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

Among Cu-containing zeolites, Cu-CHA exhibits relativelyhigh catalyticperformance in direct oxidation of CH4 to CH3OH in a continuous-flow reaction of a CH4-O-2-H2O gas mixture. The catalytic activityof Cu-CHA varies with its composition, e.g., Cu loading and Si/Alratio, suggesting formation of different Cu active-site structuresdepending on the composition. In the present study, to identify catalyticallyactive structures of Cu species, 18 different Cu-CHA catalysts havingthree Si/Al ratios and six Cu loadings at each Si/Al ratio were prepared,and their catalytic performance was evaluated. Based on the catalyticperformance, the Cu species in Cu-CHA catalysts were categorized intothree groups: those having high selectivity but low catalytic activitywere formed at a low Cu/Al ratio (group A); those having high selectivityand high catalytic activity for CH3OH production were formedat a medium Cu/Al ratio (group B); and those having high activityfor the complete oxidation of CH4 to CO2 wereformed at a high Cu/Al ratio (group C). Thus, three representativecatalysts corresponding to the three groups were selected at eachSi/Al ratio and analyzed using in situ UV-vis spectroscopy,from which the spectral features of Cu species involved in the catalyticreaction were extracted from the difference between the spectra underreduction conditions and those under oxidation conditions. Based onthe comparison of the spectral features with the simulated spectraof various Cu active-site structures in the literature, the main speciesof the three groups were suggested: Cu2+ species in thesix-membered ring of SSZ-13 zeolites and water-coordinated CuOH+ in the eight-membered ring of SSZ-13 zeolite for group A;mono-(mu-oxo) di-Cu species for group B; and mu-eta 2:eta 2peroxide di-Cu, trans-mu-1,2-peroxo di-Cu and/ormono-/bis-(mu-hydroxo) di-Cu species for group C. Therefore,the mono-(mu-oxo) di-Cu species in Cu-CHA is proposed as theactive species for the catalytic direct oxidation of CH4 to CH3OH.

Automated summary

Cu-CHA exhibits high catalytic performance in the direct oxidation of CH4 to CH3OH. The catalytic activity varies with the composition of Cu-CHA. Three groups of Cu species were identified based on their catalytic performance: low Cu/Al ratio for high selectivity and low activity, medium Cu/Al ratio for high selectivity and high activity in CH3OH production, and high Cu/Al ratio for high activity in complete oxidation of CH4 to CO2. The mono-(mu-oxo) di-Cu species in Cu-CHA is proposed as the active species for the catalytic reaction.