A Comparison of the Reactivity of “Nonequilibrated” and “Equilibrated” V–P–O Catalysts: Structural Evolution, Surface Characterization, and Reactivity in the Selective Oxidation ofn-Butane andn-Pentane

Abstract Changes occurring on thermal treatment of the precursor of vanadium/phosphorus mixed oxide, the industrial catalyst for the oxidation of n -butane, were studied. The precursor was mixed with stearic acid, used as an organic binder for pelletization of the powder. The calcination of the precursor leads to a partially oxidized compound, constituted of an amorphous V IV –P mixed oxide and a crystalline hydrated V V –P–O phase. The calcined compound, when left in a 1% hydrocarbon/air stream for 100 h leads to a “nonequilibrated” catalyst, and after 1000 h to the “equilibrated” catalyst. The catalytic activity of the nonequilibrated and equilibrated catalysts in n -butane and n -pentane oxidation was studied and compared; the chemical–physical features of the two catalysts were studied by means of XRD, FT-IR, chemical analysis, TGA, XPS, and TPD. Only well crystallized (VO) 2 P 2 O 7 was detected in the equilibrated catalyst and a homogeneous distribution of surface centers seems to be present on its surface. In the case of nonequilibrated catalyst, a poorly crystallized (VO) 2 P 2 O 7 is present together with an amorphous V IV –P–O phase and γ-VOPO 4 ; these phases define a heterogeneous distribution of at least two kind of surface centers. This surface heterogeneity gives rise to a catalyst less selective in n -butane oxidation to maleic anhydride and less specific in the conversion of n -pentane to phthalic anhydride.