n-Butane Oxidation over γ-Al2O3 Supported Vanadium Phosphate Catalysts

Four vanadium phosphate catalysts supported on γ-Al2O3 (20 wt%) were synthesized via wetness impregnation of VOHPO4·0.5H2O precursor and calcined for different durations (6, 10, 30 and 75 h) at 673 K in a reaction flow of n-butane/air mixture. The samples calcined for 6 and 10 h produced only a single phase of (VO)2P2O7. However, the VOPO4 phase (β-VOPO4) was detected and became more prominent with only a minor pyrophosphate peaks were found after 30 h of calcination. All these pyrophosphate peaks disappeared after 75 h of calcination. The formation of V5+ phase was also observed in the SEM micrographs. The redox properties and the nature of oxidants of the catalysts employed in this study were investigated by H2-TPR analysis. Selective oxidation of n-butane to maleic anhydride (MA) over these catalysts shows that the percentage of n-butane conversion decreases with the transformation of the catalysts from V4+ to V5+ phases. An appropriate ratio of V5+/V4+ can enhance the performance of the VPO catalyst. However, a higher amount of V5+ and its associated oxygen species are responsible to promote the MA selectivity.