Isolation and Structural Characterization of Lignin from Cotton Stalk Treated in an Ammonia Hydrothermal System

To investigate the potential for the utilization of cotton stalk, ammonia hydrothermal treatment was applied to fractionate the samples into aqueous ammonia-soluble and ammonia-insoluble portions. The ammonia-soluble portion was purified to yield lignin fractions. The lignin fractions obtained were characterized by wet chemistry (carbohydrate analysis) and spectroscopy methods (FT-IR, C-13 and H-1-C-13 HSQC NMR spectroscopy) as well as gel permeation chromatography (GPC). The results showed that the cotton stalk lignin fractions were almost absent of neutral sugars (0.43%-1.29%) and had relatively low average molecular weights (1255-1746 g/mol). The lignin fractions belonged to typical G-S lignin, which was composed predominately of G-type units (59%) and noticeable amounts of S-type units (40%) together with a small amount of H-type units (similar to 1%). Furthermore, the ammonia-extractable lignin fractions were mainly composed of beta-O-4' inter-unit linkages (75.6%), and small quantities of beta-beta' (12.2%), together with lower amounts of beta-5' carbon-carbon linkages (7.4%) and p-hydroxycinnamyl alcohol end groups.