Comparative Analysis of Catalytic Hydrogenolysis of Technical Lignins and Lignin Model Compounds

Lignin valorization is essential for the economic viability of modern biorefineries and addresses the increasing demand for renewable energy and sustainable materials. Despite lignin's potential, its inherent recalcitrance complicates efficient conversion into valuable products. This study explores catalytic hydrogenolysis as a method for depolymerizing polymeric lignin into high-value monomeric phenols. We investigated the hydrogenolysis of three lignin samples - milled wood lignin (MWL), ethanol organosolv lignin (OL), and residual lignin obtained after supercritical water treatment (SCWL) - extracted from Mongolian oak using isopropyl alcohol and an Ru/C catalyst. Structural analysis revealed significant differences, with MWL exhibiting abundant beta-O-4 linkages, resulting in the highest monomeric phenol yield (28.4 wt %), compared to OL (10.5 wt %) and SCWL (5.6 wt %). Additionally, a lignin model dimer was used to provide mechanistic insights into the hydrogenolytic cleavage of lignin. Density functional theory (DFT) calculations of bond dissociation energies (BDEs) for C-O and C-C bonds highlight the challenges of breaking recalcitrant C-C bonds. Our findings emphasize the need to tailor hydrogenolysis conditions based on lignin structural features to maximize monomer yields, advancing strategies for lignin valorization.