Formation and structural characterisation of ammonium vanadyl phosphates prepared by solid state reactions of vanadyl(IV) phosphates in the presence of ammonia
A. Martin et al., Formation and structural characterisation of ammonium vanadyl phosphates prepared by solid state reactions of vanadyl(IV) phosphates in the presence of ammonia, J MAT CHEM, 10(10), 2000, pp. 2368-2374
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.