Vanadium phosphates three-dimensional structures

The selective oxidation of ra-butane to give maleic anhydride (MA) catalyzed by vanadium phosphorus oxides is an important commercial process (99). MA is subsequently used in catalytic processes to make tetrahydrofurans and agricultural chemicals. The active phase in the selective butane oxidation catalyst is identified as vanadyl pyrophosphate, (V0)2P207, referred to as VPO. The three-dimensional structure of orthorhombic VPO, consisting of vanadyl octahedra and phosphate tetrahedra, is shown in Fig. 17, with a= 1.6594 nm, b = 0.776 nm, and c = 0.958 nm (100), with (010) as the active plane (99). Conventional crystallographic notations of round brackets (), and triangular point brackets (), are used to denote a crystal plane and crystallographic directions in the VPO structure, respectively. The latter refers to symmetrically equivalent directions present in a crystal. 

Fig. 17. Structure of complex (VO)2P2C>7 in (010), viewed down the fr-axis. Vanadium octahedra and phosphate tetrahedra link together forming a three-dimensional network. Front (bold) and back (faint) layers are shown.

The products of such reactions depend upon the pressure, temperature, pH, phosphate, and cation concentrations, and may be difficult to predict or rationalize. For example, the equation (3) produces an acid phosphate and a phosphate hydroxide. Microporous aluminophosphates and related phases (see Section 5.1.2) are prepared in hydrothermal bombs using hydrated cations or molecular templates such as organic amines or ammonium cations to direct the porous framework. Many new structures with metal phosphate chains, layers, or three-dimensional networks have been prepared hydrothermally in recent years, for example, templated vanadium phosphates and iron phosphates. ... 

All of the above compounds are oxides. The application of the fluorine route already described for AlPOs and GaPOs has shown that it is also possible to obtain open framework structures with fluoride in the VPOs. This led to the discovery by Riou and Ferey [72] of the first open-framework fluorovanado(V)phosphate, ULM-7 or (H3N(CH2-CH2)NH3)[(V 02)2(P04)F], in which the building units are V2P trimers. Vanadium exhibits both octahedral and square pyramidal coordination, and fluoride ion is shared between them. The three-dimensional network generated by comer-sharing of these trimers, exhibits 10-membered rings occupied by diprotonated amines. 

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