Abstract: The performance variation of industrial vanadium phosphorus oxide (VPO) catalyst for n-butane oxidation to maleic anhydride under phosphorus replenishment conditions was investigated. By comparing the fresh catalyst with the catalyst after four years of industrial operation, various physicochemical characterization techniques were employed to analyze the crystal phase structure, surface acidity, elemental composition, and valence state of the catalyst. The results indicated that the specific surface area, pore volume, and pore size of the catalyst after long-term operation changed insignificantly, but the crystal phase structure underwent significant changes. The vanadium(V) phosphate (VOPO₄) phase gradually disappeared, and the catalyst mainly existed in the form of the vanadium(Ⅳ) pyrophosphate ((VO)₂P₂O₇) phase. Meanwhile, severe phosphorus loss occurred on the catalyst surface, leading to a decrease in acidity, a reduction in the average vanadium valence, and a decrease in surface oxygen concentration. In the reactor, as the operation time extended, the average reaction temperature gradually increased, and the molar yield of maleic anhydride gradually decreased.

Key words: VPO catalyst, crystal composition, n-butane, selective oxidation