*2.7. CO2 Absorption Measurement*

Carbon dioxide absorption measurements have been performed to investigate the potential ability of the ammonium-based PILs as solvents for CO2 capture. The measurements were conducted in the pressure range of 1-29 bar at room temperature and the results are plotted in Figures 6 and 7. From the plots, the CO2 uptake by the ammonium-based PILs shows a trend of polynomial increment with CO2 pressure. Generally, ammonium-based PILs with [BEHA] cation exhibited marginal difference in CO2 absorption values than that of ammonium-based PILs with [EHA] cations as shown in Figure 7. At a constant pressure of 29 bar, [BEHA][C7] displayed the highest CO2 absorption with the value of 0.78 mol fraction when compared to [BEHA][C6] and [BEHA][C5] with the CO2 mol fractions of 0.68 and 0.64, respectively. This behavior can be explained by using the data reported of density and molar volume of the ammonium-based PILs. The increment in the density value of the PIL increases the molar volume of the PILs which thus in turn causes an increase in the fractional free volume and consequently enhances the CO2 uptake by the ammonium-based PILs [67,68]. Based on the analysis and comparison of FTIR and 13C NMR, Xu and Oncsik et al. proposed that the mechanism of CO2 absorption is via the interaction between gas and the basic anion [32,69]. At approximately 20 bar and 25 ◦C, the CO2 uptake by the [BEHA][C7] is about 40% higher than that of bis(2-ethylhexyl)ammonium butyrate protic ionic liquid [37]. On the other hand, some researchers have performed investigations on the relationship between the viscosity of PILs and performance of CO2 absorption by the PILs and found that PIL with low viscosity value has a high absorption capacity of CO2 [70]. ILs with low viscosities can result in low mass transfer resistance between liquid and gas phases, and this eventually increases the CO2 absorption rate. The viscosities of [BEHA][C5], [BEHA][C6] and [BEHA][C7] were recorded to have values between 19.64 and 21.70 mPa·s at 30 ◦C, which are much lower when compared to conventional ILs, for example [Bmim][BF4] with the viscosity value of 68.90 mPa·s at the same temperature [71]. Regardless of the cation, there is an increasing trend of CO2 absorption in the order of [C5] < [C6] < [C7] anion. As such, both [EHA][C7] and [BEHA][C7] show the highest CO2 absorption capacity at 29 bar and room temperature with the values of CO2 mol fractions of 0.77 moles and 0.78 moles, respectively. These results could be considered an indication of the potential ability of the ammonium-based PILs as solvents for CO2 capture. However, more thorough studies must be conducted for further evaluation before the ammonium-based PILs can be fully used as new solvents in the field of CO2 removal.

**Figure 6.** Plot of CO2 absorption in ammonium-based PILs with (**a**) [EHA] cation, and (**b**) [BEHA] cation.

**Figure 7.** Plot of CO2 absorption in ammonium-based PILs at 298.15 K.

#### **3. Materials and Methods**

#### *3.1. Chemicals*

To synthesize all six ammonium-based PILs, analytical grade chemicals from Merck, Darmstadt, Germany were used. The CAS numbers, abbreviations, grade percentage, density, viscosity, flash point and melting points of all chemicals are as follows: 2-ethylhexylamine (104-75-6, [EHA], 99.0%, 0.789 g.cm−3, 1.1 cP, 140 ◦C, −<sup>76</sup> ◦C), bis-(2-ethylhexyl)amine (106-20-7, [BEHA], 99.0%, 0.805 g.cm−3, 3.7 cP, 130 ◦C, <−<sup>20</sup> ◦ C), pentanoic acid (109- 52-4, [C5], 98.0%, 0.939 g.cm−3, 2.3 cP, 96 ◦C, −34.5 ◦C), hexanoic acid (142-62-1, [C6], 98.0%, 0.927 g.cm<sup>−</sup>3, 3.2 cP, 102 ◦C, −2.78 ◦C) and heptanoic acid (111-14-8, [C7], 99.0%, 0.917 g cm<sup>−</sup>3, 3.4 cP, 113 ◦C, −7.5 ◦C)).

#### *3.2. Synthesis of PILs*

The synthesis of PILs was carried out by using a one-step neutralization reaction and the reaction is written as follows:

$$\text{(R}\_{\text{x}}\text{)}\_{2}\text{NH} + \text{HOOC(R}\_{\text{Y}}) \rightarrow \text{(R}\_{\text{x}}\text{)}\_{2}\text{NH}\_{2}{\text{ }^{\text{+}}}\text{-OOC(R}\_{\text{Y}})$$

RX is 2-ethylhexyl or bis-(2-ethylhexyl and RY is either pentyl, hexyl or heptyl. In this work, 2-ethylhexylamine and bis-(2-ethylhexyl)amine, abbreviated as [EHA] and [BEHA], respectively, were the cations while the acids with the variation of alkyl chain length from pentyl [C5], hexyl [C6] and heptyl [C7] were the anions of the PILs.

In a specific procedure, a 1:1 mol ratio of acid was added to the base with continuous stirring at 250 rpm for 24 h at room temperature. The resulting product was dried under vacuum at 80 ◦C for 6 h to remove any water traces and impurities that might be present resulting from starting reagents as well as surrounding atmosphere. The PILs which were in liquid forms without noticeable solid crystal or precipitation after the purification step were kept in sealed containers until further analysis. The proton transfer reaction had resulted in the formation of six PILs as tabulated in Table 5. Figure 8 depicts the reaction for the synthesis of [EHA][C5].

**Table 5.** Structures of acids and bases, their names, and abbreviations used.

**Figure 8.** Synthesis reaction for [EHA][C5].
