2.4.3 Effects of ILs structures on CO 2 solubility
2.4.3.1 Effect of the cation
Experimental and molecular modeling studies to investigate the underlying mechanisms for the high solubility of CO2 in imidazolium-based ILs was conducted by Cadena, C. et al.[143]. The CO2 absorption in six different ILs at 10, 25, and 50 °C are reported, two different cations differ only in the nature of the ―acidic‖ site at the 2- position on the imidazolium ring and NTf2, PF6 and BF4 anions were used. The simulation results are consistent with the experimental finding that, for a given anion, there are only small differences in CO2 solubility for the two cations [143]. Moreover, Huang, J. and Ruether, T. [144] reported that CO2 is absorbed in ILs by occupying the free space between the ions through physical absorption mechanisms. Moreover, the overall absorption capacity could be improve by attaching functional group to the ions.
The solubility of CO2 in imidazolium-based ILs incorporating alkyl chains varied between ethyl and octyl was tested [142, 145]. It was observed that Henry‘s constant for the studied ILs decrease gradually from (39 to 30) bar. Also, Peters et al. showed that the solubility of CO2 in [Hmim] is higher than that in [Emim] salt [146]. The solubilities of CO2 in n-alkyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl) imide ILs (n = 2, 6, 10) were measured in a temperature range from (298.15 to 343.15) K and pressure up to 25 MPa. The effects of the cation on the phase behavior and CO2 solubility showed that the longer alkyl chain lengths increase the CO2
solubility (Fig 2-3) [142, 147]. Moreover, other studies showed that CO2 is more soluble in PF6 - salt of the [Bmim] compound than that of the [Emim] derivative [146].
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Fig 2-3 Carbon dioxide solubility in 1-alkyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)amide ionic liquids as a function of the number of carbon atoms, n, in the alkyl-side chain. On the right: carbon dioxide solubility at 303 K in light gray and carbon dioxide solubility at 323 K in dark gray [142].
Cadena, et al. studied a series of imidazolium-based ILs incorporating methyl group at the C2 atom to investigate the responsibility of the acidic proton of the imidazolium ring located at C2 in the CO2- IL interaction [143]. The modification of the structure of the cation leads to a small decrease (1-3 kJ/mol) in the enthalpy of absorption [12]. The influence of changing the cation of the IL (IL) on CO2 solubility was also studied by G. Hong, G. and coworkers [148]. The solubility of carbon dioxide in three ILs based on the [NTf2] anion and 1-ethyl-3-methylimidazolium [C1C2im], 1-butyl-1-methylpyrrolidinium [C1C4pyrr] and propylcholinium [N1132-OH]
was investigated experimentally between 300 and 345 K. The effect of changing the cation is small but significant. The solubility of CO2 in imidazolium-, and ammonium-based RTILs incorporating the [NTf2] anion were studied by Kilaru, P.
and Scovazzo, P. [149]. The results showed that the solubility increases in the order;
[C6mim] > [Emim] for imidazolium, and [N1,8,8,8] > [N1,1,3,10] >[N1,1,1,10] >[N2,2,2,6] >
[N1,1,1,4] for ammonium. In addition, the solubility of CO2 in a Brønsted acid–base IL, [DMFH][NTf2] was investigated at high pressures and at different temperatures. The results showed that the mole fraction solubility of CO2 in [DMFH]NTf2 was a slight higher than [Bmim]NTf2 [150].
Fenga and coworkers [151] studied the solubility of CO2 in a new type of solvents constitute of tetramethylammonium glycinate ([N1,1,1,1]Gly), tetraethylammonium
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glycinate ([N2,2,2,2]Gly), tetramethylammonium lysinate ([N1,1,1,1]Lys) and tetraethylammonium lysinate ([N2,2,2,2]Lys) mixed with N-methyldiethanolamine (MDEA) for the uptake of CO2 (Fig 2-4).
Fig 2-4 Absorption amount of CO2 in aqueous solution of IL +MDEA with 30% total amines [151]
The solubility of CO2 in these IL+MDEA aqueous solutions was investigated over a wide range of IL concentrations (5100%), temperature (298–318 K) and partial pressure of CO2 (4–400 kPa). It was found that the aqueous solutions of 15% IL and 15% MDEA had higher absorption rate and larger uptake than other IL + MDEA solutions of 30% total amines. The results indicated that IL could greatly enhance the absorption and increased the absorption rate of CO2 in MDEA aqueous solutions.
