The oxidation of toluene and o-xylene on alpha-Ti(HPO4)(2) . H2O-supported
vanadium oxide catalysts is reported. Vanadium is incorporated at different
loadings by VOCl3 grafting and by vanadyl oxalate wet impregnation. Their
catalytic properties and the results of the physico-chemical characterisati
on (Laser Raman, XRD, SEM, Na adsorption, XPS, and thermal analysis) were c
ompared with those of a conventional V2O5/TiO2 catalyst (with an 8 V2O5 wt%
loading). It is shown that toluene oxidation is as selective in Tiphosphat
e-based catalysts as in the conventional system, whereas o-xylene oxidation
is less selective: a much lower phthalic anhydride yield is obtained. Vana
dium uptake on Ti phosphate reaches a limit value when the grafting method
is used: V2O5 loadings higher than 5 wt% cannot be surpassed. A vanadium ox
ide-Ti phosphate support interaction exists at V loadings lower than 5 V2O5
wt% as suggested by the fact that the same VOPO4-like environment is detec
ted irrespective of the method used to incorporate V. Higher vanadium loadi
ngs by using wet impregnation result in the formation of larger V2O5 crysta
llites than in the case of the conventional V2O5/TiO2 catalyst. The lack of
interaction at high V loadings leaves the support uncovered. The poorly co
vered Ti phosphate surface exposing acid surface sites must account for the
lower o-xylene oxidation to phthalic anhydride. (C) 1999 Academic Press.