Catalytic in Situ Hydrogenolysis of Lignin in Supercritical Ethanol: Effect of Phenol, Catalysts, and Reaction Temperature

This study aimed to explore the in situ hydrogenolysis of alkali lignin into bio-oil over three kinds of heterogeneous catalysts with varied catalytic properties (Ru/C, Ni/ZSM-5, and CuNiAl hydrotalcite-based catalyst) in supercritical ethanol. Phenol was first introduced to the in situ hydrogenolysis system to form a complex solvent to improve the lignin depolymerization over heterogeneous catalysts. The promotion effect of phenol was obviously observed during the hydrogenolysis process, leading to improved bio-oil yield and decreased solid residue yield, due to the unique dissolution and diffusion properties of phenol-containing solvent. The synergistic effects of basic sites and complex solvents were observed; thus, herein, the effect of catalysts, reaction temperature, and time on the hydrogenolysis, repolymerization, and coking on catalysts was investigated in detail, considering the molecular weight, elemental composition, and higher heating value (HHV) of bio-oil. The highest bio-oil yield was up to 81.8%, with an improved HHV of 30.09 MJ/kg, when the hydrogenolysis reaction was carried out at 290 °C for 3 h over CuNiAl catalyst, in ethanol–phenol solvent (phenol/lignin ratio of 0.8). This study could provide a beneficial reference for the hydrogenolysis of lignin over heterogeneous catalyzed systems in complex solvent.