Investigation of the Active-Site Structure of Cu-CHA Catalysts for the Direct Oxidation of Methane to Methanol Using In Situ UV–Vis Spectroscopy

Abstract

Among Cu-containing zeolites, Cu-CHA exhibits relatively high catalytic performance in direct oxidation of CH4 to CH3OH in a continuous-flow reaction of a CH4–O2–H2O gas mixture. The catalytic activity of Cu-CHA varies with its composition, e.g., Cu loading and Si/Al ratio, suggesting formation of different Cu active-site structures depending on the composition. In the present study, to identify catalytically active structures of Cu species, 18 different Cu-CHA catalysts having three Si/Al ratios and six Cu loadings at each Si/Al ratio were prepared, and their catalytic performance was evaluated. Based on the catalytic performance, the Cu species in Cu-CHA catalysts were categorized into three groups: those having high selectivity but low catalytic activity were formed at a low Cu/Al ratio (group A); those having high selectivity and high catalytic activity for CH3OH production were formed at a medium Cu/Al ratio (group B); and those having high activity for the complete oxidation of CH4 to CO2 were formed at a high Cu/Al ratio (group C). Thus, three representative catalysts corresponding to the three groups were selected at each Si/Al ratio and analyzed using in situ UV–vis spectroscopy, from which the spectral features of Cu species involved in the catalytic reaction were extracted from the difference between the spectra under reduction conditions and those under oxidation conditions. Based on the comparison of the spectral features with the simulated spectra of various Cu active-site structures in the literature, the main species of the three groups were suggested: Cu2+ species in the six-membered ring of SSZ-13 zeolites and water-coordinated CuOH+ in the eight-membered ring of SSZ-13 zeolite for group A; mono-(μ-oxo) di-Cu species for group B; and μ-η2:η2 peroxide di-Cu, trans-μ-1,2-peroxo di-Cu and/or mono-/bis-(μ-hydroxo) di-Cu species for group C. Therefore, the mono-(μ-oxo) di-Cu species in Cu-CHA is proposed as the active species for the catalytic direct oxidation of CH4 to CH3OH.