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

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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 µmoltol h-1 m-2) but high nitrile selectivities of up to 95%.

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