Oxidative dehydrogenation of ethane over novel Li/Dy/Mg mixed oxides: structure-activity study

Oxidative dehydrogenation of ethane to ethylene was studied using variously prepared Li/Dy/Mg/Cl mixed metal oxides as catalysts. The catalytic performance was found to be strongly dependant on the method of preparation and the LiCl content of the solids. Ethylene yields of up to 77% were obtained with catalysts prepared by precipitation of the catalyst precursors with an equimolar mixture of NH4Cl and HCl, and subsequent calcination in synthetic air (i.e., absence of CO2). Both highest ethylene yields and best long-term stability were achieved with catalysts having the highest chloride loading. Based on kinetic data and high-temperature XRD measurements (under controlled atmosphere), a new reaction mechanism is proposed wherein the active sites of the catalytic system are postulated to reside in molten LiCl, supported on Dy2O3/MgO. Oxygen is solved dissociatively in the LiCl melt forming the catalytically active hypochlorite OCl-. With increasing temperature, OCl- decomposes to O-. + Cl- or O- + Cl-.. The two radical species are highly oxidative and can readily activate an alkane by homolytic hydrogen abstraction. The so-created alkane radicals react further with OH to form an olefin and H2O. At low temperatures, a regime of high apparent activation energy has been determined for high chloride loadings, while at high temperatures and low chloride loadings, a second regime with lower activation energy was found. It is suggested that the first regime is controlled by reaction kinetics, whereas the second regime is diffusion-controlled. Which of the two regimes predominates is strongly dependent on the reaction temperature and the structure and composition of the catalyst.