Thermophysical Properties and CO 2 Absorption of Ammonium-Based Protic Ionic Liquids Containing Acetate and Butyrate Anions

: Ionic liquids, which are classiﬁed as new solvents, have been identiﬁed to be potential solvents in the application of CO 2 capture. In this work, six ammonium-based protic ionic liquids, containing ethanolammonium [EtOHA], tributylammonium [TBA], bis(2-ethylhexyl)ammonium [BEHA] cations, and acetate [AC] and butyrate [BA] anions, were synthesized and characterized. The thermophysical properties of the ammonium-based protic ionic liquids were measured. Density, ρ , and dynamic viscosity, η , were determined at temperatures between 293.15 K and 363.15 K. The density and viscosity values were correlated using empirical correlations and the thermal coe ﬃ cient expansion, α p , and molecular volume, V m , were estimated using density values. The thermal stability of the ammonium-based protic ionic liquids was investigated using thermogravimetric analyzer (TGA) at a heating rate of 10 ◦ C · min -1 . The CO 2 absorption of the ammonium-based ionic liquids were measured up to 20 bar at 298.15 K. From the experimental results, [BEHA][BA] had the highest a ﬃ nity towards CO 2 with the mol fraction of CO 2 absorbed approaching 0.5 at 20 bar. Generally, ionic liquids with butyrate anions have better CO 2 absorption than that of acetate anions while [BEHA] ionic liquids have higher a ﬃ nity towards CO 2 followed by [TBA] and [EtOHA] ionic liquids. and N.M.Y.; validation, N.H.H..; formal analysis, T.M.; resources, C.D.W. and T.M.; data curation, N.M.Y. and T.M.; writing—original draft preparation, N.M.Y. and N.H.H.; writing—review and editing, J.W.L. and P.L.S.; supervision, N.M.Y.; project administration, N.M.Y. and C.D.W.; funding acquisition, N.M.Y.


Introduction
Natural gas consists mainly of methane as well as other higher alkanes in varied amounts. It is mainly used as a fuel and as a raw material in petrochemical industries [1]. While natural gas is principally a mixture of combustible hydrocarbons, many natural gases also contain impurities, such as carbon dioxide, CO 2 , hydrogen sulfide, H 2 S, and water. Refining processes are required to remove all of these unwanted impurities from natural gas. Besides water and higher-molecular-weight hydrocarbons, one of the most crucial parts of gas processing is the elimination of CO 2 and this process is normally done by means of chemical absorption techniques using alkanolamine solutions. Despite the successful practice of using alkanolamines for CO 2 removal, several disadvantages have

Synthesis
For the synthesis of each of the ammonium-based protic ionic liquids, an equimolar amount of the acid was added dropwise to the amine at ambient conditions and the mixture was consistently stirred for 24 h to facilitate mixing. The resulting solution was dried under vacuum at 65 • C for 6 h to remove remaining reactants. The final product was kept in a seal container until further use. The combinations of two acids and three amines produce six ammonium-based protic ionic liquids. Table 1 shows the structures and the abbreviation used for the ionic liquids. All ionic liquids exist as liquids except [BEHA][AC], which exists as a solid compound at room temperature. Table 1. Structures of cations and anions, names and abbreviations.

Synthesis
For the synthesis of each of the ammonium-based protic ionic liquids, an equimolar amount of the acid was added dropwise to the amine at ambient conditions and the mixture was consistently stirred for 24 h to facilitate mixing. The resulting solution was dried under vacuum at 65 °C for 6 h to remove remaining reactants. The final product was kept in a seal container until further use. The combinations of two acids and three amines produce six ammonium-based protic ionic liquids. Table  1 shows the structures and the abbreviation used for the ionic liquids. All ionic liquids exist as liquids except [BEHA][AC], which exists as a solid compound at room temperature.

NMR and Water Content
The structure of the ammonium-based protic ionic liquids was analyzed and confirmed via nuclear magnetic resonance (NMR) spectroscopy. About 5 mg sample of ionic liquid was dissolved in 6 mL deuterated solvent and the sample's purity was determined using 500 MHz Bruker NMR Oxford Instrument. Coulometric Karl Fischer autotitrator DL39 from Mettler was used to determine the water content of the ionic liquids.

