Structural Transformations of VOHPO₄·¹/₂H₂O in the Presence of Ammonia

Abstract

The structural transformation of vanadium phosphate hemihydrate, VOHPO 4 ‚ 1 / 2 H 2 O, into ammonium vanadyl pyrophosphate, (NH 4 ) 2 [(VO) 3 (P 2 O 7 ) 2 ], was studied by XRD and spectroscopic methods such as FTIR and Raman spectroscopy. The reaction was carried out at 673 K in the presence of the ammoxidation feed (toluene, oxygen (air), ammonia, and water vapor), proceeding through an intermediate crystalline phase. The main product of the transformation was R-. Both materials are characterized by the orthorhombic space group Pna2 1 and an intersecting tunnel structure. Apart from R-(NH 4 ) 2 [(VO) 3 (P 2 O 7 ) 2 ], a vanadium-rich phase was formed according to the stoichiometry which is probably a mixed valent vanadium oxide. The structural transformation proceeded also in the same direction without participation of the aromatic substrate. The reaction of VOHPO 4 ‚ 1 / 2 H 2 O carried out only under an ammonia/ oxygen (air) flow led to the generation of R-(NH 4 ) 2 [(VO) 3 (P 2 O 7 ) 2 ] as well, but to a minor degree. An XRD-amorphous material was obtained, being the main phase. The treatment of the hemihydrate with an ammonia/nitrogen flow generated an XRD-amorphous product and a deep reduction of V IV w V III proceeded.

Structural Investigations of Ammonium Vanadium Diphosphates by X-Ray Powder Diffraction

“…For the preparation of NH VP O a mixture with a V : P : N mole ratio of 1 : 4 : 8 was heated up to 633 K for about 10 h. The obtained product, a green powder, was then leached with 50% concentrated HCl, washed with water, and dried in air. In the case of ammonium vanadium(IV) diphosphates, (NH ) VOP O was obtained from a V : P : N mole ratio of 1 : 6 : 12 at a temperature of about 553 K, whereas -(NH ) (VO) (P O ) was synthesized from a mixture with V : P : N of 1 : 8 : 16 at 598 K for at least 2 h (normally about 10 h) in air (5,18). The light green powders were eluted with water and dried in air.…”

“…In the latter case under ammoxidation conditions (ammonia, air, water vapor, and with or without a methylaromatic), various precursor compounds of the vanadium phosphates ( -and -VOPO , VOHPO ) 0.5H O, VOHPO ) are transformed and -(NH ) (VO) (P O ) is found as major phase (2,5,6). Beside the generation of this ammonium vanadyl diphosphate during the ammoxidation, the phase can also be synthesized directly by heating a mixture of V O and (NH ) HPO (5). Another ammonium vanadyl diphosphate, (NH ) VO P O , was also used as a precursor for investigations of the ammoxidation of aromatic compounds; in this case the To whom correspondence should be addressed.…”

“…Until now none of these structures has been ascertained. From a comparative analysis (5,6) of preliminary crystal data, such as lattice constants, space group, etc., it has been assumed that there is a structural relationship to known alkali compounds: MVP O (7-9), M VOP O (10), M (VO) (P O ) (11-13) (M"K, Rb, Cs).…”

Ammoxidation of toluene on vanadyl polyphosphates-VO(PO3)2, 2. Catalytic properties

“…[4,10]). For example, VOHPO4 .1/2 H20 is mainly transformed into (NH4)z(VO)3(P207) 2 under ammoxidation conditions [10,11].…”

“…[4,10]). For example, VOHPO4 .1/2 H20 is mainly transformed into (NH4)z(VO)3(P207) 2 under ammoxidation conditions [10,11]. Recently, the effect of a varying P/V ratio of VPO's on their catalytic performance [12,13] as well as the use of P/V = 2 VPO specimens (e.g.…”