Phosphorus Atoms Repair Carbon Defects in Carbon Nitride to Cleave Lignin C–C Bonds

Photocatalytic cleavage of lignin C–C bonds is a green and sustainable strategy to promote high-value utilization of lignin, which is of great significance for alleviating the shortage of fossil resources. However, due to the low lignin conversion and product selectivity, there is still much room for improvement in the high-value conversion of lignin by photocatalytic technology. Herein, carbon nitride with carbon defects is first prepared. Red phosphorus (RP) is then used to repair carbon defects in carbon nitride. Based on this method, a series of photocatalysts XRP/CN have also been successfully synthesized. 8RP/CN was found to have both the advantages of carbon-deficient carbon nitride and the π-conjugate structure in the heptazine units, which played a very positive role in accelerating the transfer of photogenerated carriers, enhancing the capture capacity of visible light, and promoting the adsorption and activation of lignin macromolecules. With these advantages, the C–C bonds in both lignin models and real lignin are broken efficiently. In particular, 0.1564 g of lignin oil was obtained from 0.3 g of ethanol-extracted organic lignin using photocatalyst 8RP/CN. Moreover, the main products formed by lignin C–C bond breakage are aromatic aldehydes and aromatic acids. The mechanism of photocatalytic cleavage of lignin C–C bonds was also studied in detail. Solvent H2O has been shown to play an important role. 8RP/CN first converts H2O to H2O2. Subsequently, H2O2 is broken down into hydroxyl radicals, which are critical for the photocatalytic breaking of lignin C–C bonds, as hydroxyl radicals act as intermediates to provide the necessary O and H atoms for the photocatalytic breaking of lignin C–C bonds. Moreover, hydroxyl radicals were shown to be the most important active radicals. This study provides a new strategy for the green and sustainable transformation of lignin by constructing heteroatomic, defect-repairing carbon nitride photocatalysts.