On the Mechanistic Aspects of Mg6MnO8-Based Redox Catalysts for Oxidative Dehydrogenation of Ethane Via a Chemical Looping Scheme

Redox catalysts, which perform selective or deep oxidation reactions utilizing lattice oxygen, have become increasingly attractive due to their potential to facilitate high energy conversion efficiency with low CO₂ and NO_x emissions_. Oxidative dehydrogenation of ethane (ODH) is one very promising application of chemical looping. In chemical looping-ODH, ethane is partially oxidized by a redox catalyst to form water and ethylene. The regeneration of lattice oxygen lost to water formation provides the heat necessary to drive the endothermic dehydrogenation reaction. Although such an approach possesses numerous advantages relative to conventional cracking, achieving industrially relevant conversion in ODH while suppressing deep oxidation is not trivial. Additionally, thermal cracking of ethane to ethylene and hydrogen can be significant at temperatures where chemical looping normally operates. It is therefore important to ensure facile supply of oxygen from the lattice without permitting significant oligomerization or deep oxidation of ethylene. As such, detailed mechanistic understanding of redox catalyst in ODH is highly desirable to support rational catalyst design and optimization.

Previously, we have identified that Mg₆MnO_8­, a mixed oxide with a cation deficient rocksalt structure, is an excellent model redox catalyst for ODH. It is capable of supplying lattice oxygen at rates comparable to the rate of hydrogen formation via thermal cracking of ethane. Unpromoted Mg₆MnO₈ is highly efficient for deep oxidation of ethane, producing primarily of CO₂and water. When promoted with alkali salts, however, it can produce ethylene with exceptional selectivity.

The current work investigates the mechanistic aspects of (promoted) Mg₆MnO₈ redox catalysts. Extensive characterizations including TPR, TPD, XPS, in-situ XRD, in-situ DRIFTS, Raman Spectroscopy, and pulse reactions are performed. The data indicate that alkali salt promoters induce changes in the bulk and near surface properties of the Mg/MnO system and suppress activation and deep oxidation of ethane and ethylene. Meanwhile, the promoted redox catalysts allow facile combustion of hydrogen, leading to high ethylene yield.