Nonoxidative Couplings of Methane to Form Ethylene Catalyzed by Coinage Metal-Containing ZSM-5 Zeolites: A Theoretical Study

Abstract

DFT-based ONIOM calculations were employed to investigate the potential energy surfaces (PESs) for the conversion of methane to ethylene by a ZSM-5 zeolite containing a single coinage metal atom whose formal charge is +1 (M-ZSM-5). According to the ONIOM calculations, two methanes are activated by M-ZSM-5 to form ethylene and H2 via ethane. The mechanisms consist of the following elementary steps: activation of a C–H bond of two methanes by M-ZSM-5 to form CH3–M–CH3, a C–C bond formation in CH3–M–CH3 to form ethane and M-ZSM-5, and sequential activation of two C–H ethane bonds by the regenerated M-ZSM-5 to form ethylene. Depending on the metal cations, various active sites appear, reflecting whether Brønsted acid sites (BASs) are formed after the alkane C–H activation. Three types of active sites are present in Cu- and Ag-ZSM-5: metal cations, methyl-metal (M–CH3) species, and BASs. In contrast, the active sites in Au-ZSM-5 consist of Au cations or H–Au–alkyl species. Looking at their PESs, we found two energy-demanding steps: the second methane activation to form CH3–M–CH3 for all M-ZSM-5 and the second C–H bond activation of ethane by BASs (M = Cu and Ag). We compared their activation energies to find that Cu- and Au-ZSM-5 are catalysts that are more suitable for forming ethylene from methane and ethane, respectively.