Elucidation of the Structure-Activity Relationship for Cu-Erionite in the Direct Conversion of Methane to Methanol: An Operando XAS Study.

The partial oxidation of methane to methanol over copper-exchanged zeolites offers a promising avenue for methane valorization. Numerous zeolites have been demonstrated to be active for the selective oxidation of methane, with the methanol yield varying significantly depending on the zeolite framework, Si/Al ratio, and copper loading. Herein, we present a comprehensive study of one of the most active Cu-erionite (Cu-ERI) zeolites with different compositions for the stepwise conversion of methane to methanol, aiming to elucidate the relationship between the methanol yield and the nature of copper species in Cu-ERI zeolites. Operando X-ray absorption spectroscopy (XAS), combined with Fourier-transform infrared spectroscopy (FTIR), allows us to establish a correlation that reveals the dependence of the methanol yield on the reduction rate of copper species. Our findings demonstrate that the Cu/Al ratio plays a crucial role in determining the reducibility of copper species in Cu-ERI zeolites, which in turn governs methanol yield normalized to the copper content. While the Si/Al ratio of the parent zeolite determines the achievable copper loading and the maximal methanol yield, it does not influence the normalized methanol yield. This work suggests that controlling the Cu/Al ratio is essential for maximizing copper efficiency and achieving selective methane partial oxidation. At a fixed optimal Cu/Al ratio, increasing the Al content enhances the total methanol yield by providing more copper exchange sites. The structure-activity relationship of Cu-ERI zeolites in the direct conversion of methane to methanol offers valuable insights into the interplay between the zeolite host and copper species, highlighting the importance of both Cu/Al and Si/Al ratios in designing selective, high-performance materials for this challenging reaction.