Unveiling Acidity–Dependent Delignification in Choline Chloride–Derived DESs: Efficiency, Mechanism, and Lignin Structural Evolution

Abstract

The delignification mechanisms and structural evolution of lignin from poplar wood (PW) and corn stover (CS) were investigated during pretreatment with acidic, neutral, and alkaline choline chloride (ChCl)-based deep eutectic solvents (DESs). The delignification process was found to be primarily governed by the multi-level hydrogen bonding interactions. The hydrogen bond donors (HBDs) of DESs acted as “molecular crowbars” to preferentially disrupt the lignin hydrogen bonding network, while Cl^– ions further extended the lignin conformation through stronger hydrogen bonding. Alkaline DESs formed the strongest hydrogen bonding interactions, achieving the highest delignification efficiencies (78.8% for PW; 89.3% for CS). They preferentially cleaved G-type β-O-4 bonds via a “deprotonation–cyclization–cleavage” mechanism, yielding high–molecular–weight lignin with excellent β-O-4 retention (∼50/100 Ar), abundant aliphatic hydroxyls, and low condensation degree. Acidic DESs, despite possessing the strongest individual hydrogen bond strengths, exhibited lower total interaction energies and delignification efficiencies (72.4% for PW; 71.3% for CS). They preferentially cleaved S-type β-O-4 bonds through a “protonation–elimination–cleavage” mechanism via benzyl carbocation intermediates, producing low–molecular–weight lignin with high phenolic hydroxyl content (∼3.5 mmol/g) but the highest condensation degree. Neutral DESs showed the lowest efficiencies (53.2% for PW; 64.2% for CS) due to weak hydrogen bonding interactions. Although following a similar cleavage mechanism to acidic DESs, their alcoholic HBDs readily captured carbocations to form α-etherification byproducts, hindering β-O-4 cleavage and yielding lignin with “intermediate-state” characteristics. This study provides theoretical guidance for targeted lignin depolymerization and high-value utilization.