Exchanged oxo-Cu2+ cations in mordenite zeolites: Evidencing the truly active in the stepwise conversion of methane to methanol from in situ DRS-UV–vis analyses performed during activation and reaction

Abstract

After activation, Cu²⁺-exchanged zeolites generate exchanged oxo-Cu²⁺ cations that absorb UV–vis radiation in all of the d-d and LMCT transitions range. On the other hand, based on great experimental and/or theoretical data, bi- or trinuclear exchanged oxo-Cu²⁺-zeolites with attributed absorption UV–vis bands centered between 20,000 and 35,000 cm^-1, are the mainly considered as the potential active species to directly convert methane to methanol (MTM) in a stepwise process, which obviously leads to low copper specific activities (TON). What about the others generated oxo-Cu²⁺ cations? To elucidate this question, we developed a systematic study using catalytic evaluation data of oxo-Cu²⁺ mordenites and in situ DRS-UV–vis analyses performed during air activation at 550 °C and MTM conversion at 200 °C. The obtained data allowed to confirm that the less-generated oxo-Cu²⁺ cations related to absorption DRS-UV–vis bands centered at wavenumbers lower than 33,000 cm^-1, are the effectively active species to directly oxidize the methane molecule to methanol, but also evidencing that the most generated oxo-Cu²⁺ cations are poorly active, thus explaining the reported low copper TON values of activated Cu²⁺-exchanged zeolites. This finding must stimulate the involved scientific community to find more effective activation procedures that can promote the formation of such more active oxo-Cu²⁺ cations.