Stepwise methane to methanol conversion: Effect of copper loading on the formation of active species in copper-exchanged mordenite

• Isolated Cu 2+ and [CuOH] + species were the precursors of Cu sites with high activity for CH 4 activation. • The optimum activity was strongly dependent on the Cu/Al ratio. • The Si/Al ratio of the zeolite framework determined the Cu/Al ratio for higher activity. • The copper active species were identified throughout in situ Diffuse Reflectance spectroscopy (DRS). • DRS revealed different copper actives species and their dynamic behavior with temperature. This work investigates the influences of Cu content, Si/Al ratio, and sample preparation method on the active Cu species in mordenite (MOR). The Cu/MOR prepared by multiple Cu-exchange procedures showed an optimum Cu/Al ratio for the formation of the most active species for methanol formation. XANES spectra indicated that isolated Cu + species were formed after CH 4 interaction and were re-oxidized under a flow of water steam. The Cu-MOR samples obtained by ion-exchange method presented well-dispersed CuO in the zeolite pores, while samples obtained by impregnation method presented segregated CuO and were inactive for CH 4 activation. The methodology used for in situ UV–vis spectra analyses indicated that different Cu species were active for CH 4 oxidation. Isolated Cu 2+ species were not involved in CH 4 activation at temperatures lower than 300 °C. The isolated Cu 2+ species were partially hydrolyzed by O 2 treatment and species like [CuOH] + and [CuO x ] n or polynuclear Cu 2+ -oxyhydroxide were formed. The formation of polynuclear species in substitution of di-copper would impact the specific activity with the Cu content and decrease the activity for methanol production.