Molecular Weight Distribution and Dissolution Behavior of Lignin in Alkaline Solutions

Lignin, as the sole renewable aromatic resource in nature, has great potential for replacing fossil resources. However, the complexity of its structure limits its high value utilization, and the molecular weight distribution and dissolution behavior of lignin in alkaline solutions is still unclear. In this study, a conventional lignin separation during the pulping process in an alkaline hydrothermal system was performed by controlling the amount of NaOH, reaction temperature and holding time. Various analysis methods, including GPC, 2D–HSQC NMR and FTIR were used to study the characteristics of lignin fragments dissolved from wood. We were aiming to understand the rule of lignin dissolution and the recondensation mechanism during the process. The results showed dissolution of lignin due to ether bond fracturing by OH(−) attacking the Cα or Cβ positions of the side chain with penetration of NaOH, and the lignin fragments in solution recondensed into complex lignin with more stable C–C bonds. The experimental results also prove that the average molecular weight increased from 4337 g/mol to 11,036 g/mol and that holding time from 60 min to 120 min at 150 °C with 14 wt% of NaOH.