Methane oxidation to ethanol by a molecular junction photocatalyst

Abstract

<p>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^{<a title="Sushkevich, V. L., Palagin, D., Ranocchiari, M. & van Bokhoven, J. A. Selective anaerobic oxidation of methane enables direct synthesis of methanol. Science 356, 523–527 (2017)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR1">1</a>,<a title="Agarwal, N. et al. Aqueous Au-Pd colloids catalyze selective CH4 oxidation to CH3OH with O2 under mild conditions. Science 358, 223–227 (2017)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR2">2</a>,<a title="Shan, J., Li, M., Allard, L. F., Lee, S. & Flytzani-Stephanopoulos, M. Mild oxidation of methane to methanol or acetic acid on supported isolated rhodium catalysts. Nature 551, 605–608 (2017)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR3">3</a>} 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^{<a title="Okolie, C. et al. Conversion of methane into methanol and ethanol over nickel oxide on ceria–zirconia catalysts in a single reactor. Angew. Chem. Int. Ed. Engl. 56, 13876–13881 (2017); retraction 58, 10785 (2019)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR4">4</a>,<a title="Zhou, Y., Zhang, L. & Wang, W. Direct functionalization of methane into ethanol over copper modified polymeric carbon nitride via photocatalysis. Nat. Commun. 10, 506 (2019)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR5">5</a>}. Whereas selective partial methane oxidation processes^{<a title="Sushkevich, V. L., Palagin, D., Ranocchiari, M. & van Bokhoven, J. A. Selective anaerobic oxidation of methane enables direct synthesis of methanol. Science 356, 523–527 (2017)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR1">1</a>,<a title="Agarwal, N. et al. Aqueous Au-Pd colloids catalyze selective CH4 oxidation to CH3OH with O2 under mild conditions. Science 358, 223–227 (2017)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR2">2</a>,<a title="Shan, J., Li, M., Allard, L. F., Lee, S. & Flytzani-Stephanopoulos, M. Mild oxidation of methane to methanol or acetic acid on supported isolated rhodium catalysts. Nature 551, 605–608 (2017)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR3">3</a>,<a title="Tomkins, P., Ranocchiari, M. & van Bokhoven, J. A. Direct conversion of methane to methanol under mild conditions over Cu-zeolites and beyond. Acc. Chem. Res. 50, 418–425 (2017)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR6">6</a>,<a title="Xie, J. et al. Highly selective oxidation of methane to methanol at ambient conditions by titanium dioxide-supported iron species. Nat. Catal. 1, 889–896 (2018)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR7">7</a>,<a title="Du, X. et al. Efficient photocatalytic conversion of methane into ethanol over P-doped g-C 3 N 4 under ambient conditions. Energy Fuels 36, 3929–3937 (2022)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR8">8</a>,<a title="Li, N., Li, Y., Jiang, R., Zhou, J. & Liu, M. Photocatalytic coupling of methane and CO2 into C2-hydrocarbons over Zn doped g-C3N4 catalysts. Appl. Surf. Sci. 498, 143861 (2019)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR9">9</a>} have thus typically generated C₁ oxygenates^{<a title="Tomkins, P., Ranocchiari, M. & van Bokhoven, J. A. Direct conversion of methane to methanol under mild conditions over Cu-zeolites and beyond. Acc. Chem. Res. 50, 418–425 (2017)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR6">6</a>,<a title="Xie, J. et al. Highly selective oxidation of methane to methanol at ambient conditions by titanium dioxide-supported iron species. Nat. Catal. 1, 889–896 (2018)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR7">7</a>}, recent reports have documented photocatalytic methane conversion to the C₂ oxygenate ethanol with low conversions but good-to-high selectivities^{<a title="Okolie, C. et al. Conversion of methane into methanol and ethanol over nickel oxide on ceria–zirconia catalysts in a single reactor. Angew. Chem. Int. Ed. Engl. 56, 13876–13881 (2017); retraction 58, 10785 (2019)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR4">4</a>,<a title="Zhou, Y., Zhang, L. & Wang, W. Direct functionalization of methane into ethanol over copper modified polymeric carbon nitride via photocatalysis. Nat. Commun. 10, 506 (2019)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR5">5</a>,<a title="Du, X. et al. Efficient photocatalytic conversion of methane into ethanol over P-doped g-C 3 N 4 under ambient conditions. Energy Fuels 36, 3929–3937 (2022)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR8">8</a>,<a title="Li, N., Li, Y., Jiang, R., Zhou, J. & Liu, M. Photocatalytic coupling of methane and CO2 into C2-hydrocarbons over Zn doped g-C3N4 catalysts. Appl. Surf. Sci. 498, 143861 (2019)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR9">9</a>,<a title="He, C. et al. Photocatalytic conversion of methane to ethanol at a three-phase interface with concentration-matched hydroxyl and methyl radicals. J. Am. Chem. Soc. 146, 11968–11977 (2024)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR10">10</a>,<a title="Yang, Z. et al. Efficient photocatalytic conversion of CH4 into ethanol with O2 over nitrogen vacancy-rich carbon nitride at room temperature. Chem. Commun. 57, 871–874 (2021)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR11">11</a>,<a title="Du, J. et al. Evoked methane photocatalytic conversion to C2 oxygenates over ceria with oxygen vacancy. Catalysts 10, 196 (2020)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR12">12</a>}. Here we show that the intramolecular junction photocatalyst covalent triazine-based framework-1 with alternating benzene and triazine motifs^{<a title="Schwinghammer, K., Hug, S., Mesch, M. B., Senker, J. & Lotsch, B. V. Phenyl-triazine oligomers for light-driven hydrogen evolution. Energy Environ. Sci. 8, 3345–3353 (2015)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR13">13</a>,<a title="Xie, J. et al. Efficient visible light-driven water oxidation and proton reduction by an ordered covalent triazine-based framework. Energy Environ. Sci. 11, 1617–1624 (2018)." href="https://www-nature-com.eproxy.lib.hku.hk/articles/s41586-025-08630-x#ref-CR14">14</a>} 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.<br></p>