3.3 Impact of the third constituent in ChCl-DHBA DES system on enzymatic hydrolysis of sorghum stover
The pretreatment efficiency of each DES was evaluated by measuring fermentable sugar production via enzymatic hydrolysis. The average value and standard deviation are in Figure 3 and Supplementary Materials with Table S3 for glucan conversion and Table S4 for xylan conversion, respectively. Overall, both binary and ternary ChCl-DHBA DESs improved the glucan and xylan conversion in sorghum stover. Over 80% of glucan conversion and over 40% of xylan conversion were obtained with the DES pretreated WT and TG biomass. Especially the glucan conversion yields of TG biomass pretreated by both binary and ternary DESs (ChCl-DHBA, ChCl-DHBA-W, and ChCl-DHBA-EG) after 72 h-enzymatic hydrolysis ranged 98-100%, which were higher than pretreated WT biomass had. It is well-known that xylan removal and delignification are well correlated with glucan conversion in previous studies.40,41 However, the enzymatic hydrolysis results in this study were not well correlated with their delignification and xylan removal efficiency. Other factors, such as possible pseudo lignin deposition and cellulose crystallinity, could be possible reasons.42-44
The glucan conversions of binary DES pretreated biomass were 85% (pretreated WT) and 98% (pretreated TG). The addition of the solvent-based third constituents further improved the glucan conversion of the binary DES pretreated biomass. ChCl-DHBA-W pretreated WT sorghum stover showed 97% of glucan conversion, while ChCl-DHBA-EG pretreated one showed a similar yield (89%) with binary ChCl-DHBA DES. The effect of ternary DES on xylan convolution was more pronounced. The ChCl-DHBA-W and ChCl-DHBA-EG DES pretreated WT biomass had 11% and 15% higher xylan conversion than the binary DES pretreated one. The ternary DES pretreated TG biomass also had 17-35% enhancement in xylose production compared to binary DES pretreated one.
Two types of solvent-based third constituents, water in ChCl-DHBA-W DES and ethylene glycol in ChCl-DHBA-EG DES, further increased glucan and xylan efficiency of the ChCl-DHBA DES pretreated biomass in both WT and TG biomass. ChCh-DHBA-W DES pretreated biomass showed similar glucan conversion to WT (97%) and TG (98%). The ChCl-DHBA-W DES exhibited the highest glucan and xylan conversion with WT sorghum stover. This DES also resulted in notably high xylan conversion (81%) with TG biomass. Another ternary DES, ChCl-DHBA-EG DES, showed higher glucan and xylan conversion (89% and 57%) than ChCl-DHBA DES (85 and 42%) with the pretreated WT. The ChCl-DHBA-EG DES provided superior sugar conversion with the pretreated TG biomass (99%) compared to the WT biomass (88%) even though the chemical composition changes in WT and. TG biomass were similar (Figure 1 , Table 1, and Table S1 )