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

[Display omitted] •After choline chloride (ChCl)-oxalic acid (OA)-AlCl3·6H2O pretreatment, lignin yield was 55.43 %.•After ChCl-lactic acid (LA)-AlCl3·6H2O pretreatment, the yields of lignin was 65.40 %.•After ChCl-LA-AlCl3·6H2O pretreatment, the total phenolic hydroxyl was 2.22 mmol g−1.•After ChCl-OA- AlCl3·6H2O pretreatment, the total phenolic hydroxyl was 2.84 mmol g−1.•After ChCl-OA- AlCl3·6H2O pretreatment, the aliphatic hydroxyl was 0.79 mmol g−1. 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.