Hydrothermal Depolymerization of Kraft Lignins with Green C₁–C₃ Alcohol–Water Mixtures

Lignin is a potential renewable raw material for synthesis of aromatic molecules and materials. Particularly, Kraft-type lignins are abundantly available as a byproduct of wood and paper industries. Currently, there are practically no valuable products made using this highly complex polymer. Within this work, we have investigated the straightforward, high-yield depolymerization of two different technical lignins (Indulin AT and alkaline lignin) using only a mixture of green short-chain (C₁–C₃) alcohols and water as solvents. As shown here, at a temperature of 220 °C, autogenous pressure, and isopropanol/water as a co-solvent medium, it is possible to cleave Kraft lignin without char-forming side reactions. The obtained so-called “light” oil contains guaiacol-based monomeric units of 23 wt % yield together with liquid oligomers of 13 wt % yield combining liquid–liquid and solid–liquid extraction. Two-dimensional nuclear magnetic resonance analysis of lignin residues showed that the isopropanol/water treatment caused a marked breakdown of the intermolecular β–O–4 and β–β bonds; thus, the depolymerization produced monomers and lignin residues with lower molecular weight. The results suggested a synergistic effect between isopropanol and water. No sign of repolymerization reactions could be observed with this process.