Synergy of Single Atoms and Lewis Acid Sites for Efficient and Selective Lignin Disassembly into Monolignol Derivatives.

Lignin is the most abundant aromatic polymer from the natural and renewable lignocellulosic biomass resource. Developing highly efficient catalysts for lignin depolymerization to produce valuable monophenols with high yield and selectivity remains a desirable but challenging target in this field. Here, we design a synergistic catalyst combining atomically dispersed Mo centers and Al Lewis acid sites on a MgO substrate (Mo1Al/MgO) for the depolymerization of Eucalyptus lignin via the ß-aryl ether bond cleavage. A near-theoretical monophenol yield of 46% with an ultrahigh selectivity of 92% for coniferyl and sinapyl methyl ether, as well as good cycling durability, was achieved simultaneously by Mo1Al/MgO in an inert N2 atmosphere. First-principles calculations and control catalytic experiments confirmed the synergistic catalysis mechanism between Mo1-O5 single-atom centers and the neighboring Al Lewis acid sites with the participation of a methanol solvent. This study validates the feasibility of designing better-performing catalysts with synergistic multiactive sites for the efficient and selective disassembly of complex renewable biopolymers into highly value-added products with lower cost and greater security.

Isolation, Characterization, and Depolymerization of l-Cysteine Substituted Eucalyptus Lignin.

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 ß-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, 31P NMR, thioacidolysis, etc. Compared to the two-step control treatment, a much higher ß-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.