A study of M1/M2 phase synergy in the MoVTe(Nb,Ta)O catalysts for propane ammoxidation to acrylonitrile

Abstract

The bulk mixed MoVTe(Nb,Ta)O catalysts containing nanoscale intergrowths of so-called M1 and M2 phases display uniquely high reactivity in propane ammoxidation to acrylonitrile (ACN) associated with the surface ab planes of the M1 phase. The current controversy surrounding this catalytic system is focused on the role of the M2 phase, which is unable to activate propane but was suggested in some studies to function in synergy with the M1 phase by efficiently converting the propylene intermediate to ACN. The present study systematically examined the catalytic behavior of pure M1 phases prepared by selective dissolution of the M2 phase in MoVTe(Nb,Ta)O catalysts in aqueous hydrogen peroxide. It confirmed that the MoVTe(Nb,Ta)O M1 phases are highly active and selective for propane and propylene ammoxidation, while the MoVTe(Nb,Ta)O M2 phases were active and selective in propylene ammoxidation only. Crucially, the present study of the pure MoVTe(Nb,Ta)O M1 and M2 phases in propylene ammoxidation revealed for the very first time that the M2 phases are significantly less active than the corresponding M1 phases in propylene ammoxidation. The findings of this study do not support the existence of the synergy effect for the MoVTe(Nb,Ta)O M1 and M2 phases. Instead, the trends previously associated with the M1 and M2 phase synergy in propane ammoxidation can be explained by two effects that were unaccounted for previously: (i) the preferential loss of surface active TeOx species from the M1 phase during the H2O2 treatment, and (ii) the generation of fresh ab planes via mechanical crushing of the H2O2-treated M1 phase. These findings confirmed that the M1 phase is the only phase required for the activity and selectivity of the MoVTe(Nb,Ta)O catalysts in propane ammoxidation to ACN.