Vanadyl pyrophosphate (VPP) is the active catalytic phase that converts n-butane to maleic anhydride in fixed beds, fluidized and DuPont’s circulating bed (CFB). The original CFB concept was based on Mars van-Krevelen reaction mechanism which solid lattice contributes all of oxygen partially oxidize n-butane. However, maintain high activity selectivity requires surface oxidized; thus, DuPont co...