Enhanced Chemical looping oxidative coupling of methane by Na-doped LaMnO3 redox catalysts

Abstract

Chemical looping oxidative coupling of methane (CLOCM) describes an energy-saving and redox approach to convert methane into C2 products, which utilizes lattice oxygen in redox catalysts in place of molecular oxygen, to facilitate the oxidative coupling of methane reaction. This study synthesized a series of alkali Na-doping LaMnO3 as the redox catalysts for chemical looping oxidative coupling of methane. Incorporation of Na into LaMnO3 lattice induced the higher molar ratio of Mn4+/Mn3+ and more oxygen vacancies, which would be correlated to defects inside the material lattice and consequently promote the redox performance of Mn-based oxygen carriers. Characterizations and consecutive pulse tests showed that the surface oxygen species would be the active center for the methane activation, while a kind of un-fully reduced lattice oxygen (Oα-, 0 < α < 2) caused by the doping of Na benefits the methane oxidative coupling reaction to form C2 hydrocarbons. Among all the samples, 018Na-doped redox catalysts exhibited the highest C2 product yield and 7.44 times increase in C2 yield can be obtained compared to the un-doped LaMnO3. Good regenerability of 018Na-LaMnO3 was demonstrated through 20 consecutive cyclic redox tests, exhibiting above 30% methane conversion, ~55% C2 product selectivity and 20% of C2 hydrocarbon yield, which will substantially provide a screening strategy for oxygen carrier with good recyclability.