Paving the Way for the Lignin Hydrogenolysis Mechanism by Deuterium-Incorporated β-O-4 Mimics

Gaining more insight into the lignin hydrogenolysis mechanism is of great importance for developing next-generation catalysts and regulating product distribution because lignin is the only renewable aromatic source leading to aromatic chemicals. However, the inherent complexity of the lignin structure and the multiple pathways in lignin hydrogenolysis make gaining insight into the lignin hydrogenolysis mechanism even more challenging. In this report, a β-O-4 polymer with deuterium incorporated at the α, β, and γ positions which can better model the plant lignin structure was prepared for mechanistic study. The location and retention of deuterium in the monomers resulting from the hydrogenolysis and transfer hydrogenolysis of the deuterated β-O-4 mimics with Ru/C, Pd/C, and Pd/Zn/C in MeOH indicated that the β-O-4 linkage protons did not participate in the cleavage process, which indicated that pathways involving dehydrogenation and/or dehydration reactions are infeasible for these catalysts under these conditions. A concerted process, wherein the Cα–O and Cβ–O bonds in β-O-4 structures were cleaved concurrently, was proposed as a potential hydrogenolysis mechanism in the Pd/C and Ru/C systems. Dehydroxylation at the α position was identified as a side reaction in the Pd/C-catalyzed hydrogenolysis of dimer models, from which the same concurrent cleavage mechanism was also inferred. The introduction of a Lewis acid center in Pd/C was conducive to β-O-4 hydrogenolysis, as confirmed by the abatement of the side reaction and the enhanced monomer yield in the polymeric model reactions. The use of deuterium-incorporated β-O-4 mimics paved the way for clearly elucidating the lignin hydrogenolysis mechanism.