Thermophysical Characterization
The viscosity and density of the ammonium-based protic ionic liquids were determined simultaneously using Anton Parr Stabinger Viscometer SVM3000 in the temperature range of 293.15 K to 363.15 K. The temperature measurement's accuracy was within 0.02 K while the reproducibility of the viscosity and density measurements were 0.35% and ±5.10 −4 g.cm −3 , respectively [28]. The decomposition temperatures of the ionic liquids were examined by means of thermogravimetric analyzer, TGA Perkin Elmer STA 6000. About 10 mg of sample was loaded into a platinum pan and the sample was heated at a heating rate of 10 °C.min -1 under nitrogen flow.

Synthesis
For the synthesis of each of the ammonium-based protic ionic liquids, an equimolar amount of the acid was added dropwise to the amine at ambient conditions and the mixture was consistently stirred for 24 h to facilitate mixing. The resulting solution was dried under vacuum at 65 °C for 6 h to remove remaining reactants. The final product was kept in a seal container until further use. The combinations of two acids and three amines produce six ammonium-based protic ionic liquids. Table  1 shows the structures and the abbreviation used for the ionic liquids. All ionic liquids exist as liquids except [BEHA][AC], which exists as a solid compound at room temperature.

NMR and Water Content
The structure of the ammonium-based protic ionic liquids was analyzed and confirmed via nuclear magnetic resonance (NMR) spectroscopy. About 5 mg sample of ionic liquid was dissolved in 6 mL deuterated solvent and the sample's purity was determined using 500 MHz Bruker NMR Oxford Instrument. Coulometric Karl Fischer autotitrator DL39 from Mettler was used to determine the water content of the ionic liquids.

Thermophysical Characterization
The viscosity and density of the ammonium-based protic ionic liquids were determined simultaneously using Anton Parr Stabinger Viscometer SVM3000 in the temperature range of 293.15 K to 363.15 K. The temperature measurement's accuracy was within 0.02 K while the reproducibility of the viscosity and density measurements were 0.35% and ±5.10 −4 g.cm −3 , respectively [28]. The decomposition temperatures of the ionic liquids were examined by means of thermogravimetric analyzer, TGA Perkin Elmer STA 6000. About 10 mg of sample was loaded into a platinum pan and the sample was heated at a heating rate of 10 °C.min -1 under nitrogen flow.

Synthesis
For the synthesis of each of the ammonium-based protic ionic liquids, an equimolar amount of the acid was added dropwise to the amine at ambient conditions and the mixture was consistently stirred for 24 h to facilitate mixing. The resulting solution was dried under vacuum at 65 °C for 6 h to remove remaining reactants. The final product was kept in a seal container until further use. The combinations of two acids and three amines produce six ammonium-based protic ionic liquids. Table  1 shows the structures and the abbreviation used for the ionic liquids. All ionic liquids exist as liquids except [BEHA][AC], which exists as a solid compound at room temperature.

NMR and Water Content
The structure of the ammonium-based protic ionic liquids was analyzed and confirmed via nuclear magnetic resonance (NMR) spectroscopy. About 5 mg sample of ionic liquid was dissolved in 6 mL deuterated solvent and the sample's purity was determined using 500 MHz Bruker NMR Oxford Instrument. Coulometric Karl Fischer autotitrator DL39 from Mettler was used to determine the water content of the ionic liquids.

Thermophysical Characterization
The viscosity and density of the ammonium-based protic ionic liquids were determined simultaneously using Anton Parr Stabinger Viscometer SVM3000 in the temperature range of 293.15 K to 363.15 K. The temperature measurement's accuracy was within 0.02 K while the reproducibility of the viscosity and density measurements were 0.35% and ±5.10 −4 g.cm −3 , respectively [28]. The decomposition temperatures of the ionic liquids were examined by means of thermogravimetric analyzer, TGA Perkin Elmer STA 6000. About 10 mg of sample was loaded into a platinum pan and the sample was heated at a heating rate of 10 °C.min -1 under nitrogen flow.

