Formation and structural characterisation of ammonium vanadyl phosphates prepared by solid state reactions of vanadyl(IV) phosphates in the presence of ammonia

The structural transformation of various vanadyl(IV) phosphates into vanadi um oxide-containing crystalline (NH4)(2)(VO)(3)(P2O7)(2) in the presence of ammonia-containing gas flows and under ammoxidation of toluene reaction co nditions has been investigated using a coupled hot stage XRD and on-line GC apparatus. Two different types of crystalline layered NH4+-containing vana dium phosphates were generated as intermediate phases during the formation process: (i) NH4VOPO4. nH(2)O, n = 0.5, 2, 4, a kind of intercalation compo und formed between 293 and 390 K, and (ii) beta-(NH4)(2)(VO)(3)(P2O7)(2), o bserved during a period where the temperature was held constant at 713 K. B esides the formation of these crystalline intermediates, the generation of X-ray amorphous states was noted. The final product, consisting of distorte d ammonium vanadyl(IV) pyrophosphate [alpha-(NH4)(2)(VO)(3)(P2O7)(2)], addi tionally contains various amounts of X-ray amorphous vanadium oxides. The t ransformation process is discussed on the basis of the precursor structures . Measurable catalytic activity was not observed until the formation of bet a-(NH4)(2)(VO)(3)(P2O7)(2). Increased catalytic activity is associated with the formation of alpha-(NH4)(2)(VO)(3)(P2O7)(2) and the generation of vana dium oxide co-phases. The in situ XRD data obtained during the phase transf ormation and the evaluation of the simultaneously-received data on catalyti c performance at different transformation stages provide a more detailed in sight into the formation of an effective catalyst and its catalytic perform ance from the position of the bulk structure.