Macromolecular structural characteristics and functional potential of tobacco stalk lignin from the phosphotungstic acid-assisted delignification process

Exploring the macromolecular structures of lignin from agriculture biomass waste will significantly facilitate its valorization in the biorefinery process, which also strengthens the sustainable development of lignocellulose conversion. Herein, lignin polymers from raw and pre-hydrolyzed tobacco stalk were firstly isolated to characterize the structural differences before and after the proposed pre-hydrolysis processes. Then, the structural evolution of lignin macromolecules during the pre-hydrolysis and further alkaline delignification was elucidated by comparing the native and recovered lignins that evaluated by 2D-HSQC, 31P NMR, and GPC as well as FTIR spectroscopic techniques. Further, UV–vis absorption and thermal properties of lignins before and after the integrated processes were also investigated, respectively. It was found that the tobacco stalk lignin structurally resembles hardwood lignin, which was extensively depolymerized (molecular weights and β-O-4 contents decreased from 2110 g/mol and 55.8/100 Ar to 1130–1590 g/mol and 23.1–38.7/100 Ar, respectively) and recondensed during the pre-hydrolysis and delignification process. A possible pathway of chemical structure evolution of lignin was revealed in the proposed deconstruction process. Recovered lignins also exhibited abundant hydroxyl groups (3.11 mmol/g) and high carbon content (up to 45 wt%) that can be chemically tailored into functional biobased materials.