Co-Producing Reducing Sugars, Furfural and Xylooligosaccharides from Phragmites australis Straw via Efficient Pretreatment by Deep Eutectic Solvent Lactic Acid: Cetyltrimethylammonium Bromide

In this work, five monocarboxylic-acid/Cetyltrimethylammonium Bromide (CTAB) solvent formulations containing formic acid, acetic acid, propionic acid, butyric acid, or lactic acid were systematically compared under identical pretreatment conditions using Phragmites australis straw (PAS) as the feedstock. The lactic acid (LA):CTAB formulation was ultimately selected for further pretreatment of PAS, because it retained substantially more of the initial glucan than formic acid (FA):CTAB, while maintaining a statistically comparable saccharification efficiency. Under the selected conditions (LA:CTAB = 4:1 mol/mol, 170 °C, 60 min), the removal rates of xylan and lignin reached 74.3% and 71.4%, respectively, while an enzymatic hydrolysis efficiency of 72.8% was acquired. In the LA:CTAB-treated liquor, xylooligosaccharides (XOS) and furfural formed in the deep eutectic solvent (DES)-pretreatment liquor contained 4.1 g/L and 1.7 g/L, respectively. Linear fitting between component removal (e.g., xylan removal and delignification) and severity factor (LogR₀) were explored. Molecular analyses of the selected LA system qualitatively indicated attractive interactions around the polar regions of lignocellulosic model compounds and possible dispersive interactions involving the alkyl chain of CTAB. These results illustrate potential noncovalent interaction modes within LA but do not establish a comparative interaction ranking among the five solvent formulations. This study offers theoretical insights into the mechanism of CTAB-based acidic DESs in biomass pretreatment and valorization and suggests a potential direction for the development of efficient and tunable pretreatment solvents for valorization of biomass and co-producing valuable biobased chemicals.