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

Abstract Selective coupling of methyl radicals to produce C2 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 O2-containing gases are uncontrollable, resulting in limited C2 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 C2 selectivity by three times and achieves a C2 yield as high as 10.9% at bed temperature of only 570 °C. Structure-property studies suggest that Na2WO4 nanoclusters are the active sites for methyl radical coupling. The strong CH3· 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· coupling center, may deliver promising OCM performance at reaction temperatures below the ignition temperature of C2H6 and C2H4 (~600 °C) and is therefore more controllable, safer, and certainly more attractive as an actual process.