(758b) Influence of Copper Site Structure and Proximity in Cu-CHA Zeolites on Partial Methane Oxidation to Methanol

We interrogate Cu-zeolites with well-defined Cu speciation and proximity, prepared using judiciously chosen synthetic and treatment procedures, to facilitate identifying partial methane oxidation (PMO) active sites. The distribution of framework Al in chabazite (CHA) zeolites influences the speciation of mononuclear Cu²⁺ (Z₂Cu) and [Cu(II)OH]⁺ (ZCuOH) [1] and binuclear Cu²⁺ di-oxo complexes [2]. Cu-CHA were synthesized to contain solely Z₂Cu or ZCuOH sites of varying density, or a mixture of Z₂Cu and ZCuOH sites [1], and studied for stoichiometric PMO (oxidative treatment: 20 kPa O₂, 723 K, 6 h, methane exposure: 21 kPa CH₄, 473 K, 0.5 h, methanol extraction: 2.5 kPa H₂O, 473 K, 1 h). Methanol yields (per Cu) do not depend systematically on total Cu or Z₂Cu content, but increase systematically with ZCuOH density. ZCuOH sites are precursors to polynuclear Cu-oxo structures that form after O₂ treatments [2] and behave as PMO active sites [3]. In-situ X-ray absorption spectroscopy showed that inert (He, 723 K) and reducing (CH₄, 473 K) environments led to increasing fractions of Cu(I) with ZCuOH density. In-situ UV-Visible spectroscopy identified mononuclear Cu²⁺ species from d-d transitions (~8,000-16,000 cm^-1) and binuclear Cu²⁺ di-oxo from LMCT (24,000-30,000 cm^-1), and spectral changes were monitored while reducing Cu di-oxo species by CO (523 K) [2] and CH₄ (473 K). O₂ activation of Cu-CHA forms bis(μ-oxo) dicopper(II), trans-1,2-peroxo dicopper(II), and μ-(η²:η²)peroxo dicopper(II) intermediates identified by Raman spectra, whose assignments were confirmed with computational modeling. Multivariate curve resolution-alternative least squares was used to extract time-resolved contributions from Raman spectra to investigate O₂ activation kinetics, which reveal two Cu site types with distinct kinetic behavior.

[1] Paolucci, C. et al., J. Am. Chem. Soc., 2016, 138, 6028.

[2] Li, H., et al., Chem. Sci., 2019, 10, 2373.

[3] Pappas, D. K. et al., J. Am. Chem. Soc., 2017, 139, 14961.