Surface coupling of methyl radicals for efficient low-temperature oxidative coupling of methane

Selective coupling of methyl radicals to produce C-2 species (C2H4 and C2H6) is a key challenge for oxidative coupling of methane (OCM). In traditional OCM reaction systems, homogeneous transformation of methyl radicals in O-2-containing gases are uncontrollable, resulting in limited C-2 selectivity and yield. Herein, we demonstrate that methyl radicals generated by La2O3 at low reaction temperature can selectively couple on the surface of 5 wt% Na2WO4/SiO2. The controllable surface coupling against overoxidation barely changes the activity of La2O3 but boosts the C-2 selectivity by three times and achieves a C-2 yield as high as 10.9% at bed temperature of only 570 degrees C. Structure-property studies suggest that Na2WO4 nanoclusters are the active sites for methyl radical coupling. The strong CH3 center dot affinity of these sites can even endow some methane combustion catalysts with OCM activity. The findings of the surface coupling of methyl radicals open a new direction to develop OCM catalyst. The bifunctional OCM catalyst system, which composes of a methane activation center and a CH3 center dot coupling center, may deliver promising OCM performance at reaction temperatures below the ignition temperature of C2H6 and C2H4 (similar to 600 degrees C) and is therefore more controllable, safer, and certainly more attractive as an actual process. (C) 2021, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

Automated summary

In this study, methyl radicals generated by La2O3 were shown to selectively couple on the surface of Na2WO4/SiO2 at low temperatures, significantly improving the C-2 selectivity and yield. Na2WO4 nanoclusters were identified as the active sites for methyl radical coupling. This new bifunctional OCM catalyst system holds promise for achieving high OCM performance at lower reaction temperatures.