| The research drive to develop an oxidative dehydrogenation (ODH) process for propane and ethane comes from the fact that the chemical industry depends heavily on propylene and other alkene feedstocks. Research efforts on ethane ODH have led to catalytic systems with yields approaching those that could compete with the current non-oxidative technology, indicating the high probability of a viable process. However, in propane ODH, the development of catalysts able to achieve higher yields is of current interest in the literature and warrants further investigation. The use of a silica titanic mixed-oxide-supported molybdenum catalysts has been studied in regard to their activity for the oxidative dehydrogenation (ODH) of propane and ethane. By varying the K/Mo molar ratio a maximum in activity was obtained for propane ODH.; The effect of modifiers (alkali and halide) on the catalyst surface characteristics and, in turn, on the catalytic performance in ethane and propane ODH has been examined. The catalysts used in this study have been synthesized by a “one-pot” sol gel/co-precipitation technique. The main focus of the work has been characterization of the surface molybdena species, physical-chemical properties of the Si:Ti support, surface acidity, reducibility, adsorption/desorption behavior, and surface intermediates present during the reaction. Catalysts were characterized by BET surface area measurements, X-ray diffraction (XRD), laser Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), Transmission Electron Microscopy (TEM), Electron-Spin Resonance (ESR), 29Si CP-MAS NMR, transient/steady-state isotopic labeling studies using 18O, and propane temperature programmed desorption (TPD).; Insight into the supported MoOx structure is obtained. This allows characterization of the surface molybdena species, physical-chemical properties of the Si:Ti support, surface acidity, reducibility, adsorption/desorption behavior, and surface intermediates present during the reaction, and correlation of these characteristics with the level of alkali or halide doping. It was found that potassium suppresses the reducibility of the molybdenum species at low K/Mo ratios while possibly stabilizing the Mo(V) oxidation state during reaction. The trends seen in XPS, and X-ray diffraction patterns indicate that there are additional factors due to the interaction of the support that control catalytic behavior. Further characterization has also been performed over un-promoted Mo/Si:Ti catalysts with various Mo loadings to elucidate the effects of surface coverage. |
