Easy and Efficient Recovery of EMIMCl from Cellulose Solutions by Addition of Acetic Acid and the Transition from the Original Ionic Liquid to an Eutectic Mixture

Abstract

Ionic liquids (ILs) and deep eutectic solvents (DESs) are the two most widely used neoteric solvents. Recently, our group described how the simple addition of acetic acid (AcOH) to 1-Ethyl-3-methylimidazolium chloride (EMIMCl) could promote the transition from the original IL to an eutectic mixture of EMIMCl and AcOH. Herein, we studied how cellulose regeneration and EMIMCl recovery from EMIMCl solutions of cellulose could be benefited by the significant differences existing between EMIMCl- and EMIMCl·AcOH-based mixtures and the easy switching from one to the other. Finally, we also demonstrated that the transition could also be accomplished by addition of acetic anhydride and water so that the process could be eventually useful for the achievement of highly acetylated cellulose.

Keywords: cellulose acetylation; cellulose dissolution; cellulose regeneration; deep eutectic solvents; ionic liquid recovery; ionic liquids.

Figure 1

Picture of the supernatant phase obtained after addition of H₂O at 20 °C (1), H₂O at 4–5 °C (2), or AcOH at either 20 or 60 °C (3) to EMIMCl solutions with 2 wt% (left) and 8 wt% (right) cellulose contents.

Figure 2

(a,b) ¹H and (c,d) ¹³C NMR spectra of recovered EMIMCl (a,c) using H₂O as the antisolvent or (b,d) adding first AcOH and promoting the transition from EMIMCl- to EMIMCl·HOAc-based mixtures. ¹H and ¹³C NMR spectra were performed at 85 and 25 °C, respectively.

Figure 3

FTIR spectra of cellulose regenerated from EMIMCl solutions. The cellulose contents were 2 wt% (a) and 8 wt% (b). The antisolvent used for cellulose precipitation was either water (10 mL) at 20 °C (a2, b2, red line) or 1 equivalent of AcOH (a3, b3, green line), 2 equivalents of AcOH (a4, b4, blue line), or 3 equivalents of AcOH (a5, b5, light blue line) at 60 °C. The spectrum of MCC was included in both graphics for comparison (a1, b1, black line).

Figure 4

¹H NMR spectra of EMIMCl·HOAc-based DESs; (a) EMIMCl·1HOAc obtained by addition of 0.5 equivalents of Ac₂O and 0.5 equivalents of H₂O to 1 equivalent of EMIMCl, (b) EMIMCl·2HOAc obtained by addition of 1 equivalent of Ac₂O and 1 equivalent of H₂O to 1 equivalent of EMIMCl, and (c) EMIMCl·3HOAc obtained by addition of 1.5 equivalents of Ac₂O and 1.5 equivalents of H₂O to 1 equivalent of EMIMCl. All ¹H NMR spectra were performed at 25 °C.

Figure 5

FTIR spectra of cellulose regenerated from EMIMCl solutions with 2 wt% cellulose content. Acetylation was carried out at 60 °C in the presence of 0.5 equivalent of Ac₂O (4, dark blue line), 1 equivalent of Ac₂O (5, light blue line), or 1.5 equivalents of Ac₂O (6, pink line), followed by the addition of 10 mL of H₂O to promote the EMIMCl- to EMIMCl·HOAc-based DESs transition and cellulose precipitation. The FTIR spectra of precipitated cellulose obtained upon the addition at 60 °C of 1 equivalent of AcOH (1, black line), 2 equivalents of AcOH (2, red line), or 3 equivalents of AcOH (3, green line) were also included for comparison.

Figure 6

Glucose unit with labelled atoms (a) used for the assignment of peaks in the ¹H NMR spectra of cellulose obtained after acetylation at 60 °C in the presence of (b) 0.5, (c) 1, or (d) 1.5 equivalents of Ac₂O. DSs of ca. 2.5, 2.9, and 3 were obtained from the integrals of peaks assigned to protons of the acetyl group (CH₃) and the anhydroglucose unit (AGU)—e.g., $DS=7\times I_{(C{H}_3, H)}/3\times I_{\left(AGU,H\right)}$, where 7 and 3 come from the number of protons in the AGU and the number of hydroxyl groups per AGU. All ¹H NMR spectra were performed at 25 °C.

Figure 6

Glucose unit with labelled atoms (a) used for the assignment of peaks in the ¹H NMR spectra of cellulose obtained after acetylation at 60 °C in the presence of (b) 0.5, (c) 1, or (d) 1.5 equivalents of Ac₂O. DSs of ca. 2.5, 2.9, and 3 were obtained from the integrals of peaks assigned to protons of the acetyl group (CH₃) and the anhydroglucose unit (AGU)—e.g., $DS=7\times I_{(C{H}_3, H)}/3\times I_{\left(AGU,H\right)}$, where 7 and 3 come from the number of protons in the AGU and the number of hydroxyl groups per AGU. All ¹H NMR spectra were performed at 25 °C.