Structural Changes of Sugar Cane Bagasse Lignin during Cellulosic Ethanol Production Process

Abstract

Dilute acid pretreatment is one of the most studied biomass deconstructing technologies due to its relatively low process cost. Typical cellulosic ethanol production process involves three main stages: (1) dilute acid pretreatment, (2) enzymatic hydrolysis, and (3) fermentation/distillation. In this study, three lignins, L1, L2, and L3, were extracted using 1% (w/w) NaOH solution at 80 °C following each stage, respectively. Lignin characterization shows the reduction of aliphatic, guaiacyl, and carboxylic OH groups of the L2 and L3 compared to those of the L1 but increases in both the weight and number molecular weights as well as slight increases in the polydispersity index, probably because, among other reasons, of the removal of a higher molecular weight lignin fraction from the pretreated biomass during enzymatic hydrolysis. While 2D HSQC NMR and Py-GC/MS did not reveal differences in the proportions of syringyl and p-hydroxyphenyl units among the lignins, there is a predominance of the phenylcoumaran substructures and guaiacyl lignin units in L1 and the absence of some substructures/linkages associated with L2 and L3. Lignin extraction following dilute acid pretreatment not only produced the lignin with the highest purity but also resulted in improved glucose yield from 71% to 77%.