Oxidation of hydrocarbons with dioxygen via peroxide intermediates

Abstract

Cyclohexane, cyclohexene, and α-pinene react with dioxygen in the liquid phase in the presence of catalysts based on platinum, heteropoly compounds (HPCs), metal-containing HPCs, and combinations of these components. In cyclohexane and α-pinene oxidations occurring by an autooxidation mechanism at 160–170 and 80– 90°C, respectively, the catalysts serve to control free-radical processes. The simultaneous action of a Ru-containing phosphotungstate as a hydroperoxide decomposition catalyst and of a V-containing phosphotungstate as a scavenger of hydroxyl and alkoxyl radicals increases the cyclohexanol + cyclohexanone selectivity of cyclohexane oxidation without yielding a hydroperoxide. A Pt/C catalyst affords an increase in α-pinene conversion in a fixed time. In combination with ammonia or tetrahexylammonium chloride admixtures, it retards side reactions and raises the yield of verbenol and verbenone, which are the most valuable products. During cyclohexane, cyclohexene, and α-pinene oxidation with an O2-H2 mixture at room temperature, no free-radical chain reaction develops in the Pt-HPC system and reactive intermediates form and interact, involving the HPC, with hydrocarbons on the surface of the platinum catalyst. Analysis of reactivity and of the composition of substrate oxidation products suggests a mechanism for the conjugate oxidation of hydrocarbons in systems with various HPCs. In this mechanism, HPC composition determines, to a large extent, the nature of reactive intermediates, which may be peroxides or radicals bound to platinum or HPC. The properties of catalytic systems in oxidation with O2-H2 mixtures can be controlled by selecting an appropriate HPC as the modifying component.