SOLID-STATE REACTIONS OF VO(H2PO4)(2) AND (NH4)(2)VOP2O7 IN THE PRESENCE OF AMMONIA

The structural transformation of vanadyl bis(dihydrogen phosphate), VO (H2PO4)(2), and diammonium vanadyl pyrophosphate, (NH4)(2)VOP2O7, in t he presence of NH3, mainly into ammonium vanadium pyrophosphate, NH4VP 2O7, 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 microscop y), TG/DTA, FTIR spectroscopy and chemical analysis. The transformatio n reaction of both parent samples led mainly to crystalline NH4VP2O7 o nly if additional H2O vapour was present during the transformation, ot herwise XRD amorphous products were obtained. The reaction also led to amorphous materials under an NH3-N-2 flow. Apart from NH4VP2O7, small amounts of (NH4)(2)(VO)(3)(P2O7)(2) were detected. The catalytic prop erties 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 ap plication of the as-synthesized NH4VP2O7 as well as to an (NH4)(2)(VO) (3)(P2O7)(2)-containing catalyst. All NH4VP2O7-containing solids showe d rather poor activity (up to ca. 700 mu mol(tol) h(-1) m(-2)) but hig h nitrile selectivities of up to 95%.