The use of chemical looping redox catalysts for oxidative dehydrogenation of ethane (ODH) has the potential to greatly reduce CO2 and NOx emissions from ethylene production. Steam cracking/pyrolysis of ethane, the traditional industrial process for ethylene production, is high temperature (> 1100 K), highly endothermic, and equilibrium-limited. It also requires a significant steam generation load, making the process energy and carbon intensive. Additionally, the high temperatures needed in the reactor lead to significant NOx emission, and the reactor requires periodic shutdowns to burn out coke. These issues can be addressed in ODH, where ethane is partially oxidized to water and ethylene. The production of water as opposed to hydrogen can push the net reaction to exothermic, removing the need for an external heat source and reducing the corresponding CO2 and NOxemissions. Furthermore, the need for energy intensive air separation in ODH can be eliminated through a chemical looping strategy, in which active lattice oxygen of a redox catalyst is reversibly donated (to ethane) and replenished (by air) in a cyclic manner.
The reactor design for chemical looping-ODH is complicated by complex reaction kinetics combined with a circulating fluidized bed (CFB) reactor. At the reaction temperatures above 750 °C, significant amounts of thermal cracking in the gas phase occur. The catalyst should be selective for dehydrogenation and/or hydrogen oxidation over the deep oxidation of ethane and olefin products. The ODH reactor catalyst inventory and circulation rate must be sufficient to consume adequate hydrogen to produce the necessary energy without oxidizing significant amounts of olefins. Our work has shown that doped magnesia-supported manganese oxide can achieve high activity and reasonable selectivity for ethylene vs. deep oxidation. In this work, 15 wt.% manganese is impregnated onto a magnesia support with and without sodium or sodium pyrophosphate doping. The reduction kinetics of the catalysts in ethane, ethylene, and hydrogen are characterized in a Setaram SETSYS TGA. An empirical fit is obtained for the reduction kinetics and correlated to the performance of the catalyst for ODH in a packed bed reactor.