Structural Transformations of VOHPO 4 · 1 / 2 H 2 O in the Presence of Ammonia
1996; American Chemical Society; Volume: 8; Issue: 5 Linguagem: Inglês
10.1021/cm950557r
ISSN1520-5002
AutoresYue Zhang, Andreas Martin, Gert‐Ulrich Wolf, Stefan Rabe, H. Worzala, Bernhard Lücke, M. Meisel, Klaus Witke,
Tópico(s)Catalytic Processes in Materials Science
ResumoThe structural transformation of vanadium phosphate hemihydrate, VOHPO4·1/2H2O, into ammonium vanadyl pyrophosphate, (NH4)2[(VO)3(P2O7)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 α-(NH4)2[(VO)3(P2O7)2], which is isostructural to α-K2[(VO)3(P2O7)2]. Both materials are characterized by the orthorhombic space group Pna21 and an intersecting tunnel structure. Apart from α-(NH4)2[(VO)3(P2O7)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 VOHPO4·1/2H2O carried out only under an ammonia/oxygen (air) flow led to the generation of α-(NH4)2[(VO)3(P2O7)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 VIV ⇒ VIII proceeded.
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