Structural characterization of vanadiumphosphate catalysts generated under ammoxidation conditions

(NH4)(2)[(VO)(3)(P2O7)(2)]/(AVP, V/P-ratio = 0.75) is formed from various precursor phases VOHPO4. n H2O (n =0, 0.5, 2, 4) (V/P-ratio = 1) under ammoxidation feed (toluene, ammonia, air, water vapour) at temperatures of about 670 K. AVP, which is generated as mentioned above (AVP(gen)), revealed a much higher catalytic activity in the ammoxidation of toluene than pure as-synthesised AVP (AVP(syn)). Nevertheless, significant differences in the catalytic activity of such generated AVP(gen) were observed, depending on the precursor phase. This is the result of different transformation processes. The catalytic activity is obviously due to vanadium-rich, mixed-valent V-IV/V - oxide phases (VxOy), that are generated beside AVP by the remaining vanadium proportion. However, the components of AVP(gen) (AVP and VxOy) do not form a simple mixture, but apparently consist of microdomains of AVP and VxOy which are intimately intergrown. Probably, the AVP structure seems to act only as a matrix for the catalytic active VxOy-phase. The AVP structure in AVP,, is distorted compared to AVP(syn). The generated AVP was intensively studied by means of XRD and EDX. The reasons for the differences in the catalytic activity are discussed.