Selectivity patterns in alkane oxidation over Mg₃(VO₄)₂-MgO, Mg₂V₂O₇, and (VO)₂P₂O₇

The catalytic oxidation of ethane, propane, 2-methylpropane, butane, pentane, and cyclohexane has been studied over Mg₃(VO₄)₂-MgO, Mg₂V₂O₇, and (VO)₂P₂O₇. These reactions produced a wide range of products ranging from alkenes, dienes, anhydrides, acids, and carbon oxides. Broadly speaking, the major products were dehydrogenation products over Mg₃(VO₄)₂, combustion products over Mg₂V₂O₇, and oxygen-containing organic products over (VO)₂P₂O₇, although there are exceptions to this broad generalization. From the product selectivities, the moles of oxygen consumed per mole of alkane reacted could be calculated. These were the average oxygen stoichiometries for the reactions. They had values of about unity for ethane oxidation over VPO, about two when oxidative dehydrogenation was the predominant reaction, and about four for the other reactions studied. The selectivity patterns in terms of the formation of dehydrogenation products versus oxygen-containing products including carbon oxides were explained by assuming that there existed a selectivity-determining step, which could be the reaction of the surface alkyl species or the adsorbed alkene before its desorption. The formation of oxygen-containing products would be facilitated if the surface alkyl or adsorbed alkene could interact with the vanadium ions in two adjacent VO_x units such that it could easily react with the reactive oxygen in the V-O-V bridge. The results of ethane oxidation, however, could not be explained simply by this model, and other factors needed to be considered.