Room-Temperature Activation of Methane and Direct Formations of Acetic Acid and Methanol on Zn-ZSM-5 Zeolite: A Mechanistic DFT Study

Zn-ZSM-5 zeolite is a promising catalyst that activates methane at room temperature without the need of a high-temperature pre-oxidation step, which is required for Fe- and Cu-ZSM-5 to form Fe- and Cu-oxo active sites. While two distinct structures of Zn active site, namely [Zn-O-Zn](2+) and Zn2+, were experimentally proposed, the mechanism of how the C-H bond of methane is cleaved is still an intense debate. In addition, the mechanism for moderate-temperature formation of acetic acid by CO2 insertion to the CH4-reacted Zn-ZSM-5 is unclear and the possibility of methanol formation in the presence of an oxidant has never been explored. In the present study, we performed density functional theory (DFT) calculations on the periodic structure of Zn-ZSM-5 zeolite to investigate and clarify these issues. We found that the C-H bond of methane is preferably cleaved on the mononuclear Zn2+ active site through a heterolytic, non-radical mechanism, where the resultant CH3 is bound to the Zn center (Zn-CH3) in the closed-shell singlet state. A good agreement with the reported experimental C-H activation barrier is achieved and plausible mechanisms for the CO2 insertion to and N2O decomposition on the Zn-CH3 bond forming acetic acid and methanol, respectively, are discussed. This study provides a theoretical prediction of an alternative metal-exchanged zeolite catalyst for the low-temperature continuous process of methane selective oxidation to methanol.