Comparative Evaluation of Dilute Acid, Alkaline, and Deep Eutectic Solvent Pretreatments on Enzymatic Hydrolysis of Sunflower Stalk Bark.

In this study, in order to overcome the recalcitrance of sunflower stalk bark (SSB) residues, three kinds of pretreatment methods, i.e., dilute acid pretreatment, alkaline pretreatment, and deep eutectic solvent (DES) pretreatment, were employed to pretreat the SSB material. The effects of the different pretreatment methods for enzymatic hydrolysis efficiency and sugar recovery were evaluated. The enzymatic hydrolysis yield (EHY) of SSB after dilute sulfuric acid, NaOH, and DES pretreatment could be improved from 15.2% to 26.8%, 96.7%, and 93.3%, respectively. Among the three pretreatment methods, dilute acid could effectively decompose hemicellulose while preserving more lignin in the pretreated solids, and it could only enhance EHY within a limited range (< 30%). Compared with dilute sulfuric acid pretreatment, NaOH and choline chloride/lactic acid pretreatments could effectively remove not only hemicellulose but also lignin. Results showed that satisfactory EHY (> 90%) was observed in both alkaline and DES pretreated SSB solids. Besides, DES method was much more efficient in the extraction of lignin (95%) compared with NaOH pretreatment (80.2%). Lignin could be recovery by DES pretreatment as value-added by-product from SSB and was more environmentally friendly and sustainable during production. Finally, under the condition of 1:10 (w/w) solid to DES solution and 150 °C pretreatment temperature, the mass balance of DES-150 °C showed that 28.1 g glucose was obtained from 100 g SSB, and the glucose recovery yield was 77.6% of theoretical, demonstrating the potential of environmentally friendly DESs in biorefinery.