Tuning Copper Active Site Composition in Cu-MOR through Co-Cation Modification for Methane Activation

The industrial implementation of a direct methane to methanol process would lead to environmental and economic benefits. Copper zeolites successfully execute this reaction at relatively low temperatures, and mordenite zeolites in particular enable high methanol production. When loaded to a Cu/Al ratio of 0.45, mordenite (Si/Al 5 to 9) has been shown to host three active sites: two [CuOCu]²⁺ sites labeled MOR1 and MOR2, and a mononuclear [CuOH]⁺ site. Also at low copper loadings (Cu/Al < 0.20), mordenite has been demonstrated to activate methane, but its active site has never been reported. Here, we investigate Na⁺ mordenite with varying copper loadings to better understand copper speciation in mordenite. At low copper loadings, we uncover an unidentified active site ('MOR3') with a strong overlap with the [CuOH]⁺ site's spectroscopic signal. By changing the co-cation, we selectively speciate more MOR3 relative to [CuOH]⁺, allowing its identification as a [CuOCu]²⁺ site. Active site identification in heterogeneous catalysts is a frequent problem due to signal overlap. By changing cation composition, we introduce an innovative method for simplifying a material to allow better analysis. This has implications for the study of Cu zeolites for methane to methanol and NO_x catalysis, but also for studying and tuning heterogeneous catalysts in general.