The Oxidative Dehydrogenation of Cyclohexane and Cyclohexene over Unsupported and Supported Molybdena Catalysts Prepared by Metal Oxide Vapor Deposition

Abstract

A novel molybdena catalyst has been prepared by metal oxide vapor synthesis (MOVS) and used to promote the vapor phase oxidative dehydrogenation of cyclohexane and cyclohexene. At temperatures in excess of 553 K, the conversion of cyclohexane over γ-alumina supported and unsupported MOVS catalysts and a commercial MoO3/Al2O3sample yielded cyclohexene, benzene, carbon monoxide, and carbon dioxide as the only detected products. The unsupported MOVS sample behaves as an active heterogeneous catalyst but the specific activities are increased by a factor of up to 8 upon supporting the precursor on alumina. The supported MOVS system also exhibits appreciable higher activity than the commercial catalyst and this is attributed to the unique dual dioxo-molybdenum sites present in the active MOVS catalyst. The oxidative dehydrogenation of cyclohexane is viewed as occurring in a stepwise fashion with cyclohexene appearing in the product mixture and the generation of carbon oxides occurring principally from the direct combustion of cyclohexane. Oxygen consumption, specific activities, and molar selectivities for the conversion of both reactants in the overall temperature range 488 K≤T≤673 K over the three molybdena systems are provided. Cyclohexene conversion to benzene exhibits zero and first-order behavior with regard to the hydrocarbon and oxygen concentrations, respectively, and apparent activation energies are recorded for the supported (89 kJ mol−1) and unsupported (126 kJ mol−1) MOVS and commercial (103 kJ mol−1) catalysts. The nature of the reactive oxygen and catalytic sites are considered and the reactivity and stoichiometry of the deep oxidation of both cyclic hydrocarbons are presented.