(618d) The Microstructure and Catalytic Behavior of M1 and M2 Phases With Bulk Mo-V-M-O Compositions (M= combination of Sb, Nb, Te and Ta) in Propane Ammoxidation to Acrylonitrile

Abstract

Acrylonitrile, ACN, is a top 50 chemical produced globally. The bulk mixed metal Mo-V-M oxides (M= combination of Nb, Te, Sb, and Ta) consisting of so-called “M1” and “M2” phases are highly promising catalysts for a one-step propane ammoxidation to ACN. The M1 phase has been proposed to be responsible for propane activation and its selective transformation to ACN. The ab planes [001] in M1 phase have been proposed to contain the active and selective sites for propane ammoxidation. However, the structure-directing (in a sense of the Mo/V distribution in surface ab planes) and catalytic roles of these M substituents in the ab planes are poorly understood.  Specifically, there is a lack of the fundamental understanding of the Sb functions in the Mo-V-Sb-O, Mo-V-Sb-Nb-O, and Mo-V-Sb-Ta-O catalysts. Moreover, the M2 phase has not been studied as extensively as the M1 phase, whereas this phase was found to play a role in the ammoxidation of the propylene intermediate. The synergy between the Mo-V-Te-Nb-O M1 and M2 phases was reported previously. However, very little is known about synergistic effects for the Mo-V-Sb-O, Mo-V-Sb-Nb-O, and Mo-V-Sb-Ta-O compositions, which are also active and selective for propane ammoxidation to ACN.

In this study, the M1 and M2 phases possessing the Mo-V-Sb-Ta-O, Mo-V-Te-Ta-O, Mo-V-Sb-Nb-O, Mo-V-Te-Nb-O, and Mo-V-Sb-O compositions were prepared by hydrothermal (HT), microwave (MW), and slurry-evaporation (SE) syntheses, which usually resulted in M1/M2 phase mixtures. These mixed phases were treated by H₂O₂ to selectively remove the M2 phase and obtain pure M1 phases.  

The M1 phases for STEM characterization were prepared by sectioning epoxy-embedded M1 crystals producing slices of uniform thickness (ca. 50 nm) which eliminated significant uncertainty in the atomic contrast of various metal lattice sites encountered in previous studies of mechanically crushed M1 crystals. The HAADF STEM images obtained were quantified and analyzed statistically in terms of atomic contrast of metal lattice sites, atomic column by column, using Gatan Digital Micrograph (DM) software running DM scripts. A large number of HAADF-STEM images were analyzed with high accuracy using DM scripts. The statistically analyzed intensity of atomic columns suggested that the Mo-V-M-O catalysts show different Mo/V distribution in the ab planes. The atomic contrasts of various metal lattice sites confirmed the location of Ta in the center of pentagonal site and Sb in the hexagonal channel. The STEM-based partial occupancy data, especially the Mo/V distribution in ab planes from Mo-V-M catalysts, were correlated to their experimentally observed catalytic performance in propane ammoxidation. Additionally, mixtures of M1 and M2 and pure M2 phases with various Mo-V-M-O compositions were also investigated in propane and propylene ammoxidation to understand the catalytic behavior of the M2 phases and the M1/M2 synergy in this selective ammoxidation reaction.