Abstract
The structural transformation of various vanadyl(IV) phosphates into vanadium oxide-containing crystalline (NH4)2(VO)3(P2O7)2 in the presence of ammonia-containing gas flows and under ammoxidation of toluene reaction conditions has been investigated using a coupled hot stage XRD and on-line GC apparatus. Two different types of crystalline layered NH4+-containing vanadium phosphates were generated as intermediate phases during the formation process: (i) NH4VOPO4·nH2O, n = 0.5, 2, 4, a kind of intercalation compound formed between 293 and 390 K, and (ii) β-(NH4)2(VO)3(P2O7)2, observed during a period where the temperature was held constant at 713 K. Besides the formation of these crystalline intermediates, the generation of X-ray amorphous states was noted. The final product, consisting of distorted ammonium vanadyl(IV) pyrophosphate [α-(NH4)2(VO)3(P2O7)2], additionally contains various amounts of X-ray amorphous vanadium oxides. The transformation process is discussed on the basis of the precursor structures. Measurable catalytic activity was not observed until the formation of β-(NH4)2(VO)3(P2O7)2. Increased catalytic activity is associated with the formation of α-(NH4)2(VO)3(P2O7)2 and the generation of vanadium oxide co-phases. The in situ XRD data obtained during the phase transformation and the evaluation of the simultaneously-received data on catalytic performance at different transformation stages provide a more detailed insight into the formation of an effective catalyst and its catalytic performance from the position of the bulk structure.
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