Selective oxidation with air on metal catalysts

Oxidation of organic molecules with air on metal catalysts has been known for a long time but there has been a renewed interest in recent years because these catalytic reactions are environmentally safe and could replace stoichiometric oxidations. This paper describes several oxidation reactions conducted either at high temperatures in the gas phase or at moderate temperatures in the liquid phase; in both cases they proceed via a mechanism of oxidative dehydrogenation on the metal surface. Ethylene glycol was converted to glyoxal at 550°C on Ag/SiC catalyst with a 70% yield provided promoters were added to the reaction feed (diethylphosphite or iodine) or deposited on the catalyst (LiPO₄ or H₃PO₄). The promoters improve the conversion and selectivity by modifying the structure and the oxygen concentration on the surface of silver. Oxidation of glyoxal to glyoxylic acid, glucose to gluconic acid and glycerol to various oxygenated derivatives were conducted in water at 60°C in the presence of carbon-supported palladium or platinum catalysts. Bismuth promoter, deposited on the platinum metals by redox reaction, improves the catalyst activity by preventing over-oxidation of the metal surface and favors the oxidation of secondary alcohol functions into keto-derivatives. At higher reaction temperatures, platinum catalysts produce C-C bond rupture with the formation of carboxylic acids with smaller chains. Thus, cyclohexanol was converted into C₆, C₅, and C₄ diacids with a 45% selectivity to adipic acid on Pt/C catalysts at 150°C.