Yixin Li,Jianhao Qiu,Jie Xu,Yong Tang,Dingliang Dai,Guanglu Xia,Biyao Fang,Jianfeng Yao
The utilization of solar energy to break the C α C β bonds in lignin under mild conditions is highly important for lignin valorization. This work presents an MIL-125-derived titanium oxide (M-TiO 2 )-modified carbon nitride (g-C 3 N 4 ) heterostructure for selective photocatalytic lignin C α C β bond cleavage at room temperature. The conversion of lignin model 2-phenoxy-1-phenylethanol (PP-ol) reached 90.8 % over M-TiO 2 /g-C 3 N 4 composites, which is visibly higher than that of bare M-TiO 2 (33.2 %) and g-C 3 N 4 (57.3 %). Furthermore, the selectivity of C α C β bond breakage reached 96.8 %, and the yield of benzaldehyde was as high as 86.4 %. A mechanistic study innovatively demonstrated that lignin C α C β bond cleavages are achieved by the cooperative effect between holes and 1 O 2 , where holes dominate PP-ol conversion, whereas 1 O 2 governs C α C β bond cleavages. M-TiO 2 has relatively large surface areas and high porosities, and the Z-scheme heterojunctions retard charge carrier recombination and outfit strong redox abilities, which separately prompt O 2 adsorption and evolution to generate 1 O 2 , accounting for the high selectivity of C α C β bond breakages over M-TiO 2 /g-C 3 N 4 composites. This study sheds new light on the potential of metal–organic framework-derived materials in photocatalysis and provides valuable insights into the photocatalytic cleavage of lignin C α C β bonds.
Lignin is an alternative renewable feedstock for the production of low molecular weight aromatics. The cleavage of phenyl side-chain C α C β bond is a valid depolymerization pathway to obtain aromatic monomers . The selective cleavage of the C α C β bonds in two lignin model dimers and lignin with ferric sulfate as the catalyst was investigated to narrow the distribution of aromatic monomers. The C α C β bonds in phenolic and non-phenolic dimers were selectively cleaved under microwave irradiation . The acidity promoted the etherification of C α -hydroxyl groups with methanol solvent. Both the etherification and the decomposition of alkenyl products occurred in fast reaction steps, but the formation of alkenyl products was a decomposition-determining step. Density functional theory (DFT) calculations indicated that the phenolic dimer was more thermodynamically favorable to form alkenyl product than non-phenolic dimer, which resulted in the C α C β bond cleavage reactivity of phenolic dimer being higher than that of non-phenolic dimer. Microwave heating contributed to 96.3% of C α C β bond cleavage of organosolv lignin (OL), and a narrow distribution of aromatic monomers which mainly contained vanillin , syringaldehyde , methyl vanillate , and methyl syringate. This work highlights the prospect of microwave-assisted selective cleavage of the C α C β bond to achieve the selective production of aromatic monomers from lignin.
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