Solid-state reactions of vanadium(v) phosphates in the presence of ammonia

The structural transformation of vanadyl(V) phosphate dihydrate ((VOPO4)-O- V. 2H(2)O, V/P=1) and a Ga-containing vanadyl(V) phosphate dihydrate ([Ga(H 2O)](x)((VO)-O-V)(1-x)PO4. 2H(2)O, V/P=1-x) in the presence of ammonia have been investigated. (VOPO4)-O-V. 2H(2)O was transformed in the presence of an NH3-air-water vapour flow at temperatures of ca. 670 K mainly into disto rted (NH4)(2)((VO)-O-IV)(3)(P2O7)(2) (V/P=0.75). Additionally, the generati on of crystalline V2O5 (up to 10%) was observed, mainly representing the re mainder of the vanadium of the precursor compound. [Ga(H2O)](x)((VO)-O-V)(1 -x)PO4. 2H(2)O was synthesised by the replacement of a number of the ((VO)- O-V)(3+) groups of the parent (VOPO4)-O-V. 2H(2)O by [Ga(H2O)](3+). A simil ar solid-state transformation was observed when this material was treated u nder the same gas flow but, besides crystalline V2O5, a significant proport ion of GaPO4 was also formed. The heterogeneous catalytic ammoxidation of t oluene to benzonitrile was applied as a test reaction in the temperature ra nge 570-625 K for the evaluation of catalytic performance. The (VOPO4)-O-V. 2H(2)O derived catalyst revealed an improved catalytic activity in compari son to similar catalysts obtained by the transformation of V(IV)-containing precursor compounds. It seems very likely that this is due to the existenc e of a proportion of crystalline V2O5. The catalytic activity of the Ga-con taining material is much lower, but is still in the range of the V(IV)-deri ved catalysts. Characterisation of the parent samples and the generated pro ducts (after equilibration as well as catalytic runs) carried out by means of XRD, XPS and FTIR spectroscopy.