A comparison of the reactivity of "nonequilibrated" and "equilibrated" V-P-O catalysts: Structural evolution, surface characterization, and reactivity in the selective oxidation of n-butane and n-pentane

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 VIV-P mixed oxide and a crystalline hydrated VV-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)2P2O7 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)2P2O7 is present together with an amorphous VIV-P-O phase and γ-VOPO4; 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. © 1996 Academic Press, Inc.