Inhomogeneities in the Chemical Structure of Sugarcane Bagasse Lignin

Sequential treatments of dewaxed bagasse with distilled water, 0.5 M NaOH, 0.5, 1.0, 1.5, 2.0, and 3.0% H₂O₂ at pH 11.5, and 2 M NaOH at 55 °C for 2 h solubilized 2.8, 52.5, 14.9, 3.3, 5.5, 5.0, 2.8, and 2.2% of the original lignin, respectively. The eight isolated lignin fractions were subjected to a comprehensive structural characterization by UV, FT-IR, and ¹H and ¹³C NMR spectroscopies and thermal analysis. The nitrobenzene oxidation method was also applied to the in situ lignins. The seven lignin fractions, isolated successively with alkali and alkaline peroxide, were all SGH-type lignins, with a small amount of esterified p-coumaric acid and mainly etherified ferulic acid. No significant differences were found in the weight-average molecular weights (1680−2220 g/mol) of the seven alkali and alkaline peroxide dissolved lignins. However, the first four lignin fractions, isolated with 0.5 M NaOH and 0.5, 1.0, and 1.5% H₂O₂ at pH 11.5, were rich in syringyl units and contained large amounts of noncondensed ether structures, whereas the last three lignin fractions, isolated sequentially with 2.0 and 3.0% H₂O₂ at pH 11.5 and 2 M NaOH at 55 °C for 2 h, had a higher degree on condensation and were rich in guaiacyl lignins. Keywords: Bagasse lignins; sugars; molecular weight; UV; FT-IR; ¹H and ¹³C NMR; nitrobenzene oxidation; phenolic acids