A bifunctional brønsted acidic deep eutectic solvent to dissolve and catalyze the depolymerization of alkali lignin

Abstract

Lignin is an abundant renewable macromolecular material in nature, and degradation of lignin to improve its hydroxyl content is the key to its efficient use. Alkali lignin (AL) was treated with Brønsted acidic deep eutectic solvent (DES) based on choline chloride and p-toluenesulfonic acid at mild reaction temperature, the structure of the lignin before and after degradation, as well as the composition of small molecules of lignin were analyzed in order to investigate the chemical structure changes of lignin with DES treatment, and the degradation mechanism of lignin in this acidic DES was elucidated in this work. FTIR and NMR analyses demonstrated the selective cleavage of the lignin ether linkages in the degradation process, which was in line with the increased content of phe-nolic hydroxyl species. XPS revealed that the O/C atomic ratio of the regenerated lignin was lower than that of the AL sample, revealing that the lignin underwent decarbonylation during the DES treatment. Regenerated lignin with low molecular weight and narrow polydispersity index was obtained, and the average molecular weight (Mw) decreased from 17680 g/mol to 2792 g/mol (130°C, 3 h) according to GPC analysis. The lignin-degraded products were mainly G-type phenolics and ketones, and small number of aldehydes were also generated, the pos-sible degradation pathway of lignin in this acidic DES was proposed.