The Kinetic Significance of V 5+ in n -Butane Oxidation Catalyzed by Vanadium Phosphates

Maleic anhydride, a precursor to polyester resins, is made by oxidation of n -butane over vanadium phosphate catalysts. This system is of general interest because it is the only heterogeneously catalyzed, alkane-selective oxidation reaction in commercial use. Time-resolved in situ x-ray absorption spectroscopy shows that when either α I -VOPO 4 /SiO 2 or (VO) 2 P 2 O 7 /SiO 2 catalysts are exposed to n -butane, the rate of maleic anhydride formation is proportional to the rate of decay of V 5+ species in the catalyst. Thus V 5+ species are kinetically significant for the production of maleic anhydride and not just for the production of by-products. The results also suggest that V 5+ species may play a role in the initial hydrogen abstraction from n -butane, the rate-determining step in the reaction sequence. V 4+ sites appear to be responsible for by-product formation.