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/ph osphorus mixed oxide, the industrial catalyst for the oxidation of n-b utane, were studied. The precursor was mixed viith 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 ph ase. The calcined compound, when left in a 1% hydrocarbon/air stream f or 100 h leads to a ''nonequilibrated'' catalyst, and after 1000 h to the ''equilibrated'' catalyst. The catalytic activity of the nonequili brated and equilibrated catalysts in n-butane and n-pentane oxidation was studied and compared; the chemical-physical features of the two ca talysts were studied by means of XRD, FT-IR, chemical analysis, TGA, X PS, and TPD. Only well crystallized (VO)(2)P2O7 was detected in the eq uilibrated catalyst and a homogeneous distribution of surface centers seems to be present on ias surface, In the case of nonequilibrated cat alyst, a poorly crystallized (VO)(2)P2O7 is present together with an a morphous V-IV-P-O phase and gamma-VOPO4; these phases define a heterog eneous distribution of at least two kind of surface centers. This surf ace 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. (C) 1996 Academic Press, Inc.