Revealing structural and functional specificity of lignin from tobacco stalk during deep eutectic solvents deconstruction aiming to targeted valorization

Achieving efficient exploitation of agricultural waste is of great significance for future biorefinery. A green deconstruction strategy of lignocellulose exerts an immense impact on their conversion and lignin valorization. Deep eutectic solvent (DES) as eco-friendly solvents can sufficiently fractionate lignin from lignocellulose, and the considerate comprehension of fundamental chemistry of lignin would facilitate the process optimization and tailored upgrade of lignin in biorefinery. Herein, three different DESs, including acidic and alkaline systems, were adopted to deconstruct raw and hydrothermal tobacco stalk under certain conditions, gaining different lignin fractions. Structural transformation of recovered lignin macromolecules during acidic and alkaline DES delignification has been investigated in comparison with mill wood lignin (MWL) via state-of-the-art NMR spectra, GPC, FTIR, and TGA techniques. Results showed that hydrothermal pretreatment facilitated the yields (from 35.6% to 46.0% to 43.1–58.5%) and molecular weights (from 1310 to 2280 g/mol to 1600–2400 g/mol) of the recovered DES lignin. Moreover, acidic DES lignin presented significantly fragmented (without β-O-4 linkage) and condensed structures as well as the lower molecular weight, while a well-preserved structure (~50/100 Ar β-O-4 linkage) and fewer hydroxyl groups were observed in alkaline DES ones. Furthermore, these lignins displayed structure-dependent nanoparticle size, excellent UV absorption, and antioxidant activity, offering a crucial foundation for the targeted valorization of lignin from agricultural waste.