Effect of solvent systems on the synergistic catalytic hydrogenolysis of alkaline lignin over Pd/C-NaOH

Abstract

Solvents and catalysts are two of the most critical factors for the depolymerization of lignin. This paper investigated the effect of different pure solvents on the hydrogenolysis of the β-O-4 MC and one of the mixed solvents on the hydrogenolysis of alkaline lignin. It was found that MC had higher conversion (96.5% and 94.3%) and monophenol yield (83.4 wt% and 77.9 wt%) in ethanol and 1,4-dioxane. The effects of different ratios of mixed solvents (ethanol-H2O and 1,4-dioxane-H2O), reaction conditions (temperature, time, pressure), and the synergistic effect of alkali and Pd/C catalysts on the hydrogenolysis of alkaline lignin were investigated in this paper. The composition, structure, and distribution of the lignin degradation products were also analyzed by GC, GC–MS, GPC, FT-IR, and 2D-HSQC. The results showed that the optimum monomer yields were 37.5 wt% and 24.7 wt% for ethanol-H2O and 1,4-dioxane-H2O at 3:2 (v:v) and 2:3 (v:v), respectively. The optimum yields were obtained at 260 °C, 2 MPa H2, and 4 h. The properties of solvent (protic polar solvent, nonpolar aprotic solvent) could influence the hydrogenolysis greatly and selectively break bonds. The different ratios of solvents in the mixed solvent systems influenced the solubility of alkaline lignin and its hydrogenolysis results. In addition, the synergistic effect of Pd/C and NaOH could improve the monomer yield and inhibit the polymerization of intermediates in hydrogenolysis.