| The modern petrochemical industry depends heavily on ethylene, propylene and other alkene feedstocks, providing the research drive to develop an oxidative dehydrogenation (ODH) process for ethane and propane. Although it offers many advantages as compared to steam cracking and direct dehydrogenation, catalytic oxidative dehydrogenation (ODH) of alkanes continues to pose major challenges as a research area. 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. While the positive effects of chlorine to enhance the homogeneous reaction in ethane ODH have been the focus of many studies, the role of chlorine for the surface redox mechanism over easily reducible metal oxide catalysts at lower temperatures is still not well understood. Furthermore, the use of chlorine as a dopant over Mo/Si:Ti catalysts for the ODH of propane has not been previously reported.; In this study, the use of molybdenum catalysts supported over silica-titania mixed-oxides has been investigated in regard to their activity for the oxidative dehydrogenation (ODH) of ethane and propane. The incorporation of chlorine was able to improve the ethylene and propylene formation rates up to Cl/Mo = 2.0. The effect of chlorine dopants 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 modified "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, reducibility, adsorption/desorption behavior, and surface intermediates present during the reaction. Catalysts were characterized by physical adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), laser Raman spectroscopy (LSR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), temperature programmed reduction (TPR), temperature programmed desorption/reaction (TPD/TPrxn), electron spin resonance (ESR), secondary ion mass spectrometry (SIMS), and steady-state isotopic transient kinetic analyses (SSITKAS).; The structural modification on the supported MoOx species over the silica-titania mixed oxide support induced by chlorine was obtained. It appears the addition of chlorine causes a stronger interaction of Mo species with silica support while the titania begins to segregate from silica network forming anatase crystal phase. These modifications mainly come from an electronic interaction while the chemical location of chlorine was also pinpointed. For alkane ODH, chlorine improves the olefin yield not only by hindering its further reaction, but also through affecting the interaction of alkane with the catalyst surface and altering its activation steps. |
