Methane to Methanol Transformation on Cu2+/H-ZSM-5 Zeolite. Characterization of Copper State and Mechanism of the Reaction

Cu-modified zeolites provide methane conversion to methanol with high selectivity under mild conditions. The activity of different possible Cu-sites for methane transformation is still under discussion. Herein, ZSM-5 zeolite has been loaded with Cu2+ cations (1.4 wt% Cu) as characterized by UV-vis DRS, EPR, EXAFS, and 1H MAS NMR. It is inferred that Cu2+ cations, attached to the cation-exchange Al-O--Si sites of the zeolite framework, can exist in the form of either isolated or paired Cu2+ sites. The transformation of methane to methanol on these sites has been verified by the observation of the methoxy species formation using 13C MAS NMR and FTIR spectroscopy. The related mechanisms have been analyzed by DFT calculations. It is obtained that Cu2+ sites enable heterolytic C-H bond dissociation via the "alkyl" pathway resulting in methylcopper species which however are not detected experimentally due to further rapid transformation to surface methoxy species. For the paired Cu2+ sites, such reaction occurs through methyl radical formation and recombination with Si-O--Al site. Based on the obtained data, it has been concluded that methane transformation to methanol on Cu2+ sites, having no extra-framework oxygen ligand, is possible in Cu-modified zeolites.