Methane to Methanol Transformation on Cu/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 Cu-sites for methane transformation is still under discussion. Herein, ZSM-5 zeolite has been loaded with Cu cations (1.4 wt % Cu) as characterized by UV-vis DRS, EPR, EXAFS, and H MAS NMR. It is inferred that Cu cations, attached to the cation-exchange Al-O-Si sites of the zeolite framework, can exist in the form of either isolated or paired Cu sites. The transformation of methane to methanol on Cu/H-ZSM-5 has been verified by the observation of the methoxy species formation with C MAS NMR and FTIR spectroscopy. The related mechanisms have been analyzed by DFT calculations. The calculations show that the paired Cu 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 through methyl radical formation and recombination with Si-OAl site. Based on the obtained data, it has been concluded that methane transformation to methanol on paired Cu sites, having no extra-framework oxygen ligand, is possible in Cu-modified zeolites. The pathways of Cu cations regeneration with O and HO have been experimentally explored.