Successive Organic Solvent Fractionation and Characterization of Heterogeneous Lignin Extracted by p-Toluenesulfonic Acid from Hybrid Poplar

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.