Mechanism Investigation on Direct Conversion of Methane over a Mononuclear Rh-ZSM-5 Catalyst: Multiple Roles of CO

In the present contribution, we investigated possible reaction mechanisms for the direct conversion of methane into methanol and acetic acid over a mononuclear rhodium species anchored on a zeolite support (Rh-ZSM-5) by density functional theory (DFT) calculations. To elucidate the role of CO, we systematically compared the reaction mechanisms in the absence/ presence of CO. The most favored mechanism is shown to be via a CO-assisted oxygen activation, and the formation of the CO-O bond is found to be the rate-determining step. It is clearly shown from our calculations that the presence of CO will promote the formation of a more active Rh-oxo species, which can easily catalyze the conversion of methane so that the direct transformation of methane can be achieved under mild conditions. Apart from forming a stable catalytic precursor and promoting the formation of the active Rh-oxo species, CO can also be directly involved in the reaction, resulting in the formation of acetic acid.