Fundamentals of enhanced oxygen releasability of Mn-Na2WO4/SiO2 through cofed water for efficient oxidative coupling of methane in a chemical looping mode.

[Display omitted] • The performance of Mn-Na 2 WO 4 /SiO 2 catalysts in CL-OCM can be significantly improved by cofed water. • Water does not alter the scheme of product formation but influences their kinetics. • Water facilitates the releasability of lattice oxygen. • The role of MnO x and Na 2 WO 4 is shown by in situ XRD and UV–vis spectroscopy. The oxidative coupling of methane (OCM) to C 2 H 6 and C 2 H 4 (C 2 -hydrocarbons) is an industrially attractive reaction, which has not yet been commercialized. Apart from the low selectivity to C 2 -hydrocarbons at high degrees of CH 4 conversion, another substantial drawback is the necessity to use pure O 2 when this oxidant and methane are cofed. 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-Na 2 WO 4 /SiO 2 catalysts in the presence of water. In comparison to water-free CL-OCM, both methane conversion and C 2 -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 % CH 4 conversion. The total selectivity to C 2 -hydrocarbons was about 76 %. The analysis of selectivity-conversion relationships for C 2 H 4 , C 2 H 6 , CO and CO 2 proved that water does not alter their general formation pathways but influences the kinetics of their formation. In situ X-ray diffraction, in situ UV–vis spectroscopic, and temperature-programmed tests (H 2 -TPR and O 2 -TPD with or without cofed H 2 O) showed that Mn 2 O 3 acts as the oxygen carrier in CL-OCM. In-situ DRIFTS proved the presence of O 2 – and O 2 2–. They can interact with water yielding OH radicals that accelerate CH 4 conversion. [ABSTRACT FROM AUTHOR]