Structure of vanadium sites in VPO catalysts and their influence on the catalytic performance in selective O- and N-insertion reactions

The behaviour of (NH4)(V4+OP2O7)-V-2, NH4V3+P2O7 and V3+(PO3)(3) as catalysts 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 ammoxidation conditions which consists of effectively coupled VO2+ ions and governs catalytic performance. In the case of V(PO3)(3), few isolated VO2+ ions appear on the surface during toluene ammoxidation while coupled VO2+ ions are formed during n-butane oxidation. This catalyst was found to be inactive in the former but active in the latter reaction. The differences in catalytic performance are explained in terms of the different structure of the vanadium centres.