Addition of Sodium Fluoride to an Ionic Liquid Innovative System in the Reused Pretreatment to Enhance Cellulose Separation.

Eco-friendly ionic liquids (ILs) are widely used in lignocellulosic biomass pretreatment. With their rather high cost, ILs must be reused in pretreatment processes. By pretreating with reused [BMIM]BF4:DMSO: NaF and [BMIM]BF4:DMSO for 22 rounds, the residual solid lignin contents of raw corn cobs samples are 6.94% ± 0.11% and 11.87% ± 0.22%, respectively, compared with 19.19% ± 0.41% in raw corn cobs, demonstrating the superior delignification performance of [BMIM]BF4:DMSO: NaF. Moreover, the residual lignin content remained consistently low throughout the reuse cycles for [BMIM]BF4:DMSO: NaF, increasing only slightly from 6.62% ± 0.15% in the first cycle to 6.94% ± 0.11% in the 22 rounds, whereas a more pronounced increase was observed for [BMIM]BF4:DMSO, from 9.28% ± 0.19% to 11.87% ± 0.22%. The crystallinity index (CI) of a raw corn cob sample is 40.56 ± 0.02, whereas that of a corn cobs sample pretreated by reused [BMIM]BF4:DMSO: NaF and [BMIM]BF4:DMSO for 22 rounds is 22.10 ± 0.01 and 39.00 ± 0.01, respectively. This comparison suggests that the crystalline lignocellulose of a corn cob sample is regenerated into an amorphous form more by [BMIM]BF4:DMSO: NaF pretreatment than by [BMIM]BF4:DMSO pretreatment, verifying that [BMIM]BF4:DMSO: NaF performs well in delignification and cellulose separation. This conclusion is also supported by the peak of F- detected through ion chromatography analysis because F- in reused [BMIM]BF4:DMSO: NaF solution can increase lignocellulose denaturation. Consequently, the cellulose conversion of the enzymatic hydrolysis of the 22nd reuse of [BMIM]BF4:DMSO: NaF, 85.82% ± 0.12%, is higher than that of [BMIM]BF4:DMSO, 26.55% ± 0.10%. Moreover, the [BMIM]BF4:DMSO: NaF has successfully reused because the solid lignin separated at 40 °C, suggesting that the [BMIM]BF4:DMSO: NaF can be used in the next round of pretreatment. By considering the function of [BMIM]BF4:DMSO: NaF, the combination between [BMIM]BF4 and DMSO: NaF mechanism explanation of reused pretreatment is first reported in lignocellulosic pretreatment.