A study of M1/M2 phase cooperation in the MoV(Te,Sb)(Nb,Ta)O catalysts for propane ammoxidation to acrylonitrile

Abstract

The bulk mixed MoV(Te,Sb)(Nb,Ta)O catalysts containing nanoscale intergrowths of so-called M1 and M2 phases display uniquely high reactivity in propane ammoxidation to acrylonitrile 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 acrylonitrile. The present study systematically examined the catalytic behavior of pure M1 phases prepared by selective dissolution of the M2 phase in MoV(Te,Sb)(Nb,Ta)O catalysts in aqueous hydrogen peroxide. It confirmed that the MoV(Te,Sb)(Nb,Ta)O M1 phases are highly active and selective for propane and propylene ammoxidation, while the MoV(Te,Sb)(Nb,Ta)O M2 phases were active and selective in propylene ammoxidation only. Most importantly, the kinetic study of the MoV(Te,Sb)(Nb,Ta)O M1 and M2 phases in propylene ammoxidation revealed for the very first time that that the M2 phases are significantly less active than the M1 phase in propylene ammoxidation. The findings of this study do not support the existence of the synergy effect for any M1/M2 compositional variant. Instead, the observed behavior of MoV(Te,Sb)(Nb,Ta)O catalysts was consistent with partial loss of some surface active species from the M1 phase surface during the H2O2 treatment and generation of fresh ab planes of the M1 phase via mechanical grinding of the H2O2-treated M1 phase. These findings provided further evidence that the M1 phase is the only phase required for the activity and selectivity of the MoV(Te,Sb)(Nb,Ta)O catalysts in propane ammoxidation to ACN.