Synthesis
For the synthesis of each of the ammonium-based protic ionic liquids, an equimolar amount of the acid was added dropwise to the amine at ambient conditions and the mixture was consistently stirred for 24 h to facilitate mixing. The resulting solution was dried under vacuum at 65 °C for 6 h to remove remaining reactants. The final product was kept in a seal container until further use. The combinations of two acids and three amines produce six ammonium-based protic ionic liquids. Table  1 shows the structures and the abbreviation used for the ionic liquids. All ionic liquids exist as liquids except [BEHA][AC], which exists as a solid compound at room temperature.

NMR and Water Content
The structure of the ammonium-based protic ionic liquids was analyzed and confirmed via nuclear magnetic resonance (NMR) spectroscopy. About 5 mg sample of ionic liquid was dissolved in 6 mL deuterated solvent and the sample's purity was determined using 500 MHz Bruker NMR Oxford Instrument. Coulometric Karl Fischer autotitrator DL39 from Mettler was used to determine the water content of the ionic liquids.

Thermophysical Characterization
The viscosity and density of the ammonium-based protic ionic liquids were determined simultaneously using Anton Parr Stabinger Viscometer SVM3000 in the temperature range of 293.15 K to 363.15 K. The temperature measurement's accuracy was within 0.02 K while the reproducibility of the viscosity and density measurements were 0.35% and ±5.10 −4 g.cm −3 , respectively [28]. The decomposition temperatures of the ionic liquids were examined by means of thermogravimetric analyzer, TGA Perkin Elmer STA 6000. About 10 mg of sample was loaded into a platinum pan and the sample was heated at a heating rate of 10 °C.min -1 under nitrogen flow.

Synthesis
For the synthesis of each of the ammonium-based protic ionic liquids, an equimolar amount of the acid was added dropwise to the amine at ambient conditions and the mixture was consistently stirred for 24 h to facilitate mixing. The resulting solution was dried under vacuum at 65 °C for 6 h to remove remaining reactants. The final product was kept in a seal container until further use. The combinations of two acids and three amines produce six ammonium-based protic ionic liquids. Table  1 shows the structures and the abbreviation used for the ionic liquids. All ionic liquids exist as liquids except [BEHA][AC], which exists as a solid compound at room temperature.

NMR and Water Content
The structure of the ammonium-based protic ionic liquids was analyzed and confirmed via nuclear magnetic resonance (NMR) spectroscopy. About 5 mg sample of ionic liquid was dissolved in 6 mL deuterated solvent and the sample's purity was determined using 500 MHz Bruker NMR Oxford Instrument. Coulometric Karl Fischer autotitrator DL39 from Mettler was used to determine the water content of the ionic liquids.

Thermophysical Characterization
The viscosity and density of the ammonium-based protic ionic liquids were determined simultaneously using Anton Parr Stabinger Viscometer SVM3000 in the temperature range of 293.15 K to 363.15 K. The temperature measurement's accuracy was within 0.02 K while the reproducibility of the viscosity and density measurements were 0.35% and ±5.10 −4 g.cm −3 , respectively [28]. The decomposition temperatures of the ionic liquids were examined by means of thermogravimetric analyzer, TGA Perkin Elmer STA 6000. About 10 mg of sample was loaded into a platinum pan and the sample was heated at a heating rate of 10 °C.min -1 under nitrogen flow.

NMR and Water Content
The structure of the ammonium-based protic ionic liquids was analyzed and confirmed via nuclear magnetic resonance (NMR) spectroscopy. About 5 mg sample of ionic liquid was dissolved in 6 mL deuterated solvent and the sample's purity was determined using 500 MHz Bruker NMR Oxford Instrument. Coulometric Karl Fischer autotitrator DL39 from Mettler was used to determine the water content of the ionic liquids.

Thermophysical Characterization
The viscosity and density of the ammonium-based protic ionic liquids were determined simultaneously using Anton Parr Stabinger Viscometer SVM3000 in the temperature range of 293.15 K to 363.15 K. The temperature measurement's accuracy was within 0.02 K while the reproducibility of the viscosity and density measurements were 0.35% and ±5.10 −4 g·cm −3 , respectively [28]. The decomposition temperatures of the ionic liquids were examined by means of thermogravimetric analyzer, TGA Perkin Elmer STA 6000. About 10 mg of sample was loaded into a platinum pan and the sample was heated at a heating rate of 10 • C·min -1 under nitrogen flow.

