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 conditions^1-3 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 coupling^4,5. Whereas selective partial methane oxidation processes^1-3,6-9 have thus typically generated C₁ oxygenates^6,7, recent reports have documented photocatalytic methane conversion to the C₂ oxygenate ethanol with low conversions but good-to-high selectivities^4,5,8-12. Here we show that the intramolecular junction photocatalyst covalent triazine-based framework-1 with alternating benzene and triazine motifs^13,14 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.