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Unlocking Structure-Reactivity Relationships for Catalytic Hydrogenolysis of Lignin into Phenolic Monomers.

Lignin depolymerization into aromatic monomers with high yields and selectivity is essential for the economic feasibility of biorefinery. However, the relationships between lignin structure and its reactivity for upgradeability are still poorly understood, in large part due to the difficulty in quantitative characterization of lignin structural properties. To overcome these shortcomings, we use advanced NMR technologies (2D HSQC and 31 P) to accurately quantify lignin functionalities. Diverse lignin samples prepared from Eucalyptus grandis with varying β-O-4 linkages were subjected to Pd/C-catalyzed hydrogenolysis for efficient C-O bond cleavage to achieve theoretical monomer yields. Strong correlations were observed between the yield of monomeric aromatic compounds and the structural features of lignin, including the contents of β-O-4 linkages and phenolic hydroxyl groups. Notably, a combined yield of up to 44.1 wt% was obtained from β-aryl ether rich in native lignin, whilst much lower yields were obtained from technical lignins which were low in β-aryl ether contents. This work quantitatively demonstrates that the lignin reactivity for acquiring aromatic monomer yields varies depending on the lignin fractionation processess.