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DOI: 10.1039/A903491I (Paper) Reference Section for: J. Mater. Chem., 1999, 9, 2523-2527

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

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Andreas Martin, Ursula Steinike, Klára Melánová and Vítezslav Zima

Abstract

The structural transformation of vanadyl(V) phosphate dihydrate (VVOPO4·2H2O, V/P = 1) and a Ga-containing vanadyl(V) phosphate dihydrate ([Ga(H2O)]x(VVO)1–xPO4·2H2O, V/P = 1 – x) in the presence of ammonia have been investigated. VVOPO4·2H2O was transformed in the presence of an NH3–air–water vapour flow at temperatures of ca. 670 K mainly into distorted (NH4)2(VIVO)3(P2O7)2 (V/P = 0.75). Additionally, the generation of crystalline V2O5 (up to 10%) was observed, mainly representing the remainder of the vanadium of the precursor compound. [Ga(H2O)]x(VVO)1–xPO4·2H2O was synthesised by the replacement of a number of the (VVO)3+ groups of the parent VVOPO4·2H2O by [Ga(H2O)]3+. A similar solid-state transformation was observed when this material was treated under the same gas flow but, besides crystalline V2O5, a significant proportion of GaPO4 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 VVOPO4·2H2O 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 V2O5. 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.

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