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. 2022 Jan 27;27(3):851.
doi: 10.3390/molecules27030851.

Experimental Investigation on Thermophysical Properties of Ammonium-Based Protic Ionic Liquids and Their Potential Ability towards CO2 Capture

Nur Hidayah Zulaikha Othman Zailani  1 , Normawati M Yunus  1 , Asyraf Hanim Ab Rahim  1 , Mohamad Azmi Bustam  2
Affiliations

Experimental Investigation on Thermophysical Properties of Ammonium-Based Protic Ionic Liquids and Their Potential Ability towards CO2 Capture

Nur Hidayah Zulaikha Othman Zailani et al. Molecules. .

Abstract

Ionic liquids, which are extensively known as low-melting-point salts, have received significant attention as the promising solvent for CO2 capture. This work presents the synthesis, thermophysical properties and the CO2 absorption of a series of ammonium cations coupled with carboxylate anions producing ammonium-based protic ionic liquids (PILs), namely 2-ethylhexylammonium pentanoate ([EHA][C5]), 2-ethylhexylammonium hexanoate ([EHA][C6]), 2-ethylhexylammonium heptanoate ([EHA][C7]), bis-(2-ethylhexyl)ammonium pentanoate ([BEHA][C5]), bis-(2-ethylhexyl)ammonium hexanoate ([BEHA][C6]) and bis-(2-ethylhexyl)ammonium heptanoate ([BEHA][C7]). The chemical structures of the PILs were confirmed by using Nuclear Magnetic Resonance (NMR) spectroscopy while the density (ρ) and the dynamic viscosity (η) of the PILs were determined and analyzed in a range from 293.15K up to 363.15K. The refractive index (nD) was also measured at T = (293.15 to 333.15) K. Thermal analyses conducted via a thermogravimetric analyzer (TGA) and differential scanning calorimeter (DSC) indicated that all PILs have the thermal decomposition temperature, Td of greater than 416K and the presence of glass transition, Tg was detected in each PIL. The CO2 absorption of the PILs was studied up to 29 bar at 298.15 K and the experimental results showed that [BEHA][C7] had the highest CO2 absorption with 0.78 mol at 29 bar. The CO2 absorption values increase in the order of [C5] < [C6] < [C7] anion regardless of the nature of the cation.

Keywords: CO2 absorption; ammonium-based protic ionic liquids; density; lattice potential energy; phase transition; refractive index; standard entropy; thermal expansion coefficient; viscosity.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Thermal decomposition curves of (a) [EHA][C5], [EHA][C6], [EHA][C7], and (b) [BEHA][C5], [BEHA][C6], [BEHA][C7] at heating rate of 10 °C.min−1.
Figure 2
Figure 2
Differential scanning calorimetry (DSC) curves of (a) [EHA][C7], and (b) [BEHA][C6] at a heating rate of 10 °C min−1.
Figure 3
Figure 3
Density (ρ) values of (a) [EHA][C5], [EHA][C6], [EHA][C7], and (b) [BEHA][C5], [BEHA][C6], [BEHA][C7] as a function of temperature.
Figure 4
Figure 4
Viscosity (ŋ) values of (a) [EHA][C5], [EHA][C6], [EHA][C7], and (b) [BEHA][C5], [BEHA][C6], [BEHA][C7] as a function of temperature.
Figure 5
Figure 5
Refractive index (nD) values of (a) [EHA][C5], [EHA][C6] and [EHA][C7], and (b) [BEHA][C5], [BEHA][C6] and [BEHA][C7] as a function of temperature.
Figure 6
Figure 6
Plot of CO2 absorption in ammonium-based PILs with (a) [EHA] cation, and (b) [BEHA] cation.
Figure 7
Figure 7
Plot of CO2 absorption in ammonium-based PILs at 298.15 K.
Figure 8
Figure 8
Synthesis reaction for [EHA][C5].
See this image and copyright information in PMC

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