Monocopper Active Site for Partial Methane Oxidation in Cu Exchanged 8MR Zeolites
Direct conversion of methane to methanol using oxygen is experiencing renewed interest owing to the availability of new natural gas resources. Copper exchanged zeolites such as mordenite and ZSM 5 have shown encouraging results, and di and tri copper species have been suggested as active sites. Recently, small eight membered ring (8MR) zeolites including SSZ 13, 16, and 39 have been shown to be active for methane oxidation, but the active sites and reaction mechanisms in these 8MR zeolites are not known. In this work, we use density functional theory (DFT) calculations to systematically evaluate monocopper species as active sites for the partial methane oxidation reaction in Cu exchanged SSZ 13. On the basis of kinetic and thermodynamic arguments, we suggest that [CuIIOH] species in the 8MR are responsible for the experimentally observed activity. Our results successfully explain the available spectroscopic data and experimental observations including (i) the necessity of water for methanol extraction.