Near-critical water hydrothermal transformation of industrial lignins to high value phenolics

• A novel method for the hydrothermal degradation of black liquor lignin for high-value-added phenolic compounds.
• Very high yields of phenols, guaiacols, catechols, and syringols.
• Yields were dependent upon the reaction temperature and the pH of feedstock.
• Significantly valuable process for valorization of lignin feedstock for the production of renewable chemicals in a biorefinery construct.

A process for the production of high-value-added phenolics following hydrothermal depolymerization of black liquor lignins after acidification without catalysis was successfully deployed and is described herein. A high yield of liquid products up to 33.3 mg/mL (based on raw black liquor) was recovered at 290 °C, 30 min, and pH 6; meanwhile, only 12.6 mg/mL solid residue was obtained. GC–MS analyses of the liquid products confirmed value-added aromatic monomers, among which the yields of phenol, guaiacol, and 2,6-dimethoxy-phenol were high (∼60% of the identified compounds). FT–IR of the solid residues indicated that the black liquor lignin was depolymerized through cleavage of ether bonds and decarboxylation. The results suggest that black liquor may be a new resource for aromatic products.