Highly efficient CO-assisted conversion of methane to acetic acid over Rh-encapsulated MFI zeolite prepared using RhCl3 molten salt

Abstract

Direct catalytic conversion of methane to value-added chemicals with a high reaction rate and selectivity under mild conditions remains a great challenge. Rh-supported zeolite is one of the promising heterogeneous catalysts capable of selectively oxidizing methane to acetic acid using carbon monoxide and oxygen at low temperatures. However, the difficulty in the preparation of the catalytically active species, i.e., zeolite-confined Rh^I single-atom sites, has limited the mass activity. Here, we successfully prepared a highly-active Rh-containing zeolite using RhCl₃ molten salt. By high-temperature treatment of the physically-mixed H⁺-MFI zeolite and bulk RhCl₃, the RhCl₃ molten salt was formed under in situ conditions and then diffused into the zeolite pores, resulting in being trapped within the zeolite cavity via host–guest interaction and ion-exchange with H⁺. The encapsulated RhCl₃ species are efficiently dispersed with the formation of Rh^I single-atom species as the active sites in the catalytic reaction, boosting the direct conversion of methane to acetic acid with a record turnover frequency (592 mol mol_Rh⁻¹ h⁻¹) and yield (14.6 mmol g_cat⁻¹ h⁻¹) with high selectivity (76.9%).