Selective gas-phase oxidation of o-xylene to phthalic anhydride over VPO/HZSM-5 catalysts was investigated. The catalysts with various P/V ratio were facilely synthesized by impregnation method. The prepared catalysts were characterized by several techniques such as SEM, TPD-NH3, H2-TPR, FT-IR, BET and XRD. The effects of P/V ratio (1.6–2.4), reaction temperature (350–450 ◦C), Ox/O2 ratio (0.15–0.60) and gas hourly space velocity (G.H.S.V) of O2/N2 (600–1200 h-1) on yield of PA were studied by Box–Behnken design. The influence of independent factors and their quadratic interactions were examined by means of the Analysis of Variance (ANOVA). Present results indicate that P/V ratio has a different effect on the conversion and selectivity of PA. The catalyst with P/V ratio = 2.0, Ox/O2 ratio = 0.37, G.H.S.V of 770 h-1 and reaction temperature of 370 ◦C produced maximum yield of PA (80.7 %). The analysis revealed that the predicted results agree well with the experimental data (P/V ratio = 2.0, Ox/O2 ratio = 0.38, G.H.S.V of the carrier gas = 900 h-1 and reaction temperature of 400 ◦C with 74.3% yield).
