(322b) Mechanistic Insight for Oxidative Coupling of Methane on Mg6MnO8-Based Redox Catalysts in a Chemical Looping System

Methane activation has been the topic of interest for many decades for its upgradation to value added chemicals. With the high availability of shale gas due to advancements in fracking technology, understanding this C-H bond activation becomes important as it is a key reaction step in the direct synthesis of chemicals from methane. Traditionally, methane is first converted to syngas, which can then be processed and used for producing chemicals and fuels. This indirect method is both process and energy intensive and thus incentivizes direct conversion of methane to chemicals. One such method for direct conversion is oxidative coupling of methane (OCM), which provides a single step path towards synthesizing higher hydrocarbons from methane, especially ethylene. A popular way for methane oxidation in OCM is through co-feeding molecular oxygen with methane over a catalyst. Conversely, the chemical looping method utilizes the lattice oxygen in a catalytic oxygen carrier (COC) for methane oxidation. A Mg₆MnO₈-based redox system was studied under the chemical looping method for understanding the mechanism of OCM. The atomistic thermodynamics methods and density functional theory (DFT) calculations were carried out to elucidate the lattice oxygen diffusion and vacancy formation mechanism during the oxidation and reduction process.

Additionally, experiments in a fixed bed reactor were performed to validate the computational findings, by running cyclic redox reactions on a fixed amount of COC. Fixed bed experiments were performed with methane as the reducing gas and air as the oxidizing gas. Several cofeed experiments were also performed in addition to the redox experiments to get a holistic view of the process. These results were coupled with thermogravimetric analysis (TGA) to investigate the effect of temperature on the OCM reaction. A dopant screening was also investigated computationally, to potentially increase the selectivity towards the desired hydrocarbon products. Several analysis techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) were used to characterize the COC in its oxidized and reduced form. It was observed that the advent of carbon monoxide in conjunction to the desired hydrocarbons as the OCM reaction progresses gives an insight into the importance of oxygen vacancy formation on the COC surface.