(580e) Quantitative Connections between Copper Site Proximity and Binuclear Active Sites for Partial Methane Oxidation to Methanol in Cu-CHA Zeolites

Copper-zeolites facilitate partial oxidation for methane-to-methanol (PMO), with mononuclear and multinuclear active sites proposed [1,2] that form upon dioxygen activation. Here, we use a high-symmetry (single tetrahedral-site) zeolite framework (CHA) as a model support to investigate the effects of mononuclear Cu site type and proximity on the formation of binuclear O_x-bridged sites that form upon dioxygen activation, and the consumption of binuclear sites in reduction (CH₄, CO, He auto-reduction) treatments. CHA was synthesized to vary the relative amounts of 6-MR paired and isolated Al sites, which respectively exchange Cu²⁺ (Z₂Cu) or [CuOH]⁺ (ZCuOH) motifs [3], with the latter forming binuclear O_x-bridged complexes upon O₂ activation (723 K) [4]. Cu-CHA containing solely Z₂Cu do not auto-reduce (723 K), form multinuclear O_x-bridged structures upon O₂ activation (723 K), or form methanol as reported previously [1]. In contrast, O_x-bridged Cu sites that reduce in methane and undergo auto-reduction are quantitively identical, and generally increase as the mean distance between ZCuOH decreases; ZCuOH sites are thus precursors to PMO-active sites. Increasing ZCuOH proximity leads to increasing numbers of trans-μ-1,2-peroxo and mono-μ-oxo dicopper(II) (identified by in situ UV-Visible and Raman spectroscopies [5]) that form upon O₂ activation, and are quantified with CO reduction (TPR) and predicted from statistical Cu siting models that account for mononuclear and binuclear structures. In situ Cu K-edge X-ray absorption spectra show that inert (He, 723 K) and reducing (CH₄, 473 K) treatments form Cu(I) in increasing fractions with decreasing mean ZCuOH distance (Figure 1). This investigation illustrates how integrating synthesis routes to control Cu site speciation and density, in situ methods to characterize and quantify Cu sites, and theoretical Cu siting models can elucidate the mechanisms of Cu(I) oxidation and Cu(II) reduction in different gas environments and the atomic-scale properties that determine the number of Cu sites that facilitate PMO.