The challenges in high value-added utilization of lignin own to its wide molecular weight distribution and complex physicochemical properties associated with it. How to narrow the molecular weight of lignin and elucidate the differences in physiochemical properties related to molecular weight of lignins is of crucial importance for lignin conversion processing. In this study, lab-made Kraft and ethanol lignins were fractionated into several narrower molecular weight distribution components through a designed solvent system. The yield, molecular weight distribution, structural characterization and their physicochemical properties of each fractionated lignin were analyzed comparatively. The results indicated that the yield of ethanol lignin is higher (8.44 %) than that of Kraft lignin (5.26 %). Kraft lignin shows lower molecular weight (Mw=2480 g/mol), higher hydroxyl content (4.29 mmol/g), and lower content of β- O -4 and β-5 bonds compared to ethanol lignin (Mw=3682 g/mol, 2.10 mmol/g). Meanwhile, the phenolic hydroxyl content in both ethanol and Kraft lignins increased with the decrease of molecular weight. Furthermore, ethanol lignin demonstrated higher antioxidant activity and much higher (65times) tensile strength of lignin-based polyurethane elastomer than that of Kraft lignin-based polyurethane elastomer.
Yanliang Song,Xiaoyou Shi,Xue Yang,Xu Zhang,Tianwei Tan
A successive organic solvent fractionation was reported in this work to obtain lignin fractions with narrow molecular weight distribution and high purity from p-toluenesulfonic acid-extracted lignin. A decrease was found in PDI of fractionated lignin compared with parent substrate from 3.31 to 1.44 and the amount of carbohydrate derivates was detected to decline implying an increase of purity. Calculated from the results of DTG, thermal stability of lignin samples was enhanced through successive fractionation, and TG data showed that the mass of remaining char over 500 °C decreases from 51% of initial lignin to 25% of fractionated ones. There was also a decline of glass transition temperature (Tg) determined, from 142.5 to 100.8 °C. Lignin fractions that were dissolved in ethanol and acetone possessed high contents of summed aliphatic and phenolic hydroxyls might due to formation of numerous hydrogen bonds. As shown in 1H–13C HSQC NMR spectra, lignin fractions dissolving in ethyl acetate contained more C–C bonds, mainly β–β, compared with other samples.
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