The effects of acidic and basic deep eutectic solvent pretreatment of sunflower stalks on the yield of enzymatic hydrolysis and the lignin recovery

In this study, acidic (formic acid) and alkaline (urea) deep eutectic solvents (DES) were applied to sunflower stalks at different molar ratios (1:2, 1:2, 2:1), temperatures (90°C, 110°C, 130°C), and times (4 hr, 6 hr, 8 hr) to investigate the effect of pretreatment on the yield of enzymatic hydrolysis. The highest yield of glucose and removal of lignin after acid and alkaline DES pretreatment were obtained at a 1:2 molar ratio, 110°C and 6 hrs of conditional pretreatment, and these conditions were determined to provide maximum efficiency. All sunflower stalks pretreated with DES were analyzed by X-ray diffractometer (XRD), Fourier-Fourier spectroscopy (FT-IR), and scanning electron microscope (SEM). As a result of acid and alkaline DES pretreatment, solid lignin was obtained and characterized by FT-IR. The recycling process of DES was applied to the liquid obtained as a result of acid and alkaline DES pretreatment under the selected most efficient conditions. Under the most efficient conditions, lignin removal of 89.64% was achieved with ChCl/ FA. The yields of the enzymatic hydrolysis carried out after the pretreatment were determined. As a result of the most suitable conditions with ChCl/ FA, the yield of enzymatic hydrolysis is 91.94%. As a result of the recycling process, DES could be used five times. The yield of enzymatic hydrolysis when DES was reused decreased from 91.34% to 57.67% as a result of use. It was found that acid and alkaline DES could be recycled three times and used efficiently in the preatreatment.