Integrated alkaline and deep eutectic solvent-based green strategy for lignin fractionation and thermochemical valorization

In this work, an efficient and green strategy for selective extraction and high-value utilization of lignin was proposed. Most of the hemicellulose in the feedstock was first removed by alkaline pretreatment to create a favorable structural environment for subsequent lignin dissolution. Subsequently, an acid-alcohol deep eutectic solvent (DES) system based on FeCl₃-ethylene glycol (FeCl₃-EG) was developed to achieve efficient and selective lignin dissolution. The results of characterization by Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), and Brunauer-Emmett-Teller (BET) showed that the lignin was removed significantly after the treatment, the cellulose structure was preserved, and the crystallinity and specific surface area were increased to 73.46% and 2.764 m²/g, respectively. The highest delignification rate of 96.53% was achieved when EG:FeCl₃ = 1:0.3. Gas chromatography-mass spectrometry (GC-MS) detected a variety of low molecular phenolic products. In-situ Raman spectroscopy revealed the dynamic process of lignin dissolution from the cell wall. The regenerated lignin was systematically characterized by FTIR, thermogravimetry (TG), elemental analysis, and two-dimensional heteronuclear single quantum coherence-nuclear magnetic resonance (2D HSQC-NMR), and it was found that as the FeCl₃ content in DES increased, the β-O-4 content decreased, and the thermal stability of lignin increased. The pyrolysis properties were further analyzed by in-situ infrared (IR) spectroscopy, pyrolysis gas chromatography-mass spectrometry (Py GC-MS), and U-tube device, which showed that the regenerated lignin pyrolysis products had a high yield of the liquid phase and good phenol selectivity (40.94%), which demonstrated good selectivity and application prospects. This study provides a theoretical basis and process reference for the green extraction of lignin and its platform compound conversion.