Liquid–liquid phase equilibrium for the tert-butanol–methyl tert-butyl ether system with choline chloride-based deep eutectic solvents

Objectives. In comparison with methyl tert-butyl ether (MTBE) and methanol mixtures, the separation of MTBE and tert-butanol (tert-butyl alcohol, TBA) mixtures represents a relevant challenge in chemical technology due to limited research in this area. Our aim was to evaluate the potential and efficiency of using choline chloride-based deep eutectic solvents (DESs) as green extractants for the separation of the MTBE–TBA system via liquid–liquid extraction (LLE).

Methods. DESs were prepared by mixing and heating a hydrogen bond acceptor (choline chloride, ChCl) with hydrogen bond donors (HBDs) as follows: malonic acid (1 : 1 molar ratio), glutaric acid (1 : 1), urea (1 : 2), and glycerol (1 : 2). Liquid–liquid phase equilibrium was experimentally studied in four ternary systems containing MTBE, TBA, and DES at temperatures of 293.15 and 313.15 K and atmospheric pressure. The compositions of the equilibrium liquid phases were determined using ¹H nuclear magnetic resonance spectroscopy (500 MHz, dimethyl sulfoxide-d₆).

Results. The experimental data allowed the key extraction parameters to be calculated: distribution coefficients for TBA and selectivity for MTBE–TBA separation for each investigated DES. A comparative analysis of the extraction capacity of the synthesized solvents toward the separated components was performed. The experimental phase equilibrium data confirm the fundamental possibility of using the ChCl-based DESs under study as extractants for separating the MTBE–TBA system. The calculated selectivity and distribution coefficient values allow the efficiency of various HBDs within the DES composition to be evaluated for solving this separation task. The comparative analysis of the DES extraction capacity showed a dependence of selectivity on the HBD and a slight dependence of selectivity on temperature. The highest selectivity values were observed for DESs based on ChCl/glycerol (1 : 2) and ChCl/urea (1 : 2) systems.

Conclusions. The feasibility of using DESs based on choline chloride with malonic acid, glutaric acid, urea, and glycerol as extractants for separating the MTBE–TBA mixture by LLE method has been experimentally confirmed. The calculated distribution coefficients and selectivity values enable a quantitative assessment and comparison of the efficiency of different DESs for this task, identifying the most promising compositions. The results obtained can be used as the basis for further development and optimization of the LLE process for MTBE and TBA using promising green solvents.

1. Li S., Qian K., Wang S., Liang K., Yan W. Polypyrrole-Grafted Coconut Shell Biological Carbon as a Potential Adsorbent for Methyl Tert-Butyl Ether Removal: Characterization and Adsorption Capability. Int. J. Environ. Res. Public Health 2017;14(2):113. https://doi.org/10.3390/ijerph14020113

2. Levchuk I., Bhatnagar A., Sillanpää M. Overview of Technologies for Removal of Methyl Tert-Butyl Ether (MTBE) from Water. Sci. Total Environ. 2014;476–477: 415–433. https://doi.org/10.1016/j.scitotenv.2014.01.037

3. Knifton J.F., Edwards J.C. Methyl Tert-Butyl Ether Synthesis from Tert-Butanol via Inorganic Solid Acid Catalysis. Appl. Catal. A Gen. 1999;183(1):1–13. https://doi.org/10.1016/S0926-860X(98)00374-3

4. Yadav G.D., Doshi N.S. Alkylation of Aniline with Methyl-Tert-Butyl Ether (MTBE) and Tert-Butanol over Solid Acids: Product Distribution and Kinetics. J. Mol. Catal. A Chem. 2003;194(1-2):195–209. https://doi.org/10.1016/S1381-1169(02)00526-5

5. Zeng Q., Hu B., Cheng H., Chen L., Huang J., Qi Z. Liquid–Liquid Equilibrium for the System of Ionic Liquid [BMIm][HSO4] Catalysed Isobutyl Isobutyrate Formation. J. Chem. Thermodyn. 2018:122:162–169. https://doi.org/10.1016/j.jct.2018.03.015

