Conversion of Lignin to Phenol-Rich Oil Fraction under Supercritical Alcohols in the Presence of Metal Catalysts

Asian lignin was directly depolymerized to phenol-rich oil fraction (lignin-oil) over Pt/C, Pd/C, Ru/C, and Ni/C under supercritical alcohols. Ethanol and Pt/C (E-Pt) proved to be an excellent combination for producing large amounts of lignin-oil (77.4 wt %) with the smallest amount of char (3.7 wt %). Lignin-oil mainly consisting of monomeric phenols, and higher molecular phenolic was subjected to several chemical analyses. Elemental analysis indicated that hydrodeoxygenation and hydrogenation primarily occur during lignin depolymerization. H-1/2D-HSQC-NMR and GPC analysis revealed that the M-w, of lignin-oil remarkably is lower than that of Asian lignin, which is clear evidence of beta-O-4 and beta-beta bond cleavages. The top four main monomeric phenols in lignin-oil were 4-ethylphenol, guaiacol, 4-ethylguaiacol, and syringol, for which the sum was mostly produced in E-Pt (41.8 mg/g of lignin) with the highest selectivity (38.3%). With increasing catalyst dosage, we observed that excessive catalyst caused side reactions to hinder the production of monomeric phenols.