Hydrocarbon selective oxidation on vanadium phosphorus oxide catalysts: Insights from electronic structure calculations

The ability of the vanadium phosphorus oxide (VPO) catalyst to selectively activate n-butane and then perform subsequent selective oxidation to maleic anhydride was investigated using electronic structure calculations. Both active site cluster models and periodic surface models, including explicit consideration of surface relaxation and hydration, led to the same qualitative conclusions about the reactivity of the (VO)₂P₂O ₇ (1 0 0) surface in substrate adsorption and oxidation. Density functional theory (DFT) reactivity indices and Density of States (DOS) plots show that, whether stoichiometric or phosphorus-enriched, strained or relaxed, bare or hydrated, covalent reactivity at the (1 0 0) surface is controlled by vanadium species, their dual acid-base attack giving selective activation of nbutane via methylene C-H bond cleavage. 1-butene is predicted to chemisorb at the surface using a π-cation complex, the strength of which makes 1-butene an unlikely intermediate in the production of maleic anhydride from nbutane. Coordinatively-unsaturated surface P-O and inplane P-O-V oxygen species are the most nucleophilic surface oxygens, which may explain the surface-enrichment in phosphorus always seen in industrial catalysts for maleic anhydride synthesis and also recent in situ microscopy images of surface oxygen transfer to n-butane. The resistance of the maleic anhydride selective oxidation product to further transformation was shown to be dependent on its orientation in the active site, and simulation of surface hydration indicated that dissociative adsorption of water may serve to regenerate the catalyst, replenishing its supply of selective nucleophilic oxygen species for mild oxidation.