Revealing the Structural Inhomogeneity of Lignins from Sweet Sorghum Stem by Successive Alkali Extractions

To investigate the inhomogeneity of the lignin from sweet sorghum stem, successive alkali treatments were applied to extract lignin fragments in the present study. The successive treatments released 80.3% of the original lignin from the sorghum stem. The chemical structural inhomogeneity of the isolated lignins was comparatively and comprehensively investigated by UV, FT-IR, and NMR spectra. The lignins were found to be predominantly composed of β-O-4' aryl ether linkages, together with minor amounts of β-β', β-5', β-1', and α,β-diaryl ether linkages. In addition, hydroxycinnamic acid (mainly p-coumaric acid), which was found to be attached to lignin, was released and co-precipitated in the lignin fractions isolated in the initial extracting steps, whereas hydroxycinnamic acids (p-coumaric and ferulic acids) were not detected in the subsequently extracted lignin fractions. Moreover, the high proportion of carbon-carbon structures was potentially related to the high amounts of guaiacyl units in the lignin investigated. Thermogravimetric analysis revealed that the higher molecular weights of lignins resulted in relatively higher thermal stability, and the higher content of C-C structures in the lignin probably led to a higher "char residue". These findings suggested that the lignin fractions extracted from sweet sorghum stem by successive alkali extractions had inhomogeneous features in both chemical composition and structure.