CO 2 Absorption Measurement
The ability of the ammonium-based protic ionic liquids to absorb CO 2 was investigated based on a pressure drop technique using a solubility cell as described in our previous publication [29]. The Processes 2019, 7, 820 4 of 15 solubility cell consists of an equilibrium cell and a gas vessel immersed in a thermostatic bath. In a pressure drop method, the gas with a known pressure at constant volume is allowed to be in contact with the ionic liquid in the equilibrium cell and the pressure drop is monitored as the gas absorbs into the ionic liquid until equilibrium is attained. In a typical experiment, the equilibrium cell was loaded with a pre-weighed amount of the ionic liquid and the equilibrium cell was evacuated to remove any gases. In the gas vessel, CO 2 was allowed to stabilize before being quickly charged into the equilibrium cell. The CO 2 -ionic liquid system was assumed to achieve equilibrium when the pressure attained a constant value. The system was maintained in that conditions for an additional two hours to ensure equilibration. Equation (1) was used to calculate the amount of CO 2 absorbed in the ionic liquid, n 2 [30]: where P ini and T ini are the initial pressure and temperature of the system, P eq and T eq are the pressure and temperature of the system at equilibrium, V total is the volume of the equilibrium cell, V liq is the volume of ionic liquid, R is the gas constant, and Z 2 represents the compressibility factor of the gas. Z 2 can be calculated using Soave-Redlich-Kwong equation of state [31]. The mole fraction of CO 2 absorbed in the ionic liquid (x 2 ) was calculated using Equation (2): where n 2 liq represents the mole of dissolved CO 2 and n 1 liq is the mole of the ionic liquid.

