リグニン系分子素材リグノクレゾールの低温水熱環境下における構造変換

抄録

Hot water treatment of biomass has been extensively studied for autohydrolysis. The process is frequently used for fractionation of biomass components or pretreatment of enzymatic saccharification. Understanding the reaction mechanism of lignin degradation under hydrothermal conditions at lower temperatures is important for improving such fractionation or pretreatment processes. In this study, a lignin-based material: lignocresol was synthesized from native lignin and p-cresol with concentrated sulfuric acid through the phase separation system. Since the benzylic positions are partly occupied by p-cresol, lignocresol should rather be an advantageous model lignin to examine the reactions at the reactive benzylic position while avoiding drastic polymerization and insolubilization. Lignocresol was hydrothermally treated from 110 to 300 °C and then analyzed by GPC, TG and FT-IR in order to estimate the structural change of lignin during the hot water treatments. To around 200 °C, the weight-average molecular weight of lignocresol was firstly increased and then decreased probably due to the formation of benzylic aryl ether (α-O-4) linkages followed by the hydrolysis. At 250 °C, lignocresol significantly increased the thermal stability, suggesting that the active benzyl positions were consumed by self-condensation reaction. In the range from 250 to 300 °C, prompt depolymerization was observed probably due to the cleavage of β-aryl ether (β-O-4) linkages.