Investigation of vanadium phosphorus oxide catalysts (VPO) during toluene ammoxidation: new mechanistic insights by in situ EPR

Abstract

(VO)₂P₂O₇, differently prepared (NH₄)₂(VO)₃(P₂O₇)₂ samples and supported amorphous vanadium phosphorus oxide (VPO) catalysts were investigated during the ammoxidation of toluene using a self-constructed in situ EPR flow reactor in the X-band. For the unsupported catalysts, changes in the spin–spin exchange behaviour were analysed by calculating the quotient of the 4th and the square of the 2nd moment, 〈B⁴〉/〈B²〉², of the absorption signal. The catalytic activity was found to increase with exchange efficiency, while isolated vanadyl centres, as present in the supported catalysts, do not obviously participate in the catalytic process. NH₄⁺ containing catalysts can easily supply ammonia for N insertion into the hydrocarbon indicating that the latter does not react directly with NH₃ molecules from the gas phase but with those activated by adsorption on the surface of the catalyst according to a Langmuir–Hinshelwood mechanism.