Lattice oxygen behaviors of Mn-based catalytic oxygen carriers and sustainable oxidative coupling of methane in chemical-looping scheme

Developing robust catalytic oxygen carriers (COC) to make Chemical-looping Oxidation Coupling of Methane (CL-OCM) economical and practical remains a big challenge. This work modified the Mn-based catalysts for co-feed OCM and tried to adapt them into CL-OCM process. COCs were specially designed with high faction of Mn-oxides to improve oxygen storage, with supports (pelletized from SiO2, MgO and TiO2) of stable and porous structure to improve cyclic stability and to tune lattice oxygen reactivity, and with Na+ dopant to improve conversion and selectivity. Among the COCs, Mn20Na2Si and Mn20Si (20 wt% Mn2O3 supported on SiO2 with/without 2.0 wt% Na2CO3 dopants) exhibited excellent methane coupling activity. The averaged CH4 conversion, C2 selectivity and yield of Mn20Na2Si upon multiple redox cycles were 21.4 %, 79.6 % and 17.03 %, respectively. It showed improvements in C2 selectivity and yield by 187 % and 163 % compared to Mn20Si. This work tried to establish the relationships between CL-OCM performance and lattice oxygen behaviors, since COC transports lattice oxygen to promote methane coupling as it performs as a catalyst during CL-OCM reduction. It is found that high C2-selectivity and CH4 conversion are benefited from the constant oxygen transporting rate. Na+ dopant improved the concentration of surface lattice oxygens, tuned the spatial arrangement of metal oxides, and stabilized the surface lattice oxygen in cyclic reactions. Due to Na+-doping, carbon deposition on Mn20Na2Si was avoided, and catalyst deactivation as well. Finally, a mechanism was developed to reveal how lattice oxygen species and their releasing rates affected surface reaction network, CH4 conversion, C2 selectivity and cyclic stability.