Catalytic Conversion of Lignin for the Selective Preparation of Valuable Compounds

Lignin valorization is a central objective of modern biorefinery research. This study investigates the catalytic depolymerization of two technical lignins, kraft lignin from beech hardwood and natron lignin from annual plants, via two complementary routes: analytical catalytic pyrolysis (Py-GC/MS, 300–600 °C) and hydrogenolysis (250–310 °C, Ru/C, isopropanol/H2). In Py-GC/MS experiments, noble-metal catalysts on carbon supports (Ru/C, Pd/C, RuPd/C) were screened. Relative compound distributions revealed phenolic derivatives as the dominant products, with Ru/C yielding the highest conversion for lignin from annual plants at 500 °C and Pd/C proving most selective for hardwood lignin at 400 °C. Hydrogenolysis was optimized through a five-level, three-factor central composite design, varying temperature, residence time, and catalyst loading. Lignin conversion ranged from 64 to 83 wt% and bio-oil yield from 69 to 89 wt%. A regression model identified optimal conditions at 295 °C, 32 min, and 17 wt% Ru/C. Catalyst regeneration via solvent washing, H2O2 oxidation, and controlled thermal treatment resulted in only an 8% decrease in lignin conversion. The results demonstrate that lignin origin, catalyst type, and depolymerization pathway jointly govern product selectivity, highlighting clear strategies for targeted phenolic compound production.