Dependence of n-Butane Activation on Active Site of Vanadium Phosphate Catalysts

The nature and the role of oxygen species and vanadium oxidation states on the activation of n-butane for selective oxidation to maleic anhydride were investigated. Bi-Fe doped and undoped vanadium phosphate catalysts were used a model catalyst. XRD revealed that Bi-Fe mixture dopants led to formation of alpha(II)-VOPO4 phase together with (VO)(2)P2O7 as a dominant phase when the materials were heated in n-butane/air to form the final catalysts. TPR analysis showed that the reduction behaviour of Bi-Fe doped catalysts was dominated by the reduction peak assigned to the reduction of V5+ species as compared to the undoped catalyst, which gave the reduction of V4+ as the major feature. An excess of the oxygen species (O2-) associated with V5+ in Bi-Fe doped catalysts improved the maleic anhydride selectivity but significantly lowering the rate of n-butane conversion. The reactive pairing of V4+-O- was shown to be the centre for n-butane activation. It is proposed that the availability and appearance of active oxygen species (O-) on the surface of vanadium phosphate catalyst is the rate determining step of the overall reaction.