Synthesis and Investigation of Phase-pure M1 MoVTeNbOx Catalysts for Selective Oxidation of Propane to Acrylic Acid

Abstract

Hydrothermal synthesis of MoVTeNbOx catalysts has been investigated. It has been shown that the phase composition of the crystalline catalyst strongly depends on the conditions of the hydrothermal process. The X-ray amorphous product of the hydrothermal synthesis shows a long-range order of structural motifs in [001] direction. Subsequent heat treatment of the precursor in inert atmosphere leads to crystallization. The target structure is an orthorhombic bronze-like structure, denominated as M1. M1 phase has been associated with high activity and selectivity in the direct oxidation of propane to acrylic acid. Other phases formed are, e.g. the orthorhombic M2 phase (isostructural to KW3O9 phase), Mo5O14, V0.95Mo0.97O5 and TeMo5O16. Independent of the technical parameters of the autoclave used, phase-pure M1 has been successfully synthesized by optimization of the hydrothermal conditions (e.g. temperature and reaction time). As evidenced by SEM/EDX, precursor materials of M1 catalysts are characterized by a fairly homogeneous distribution of the elements and a significant higher Nb-content compared to that reported in the literature. According to EXAFS analysis, Nb is located into pentagonal bi-pyramidal units but also into octahedral coordinated positions, which is in contradiction to the established structural model. Furthermore, it was shown by XRD and TG/MS that the presence of ammonium containing phases in the precursor leads to the formation of phase mixture after the heat treatment. Additionally, the impact of phase composition on the catalytic behavior of hydrothermally prepared MoVTeNbOx catalysts in the selective oxidation of propane to acrylic acid has been investigated. Phase cooperation of M1 and M2 phase was not found in this contribution. For phase-pure M1 catalysts, the propane conversion increases with increasing the specific surface area. Other properties, such as chemical composition and aspect ratio of the needle-like M1 crystallites may influence the catalytic activity as well. In the present study the role of the (001) plane of the M1 phase in the propane oxidation is addressed by providing a M1 model catalyst that exposes preferentially this plane. The molecular structure and dynamic nature of the active moiety on the M1 surface under conditions of propane oxidation, is the key for understanding the catalytic behaviour of these catalysts. X-ray photoelectron spectroscopic experiments in presence of the reactants propane, oxygen and water indicated re-distribution of the elements at the catalyst surface, in response to changes in the gas mixture. Furthermore, phase-pure M1 showed a reasonable stability under reaction conditions in laboratory-scale experiments, which is of a great importance for their potential industrial application. The composition of multi-metal oxide catalysts has been varied. Metal substitution and/or addition of suitable diluents to the Mo-V-X-Nb (X=Te, P, Bi) oxides has been performed. The catalysts have been investigated in the selective oxidation of propane to acrylic acid. The results underlined the exceptional role of M1 phase. On the other hand, it has been proven that M1 is not necessarily required for acrylic acid formation.