Structural elucidation of tobacco stalk lignin isolated by different integrated processes

Valuable exploration of lignin is one of the feasible strategies for the development of agricultural wastes in biorefinery. In this study, five integrated treatments combining freeze-drying/alkali pre-impregnation/ultrasound-assisted alkali pretreatment/ball-milling/hydrothermal pretreatment (HTP) with dilute alkali post-treatment were employed to deconstruct tobacco stalk for lignin release. Meanwhile, double enzymatic lignin (DEL) with ultrahigh yield was successfully prepared as natural lignin to reveal the structural evolution of alkali lignins (ALs) during the different integrated treatments. Results showed that guaiacyl (G) was the predominant unit in tobacco stalk lignin and no p-coumaric and ferulic acids were found in all lignin preparations. As compared to DEL, ALs with a lower yield and higher purity had smaller molecule fragments, less beta-O-4 linkages, and more COOH groups as well as a higher thermal stability. In addition, it was observed that HTP was superior to other four pretreatments to release lignin in subsequent delignification. Accordingly, significant de-polymerization of the lignin mainly manifested as the decrease of molecular weight, cleavage of beta-O-4 linkages, and enrichment of S/G ratio occurred as a result of HTP. The structural knowledge of tobacco stalk lignin explored in this study could provide a good theoretical basis for conversion of agricultural wastes to lignin-based green products.