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.