Solid-state reactions of VO(H2PO4)2and (NH4)2VOP2O7in the presence of ammonia

Abstract

The structural transformation of vanadyl bis(dihydrogen phosphate), VO(H₂PO₄)₂, and diammonium vanadyl pyrophosphate, (NH₄)₂VOP₂O₇, in the presence of NH₃, mainly into ammonium vanadium pyrophosphate, NH₄VP₂O₇, has been investigated. The reactions were carried out at 673 K in NH₃, air and H₂O vapour-containing flows, respectively. The resulting specimens were characterized by XRD, SEM (scanning electron microscopy), TG/DTA, FTIR spectroscopy and chemical analysis. The transformation reaction of both parent samples led mainly to crystalline NH₄VP₂O₇ only if additional H₂O vapour was present during the transformation, otherwise XRD amorphous products were obtained. The reaction also led to amorphous materials under an NH₃–N₂ flow. Apart from NH₄VP₂O₇, small amounts of (NH₄)₂(VO)₃(P₂O₇)₂ were detected. The catalytic properties of the materials generated were studied using the heterogeneous catalytic ammoxidation of toluene to benzonitrile in the temperature range up to 723 K. The results were compared with those obtained by application of the as-synthesized NH₄VP₂O₇ as well as to an (NH₄)₂(VO)₃(P₂O₇)₂-containing catalyst. All NH₄VP₂O₇-containing solids showed rather poor activity (up to ca. 700 µmol_tol h^-1 m^-2) but high nitrile selectivities of up to 95%.