Results and Discussion
In this work, six ammonium-based protic ionic liquids- is not included as it exists as solid. As can be seen from Figure 1, the density of all five ammonium-based protic ionic liquids decreased gradually and linearly with increasing temperature over the range of temperature studied. An increase in temperature caused higher mobility of the ions which, in turn, weakens the intermolecular forces between the constituent ions and correspondingly increases the unit volume for these ions [32]. The density of these ammonium-based protic ionic liquids was slightly affected by the length of the alkyl chain of the anion in which the density of ionic liquids with the [AC] anion was higher than of ionic liquids with the [BA] anion for a fixed cation, as shown in Figure 2a,b. This observation is consistent with the literature in which it has been shown that the density value drops as the alkyl chain gets longer [28,[32][33][34][35][36][37]. Our experimental density value of [EtOHA][AC] is in good agreement with Kurnia et al. [35] and Hosseini et al. [38] with the value differences of less than 0.2% and 0.8%, respectively. Generally, effective arrangement of ions in a liquid can increase the density of the liquid due to a greater number of ions available in a unit volume [39]. Based on our experimental results, as shown in        The dynamic viscosity of the ammonium-based protic ionic liquids, presented in Figure 4, dropped significantly as the temperature increased and the viscosity of ionic liquids with a [BA] anion was higher than that of ionic liquids with an [AC] anion for each type of cation studied in this work, as shown in Figure 5. The longer the alkyl chain in the ionic liquid structure, the higher the viscosity of the ionic liquids due to the increase in van der Waals attraction between the aliphatic alkyl chains [35].    The dynamic viscosity of the ammonium-based protic ionic liquids, presented in Figure 4, dropped significantly as the temperature increased and the viscosity of ionic liquids with a [BA] anion was higher than that of ionic liquids with an [AC] anion for each type of cation studied in this work, as shown in Figure 5. The longer the alkyl chain in the ionic liquid structure, the higher the viscosity of the ionic liquids due to the increase in van der Waals attraction between the aliphatic alkyl chains [35].  The dynamic viscosity of the ammonium-based protic ionic liquids, presented in Figure 4, dropped significantly as the temperature increased and the viscosity of ionic liquids with a [BA] anion was higher than that of ionic liquids with an [AC] anion for each type of cation studied in this work, as shown in Figure 5. The longer the alkyl chain in the ionic liquid structure, the higher the viscosity of the ionic liquids due to the increase in van der Waals attraction between the aliphatic alkyl chains [35].   The values of density, ρ and dynamic viscosity, η were fitted using Equations (3) and (4) [28]: where ρ is the density, η is the dynamic viscosity of the ionic liquids, T is temperature in K, and A 0 , The thermal expansion coefficient, α p , for the ammonium-based protic ionic liquids can be calculated using Equation (6) [28] while the molecular volume, V m can be estimated from Equation (7) in which M is the molar mass of the ionic liquid and N A represents the Avogadro's number [32,36,37]: The calculated thermal expansion coefficients and molecular volume values of the ammoniumbased protic ionic liquids are presented in Tables 6 and 7. The calculated values lie in the range of (5.3 to 9.8).10 -4 K -1 for all five ionic liquids. The thermal expansion coefficients were found to be quite consistent over the temperature range studied and therefore are considered to be temperature independent. The pattern of the results is consistent with other types of ionic liquids [27,28,36,40,41]. The molecular volume, Vm, of the [BA] ionic liquid was greater than that of [AC] for a fixed cation and this may be attributed to the presence of additional CH2 groups [36,37]. In this work, the Vm The values of density, ρ and dynamic viscosity, η were fitted using Equations (3) and (4) [28]: where ρ is the density, η is the dynamic viscosity of the ionic liquids, T is temperature in K, and A 0 , A 1 , A 2 , and A 3 are correlation coefficients determined using the method of least squares. The calculated correlation coefficients together with standard deviations, SD are presented in Tables 4 and 5. The standard deviations, SD, were calculated using Equation (5) in which Z expt and Z calc are experimental and calculated values, respectively, while n DAT is the number of experimental points: The thermal expansion coefficient, α p , for the ammonium-based protic ionic liquids can be calculated using Equation (6) [28] while the molecular volume, V m can be estimated from Equation (7) in which M is the molar mass of the ionic liquid and N A represents the Avogadro's number [32,36,37]: The calculated thermal expansion coefficients and molecular volume values of the ammonium-based protic ionic liquids are presented in Tables 6 and 7. The calculated values lie in the range of (5.3 to 9.8)·10 -4 K -1 for all five ionic liquids. The thermal expansion coefficients were found to be quite consistent over the temperature range studied and therefore are considered to be temperature independent. The pattern of the results is consistent with other types of ionic liquids [27,28,36,40,41]. The molecular volume, V m , of the [BA] ionic liquid was greater than that of [AC] for a fixed cation and this may be attributed to the presence of additional CH 2 groups [36,37] 15 5.4 6.8 9.5 9.0 9.5 343. 15 5.4 6.8 9.6 9.0 9.6 353. 15 5.5 6.8 9.7 9.1 9.7 363. 15 5.5 6.9 9.8 9.2 9.8 Table 7. Molecular volume (V m ) of the ionic liquids calculated using Equation (7). The thermal decomposition (T d ) of the ammonium-based protic ionic liquids were measured at a heating rate of 10 • C·min -1 . The T d was approximately determined by the intersection of the baseline weight from the beginning of the measurement and the tangent of the weight against the temperature curve as the decomposition process occurs. The T d of the ionic liquids are presented in Table 8 Figure 6. The thermal stability of the ammonium-based protic ionic liquids in this study varied with the ion combination. The T d of [BA] ionic liquid was higher than that of [AC] ionic liquid for every type of cation studied while [EtOHA] ionic liquids displayed the highest T d followed by [BEHA] and [TBA]. However, ammonium-based protic ionic liquids in this work and from the literature [36] tend to possess lower thermal stability compared to other ionic liquids, such as imidazolium and pyridinium ionic liquids [28,41,42]. However, generalization must not be made as the thermal stability depends largely on the combination of the cation and anion of the ionic liquids. However, ammonium-based protic ionic liquids in this work and from the literature [36] tend to possess lower thermal stability compared to other ionic liquids, such as imidazolium and pyridinium ionic liquids [28,41,42]. However, generalization must not be made as the thermal stability depends largely on the combination of the cation and anion of the ionic liquids.

