A comparative study of effect of polyol-based and acid-based deep eutectic solvents pretreatment of rice straw under autoclave conditions on cellulose enrichment and digestibility

Abstract

Deep eutectic solvents (DESs) are attractive for lignocellulosic biomass deconstruction because of their easy synthesis, reusability, inexpensive nature, and eco-friendliness. Pretreatment of rice straw was carried out using three types of DES: choline chloride:glycerol (ChCl:Gly), choline chloride:ethylene glycol (ChCl:EG), and choline chloride:formic acid (ChCl:FA) at 120 °C, 1.5 bars in an autoclave for 1 h. Enzymatic digestibility of pretreated samples was assessed at 10% solids loading and 9.0 filter paper units g−1 cellulose of enzyme loading. A delignification efficiency of 51.6% was obtained with ChCl:FA compared to 30.4% and 36.1% obtained with ChCl:Gly and ChCl:EG respectively. The cellulose content of 58.4% was obtained from ChCl:FA pretreated samples compared to cellulose contents of < 48% from ChCl:Gly and ChCl:EG pretreated samples; cellulose content of pretreated and untreated samples correlated with their crystallinity indexes. The lowest hemicellulose content of 9.5% was obtained from the ChCl:FA pretreated sample, while ChCl:Gly and ChCl:EG were like the untreated (around 22%). As indicated by spectroscopic, microscopic, and thermogravimetric analysis, DES pretreatment caused alterations in chemical composition, structure, and surface morphology. ChCl:FA pretreated sample (Ac-PT4) was significantly hydrolysed, resulting in a glucose yield of 79.7% and a concentration of 45.56 gLl−1 after 72 h of hydrolysis. The glucose yield obtained from ChCl:Gly and ChCl:EG pretreated samples ranged from 32 to 36%, while the glucose concentrations ranged from 14 to 16 gL−1. Pretreatment using ChCl:FA effectively delignified rice straw and led to a threefold increase in enzymatic digestibility compared to untreated; hence, pretreatment using ChCl:FA could support a biorefinery concept.