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 /NaNO 2 /DDQ/NHPI system selectively oxidizes C α H–OH to C α ═O within the β-O-4 structure. In the subsequent hydrogenation step, the α-O-4 and the preoxidized β-O-4 structures are further hydrogenated over a NiMo sulfide catalyst, leading to the cleavage of C β –OPh and C α –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 α H–OH to C α ═O not only weakens the C β –OPh ether bond but also avoids the condensation reactions caused by the presence of C α + from dehydroxylation of C α H–OH. Furthermore, the NiMo sulfide prefers to catalyze the hydrogenative cleavage of the C β –OPh bond connecting with a C α ═O rather than catalyze the hydrogenation of C α ═O back to the original C α H–OH, which further ensures and utilizes the advantages of preoxidation.