(624d) Predicted Cu Ion Speciation and Correlations with Observed Continuous Partial Methane Oxidation Activity over Cu-CHA Catalysts

Valorization of methane (CH₄) by partial methane oxidation (PMO) to methanol can be carried out both step-wise and continuously over Cu-Chabazite (Cu-CHA) catalysts. Continuous PMO is attractive as a simpler process, however, the selectivity observed in the literature is below the step-wise process. A wide variety of Cu species have been proposed as possible PMO active sites, and their identities have been connected to environmental conditions, Si/Al ratio, Cu loading, and zeolite synthesis recipe. Experimental observations suggest that Cu mobility plays a role in forming active Cu sites under continuous partial methane oxidation (c-PMO) conditions. To rationalize these observations, we employed density functional theory to calculate the relative free energies of formation of candidate Cu monomers and dimers as a function of composition, conditions, and hosting site. We find Cu ions are most likely unsolvated during c-PMO. Rather, Cu ions prefer to be anchored to the CHA framework. Further, at many host sites, Cu monomers and hydroxylated Cu dimers have similar free energies, suggesting that Cu speciation during c-PMO differs from that during stepwise PMO. We used statistical models of Al distributions, combined with the free energy results, to predict Cu speciation as a function of Si/Al ratio, Al distribution, and Cu loading. We find a large fraction of Cu monomers are present at a wide range of Cu and Al loading. However, a non-negligible fraction of Cu dimers are expected to be present at some CHA locations, many of which may contribute to methane activation or over-oxidation. We compare results against experimentally observed c-PMO activity over materials of a wide range of compositions to seek correlations between computed speciation and observed product selectivity. These results highlight how the active sites are influenced by the CHA topology and composition, and subsequently impact on the catalytic activity.