(513g) The Catalytic Significance of Surface Ab Planes of M1 Phase in Propane Ammoxidation

The last few decades have seen growing interest in developing highly active and selective catalysts for selective oxidation of lower alkanes to alkenes. Many catalytic systems have been suggested for these reactions and very recently, it has been proposed that MoVTeNbO catalysts show good activity and selectivity for propane ammoxidation reaction due to presence of two major crystalline phases, called M1 and M2. Researchers have paid special attention to the M1 phase which contributes to the high activity and selectivity to produce acrylonitrile via propane ammoxidation. The surface ab (basal) planes of M1 which are oriented perpendicular to the length of the rod like crystals have been suggested to contain the active and selective sites for this reaction, however this conclusion is still being debated. Therefore, it is very important to establish the role of ab planes of M1 in this catalytic reaction.

In this study, phase pure M1 MoVTeNbO catalyst was prepared by hydrothermal method and then coated by two different procedures, solution-phase silylation and fast atomic layer deposition (ALD) of silica. Finally, the rod-like M1 crystals covered by silica were crushed carefully to preferentially expose the surface ab planes. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Low Energy Ion Scattering (LEIS) were used to analyze the microstructure the M1 phase, the thickness of the silica layer and the topmost surface composition of crushed M1 phase preferentially exposing the surface ab planes. The catalytic behavior of the ab planes of the pure M1 phase, solution-phase silylated M1, fast silica ALD M1, and crushed M1 catalysts was tested in propane ammoxidation using a fixed bed micro-reactor at atmospheric pressure at 300-430°C. The results of this study provide direct evidence that the surface ab planes exhibit much higher activity and selectivity in propane ammoxidation than the polycrystalline M1 catalysts.