**1. Introduction**

Natural gas is a naturally occurring hydrocarbon that consists of methane gas primarily followed by other mixtures of higher alkanes such as ethane, propane and butane. Generally, natural gas is widely used as a fuel and a raw material in the petrochemical industry [1,2]. Despite its mixture of combustible hydrocarbons content, trace quantities of argon (Ar), hydrogen (H), helium (He), nitrogen (N2) as well as carbon dioxide (CO2) and hydrogen sulfide (H2S) are also present in natural gas [3]. Sour gas, such as CO2, is undesirable due to its acidic property that causes corrosion in the gas pipeline [4]. Apart from that, the existence of CO2 also reduces the fuel value of natural gas due to its non-combustible nature. Therefore, CO2 removal in the refining process is crucial to improving the value of natural gas and the utilization of amine-based solvents, namely monoethanolamine (MEA), which had been widely practiced on industrial scales to capture CO2 in natural gas. This chemical absorption of CO2 by MEA is considered to be the most reliable and

**Citation:** Zailani, N.H.Z.O.; Yunus, N.M.; Ab Rahim, A.H.; Bustam, M.A. Experimental Investigation on Thermophysical Properties of Ammonium-Based Protic Ionic Liquids and Their Potential Ability towards CO2 Capture. *Molecules* **2022**, *27*, 851. https://doi.org/10.3390/ molecules27030851

Academic Editors: Reza Haghbakhsh, Sona Raeissi and Rita Craveiro

Received: 31 December 2021 Accepted: 24 January 2022 Published: 27 January 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

efficient technology for capturing CO2 [5–8]. Gómez-Díaz and his team had compared the ability of their blended amine solvent, which is diamine (N,N-dimethylethylenediamine [DMEDA]) with MEA, towards CO2 capture in which changes in the amine ratios did not lead to important changes in the absorption curve [6]. Despite the outstanding performance of amine-based solvents, it is also known to have a high vapor pressure and high energy input for regeneration. Therefore, studies related to the utilization of solid adsorbents such as zeolites, activated carbon, amine-functionalized adsorbents and metal organic frameworks (MOFs) had been conducted for CO2 adsorption due to their uniqueness as they can be personalized to capture CO2 from either post- or pre-combustion gas streams, depending upon several factors [9]. Current examples of adsorbents for CO2 adsorption are zeolites, activated carbon, amine-functionalized adsorbents and metal organic frameworks (MOFs) [10–12]. Nonetheless, further analysis using adsorbents showed poor adsorption characteristics at low CO2 partial pressures [13,14]. Furthermore, membrane separation processes are also used commercially for CO2 removal from natural gas. However, a singlestage membrane system is not capable of capturing CO2 with high efficiency [15–20]. Due to the given issue, this had encouraged researchers to find alternative solvents that can capture CO2.

Recently, ionic liquids have been recognized as promising solvents for CO2 capture from natural gas. The uniqueness of their properties, specifically their non-detectable vapor pressure, high thermal stability, and high affinity for CO2, enables ILs to be used as solvents for CO2 capture at elevated temperatures and pressures [13,21–25]. Moreover, the chemical and physical properties of ILs can be altered due to the availability of countless cation and anion combinations. Several studies involving mainly binary systems of imidazoliumbased ILs-CO2 or imidazolium-based ILs-other gas have revealed the significant solubility of CO2 in ionic liquids when compared to other gases. For example, comparison studies of CO2 absorption in individual solvents of 30 wt% of 1-(3-aminopropyl)-3-(2 aminoethyl)imidazolium hydroxide [Apaeim][OH], 30 wt% of 1-(3-aminopropyl)-3-(2 aminoethyl)imidazolium alaninate ([Apaeim][ala]) and 30 wt% monoethanolamine (MEA), have shown that both ILs displayed higher CO2 absorption capacities than that of MEA solvent by the value of 2.2-fold. This has further proven that ILs are promising solvents for CO2 capture [26]. In addition, a different class of ionic liquids namely fluorine-based protic ILs (FPILs) have displayed competitive properties for selective removal of CO2 from flue gas and natural gas [27]. Regardless of the promising performance of CO2 capture demonstrated by these types of ionic liquids, they are relatively expensive, they require several steps in the synthesis process, and the utilization of volatile organic solvents is inevitable during the purification process. Recently, protic ionic liquids (PILs) have attracted great interest because of their low cost and simple synthesis pathway. Generally, PILs can be conveniently prepared from stoichiometric neutralization between Brönsted acids and bases. Besides this, PILs display similar CO2 absorptivity with other classes of ionic liquids [28–33]. In addition, Zhu and his team have synthesized a new PIL from superbase 1,8-diazabicyclo [5.4.0]- undec-7-ene (DBU) with imidazole, and they found that the PIL could reversibly capture about 1 mole of CO2 per mole ionic liquid [34]. However, prior to utilization of ionic liquids for any applications, their precise and reliable basic thermophysical properties such as density, viscosity, thermal stability and thermal expansion data are vital for the design and scale up of process equipment. For example, density and thermal expansion data are essential for equipment sizing while thermal stability is required to ensure the practicality of the operating temperature range [35]. In addition, data on solvents' viscosity is important for the designing of industrial processes related to heat and mass transfer as well as dissolution of compounds in solvents [36]. Several research groups have also investigated and provided discussion on the temperature-dependent properties of protic ionic liquids prior to the utilization of ionic liquids in various applications [19,22,31].

Despite promising results of CO2 absorption by protic ionic liquids published in the literature [34], our current work is focusing on the utilization of much cheaper starting reagents, namely amine solutions, for the production of new ammonium-based protic ionic liquids. This work serves as a continuation from our previous work on CO2 absorption utilizing ammonium-based protic ionic liquids (PILs) [37]. Previously, the CO2 absorption of ammonium-based PILs utilizing bis (2-ethylhexyl) ammonium, tributylammonium and ethanolammonium cations coupled with acetate and butyrate anions have been reported. The motivation to further investigate this type of ionic liquid for CO2 capture has risen after we discovered that the PILs could be prepared via a simple synthesis procedure and their capability to absorb CO2 under experimental conditions. To further study the binary system of PILs–CO2, the synthesis of six new ammonium-based 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]) and their performance towards CO2 absorption in a pressure range from 1 bar to 29 bar at 298.15K, are reported in this work.
