Structure and catalytic properties of molybdenum oxide catalysts supported on zirconia

Abstract

MoO 3/ZrO 2 catalysts with different MoO 3 loadings (2–12 wt%) were prepared by the wet impregnation method. These catalysts were characterized by various techniques, such as X-ray diffraction (XRD), temperature-programmed reduction (TPR), laser Raman spectroscopy (LRS), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption of NH 3 and the catalytic properties were evaluated for vapor-phase ammoxidation of toluene to benzonitrile. XRD patterns show the presence of crystalline MoO 3 peaks above 6.6 wt% MoO 3, which corresponds to the theoretical monolayer loading of MoO 3 on the zirconia used in the present study. The TPR suggests that reduction of the catalysts occurs in two stages and indicates that the reducibility of the catalysts increases with increase in MoO 3 loading up to 6.6 wt%. The acidity of the catalysts was also found to increase up to 6.6 wt% of molybdena loading and it does not increase much at higher loadings. Raman results show that the surface molybdate species are present in low-loading samples, while crystalline MoO 3 bands are observed from 9 wt% of MoO 3 and above loadings. XPS spectra showed that molybdenum was present at Mo 6− on all fresh samples. The Mo/Zr atomic ratio shows that the dispersion of molybdena is high below 6.6 wt% MoO 3 and dispersion decreases at higher molybdena loadings. The catalytic activity of the catalysts during ammoxidation of toluene was found to increase with loading up to 6.6 wt% and did not change appreciably beyond this loading.