The kinetic significance of V⁵⁺ 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 α₁-VOPO₄/SiO₂ or (VO)₂P₂O₇/SiO₂ catalysts are exposed to n-butane, the rate of maleic anhydride formation is proportional to the rate of decay of V⁵⁺ species in the catalyst. Thus V⁵⁺ species are kinetically significant for the production of maleic anhydride and not just for the production of by-products. The results also suggest that V⁵⁺ species may play a role in the initial hydrogen abstraction from n-butane, the rate-determining step in the reaction sequence. V⁴⁺ sites appear to be responsible for by-product formation.