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

Abstract

The behaviour of (NH₄)₂V⁴⁺OP₂O₇, 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 insitu 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.