CH3•-Generating Capability as a Reactivity Descriptor for Metal Oxides in Oxidative Coupling of Methane

Methyl radicals (CH3 center dot) are the key intermediates in the heterogeneous-homogeneous reaction processes of catalytic oxidative coupling of methane (OCM). Here, by applying in situ synchrotron-based vacuum ultraviolet photoionization mass spectrometry, we quantitatively detected CH3 center dot being desorbed from various metal oxides and validated the CH3 center dot-generating capability as an effective descriptor for the catalytic performance of the metal oxides in OCM. It is found that the C-2 yield is linearly correlated to the amount and the desorption temperature of CH3 center dot, with the better OCM catalyst showing stronger CH3 center dot intensity and lower CH3 center dot desorption temperature. Furthermore, experimental characterizations together with density functional theory calculations suggest that the intrinsic electronic properties of metals and the subsequent generated electrophilic oxygen species are the decisive factors for CH3 center dot generation. Then, the CH3 center dot-generating capability can bridge the gap between the OCM performance and the structure of the catalyst and help us better understand the intrinsic structure-performance relationship in OCM over metal oxide catalysts.

Automated summary

The study highlights the importance of methyl radicals in the catalytic oxidative coupling of methane (OCM), showing a linear correlation between the amount and desorption temperature of CH3 center dot and C-2 yield. The intrinsic electronic properties of metals and the generated electrophilic oxygen species are crucial factors for the generation of CH3 center dot.