3.2 Impact of the third constituent in ChCl-DHBA DES system on the chemical compositions of sorghum stover
One of the challenges in the application of several binary DESs is its relatively high viscosity, limiting the processibility. Ternary DESs were synthesized with the selected 3:2 molar ratio of ChCl and DHBA mixture and an additional third constituent, either water (W) or ethylene glycol (EG). The molar ratio between ChCl, DHBA, and the third constituent was 3:2:5, according to our previous study.33 The prepared ternary DESs were applied to the pretreatment of two different biomass, WT and TG sorghum stover at 140 ℃ for 3 h. The contents of glucan and xylan in intrinsic WT and TG sorghum stover were similar. Although TG sorghum stover showed slightly lower lignin content (12.8%) than WT biomass did (14.4%), the values are comparable (Table 2 ), which is in accordance with the previous study.34 All the applied DES pretreatments removed over 50% of the initial mass of biomass regardless of the DES composition, resulting in solid recovery yields ranging from 43% to 46%. For the further evaluation of the performance of DHBA-based ternary DESs, the changes in the chemical composition of the pretreated biomass were measured with cellulose retention, xylan removal, and delignification, as shown in Figure 1 . The preservation of cellulose in the solid is a critical factor affecting the yield of glucose in the following enzymatic hydrolysis step. As depicted inFigure 1a , after pretreatment, over 90% of cellulose remained in the solid fraction, regardless of the biomass type and DES composition. This result indicated that ChCl in the DESs could actively bind and stabilize the cellulose fractions while the HBD facilitates the removal of hemicellulose and lignin,35 which aligns well with the current results. On the other hand, the DESs having different compositions resulted in varied xylan removal and delignification, as the xylan removal ranged from 54% to 77% and delignification ranged from 9% to 40% (Figure 1b and1c ). Detailed information on solid recovery after different pretreatment and chemical compositions of untreated and pretreated biomass are in Table 2 .
Table 2. Solid recovery and chemical compositions of WT and TG sorghum stover before and after binary & ternary DES pretreatments with different compositions.