(117a) Investigating the Influence of H2o on Cu Active Site Mobility, Structure, and Reactivity for Continuous Partial Methane Oxidation

Cu-exchanged zeolites catalyze partial methane oxidation (PMO) in a stoichiometric process, in which each reactant (O₂, CH₄, H₂O) is introduced step-wise, or a continuous process, in which all reactants are contacted simultaneously. Mononuclear ZCuOH sites condense to form binuclear Cu species (Cu₂O_y, y=1,2) heterogeneous in structure and reactivity during the stoichiometric oxidation step (e.g., 0.2 bar O₂, 723 K),¹ but it remains unclear how water, present under continuous conditions, influences Cu site mobility, speciation, and reactivity. Water was proposed by Dinh et al. to assist Cu ion diffusion between proton sites to form Cu₂O species that selectively oxidize CH₄ to CH₃OH, which can undergo sequential oxidations to form secondary CO and CO₂ products, consistent with observed total CH₄ oxidation rates independent of Cu density.² In contrast, Ohyama et al. propose varying the zeolite framework Al arrangement forms different Cu₂O_y structures that activate CH₄ with inherently distinct rates and selectivity to CH₃OH and CO₂.³ Here, we report individual product (CH₃OH, CO, CO₂, CH₃OCH₃) formation rates as a function of Cu density on the same CHA parent sample from continuous PMO data extrapolated to zero conversion to account for conversion effects on selectivity. We oxidized the samples under continuous conditions (0.07 kPa O₂, 3 kPa H₂O), performed a reduction in CH₄ or CO, quantified the fraction of CH₄- and CO-reducible sites with H₂ temperature programmed reduction (TPR), and then compared these experimental quantifications to the DFT-predicted thermodynamic Cu speciation. Kinetic and quantitative characterization experiments were then repeated on the same Cu-CHA samples titrated with Na, predicted to poison proton sites and suppress Cu ion mobility. These data provide new insight into the influence of H₂O on Cu active site formation for continuous PMO reactions and establishes relationships between Cu sites and individual product yields through a combination of kinetics and quantitative characterization.