Copper-exchanged zeolites are of interest for direct conversion of methane to methanol. As such, improving methanol yields and selectivities from methane in these zeolites is of great interest. The factors influencing methanol yields and selectivities in these zeolites have been studied by various researchers. Generally, zeolites with proton, H+, precursors provide higher methanol yields and selectivities than zeolites from sodium, Na+, precursors. These findings extend to stepwise stoichiometric as well as steady-state catalytic methane-to-methanol conversion (MMC). To provide a quantitative description of the effect of H+ and Na+ counter-ions on MMC in the mordenite zeolite, MOR, we focus on [Cu3O3]2+, [Cu2O]2+ and two [Cu2O2]2+ active sites. We used quantum-mechanical calculations with periodic density functional theory (DFT). We find that these sites are activated at 345-490 °C, with the most stable and least stable species being the bis(μ-oxo) dicopper (III)[Cu2O2]2+ moiety and the least stable is [Cu3O3]2+. Only [Cu3O3]2+ and [Cu2O]2+ are capable of activating methane, with similar C-H activation barriers in H-MOR and Na-MOR zeolites. Additionally, the structures of these active sites in both zeolites are quite similar. Specifically, the only major difference in the Na-MOR and H-MOR zeolites revolves around the fate of the methyl group produced from methane C-H activation. In both zeolites, rebound of the methyl group to an active μ-oxo atom favors over-oxidation. Interestingly, exchange of the methyl group with H+/Na+ located at remote aluminates is an alternative path to stabilization. However, Na+ exchange does not provide as much stabilization as the μ-(OCH3)-stabilized intermediate. For these reason, copper-exchanged Na-MOR zeolites are susceptible to over-oxidation and lower methanol selectivities. Remarkably, we find that Brønsted acid sites provide similar levels of stabilization to the μ-(OCH3) intermediate. This is a pathway to methanol, rather than to products of over-oxidation. Our findings are explained based on electronic-structure.

Acid sites and methane-to-methanol conversion in copper-exchanged zeolites

ACS Spring Meeting 2021

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