Design and synthesis of metal-free ethene-based covalent organic framework photocatalysts for efficient, selective, and long-term stable CO2 conversion into methane

The efficient and selective photocatalytic CO2 conversion into higher-valued hydrocarbon products (e.g., methane and ethane) over covalent organic frameworks (COFs) remains a challenge, with all previously reported attempts producing carbon monoxide as the dominant product. Herein, we report a new ethene-based COF, through polycondensation of electron-rich (E)-1,2‑diphenylethene and 1,3,6,8‑tetraphenylpyrene units. The synthesized ethene-based COF functioned as an efficient metal-free photocatalyst for the conversion of CO2 into methane under visible light irradiation, with a selectivity of 100 %, a production rate of 14.7 µmol g-1h-1, and an apparent quantum yield of c.a. 0.99 % at 489.5 nm, which are the most promising values reported for CO2 conversion by a metal-free COF photocatalyst, without any support from a co-catalyst. The carbon origin of CH4 product is confirmed by isotope tracer 13CO2 experiment. Moreover, the photocatalytic system consistently produces methane for > 14 h with recyclability.