Effects of green liquor (GL) and sodium carbonate (SC) pretreatment on structural characteristics of wheat stem lignin

The content and structure of lignin have been considered as important factors that affect both pretreatment and enzymatic hydrolysis of lignocellulosic biomass. In this work, wheat stems (WS) were pretreated using mild alkali including green liquor (GL) and sodium carbonate (SC). The results indicate that GL pretreatment exhibits better delignification selectivity and higher enzymatic digestibility than SC pretreatment. Analysis of 1H–13C HSQC NMR and FTIR on cellulolytic enzyme lignin (CEL) preparations isolated from untreated and pretreated WS also proves that a certain amount of lignin degrades which leads to a decrease of β-O-4′ linkages. Under mild alkaline conditions, more guaiacyl units in lignin are removed than syringyl units, which results in a higher condensation degree and S/G ratio of CELs isolated from GL- and SC-pretreated stems. Compared with p-coumarate structures, ferulates in lignin are more stable under mild alkaline conditions.