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 degrees C under 5 MPa H-2 with or without a catalyst. Hydrothermal treating AL at 300 degrees 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 M-w and M-n of 60,000 and 10,000 g/mol to M-w and M-n of 1010 and 415 g/mol, respectively. Compared to the 50/50 (v/v) water-ethanol treatment, the pure ethanol treatment at 300 degrees C led to a lower yield of DL ( < 15%) with smaller molecular weights (M-w = 631 g/mol and M-n = 260 g/mol). Moreover, the DLs from the pure ethanol treatment were completely soluble in THF, in comparison, only similar to 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 degrees C and 400 degrees 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). (C) 2012 Elsevier Ltd. All rights reserved.