Methane oxidation to ethanol by a molecular junction photocatalyst

Abstract

Methane, the main component of natural and shale gas, is a significant carbon source for chemical synthesis. The direct partial oxidation of methane to liquid oxygenates under mild conditions1–3 is an attractive pathway, but the inertness of the molecule makes it challenging to achieve simultaneously high conversion and high selectivity towards a single target product. This difficulty is amplified when aiming for more valuable products that require C–C coupling4,5. Whereas selective partial methane oxidation processes1–3,6–9 have thus typically generated C1 oxygenates6,7, recent reports have documented photocatalytic methane conversion to the C2 oxygenate ethanol with low conversions but good-to-high selectivities4,5,8–12. Here we show that the intramolecular junction photocatalyst covalent triazine-based framework-1 with alternating benzene and triazine motifs13,14 drives methane coupling and oxidation to ethanol with a high selectivity and significantly improved conversion. The heterojunction architecture not only enables efficient and long-lived separation of charges after their generation, but also preferential adsorption of H2O and O2 to the triazine and benzene units, respectively. This dual-site feature separates C–C coupling to form ethane intermediates from the sites where •OH radicals are formed, thereby avoiding over-oxidation. When loaded with Pt to further boost performance, the molecular heterojunction photocatalyst generates ethanol in a packed-bed flow reactor with greatly improved conversion that results in an apparent quantum efficiency of 9.4%. We anticipate that further developing the ‘intramolecular junction’ approach will deliver efficient and selective catalysts for C–C coupling, pertaining, but not limited, to methane conversion to C2+ chemicals. Mechanistic insights into a photocatalyst with an intramolecular junction and two different active sites could inspire further exploration of ‘intramolecular heterojunctions’ as the basis of selective and stable photocatalysts for C–H activation and C–C coupling.