Switchable ROS formation inhibits lignin β-O-4 models over-oxidation by CdS modified 2D g-C3N4 for highly efficient and selective producing aromatic monomers under visible light

Abstract

Highly efficient and selective cleavage of the C–C linkages in lignin is highly pursued to obtain aromatic monomers but the actual yield is always lower than the theoretical value under photocatalysis. Under the catalysis of 2D g-C₃N₄, both ·O₂^– and ·OH radicals could be generated resulting in a decreased carbon recovery for aromatic monomers (CRAM). The maximum value of CRAM and total yield of aromatic monomers are 49.2 % and 62.6 %, respectively. ·O₂^– radicals take charge of the depolymerization of 2-phenoxy-1-phenylethanol into aromatic monomers by cracking C_α-C_β bonds, while ·OH radicals account for the mineralization of aromatic monomers and the repolymerization of ·C_β intermediates. Herein, a Type-II heterojunction of CdS/2D g-C₃N₄ with modified energy-band structure was rationally designed and successfully prepared, only permitting ·O₂^– generation and inhibiting ·OH formation owing to the lower redox potentials of its photogenerated carriers, which significantly increased the CRAM value and total yield of aromatic monomers (maximum value of CRAM: 91.1 %, total yield of aromatic monomers: 96.5 %). Under the identical conditions, the CRAM value and total yield of aromatic monomers over CdS/2D g-C₃N₄ have increased by 85.2 % and 54.2 % compared to 2D g-C₃N₄ sampling at the same point in time. Moreover, the generation mechanisms for permitting ·O₂^– permit and inhibiting ·OH over CdS/2D g-C₃N₄ were analyzed based on the characterizations of catalyst structure and capture experiments of reaction oxygen species (ROS). The catalytic performances of both samples were also further verified with other lignin models and real lignin as reactants. Current work has not only provided a universal strategy for switchable ROS formation, but also developed an effective photocatalytic system for highly efficient and selective cracking C_α-C_β bonds in lignin β-O-4 linkages under mild conditions.