Thermochemical properties of lignin extracted from willow by deep eutectic solvents (DES)

In the present examination, the thermochemical behavior of the lignin extracted from willow (Salix matsudana cv. Zhuliu) by deep eutectic solvents (DES) treatment (DES-lignin) with varied times were investigated with the techniques of thermogravimetric analyzer, in situ diffuse reflectance infrared pyrolysis (in situ-FTIR), and pyrolysis–gas chromatography/mass spectrometry. The results showed that thermal degradation of DES-lignin mainly occurred at 185–470 °C with two maximum weight loss peaks at 265 °C and 380 °C respectively, and the solid residue was 40% when terminated at 800 °C. The identified cluster of aromatics from in situ-FTIR was detected earlier and stronger than those of small molecular gases, suggesting cleavage of lignin intermolecular linkages dominated the initial pyrolysis process. The DES-lignin was sensitive to pyrolysis to produce aromatics. The total phenols yield from DES-lignin pyrolysis reached a maxima of 95.1% at 450 °C, among which phenol and guaiacol accounted for the largest proportion, i.e. 20.8% and 24.2% respectively. In addition, product distribution was remarkably affected by pyrolysis temperature and the DES treatment time used for lignin isolation.