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 SiO 2 , MgO and TiO 2 ) 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% Mn 2 O 3 supported on SiO 2 with/without 2.0 wt% Na 2 CO 3 dopants) exhibited excellent methane coupling activity. The averaged CH 4 conversion, C 2 selectivity and yield of Mn20Na2Si upon multiple redox cycles were 21.4 %, 79.6 % and 17.03 %, respectively. It showed improvements in C 2 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 C 2 -selectivity and CH 4 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, CH 4 conversion, C 2 selectivity and cyclic stability.
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