Hydrothermal degradation of alkali lignin to bio-phenolic compounds in sub/supercritical ethanol and water-ethanol co-solvent

This work aimed to degrade alkali lignin (AL) for the production of bio-phenolic compounds. Effective degradation of AL was achieved in 50/50 (v/v) water-ethanol at 200-450 °C under 5 MPa H2 with or without a catalyst. Hydrothermal treating AL at 300 °C for 2 h without a catalyst led to an 89% yield of degraded lignin (DL). The molecular weights of the lignin were significantly reduced from its original Mw and Mn of 60,000 and 10,000 g/mol to Mw and Mn of 1010 and 415 g/mol, respectively. Compared to the 50/50 (v/v) water-ethanol treatment, the pure ethanol treatment at 300 °C led to a lower yield of DL (<15%) with smaller molecular weights (Mw = 631 g/mol and Mn = 260 g/mol). Moreover, the DLs from the pure ethanol treatment were completely soluble in THF, in comparison, only ∼30% soluble in THF for DL from the 50/50 (v/v) water-ethanol treatment. While reaction time had negligible effects on the DL yields and properties, reaction temperature dramatically influenced the product yield and properties: 300-325 °C and 400 °C appeared to be the optimal temperature for the process in 50/50 (v/v) water-ethanol and pure ethanol, respectively. In both solvent-systems, generally the use of a catalyst did not significantly affect the yields of DL, but slightly reduced the molecular weights of the DLs and greatly increased the solubility of DLs in tetrahydrofuran (THF).