Pretreatment of sugarcane bagasse with deep eutectic solvents affect the structure and morphology of lignin

Elucidating the structural characteristics and changes of lignin during biorefining is considerably significant for lignin valorization. Deep eutectic solvent (DES), as a green solvent, has shown excellent performance in extracting high-purity lignin from lignocellulose with high yields. However, further utilization of the multiple novels DES remains to be a severe challenge to the biorefinery process. In this study, alkaline lignin (AL), which was separated from sugarcane (Sacharum officinale L.) bagasse (SCB), was pretreated by a ternary DES (TDES) (choline chloride (ChCl)-lactic acid (LA)/oxalic acid (OA)-AlCl3·6H2O) at 120 °C for 6 h to obtain regenerated lignin. The properties and structural transformation of lignin during the TDES pretreatment were investigated by gel-permeation chromatography (GPC), two-dimensional-heteronuclear single quantum correlation nuclear magnetic resonance (2D-HSQC NMR), 31P NMR, Fourier transform infrared spectroscopy (FT-IR), and thermogravimetric analysis (TGA). Finally, the degradation products recovered from TDES pretreatment were also analyzed by pyrolytic gas chromatography-mass spectrometry (Py-GC–MS). It was found out that the yields of the regenerated lignin pretreated by ChCl-LA-AlCl3·6H2O and ChCl-OA-AlCl3·6H2O were 65.40 and 55.43 %, respectively. In contrast, the contents of phenolic hydroxyl groups in lignin were 2.54 and 2.84 mmol g−1, respectively, the content of aliphatic hydroxyl groups were 0.91 and 0.79 mmol g−1, respectively. In addition, it is found that the depolymerization of the lignin fraction was the dominant reaction during the TDES pretreatment. Based on the observed results, a possible pathway of chemical conversion of SCB lignin during the TDES pretreatment process was proposed. This study shows that the pretreatment with hydrated metal chloride-based TDES provides a promising condition for generating of the value-added lignin in the current biorefinery process.