Noticeably, due to the two amino groups in a molecular, the mole absorption of the 30
% lysine based ILs aqueous solutions was 0.98 ([N1,1,1,1]Lys) and 1.21 ([N2,2,2,2]Lys) mole CO2, being about 2–3 times the absorption capacity of MDEA under the same condition. Regeneration under the condition of temperature 353 K, 4 kPa for 240 min showed significant regeneration efficiency (over 98%).
The solubility of carbon dioxide in room temperature ILs (RTILs), dialkylimidazolium dialkylphosphates, was measured at temperature range of 313–
333 K and at pressures close to atmospheric pressure, from which Henry‘s law coefficients, standard Gibbs free energy, enthalpy, and entropy changes of solvation
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were derived. The CO2 solubility in dialkylimidazolium dialkylphosphate was found to increase with increasing chain length of the alkyl groups on the cation and/or the anion as was similarly found in other RTILs [152], the solubility capacities were lower than that of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Bmim][NTf2]) [152]. In addition, the solubilities of carbon dioxide, ethylene, ethane, methane, argon, oxygen, carbon monoxide, hydrogen, and nitrogen gases in 1- n-butyl-3-methylimidazolium hexafluorophosphate were presented. The results indicated that carbon dioxide have the highest solubility and strongest interactions with the IL, followed by ethylene and ethane. The solubilities exhibited a nonlinear trend as the CO2 pressure was increased [81].
The solubility of CO2 in phosphonium-based ILs has received little attention in spite of their interesting characteristics. The gas–liquid equilibrium of two ILs, trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide and trihexyltetradecylphosphonium chloride, in a wide range of temperatures, pressures showed that phosphonium ILs can dissolve even larger amounts of CO2 (on a molar fraction basis) than the corresponding imidazolium-based ILs [153]. Furthermore, the solubility of CO2 in sulfonate ILs (ILs), such as trihexyltetradecylphosphonium dodecylbenzenesulfonate ([P6,6,6,14]C12H25PhSO3) and trihexyltetradecylphosphonium methylsulfonate ([P6,6,6,14]MeSO3) was determined at temperatures ranging from (305 to 325) K and pressures ranging from 4 to 9 MPa. It was found that the different solubility of CO2 in the two kinds of sulfonate ILs is not dramatic on the basis of molality. The solubility of CO2 in [P6,6,6,14]MeSO3 is higher than in [P6,6,6,14]C12H25PhSO3. The Henry‘s law constant for CO2 in all the investigated ILs increases with increasing temperature [154]. Furthermore, Ferguson, L. and Scovazzo, P. [155] presented the solubility, diffusivity, and permeability data for carbon dioxide, ethylene, propylene, butene, and 1,3-butadiene gases in five phosphonium-based ILs at 30 °C. The gas solubilities and diffusivities of the phosphonium-based ILs are of the same magnitude as the gas solubilities for the more familiar imidazolium-based liquids [155].
One of the common ways to increase the solubility of CO2 in ILs is use of fluorinated cations [12]. The solubility of carbon dioxide in the IL 1-
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(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)-3-methylimidazolium bis[trifluoromethyl -sulfonyl] amide [C8H4F13mim]NTf2 was studied (Fig 2-5).
Fig 2-5 Effect of the partial fluorination of the cation on the carbon dioxide solubility in 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl) amide ionic liquids as a function of temperature expressed as mole fraction of carbon dioxide at a partial pressure of 1 bar [142]
It was reported that the solubility increases as the fluorinated alkyl chain length increases, IL with longest fluoroalkyl chain exhibit highest CO2 solubility [82, 142].
In addition, they reported that increasing the fluorinated alkyl chain in the imidazolium cation does not lead to a steady rise of the gaseous uptake by the liquid which may due to the increase of the nonpolar domains of the IL, carbon dioxide being solvated preferentially in the charged regions of the solvent [142]. Even though fluorination of cation and anion is a proven method of increasing the CO2-philicity of compounds but the disadvantages associated with these fluorinated ILs are that they are very costly and environmentally less benign (high stability and low reactivity of the fluorinated compounds lead them to being poorly biodegradable and persistent in the environment [61, 156].