Using nucleophilic naphthol derivatives to suppress biomass lignin repolymerization in fermentable sugar production

Dilute acid pretreatment of biomass decomposes hemicelluloses but cannot effectively remove lignin. Here we investigate the adding of add five nucleophilic derivatives including 2-naphthol, 6-hydroxy-2-naphthoic acid (6H2NA), 2-hydroxy-1-naphthoic acid (2H1NA), 6-hydroxy-1-naphthoic acid (6H1NA), and 3-hydroxylic-2-naphthoic acid (3H2NA) to enhance the efficiency of lignin removal. The 72-h hydrolysis yield of pretreated substrates was increased from 30.9% to 34.7%, 38.3%, 36.0%, 36.0%, and 53.2%, respectively. The chemical composition, cellulose accessibility, and surface chemistry of pretreated materials with or without additives showed the improved hydrolysis yield with 3H2NA addition being related to lower surface lignin content, higher cellulose accessibility, and delignification ratio. Furthermore, the analysis of HSQC and 31P NMR spectra of the residual lignin reflects the influence of naphthol derivatives on lignin structure. The results showed that the 3H2NA additive increased the content of lignin carboxyl groups and enhanced the helping fractionation and suppression of lignin repolymerization. These improvements through 3H2NA addition reduce lignin inhibition of fermentable sugar production from pretreated biomass.