Bioinspired Cu^II Defective Sites in ZIF-8 for Selective Methane Oxidation

Activating the C–H bonds of alkanes without further oxidation to more thermodynamically stable products, CO, and CO₂, is a long-sought goal of catalytic chemistry. Inspired by the monocopper active site of methane monooxygenase, a Cu-doped ZIF-8 metal–organic framework is synthesized with 25% Cu and 75% Zn in the nodes and activated by heating to 200 °C and dosing in stepwise fashion with O₂, methane, and steam. We found that it does oxidize methane to methanol and formaldehyde. The catalysis persists through at least five cycles, and beyond the third cycle the selectivity improves to the extent that no CO₂ can be detected. Experimental characterization and analysis were carried out by PXRD, DRUV-Vis, XANES, EXAFS, SEM, and XAS. The reaction is postulated to proceed at an open-coordination Cu^I site generated by defects, and the mechanism of methanol production was explicated by density functional calculations with the revMO6-L exchange–correlation functional. The calculations reveal a catalytic cycle of oxygen-activated Cu^I involving the conversion of two molecules of CH₄ to two molecules of CH₃OH by a sequence of hydrogen atom transfer reactions and rebound steps. For most of the steps in the cycle, the reaction is more favored by singlet species than by triplets. The singlet pathway consists of both closed-shell singlet and open-shell singlet species. The closed-shell singlet calculations were checked for wavefunction stabilities, to ensure the correctness of the calculations. The reaction mechanism along the singlet pathway goes through open-shell singlets until the final step where it passes through a closed-shell singlet transition state to give rise to closed-shell singlet final product, which is a van der Waals complex of Cu^I and CH₃OH. This work provides a bioinspired synthesis of a metal-organic framework, whose defective sites carry out selective methane oxidation to methanol. A biomimetic reaction mechanism is postulated and further validated by density functional theory calculations.