Characterization of lignin-rich residues remaining after continuous super-critical water hydrolysis of poplar wood (Populus albaglandulosa) for conversion to fermentable sugars

Poplar wood flour (Populous albaglandulosa) was treated with sub- and super-critical water (subcritical: 325, 350 °C; super-critical: 380, 400, 425 °C) for 60 s at 220 ± 10 atm. Hydrochloric acid (0.05% v/v) was added to samples as acidic catalyst. The final products were separated into water soluble fraction and undegraded solids. The yields of undegraded solids were thoroughly dependent on temperature severity and mainly composed of lignin fragments. Average molecular weights of the lignins were between 1500 and 4400 Da, which was only 1/3–1/8-fold of poplar milled wood lignin (13,250 Da). DFRC (Derivatization Followed by Reductive Cleavage) analysis revealed that C6C3 phenols (coniferyl and sinapyl alcohol) were rarely detected in the lignins, indicating occurrence of two probable lignin reactions during SCW hydrolysis: lignin fragmentation via splitting of β-O-4 linkage and loss of propane side chains. These results were also confirmed by 1H and 13C NMR spectroscopic analysis.