Structure and catalytic properties of molybdenum oxide catalysts supported on zirconia

MoO3/ZrO2 catalysts with different MoO3 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 NH3 and the catalytic properties were evaluated for vapor-phase ammoxidation of toluene to benzonitrile. XRD patterns show the presence of crystalline MoO3 peaks above 6.6 wt% MoO3, which corresponds to the theoretical monolayer loading of MoO3 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 MoO3 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 MoO3 bands are observed from 9 wt% of MoO3 and above loadings. XPS spectra showed that molybdenum was present at Mo6- on all fresh samples. The Mo/Zr atomic ratio shows that the dispersion of molybdena is high below 6.6 wt% MoO3 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.