Selective oxidation of alcohols and aldehydes on metal catalysts

Oxidation of aldehydes, alcohols or carbohydrate derivatives can be performed with air in aqueous media, in the presence of palladium and platinum catalysts under mild conditions (293-353 K and atmospheric pressure). These reactions provide valuable products and intermediates for fine chemistry. They have been known for a long time, but much effort in the last 20 years has focused on this approach, because these catalytic reactions are environmentally friendly and could replace stoichiometric oxidations with mineral oxidizing agents. An oxidative dehydrogenation mechanism on the reduced metal surface has been generally accepted. During this process, a strong deactivation of the catalysts is often reported, which is a cause of serious concern for process development. Several causes of deactivation have been put forward: oxidation of metal, blocking of active sites by strong adsorption of side-products, metal leaching and growth of platinum crystallites. The addition of certain p-electron metal promoters (e.g. Bi, Pb) has been shown to play a useful dual role in reducing catalysts deactivation and in changing the selectivity of reactions. The performances of the catalysts can also be improved by modification of the metallic surface with strongly adsorbing nitrogen-containing bases or phosphines. A few illustrative examples will be given, which show that carbohydrates, aliphatic or aromatic alcohols, and polyols can be oxidized with high selectivities into valuable products. (C) 2000 Elsevier Science B.V. All rights reserved.