Abstract
A simple and fast fractionation method was developed to obtain carbohydrate-free lignin with well-defined characteristics, such as narrowly distributed molecular weights and a tunable chemical structure for specific applications. In this study, an industrial softwood kraft lignin and a hardwood CELF lignin (coproduct lignin obtained from cosolvent enhanced lignocellulosic fractionation pretreatment) were dissolved in acetone–methanol and tetrahydrofuran–methanol cosolvents, respectively. Hexane was applied as the antisolvent for sequential precipitation of both lignin preparations. A thorough characterization including various NMR techniques (31P, quantitative 13C, and 2D-HSQC), GPC, DSC and TGA was conducted to correlate the molecular weight of obtained lignin fractions with their structural features including distributions of aliphatic and phenolic hydroxyl groups and relative abundance of interunit linkages. It was found that approximately 10% of the softwood kraft lignin was lignin carbohydrate complexes, and the latter one could be removed efficiently by decreasing polarity of the cosolvent.
