Structural properties of softwood lignin fractions: Revealed by NMR and Py-GC/MS

The structural complexity of industrial softwood kraft lignin (ISKL) strongly interrelates to the heterogeneous chemical reactions of kraft delignification process. In this work, structural properties of an ISKL were revealed by the investigation on the structural properties of fractions with gradient molar mass and trace/high amount of carbohydrates. Moreover, the variation in structural composition of ISKL enables its classification into three distinct categories, which potentially correspond to the the lignin dissolved at different phases of kraft delignification. In general, the low molar mass fraction, characterized by an abundance of reduced subliknages and a minimal presence of native linkages, are tentatively associated with the lignin dissovled at the inital phase of delignification. The medium molar mass fractions with trace amounts of carbohydrates, exhibiting similarities in the quantites of β-O-4, benzyl ether (BE), and β-5 linkages, are probably dissolved into kraft liquor in the period of the bulk phase of delignification. The lignin fraction with the highest molar mass, containing a large amount of native sublinkages and carbohydrates, is correlated with the lignification phase toward the late of delignification. This study of structural characteristics of lignin fractions and their relationship to kraft delignification chemistry is anticipated to facilitate the separation of lignin to fractions with significantly reduced structural heterogeneity, thereby enhancing the potential for high-value application based on their compositional structure.