Figure 1. (a) Cellulose retention, (b) xylan removal, and (c)
delignification of WT and TG sorghum stover before and after binary &
ternary DES pretreatments with different compositions. Pretreatment
conditions: 140 ℃, 3 h.
The primary goal of biomass pre-processing, including pretreatment and
fractionation, is reducing and removing the recalcitrance factor from
biomass. Hemicellulose and lignin physically hinder the enzyme
accessibility to cellulose and decrease the enzyme efficiency by
productive and non-productive bindings. Therefore, the removals of
hemicellulose and lignin are important parameters to measure processing
effectiveness. The numerical data for the cellulose retention, xylan
removal, and delignification (Figure 1 ) are tabulated inTable S1 in the Supplementary Materials. Cellulose retention
was over 90% after DES pretreatments with both WT and TG sorghum stover
(Figure 1a ). There was no significant difference in cellulose
retention between WT and TG as well as among three DES pretreatments. AsFigure 1b shows, xylan removals from WT and TG sorghum stover
showed similar trends. Binary DES (ChCl-DHBA) and ChCl-DHBA-water
(ChCl-DHBA-W) had approximately 20% higher than the ternary DESs with
EG (ChCl-DHBA-EG). Also, binary DES and ChCl-DHBA-W resulted in similar
xylan removal with WT sorghum stover (75% and 71%, respectively) as
well as with TG biomass (77% and 78%, respectively). However,
ChCl-DHBA-EG was not as efficient as the former two DESs in terms of
xylan removal (54-56%). Li et al. reported that the content of EG in a
ternary DES showed contradictory effects on xylan
removal.36 EG content in ChCl-oxalic acid-EG DES
showed positive effects on xylan removal when the molar ratio between EG
and ChCl was below 3:1, while xylan removal was reduced with increasing
EG content when the molar ratio of EG and ChCl was above 3:1.
The third constituent in the ChCl-DHBA DES influenced more significantly
the delignification of biomass. The additional solvents as the third
constituent enhanced the delignification. Binary ChCl-DHBA DES showed
22% of delignification with WT biomass while resulting in only 9% with
TG biomass. Water-incorporated ternary DES (ChCl-DHBA-W) increased
delignification of WT biomass to 28%, and the one with TG biomass was
improved to 34%. In the previous study, ethanol-incorporated
ChCl-polyol DES systems showed higher solubility of aromatics compared
to the binary DES.37 Since ethylene glycol leads to
higher lignin solubility than water, higher delignification with
ChCl-DHBA-EG was expected. Interestingly, delignification yields of WT
and TG sorghum stover by ethylene glycol-incorporated ternary DES
(ChCl-DHBA-EG) were 38% and 34%, respectively, which were higher than
binary DES. However, compared to the delignification with ChCl-DHBA-W,
the yield of ChCl-DHBA-EG was not notably improved, especially with TG
biomass. It is possible that other factors like the viscosity of the DES
could be more significant than lignin solubility in this ternary DES
system; therefore, the enhancement of pretreatment effects could be
explained by the improved mass and heat transfer during the pretreatment
process. Nonetheless, the third constituent clearly altered the
fractionation performance of the DES, as evidenced by the changed
delignification and xylan removal with the tested three DESs.