STRUCTURE OF VANADIUM SITES IN VPO CATALYSTS AND THEIR INFLUENCE ON THE CATALYTIC PERFORMANCE IN SELECTIVE O-INSERTION AND N-INSERTION REACTIONS

The behaviour of (NH4)(V4+OP2O7)-V-2, NH4V3+P2O7 and V3+(PO3)(3) as ca talysts in the ammoxidation of toluene and in the selective oxidation of n-butane has been studied by in situ EPR and catalytic measurements . In the first two samples an amorphous phase is formed under ammoxida tion conditions which consists of effectively coupled VO2+ ions and go verns catalytic performance. In the case of V(PO3)(3), few isolated VO 2+ ions appear on the surface during toluene ammoxidation while couple d VO2+ ions are formed during n-butane oxidation. This catalyst was fo und to be inactive in the former but active in the latter reaction. Th e differences in catalytic performance are explained in terms of the d ifferent structure of the vanadium centres.