Isolation, Characterization, and Depolymerization of l‐Cysteine Substituted Eucalyptus Lignin. Issue 4 (3rd March 2022) - Shi, Lanlan
Abstract: Lignin condensation reactions are hard to avoid or control during separation, which is a deterrent to lignin isolation and post‐conversation, especially for the full utilization of lignocelluloses. Selective protection of β‐aryl ether linkages in the isolation process is crucial to lignin valorization. Herein, a two‐step acid/alkali separation method assisted with l ‐cysteine for eucalyptus lignin separation is developed, and the isolated l ‐cysteine lignins (LCLs) are comprehensively characterized by 2D NMR, 31 P NMR, thioacidolysis, etc. Compared to the two‐step control treatment, a much higher β‐O‐4 content is preserved without reducing the separation efficiency assisted by l ‐cysteine, which is also significantly higher than alkali lignin and kraft lignin. The results of hydrogenolysis show that LCLs generate a much higher monomer yield than that of control sample. Structural analysis of LCLs suggests that lignin condensation reaction, to some extent, is suppressed by adding l ‐cysteine during the two‐step acid/alkali separation. Further, mechanistic studies using dimeric model compound reveals that l ‐cysteine may be the α‐carbon protective agent in the two‐step separation. The role of l ‐cysteine in the two‐step lignin isolation method provides novel insights to the selective fractionation of lignin from biomass, especially for the full valorization of lignocellulosic biomass. Abstract : An organic solvent‐free and toxic chemical‐free separation method assisted by l ‐cysteine is developed for lignin isolation, and the obtained lignins are comprehensively evaluated by structural characterization and depolymerization, revealing the role of l ‐cysteine as an α‐C protective agent to suppress the cleavage of β–O–4 in lignin fragments in the two‐step separation.