Promoting Lignin Depolymerization and Restraining the Condensation via an Oxidation-Hydrogenation Strategy

For lignin valorization, simultaneously achieving the efficient cleavage of ether bonds and restraining the condensation of the formed fragments represents a challenge thus far. Herein, we report a two-step oxidation-hydrogenation strategy to achieve this goal. In the oxidation step, the O-2/NaNO2/DDQ/NHPI system selectively oxidizes C alpha H-OH to C-alpha=O within the beta-O-4 structure. In the subsequent hydrogenation step, the alpha-O-4 and the preoxidized beta-O-4 structures are further hydrogenated over a NiMo sulfide catalyst, leading to the cleavage of C-beta-OPh and C-alpha-OPh bonds. Besides the transformation of lignin model compounds, the yield of phenolic monomers from birch wood is up to 32% by using this two-step strategy. The preoxidation of C-alpha-OH to C-alpha=O not only weakens the C-beta-OPh ether bond but also avoids the condensation reactions caused by the presence of C-alpha(+) from dehydroxylation of C alpha H-OH. Furthermore, the NiMo sulfide prefers to catalyze the hydrogenative cleavage of the C-beta-OPh bond connecting with a C-alpha=O rather than catalyze the hydrogenation of C-alpha=O back to the original C alpha H-OH, which further ensures and utilizes the advantages of preoxidation.