Methane (CH₄), the primary component of natural gas, plays a vital role in domestic heating, electricity generation, and transportation fuel as compressed natural gas (CNG). Despite its utility, methane is the second most abundant greenhouse gas, significantly contributing to climate change due to leaks during production, transportation, and flaring in remote areas where storage and transport are economically unviable. Converting methane into value-added oxygenates on-site offers a promising solution, enabling direct utilization at the source, reducing transportation requirements, and mitigating emissions. This approach not only addresses environmental concerns by lowering greenhouse gas emissions but also generates valuable chemicals, providing economic incentives. However, the inherent stability of methane, with its high C–H bond dissociation energy (104 kcal/mol), large HOMO–LUMO energy gap, and resistance to polarization, presents considerable challenges. Furthermore, achieving selective oxidation is complicated by the risk of over-oxidation, leading to undesired byproducts such as carbon monoxide (CO) and carbon dioxide (CO₂).

The direct transformation of methane into methanol and acetic acid represents an efficient and sustainable strategy for producing high-value chemicals from an abundant resource. Conventional industrial methanol production relies on energy-intensive steam reforming of methane to syngas, followed by subsequent catalytic conversion, while acetic acid synthesis predominantly involves methanol carbonylation using the Monsanto or Cativa processes. These methods are not only energy-demanding but also involve multiple steps, making them less sustainable. Here, we report a porous aluminium-based MOF (metal-organic framework) featuring node-supported mono iron(III)-dihydroxyl catalytic sites that enable the direct, efficient, and chemoselective oxidation of methane to methanol or acetic acid using O₂ under mild conditions. MOFs, with their high surface area, tunable pore structures, and customizable metal sites, are ideal platforms for activating methane. Their confined environments not only enhance methane concentration near the active sites but also facilitate single-site catalysis, improving selectivity and efficiency while reducing energy demands.

The catalyst features a robust and porous 3D aluminum-DUT-5 framework, constructed from Al³⁺(µ₂-OH) nodes interconnected by 4,4’-biphenyldicarboxylate linkers. Post-synthetic grafting of Fe³⁺(OH)₂ active sites at the μ₂-OH nodes generates isolated iron centers that exhibit remarkable catalytic activity. Under mild aqueous conditions at 125 °C, the catalyst achieves a methanol productivity of 2.083 mmol g_cat^-1 h^-1 with an impressive selectivity of ~ 90%, while limiting over-oxidized byproducts such as CO and CO₂ to less than 5% selectivity. Mechanistic investigations reveal that the reaction proceeds via a Fe^III–Fe^I–Fe^III catalytic cycle, which involves the formation of methyl radicals, as confirmed by EPR, EXAFS, and DFT studies. The synergistic design of the DUT-5 framework and the site-isolated Fe³⁺(OH)₂ active centers offer a sustainable, economically viable, and energy-efficient pathway for methane transformation, paving the way for industrial applications in the selective production of methanol and acetic acid.

During reaction optimization to maximize methanol productivity, a pronounced temperature-dependent shift in product selectivity was observed. At 125 °C, methanol selectivity peaked at 90%, while at 150 °C, acetic acid selectivity increased to 80%, with a productivity of 1.667 mmol g_cat^-1 h^-1. This shift highlights a temperature-driven preference for acetic acid formation at elevated temperatures. Two key insights emerged: (1) the transition from methanol to acetic acid selectivity was accompanied by decreased formic acid and CO₂ productivities, highlighting the interplay of oxygenates; (2) acetic acid productivity remained considerably less affected by radical scavengers (Na₂SO₃, TEMPO), suggesting multiple formation pathways. Control experiments, isotopic labeling, and DFT studies identified two in-situ catalytic routes for acetic acid production: methanol hydrocarboxylation and direct methane carboxylation. This research demonstrates that the MOF-supported DUT-5-Fe(OH)₂ catalyst efficiently converts methane into methanol or acetic acid, depending on the reaction temperature, using O₂ as the sole oxidant. This capability highlights the versatility and effectiveness of MOFs as catalysts in the pursuit of sustainable methane utilization. As the demand for environmentally friendly processes grows, the development of such catalysts is crucial for enhancing methane conversion technologies. This study serves not only as proof of concept for the catalytic transformation of methane with MOF-based materials but also lays the groundwork for future innovations in this area.https://www.nature.com/articles/s41467-024-54101-8