Four vanadium phosphate catalysts supported onγ-Al_2O_3(20 wt%)were synthesized via wetness impregnation of VOHPO_4·0.5H_2O 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)_2P_2O_7.However,the VOPO_4 phase(β-VOPO_4)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 H_2-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. Four vanadium phosphate catalysts supported on γ-Al_2O_3 (20 wt%) were synthesized via wetness impregnation of VOHPO_4 · 0.5H_2O 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 single phase of (VO) 2P_2O_7.However, the VOPO_4 phase (β-VOPO_4) 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 H 2 -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 performa nce of the VPO catalyst. However, a higher amount of V ~ (5+) and its associated oxygen species are responsible to promote the MA selectivity.