Carboxylic acid-based deep eutectic solvents unlock biomass recalcitrance for efficient conversion of polysaccharides...

Lignocellulosic biomass faces challenges in efficient valorization due to its recalcitrance. Herein, deep eutectic solvents (DESs) composed of ethylene glycol and carboxylic acids with varying alkyl chain lengths are systematically investigated to overcome biomass recalcitrance, thereby facilitating the green conversion of biomass polysaccharides to monosaccharides or biohydrogen. By systematically optimizing DES compositions, reaction conditions, and mechanistic interactions, an outstanding enzymatic saccharification of wheat straw cellulose (95 % glucan conversion) and a high cumulative hydrogen yield of 117.43 mL/g total solids (TS) during dark fermentation were achieved. The strong acidity and hydrogen-bond-donating capacity of maleic acid-based DES, coupled with water-induced viscosity reduction, synergistically removed 85 % lignin and 58 % xylan, while preserving cellulose crystallinity (CrI ∼ 67 %). This dual action minimized non-productive enzyme-lignin binding and maximized substrate accessibility. Crucially, the pretreatment reduced surface hydrophobicity by 75 %, enhanced electrostatic repulsion (zeta potential shift from -21.0 mV to -12.4 mV), and boosted cellulase adsorption capacity by sixfold. These structural and interfacial modifications translated to a 348 % increase in relative saccharification activity and a 3.19 mL/g TS·h peak hydrogen production rate, outperforming conventional acid-based methods. This study establishes DESs as an efficient and eco-friendly platform for lignocellulose biorefining, supporting sustainable bioenergy production.