Fractionation and characterization of lignin from steam-exploded corn stalk by sequential dissolution in ethanol–water solvent

Due to the intricate process of traditional lignin fractionation by multi-organic solvent extraction, single solvent system consisted of ethanol and water was established in this work to fractionate and purify lignin from steam-exploded corn stalk. Three alkali-extracted lignin fractions (Fraction 1, 2 and 3) were obtained by sequential dissolution and subsequently characterized by the following analysis technologies: GPC, HPLC, UV, FTIR and 1H NMR spectra and thermogravimetric analysis. The GPC analysis showed the molecular weight of lignin fractions increased gradually from Fraction 1 to 3 and their polydispersity decreased dramatically compared with the unfractionated lignin. Besides, the purity of lignin Fraction 1 and 2 increased from 87.57% to 93.12% and 91.59% respectively due to the polysaccharide elimination effect of ethanol solutions. The subsequent characterization indicated that the content of phenolic acids decreased considerably from 3.379% to 0.095% with increasing molecular weight whereas the guaiacyl unit content and thermal stability increased. Meanwhile, the results of solubility determination revealed that the solubility of lignin enhanced when the molecular weight decreased and the water content of mixed solvent increased, which successfully elucidated the fractionation mechanism. Since ethanol is the only organic solvent used in the process, this study provides a relatively simple and efficient route to fractionate lignin from steam-exploded corn stalk for the subsequent utilization to produce stable and high-value lignin products.