Lignin is a promising alternative to fossil resources due to its abundance of benzene ring monomers. However, the stability of the C α C β bond in lignin has hindered its efficient depolymerization. Electrochemical methods for breaking this bond are not well-studied. This paper presents a novel approach for catalytic depolymerization of lignin to produce acetals under mild conditions, without the need for additional catalysts. Under room temperature and in an air atmosphere, the combination of tetrabutylammonium tetrafluoroborate (TBABF 4 ) as an auxiliary electrolyte and methanol (MeOH) as a solvent has shown high selectivity in catalyzing the cleavage of C α C β bonds in lignin. Over 90.0 % of the resulting products are acetals, with the optimal conditions being a substrate concentration of 0.02 M, TBABF 4 concentration of 0.008 M, a constant current of 30 mA, and a reaction time of 3 h. This led to a substrate conversion rate of 95.8 % and a product yield of 98.0 % for benzaldehyde dimethyl acetal (Bda). The mechanism study reveals that the tributyl ammonium radical cation decomposed by TBABF 4 is adsorbed on the electrode surface. Subsequently, the adsorbed O 2 is activated to form superoxide anion radical active species through single electron transfer, which plays a crucial catalytic role. TBABF 4 acts as both an auxiliary electrolyte and a catalyst in this process. This research introduces a novel approach for electrocatalytic depolymerization of inert C α C β bonds in lignin, leading to the selective conversion into acetal chemicals.
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.
危险品化学品经营许可证（带存储）