Effect of nucleophilic additives on phosphoric acid pretreatment of lignocelluloses

The inhibition of lignin condensation during biomass pretreatment is crucial for enhancing enzymatic hydrolysis efficiency, since the formation of rigid cross-linked lignin networks hinders cellulose accessibility and enzyme activity. This study investigates the effects of nucleophilic additives, including ascorbic acid (AsA), 2-naphthol (2N), 3-hydroxy-2-naphthoic acid (3H2NA), and 2-naphthol-7-sulfonate (7S2NA), as potential agents to suppress lignin condensation on the phosphoric acid pretreatment of poplar. The phosphoric acid pretreatment demonstrated a remarkable efficacy in the removal of xylan (100%) and lignin (18.06%-31.35%) from poplar, both with and without the inclusion of nucleophilic additives. An enzymatic hydrolysis yield ranging from 71.41% to 100% was achieved with the incorporation of AsA, 2N, 3H2NA, and 7S2NA, compared to a yield of 66.15% for substrates pretreated solely with phosphoric acid. The enhancement in enzymatic hydrolysis yield upon the addition of nucleophilic additives was probably due to the improved cellulose accessibility and the enhanced proportion of cellulose II in the pretreated substrates. The analysis of total phenolic content in the prehydrolysates revealed that 3H2NA and 7S2NA, characterized by their strong hydrophilic groups within their chemical structures, significantly facilitated lignin fractionation during phosphoric acid pretreatment.