Factors That Determine Selectivity for Dehydrogenation in Oxidation of Light Alkanes

Abstract

The product distributions in the oxidation of butane over supported vanadium oxide catalysts and orthovanadates of cations of different reducibilities, and C2 to C6 alkanes over Mg orthovanadate, Mg pyrovanadate, and vanadyl pyrophosphate were compared. From these product distribution patterns, the factors that determine selectivity for dehydrogenation versus formation of oxygencontaining products (which include oxygenates and carbon oxides) were identified. It was proposed that in the formation of the primary products, the selectivity for dehydrogenation decreased with increasing probability of a surface alkyl species (or adsorbed alkene formed from the alkyl) to react with a surface lattice oxygen. This probability was higher for vanadates that had low differential heats of reoxidation or that contained cations of high reduction potential, and for vanadates whose active sites could bind the surface species at least two different carbon atoms so as to bring the hydrocarbon species close to the surface reactive lattice oxygen. These factors could be used to interpret the different selectivities for dehyrogenation in the oxidation of butane over SiO2-supported vanadia catalysts of different vanadia loadings.