Four vanadium phosphate catalysts supported on γ-A1203 （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 reactio...Four vanadium phosphate catalysts supported on γ-A1203 （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 V^5＋ 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 V^4＋ to V^5＋ phases. An appropriate ratio of V^5＋/V^4＋ can enhance the performance of the VPO catalyst. However, a higher amount of V^5＋ and its associated oxygen species are responsible to promote the MA selectivity.展开更多
文摘Four vanadium phosphate catalysts supported on γ-A1203 （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 V^5＋ 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 V^4＋ to V^5＋ phases. An appropriate ratio of V^5＋/V^4＋ can enhance the performance of the VPO catalyst. However, a higher amount of V^5＋ and its associated oxygen species are responsible to promote the MA selectivity.