CO2 Absorption
The experimental results of CO2 absorption in the ammonium-based protic ionic liquids are shown in Figure 7. Generally, the CO2 absorption in these ammonium-based protic ionic liquids increased with pressure following Henry's law; the solubility of a gas in a liquid is proportional to the partial pressure of the gas above the surface of the liquid. The mol fraction of CO2 absorbed in the ammonium-based protic ionic liquids was in the range of about 0.02 to 0.48 and up to 20 bar at 298.15 K. The effects of cation structure on the CO2 absorption in the ionic liquids are shown in Figure 8.  Figure 9 indicates a slight increase in the CO2 solubility when the anion of a common cation was changed from [AC] to [BA]. Based on our experimental results, there is a relationship between absorption of CO2 with the density and the molecular volume of the ionic liquids. As the density decreases and molecular volume increases, the fractional free volume increases and, thus, the solubility of CO2 increases [43,44]. By using a common anion, the CO2 absorption in

CO 2 Absorption
The experimental results of CO 2 absorption in the ammonium-based protic ionic liquids are shown in Figure 7. Generally, the CO 2 absorption in these ammonium-based protic ionic liquids increased with pressure following Henry's law; the solubility of a gas in a liquid is proportional to the partial pressure of the gas above the surface of the liquid. The mol fraction of CO 2 absorbed in the ammonium-based protic ionic liquids was in the range of about 0.02 to 0.48 and up to 20 bar at 298.15 K. The effects of cation structure on the CO 2 absorption in the ionic liquids are shown in Figure 8.  Figure 9 indicates a slight increase in the CO 2 solubility when the anion of a common cation was changed from [AC] to [BA]. Based on our experimental results, there is a relationship between absorption of CO 2 with the density and the molecular volume of the ionic liquids. As the density decreases and molecular volume increases, the fractional free volume increases and, thus, the solubility of CO 2 increases [43,44]. By using a common anion, the CO  capture when compared to more established type of ionic liquids. However, more experimental investigation and data are needed to further evaluate the potential ability of our ammonium-based protic ionic liquids in the application of CO2 capture.

Conclusions
Six ammonium-based protic ionic liquids were successfully synthesized via solvent-free 1-step neutralization reaction. The density, viscosity, and decomposition temperature were measured. The thermal expansion coefficient and the molecular volume were calculated using the density values. The density and viscosity values were inversely proportional with temperature in the range of temperature studied at atmospheric pressure. The density decreased when the alkyl chain of the anion increased, while the viscosity increased with the alkyl chain of the anion. The decomposition temperature of the ammonium-based protic ionic liquids was affected by the combination of cation and anion and [EtOHA] ionic liquids had the highest thermal stability when compared to the other ionic liquids. The absorption of CO2 in the six ammonium-based protic ionic liquids was measured at 298.15 K and up to a pressure of 20 bar. The CO2 absorption values in the ammonium-based protic ionic liquids increased with pressure and both the cation and anion affected the solubility of CO2 in the ionic liquids. The amount of CO2 absorbed was affected by the length of the alkyl chain of the anion while [BEHA] ionic liquids displayed higher CO2 absorption capacity compared to [TBA] and [EtOHA] ionic liquids. Results indicate the potential of the ammonium-based protic ionic liquids to be used as solvents for CO2 capture.

Conclusions
Six ammonium-based protic ionic liquids were successfully synthesized via solvent-free 1-step neutralization reaction. The density, viscosity, and decomposition temperature were measured. The thermal expansion coefficient and the molecular volume were calculated using the density values. The density and viscosity values were inversely proportional with temperature in the range of temperature studied at atmospheric pressure. The density decreased when the alkyl chain of the anion increased, while the viscosity increased with the alkyl chain of the anion. The decomposition temperature of the ammonium-based protic ionic liquids was affected by the combination of cation and anion and [EtOHA] ionic liquids had the highest thermal stability when compared to the other ionic liquids. The absorption of CO 2 in the six ammonium-based protic ionic liquids was measured at 298.15 K and up to a pressure of 20 bar. The CO 2 absorption values in the ammonium-based protic ionic liquids increased with pressure and both the cation and anion affected the solubility of CO 2 in the ionic liquids. The amount of CO 2 absorbed was affected by the length of the alkyl chain of the anion while [BEHA] ionic liquids displayed higher CO 2 absorption capacity compared to [TBA] and [EtOHA] ionic liquids. Results indicate the potential of the ammonium-based protic ionic liquids to be used as solvents for CO 2 capture.