2024 journal article
Synergistic Cooperation of Dual-Phase Redox Catalysts in Chemical Looping Oxidative Coupling of Methane
ACS Catalysis.
UN Sustainable Development Goal Categories
13. Climate Action
(Web of Science)
Source: ORCID
Added: August 22, 2024
Chemical looping oxidative coupling of methane (CL-OCM) presents a promising route for light olefin production, offering a simpler alternative to conventional methane steam reforming approaches. The selection of the redox catalyst used in CL-OCM is critical since it must achieve high C2+ yields (>25%) while maintaining longevity in harsh reaction environments. We present a comprehensive performance evaluation and characterization of an understudied, yet highly effective redox catalyst capable of achieving and maintaining a C2+ yield of 26.8% at 840 °C. Through extensive ex situ and in situ analyses, including X-ray diffraction, near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), and Raman spectroscopy, we have characterized the catalyst and identified two distinct bulk, crystalline phases: cubic LixMg6–xMnO8 and orthorhombic Mg3–xMnx(BO3)O2. Calcination at 1200 °C, as opposed to a typical calcination temperature of 900 °C, increased the orthoborate oxide phase to ∼45 wt % while reducing the BET surface area by 65%. By investigating performance differences between these catalysts in their "sintered" and "presintered" states, we have unveiled surprising cooperative effects between the two phases. Experiments with physical mixing of these two phases (granular stacking and mortar mixing) revealed that observed differences in CL-OCM efficacy cannot be solely due to sintering-induced loss of surface area but are also the result of synergistic, dual-phase interactions that enhance overall C2+ yield. H2-temperature programmed reduction measurements and ex situ XPS analysis demonstrate that the sintered catalyst has a lower average Mn-oxidation state, enabling more selective lattice oxygen release and limiting overoxidation to COx species. Additionally, NAP-XPS and in situ Raman characterization suggest that boron–oxygen coordinated sites (BOx) may also play a role in improving selectivity. Leveraging insights from our phase mixture CL-OCM performance tests, steady-state experiments with cofed O2, and corroborative in situ characterizations, we propose that the synergistic interplay between LixMg6–xMnO8 and Mg3–xMnx(BO3)O2 may be the result of facile oxygen release from the more redox-active LixMg6–xMnO8 phase combined with Li+ migration to the orthoborate oxide phase.