Reaction Behaviors and Crystal Transformation of Industrial Vanadium–Phosphorus–Oxygen (VPO) Catalysts for n-Butane Oxidation

[Image: see text] A long-time evaluation of n-butane oxidation over an industrial vanadium–phosphorus–oxygen (VPO) catalyst was implemented. The catalytic performances for n-butane oxidation during this period were obtained. It was shown that the conversion of n-butane increased with the evaluation time, but the selectivity of the maleic anhydride (MA) product decreased gradually. To investigate the crystal transformation of the VPO catalyst, the properties of fresh and evaluated VPO catalysts were measured by a series of characterization methods, including X-ray diffraction (XRD), N(2) adsorption and desorption, NH(3)-temperature programmed desorption (TPD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The results showed that the acidities and valence of vanadium increased after evaluation due to the appearance of a β-VOPO(4) phase. The crystal transformation would increase the activity for n-butane oxidation. Meanwhile, during the evaluation period, the decrease in selectivity of the MA product should be related to the decreasing percentage of Lat-O species in the VPO catalyst.