Fundamentals of Enhanced Oxygen Releasability of Mn-Na2wo4/Sio2 Through Cofed Water for Efficient Oxidative Coupling of Methane in a Chemical Looping Mode

Abstract

The oxidative coupling of methane (OCM) to C2H6 and C2H4 (C2-hydrocarbons) is an industrially attractive reaction, which has not yet been commercialized. Apart from the low selectivity to C2-hydrocarbons at high degrees of CH4 conversion, another substantial drawback is the necessity to use pure O2 when this oxidant and methane are co-fed. Original OCM studies used an approach called chemical looping OCM (CL-OCM) that consists of alternating feeding of methane and air for product formation and catalyst reoxidation, respectively. The developed catalysts, however, suffer from their low ability to provide lattice oxygen for the desired reaction. Herein, we demonstrate that this catalyst property can be significantly improved when performing CL-OCM over Mn-Na2WO4/SiO2 catalysts in the presence of water. In comparison to water-free CL-OCM, both methane conversion and C2-hydrocarbons selectivity increase resulting in at least doubling of the yield of these products. The selectivity to ethylene of about 53% was obtained at about 24% CH4 conversion. The total selectivity to C2-hydrocarbons was about 76%. The analysis of selectivity-conversion relationships for C2H4, C2H6, CO and CO2 proved that water does not alter their general formation pathways but influences their kinetics.In situ X-ray diffraction, in situ UV-vis spectroscopic, and temperature-programmed tests (H2-TPR and O2-TPD with or without cofed H2O) showed that Mn2O3 acts as the oxygen carrier in CL-OCM. Cofed water facilitates the releasability of lattice oxygen of this metal oxide. This process is suggested to occur through the formation of surface bi-atomic oxygen species. In-situ DRIFTS proved the presence of O2- and O22-. They can react with water to yield a surface mono-atomic oxygen species and H2O2. The latter decomposes to OH radicals. They and the formed oxygen species participate in homogeneous and heterogeneous CH4 conversion into CH3 radicals, respectively, which recombine to C2H6.

Keywords: chemical looping, oxidative coupling of methane, co-fed water, oxygen species, mobility, water effect, Mn-Na2WO4/SiO2