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

Abstract

The structural transformation of vanadyl(V) phosphate dihydrate (V ^V OPO ₄ ·2H ₂ O, V/P = 1) and a Ga-containing vanadyl(V) phosphate dihydrate ([Ga(H ₂ O)] _x (V ^V O) _1–x PO ₄ ·2H ₂ O, V/P = 1 – x) in the presence of ammonia have been investigated. V ^V OPO ₄ ·2H ₂ O was transformed in the presence of an NH ₃ –air–water vapour flow at temperatures of ca. 670 K mainly into distorted (NH ₄ ) ₂ (V ^IV O) ₃ (P ₂ O ₇ ) ₂ (V/P = 0.75). Additionally, the generation of crystalline V ₂ O ₅ (up to 10%) was observed, mainly representing the remainder of the vanadium of the precursor compound. [Ga(H ₂ O)] _x (V ^V O) _1–x PO ₄ ·2H ₂ O was synthesised by the replacement of a number of the (V ^V O) ³⁺ groups of the parent V ^V OPO ₄ ·2H ₂ O by [Ga(H ₂ O)] ³⁺ . A similar solid-state transformation was observed when this material was treated under the same gas flow but, besides crystalline V ₂ O ₅ , a significant proportion of GaPO ₄ was also formed. The heterogeneous catalytic ammoxidation of toluene to benzonitrile was applied as a test reaction in the temperature range 570–625 K for the evaluation of catalytic performance. The V ^V OPO ₄ ·2H ₂ O derived catalyst revealed an improved catalytic activity in comparison to similar catalysts obtained by the transformation of V(IV)-containing precursor compounds. It seems very likely that this is due to the existence of a proportion of crystalline V ₂ O ₅ . The catalytic activity of the Ga-containing material is much lower, but is still in the range of the V(IV)-derived catalysts. Characterisation of the parent samples and the generated products (after equilibration as well as catalytic runs) carried out by means of XRD, XPS and FTIR spectroscopy.