6. Castro-Muño R., Galiano F., Fíla V., Drioli E., Figoli A. Matrimid®5218 Dense Membrane for the Separation of Azeotropic MeOH-MTBE Mixtures by Pervaporation. Sep. Purif. Technol. 2018;199:27–36. https://doi.org/10.1016/j.seppur.2018.01.045

7. Villegas M., Romero A.I., Parentis M.L., Castro Vidaurre E.F., Gottifredi J.C. Acrylic Acid Plasma Polymerized Poly- (3-Hydroxybutyrate) Membranes for Methanol/MTBE Separation by Pervaporation. Chem. Eng. Res. Des. 2016;109:234–248. https://doi.org/10.1016/j.cherd.2016.01.018

8. Esfahani H.S., Khoshsima A., Pazuki G., Hosseini A. Separation of Methanol and Ethanol from Azeotropic MTBE Mixtures Using Choline Chloride-Based Deep Eutectic Solvents. J. Mol. Liq. 2023;381:121641. https://doi.org/10.1016/j.molliq.2023.121641

9. Gao Y., Fan M., Cheng X., Liu X., Yang H., Ma W., Guo M., Li L. Deep Eutectic Solvent: Synthesis, Classification, Properties and Application in Macromolecular Substances. Int. J. Biol. Macromol. 2024;278(Part 2):134593. https://doi.org/10.1016/j.ijbiomac.2024.134593

10. Ganorkar S.B., Hadole P.M., Patil M.R., Pardeshi C.V., Bobade P.S., Shirkhedkar A.A., Vander Heyden Y. Deep Eutectic Solvents in Analysis, Delivery and Chemistry of Pharmaceuticals. Int. J. Pharm. 2025;672:125278. https://doi.org/10.1016/j.ijpharm.2025.125278

11. Cardona Jimenez M.E., Rodríguez-Ruiz A.C., Velez A., Agudelo-Laverde L.M., Bodoira R.M., Santagapita P.R. An Approach to Subcritical Fluid and Natural Deep Eutectic Solvent-Ultrasound Assisted Extractions as Green Alternatives for Pecan Nutshell Phenolic Compounds Recovery. Food Bioprod. Process. 2025;151:137–147. https://doi.org/10.1016/j.fbp.2025.03.005

12. Alqahtani A.S. Indisputable Roles of Different Ionic Liquids, Deep Eutectic Solvents and Nanomaterials in Green Chemistry for Sustainable Organic Synthesis. J. Mol. Liq. 2024;399:124469. https://doi.org/10.1016/j.molliq.2024.124469

13. Samarov A., Volodina N., Prikhodko I., Toikka A. Liquid–Liquid Equilibrium in Systems with Transesterification of Alcohol (n-Propanol Or n-Butanol)–Ethyl Formate with Deep Eutectic Solvent Based on Choline Chloride at 293.15 and 313.15 K. J. Chem. Eng. Data 2024;69(9):3103–3110. https://doi.org/10.1021/acs.jced.4c00281

14. Lan S., Li Y., Xue S., Zhu T., Yan H. Application of Deep Eutectic Solvents Modified Oxidized Hydrogen-Substituted Graphyne in Adsorption and Electrochemistry. J. Mol. Liq. 2021;335:116532. https://doi.org/10.1016/j.molliq.2021.116532

15. Samarov A., Prikhodko I., Liubichev D., Toikka M. Liquid–Liquid Equilibrium of Alcohol–Ester Systems with Deep Eutectic Solvents Based on Choline Chloride and Alkanediols (Ethylene Glycol or Propylene Glycol). J. Chem. Eng. Data 2022;67(3):707–716. https://doi.org/10.1021/acs.jced.1c00913

16. Golikova A., Samarov A., Trofimova M., Rabdano S., Toikka M., Pervukhin O., Toikka A. Chemical Equilibrium for the Reacting System Acetic Acid–Ethanol–Ethyl Acetate– Water at 303.15 K, 313.15 K and 323.15 K. J. Solution Chem. 2017;46:374–387. https://doi.org/10.1007/s10953-017-0583-1