Reaction pathways of oxidative coupling of methane on lithiated lanthanum oxide

Abstract

The reaction pathways of oxidative coupling of methane (OCM) on lithium-doped lanthana were investigated using CH4CH4/O2 step reactions. As compared with those of the undoped catalysts, the results demonstrate that the lithiated lanthana first initiates activation of methane on surface oxygen species and then catalyzes the OCM, followed by production of oxidation products via chain reaction pathways. The role of doped lithium and effect of CH4/O2 ratios for the OCM were also studied. The presence of lithium was believed to promote the reaction via formation of carbonates, which gives rise to the oxidized surface that catalyzes conversion of ethane to ethylene together with complete oxidation of methane. This was also confirmed by experimental results and theoretical DFT calculations, which indicates that C2H6 and C2H4 are favored in O2-lean and O2-rich atmospheres, respectively. The activation energy of deoxyhydrogenation for C2H6 to C2H4 follows an order of La2O3 > Li/La2O3 > O2-Li/La2O3, explaining the variation of product selectivity in different CH4/O2 compositions.