Synergetic Ni–Ce Active Sites in Mixed Cerium/Zirconium Metal–Organic Framework Nodes for Selective Methane Oxidation into Ethanol

The direct oxidation of methane into ethanol with high productivity under mild conditions is a grand challenge. We report the development of mixed cerium/zirconium metal–organic framework (MOF) nodes-supported mononuclear nickel(II)-hydroxyl species [Cex/Zry–UiO–Ni(OH)] as efficient heterogeneous catalysts for direct transformation of methane into ethanol. The Ni2+ ion in Cex/Zry–UiO–Ni(OH) MOFs coordinates with a μ4–O–, one hydroxy group, and two neutral carboxylate oxygens, which are directly bonded to the Ce4+ ion at the mixed metal-oxo nodes. The spectroscopic and control experiments and theoretical calculations reveal that the precise composition of the mixed-metal node, the isolation of mono Ni-hydroxyl species at the node, and the cooperative Ni–Ce active sites confined within the porous UiO-MOFs promote the facile C–H activation of methane at 80 °C, leading to the formation of •CH3 radicals and subsequent C–C coupling within the pores to produce ethanol in an extraordinarily high yield of 6521 mmol gNi–1 with >93% selectivity, outperforming most of the current reports. Our mechanistic investigation suggests that the direct methane oxidation into ethanol proceeds via a dual catalytic cycle, in which the doping of Ce4+ ion within MOF's node and the proximity between Ce4+ and Ni2+ ions lead to the reversible Ce–Ocarboxylate bond dissociation and Ni–(μ2–OH)–Ce bond formation, which is the key for efficient formation of •CH3 radical in the turnover limiting step. This work highlights the importance of mixed metal-MOFs in designing well-defined heterobimetallic-supported catalysts for the valorization of methane and light alkanes via cooperative catalysis.