Novel Method for Production of Phenolics by Combining Lignin Extraction with Lignin Depolymerization in Aqueous Ethanol

A method for production of high-value-added phenolics by combining organosolv lignin extraction with lignin hydrothermal depolymerization without catalysts under mild conditions was successfully developed. During lignin extraction, the optimal ethanol concentration in mixed solvent is 65 vol %, with which 78% purity (based on Klason lignin) of crude lignin was obtained. High yield of liquid products up to 65.5% was recovered from lignin depolymerization under conditions of 523 K, 90 min, 65 vol % ethanol, and 3% lignin; meanwhile, only 17% solid residue was obtained. The gas chromatography-mass spectrometry (GC-MS) analyses of the obtained liquid products confirmed the presence of value-added phenolics, among which the yields of 4-ethylphenol, 4-vinylphenol, guaiacol, 4-ethylguaiacol, and 4-vinylguaiacol were very high (similar to 30% of the identified compounds). Gel permeation chromatography (GPC) and Fourier transform infrared (FT-IR) spectroscopy of these liquid products indicated that lignin was really depolymerized to low molecular weight compounds and that the cleavage of ether bonds and decarbonylation were the major reactions during lignin depolymerization.