level, Hutchings says. This approach needs a lot of thought on catalyst design but is the way forward for this type of oxidation.