C₂ Selectivity Enhancement in Chemical Looping Oxidative Coupling of Methane over a Mg–Mn Composite Oxygen Carrier by Li-Doping-Induced Oxygen Vacancies

Abstract

Chemical looping oxidative coupling of methane (CLOCM) is a promising process for direct methane conversion to C 2 products. Under the chemical looping approach, the oxygen carrier that provides lattice oxygen, in place of molecular oxygen, is used for methane oxidation. This study performs redox experiments that probe the C 2 selectivity enhancement properties of a Mg−Mn composite oxygen carrier through the use of a low concentration of Li dopant. It was found that the C 2 selectivity of the Li-doped oxygen carrier in CLOCM is universally higher than that of the undoped Mg 6 MnO 8 oxygen carrier with a maximum improvement in selectivity of ∼50% . Density functional theory simulation reveals that the Li dopant has a short-range effect on the formation of oxygen vacancies. The Li-doping-induced oxygen vacancy reduces the adsorption energy of methyl radicals and increases the C−H activation barrier. These findings provide a catalytic dopant screening strategy for CLOCM, which will substantially enhance the C 2 selectivity with desired oxygen carrier recyclability.

Chemical looping beyond combustion – a perspective

“…225,228 Experiments with doped-Mg 6 MnO 8 and Mn/Na 2 WO 4 /SiO 2 , a catalyst previously shown to attain single pass C 2+ product yields of B25% in co-feed mode, 229,230 gave C 2+ product yields of 23.2% and 25%, respectively under a chemical looping mode. [231][232][233][234] In these studies, the selectivity toward the coupling reaction was high, but the H 2 molecules formed through the dehydrogenation reaction of ethane competed with CH 4 molecules for reacting with activated oxygen on the surface of the catalyst, resulting in a decreasing rate of CH 4 activation and consequently lower C 2+ yields than expected. 222 Hence, improving the activity of the redox catalysts without negatively affecting their selectivity is one important task.…”

Synthesizing High‐Volume Chemicals from CO2 without Direct H2 Input

“…Despite relatively low ethylene yield (8.5 % at 550 °C and 1 atm pressure), their finding signifies the feasibility of the CL−CO 2 −ODHE concept and suggests potential for further improvements through oxygen carrier design. The application of chemical looping technology to oxidative coupling of methane (CL‐OCM) has also been demonstrated using the state‐of‐the art OCM catalyst, Na 2 WO 4 /Mn/SiO 2 , [185–187] as well as a Li‐doped Mg 6 MnO 8 redox catalyst [188] . For the synthesis of ethylene oxide, Chan et al.…”

A tailored multi-functional catalyst for ultra-efficient styrene production under a cyclic redox scheme

“…1a ). Facilitated by the redox catalyst, redox-ODH follows a chemical-looping-based approach, which has been previously reported for CO 2 capture from fossil fuel combustion 20 – 22 , methane-selective oxidation 23 – 29 , thermochemical water/CO 2 splitting 30 – 37 , air separation 38 – 40 , and ODH of light alkanes 41 – 43 . To our knowledge, previous chemical-looping studies have yet to investigate catalytic conversion to hydrocarbon molecules containing more than four carbon atoms, mainly due to the high operating temperature required by conventional redox catalysts and/or lack of product selectivity 44 , 45 .…”