(651b) Alkali Promoted Mg6MnO8 Redox-Catalysts for Chemical Looping-Oxidative Dehydrogenation of Ethane: Mechanistic Investigations

Steam cracking of ethane to ethylene is one of the most energy and carbon intensive processes in the chemical industry. While several decades of process improvements have pushed cracking technologies to near 95% thermal recovery, high reaction endothermicity, rapid temperature swings, and significant downstream separation costs, result in significant exergy loss (lost work). Oxidative dehydrogenation (ODH), which partially oxidizes ethane to ethylene and water, has been suggested as a pathway to process intensification. Conventional ODH, which uses gaseous oxygen, has significant challenges to overcome due to the difficulty of achieving high per-pass yields in a controllable manner and the high capital and energy costs imposed by air separation. We have introduced a chemical looping approach to ODH (CL-ODH), which uses the lattice oxygen of a redox catalyst as an oxidant. The redox catalyst lattice oxygen is subsequently regenerated in air in a separate reaction step, producing heat to drive the process. Such a cyclic redox scheme eliminates the needs for O₂ co-feeding and hence cryogenic air separation. Redox catalyst performance is vital to this process; the redox catalyst must be highly selective towards water vs. CO_x formation. It is also important to keep hydrocarbon residence times low to prevent the formation of undesired hydrocarbon byproducts such as benzene. As such, a detailed mechanistic understanding of redox catalyst in ODH is highly desirable to allow rational catalyst design and optimization.

We have reported a model Mg₆MnO₈ catalyst system with excellent CL-ODH properties. This system is interesting due to its facile lattice oxygen (O^2-) transport ability, and the very strong effects of alkali promoters on product selectivity. Without promotion Mg₆MnO₈ is highly active for deep oxidation, but the use of alkali salts as promoters significantly shifts the product selectivity towards ethylene even at high ethane conversions (>70% single pass). In this work, we describe the mechanistic aspects of promoted and unpromoted Mg₆MnO₈ redox catalysts. Extensive characterizations, including surface characterization, temperature programed reactions, and in-situ DRIFTS were performed. We also explored the lattice oxygen behavior with TGA, O¹⁸ labeling experiments, and in-situ XRD. These experimental data indicate that alkali salt promoters suppress ethane and ethylene surface activation while the bulk structure maintains fast lattice oxygen transport during the redox reactions.