Extraction and oxypropylation of lignin by an efficient and mild integration process from agricultural waste

Abstract An efficient and mild integration process for the extraction of lignin and preparation of lignopolyol from agricultural waste corn stalk was presented in this study. Firstly, the conditions of the hydrothermal coupled with dilute alkali treatment were optimized to obtain lignin fractions, the structures of which were investigated by high-performance anion-exchange chromatography (HPAEC), gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FT-IR) and 2D HSQC NMR spectroscopy. Results showed that the optimal condition was hydrothermal reaction at 150 °C for 1 h, and coupled with alkali extraction with 1 M NaOH at 70 °C for 2 h in this study, under which the lignin component with higher yield, better purity and intact lignin structure could be extracted. Subsequently, the obtained high-quality lignin was modified with propylene oxide in the presence of sodium hydroxide under mild conditions, which was characterized by scanning electron microscope (SEM), FT-IR, 31P NMR and thermogravimetric analysis. From the optimization results, it was concluded that when the ratio of lignin to propylene oxide was 2:1 mg/mL, the lignopolyol with the largest molecular weight, the most secondary hydroxyl and the best thermal stability could be prepared. This integration process including the isolation of lignin and preparation of lignopolyol from agricultural wastes was conducive to the synthesis of polyurethane materials, and promoted the development of lignin application technology.