2.4.3 Effects of ILs structures on CO 2 solubility
2.4.3.3 Effect of functional group
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constants of 33.3 and 30.7 for [C8mim]NTf2, and [C10mim]NTf2 respectively at 303 K [142]. Moreover, other studies showed the effect of other fluorinated anions on the CO2 solubility in the following order; [CF3SO3] < [(CF3SO2)2N] [82].
Muldoon, et al. [82] reported that [N4,4,4,4]DOSS which contains an anion of known low toxicity, has several features that lead to good solubility of CO2 the highest pressures. The solubility of CO2 in [N4,4,4,4]DOSS was nearly identical to that in [hmim]eFAP [82], one of the best performing fluorinated ILs.
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The fluorinated compounds are less environmentally friendly because of their high toxicity values which weaken their potential use as green solvents [12]. Additionally, they contributes significantly to the cost of CO2 capture because it increases the experimental cost.
Incorporation of oxygen-containing functional (ether and ester groups) to the cation or anion to increase the solubility of CO2 in ILs received much attention because the electron-poor carbon atom of CO2 has ability toward electronegative atom [167, 168]. The ideal solubility selectivities of CO2, N2, and CH4 in a series of imidazolium-based room-temperature ILs (RTILs) with one, two, or three oligo(ethylene glycol) substituents were investigated [169]. The results showed that these RTILs reveals similar levels of CO2 solubility but lower solubilities of N2 and CH4. As a consequence, RTILs with oligo(ethylene glycol) substituents were observed to have 30-75% higher ideal solubility selectivities for CO2/N2 and CO2/CH4. .Additionally, Yuan and co-workers [166] studied the solubility of CO2 in a group of hydroxyl ammonium ILs and obtained similar solubility values to those for imidazolium-based ILs. One of the clear application of the design of task-specific ILs (TSILs) is the incorporation of amine functional groups to ILs because they are specifically designed to increase the CO2-IL interaction [170]. The absorption of CO2 in the amine functionalized ILs is carried out through chemisorption, and the experimental results, showed that the mole ratio of CO2/TSIL approaches a maximum of 0.5 (Fig 2-7) [14].
Increasing the number of potential sites for the CO2 interaction by obtaining ILs with more than one specific functional group is another approach to obtain a low Henry‘s law constant. Amino-acid-based ILs possess both carboxyl and amine functional groups, it was found that they react with CO2 in a 1:1 stoichiometric ratio [171]; [P66614]Met and [P66614]Pro were used for this study and the absorption is in good agreement with the results obtained from IR spectroscopy and CO2 absorption studies. A developed (3-aminopropyl)tributylphosphonium amino acid ILs for CO2
capture, leading to 1 mol of CO2 captured/1 mol of IL, and the ILs can be repeatedly recycled for CO2 uptake [172]. Nevertheless, the high viscosities of these compounds, which are even larger upon complexation with CO2, would hinder the
46
CO2 diffusion and uptake rate. Furthermore, the ability of 1,1,3,3- tetramethylguanidinium-based ILs to reversibly absorb gaseous was investigated. It was found the gas almost completely desorbed by heating or lowering of the pressure and no change in absorbing capacity of the ILs was observed after several cycles [173].
Fig 2-7 CO2 absorption in NH2-cation functionalized RTILs at 303 K. Lines plotted as view aid. Points represent experimental measurements: (■)APMim[BF4], (□) APMim[DCA], (▲) AEMPyrr[BF4] and () APMim[NTf2]. Standard Ionic liquids:() emim[NTf2], (+) bmim[BF4] and () bmim[DCA].
Zhang, J. and coworkers [174] studied the solubilities of CO2 and the interactions of polar and nonpolar solutes with the ILs 1-butyronitrile-3-methylimidazolium bis(trifluoromethylsulfonyl) imidate [Cpmim]NTf2, 1-butyronitrile-2,3- dimethylimidazolium bis(trifluoromethylsulfonyl) imidate [Cpmmim]NTf2, 1- butyronitrile-3-methylimidazolium dicyanamide [Cpmim]N(CN)2, 1-butyronitrile- 2,3-dimethylimidazolium dicyanamide [Cpmmim]N(CN)2, 1-butyl-3- methylimidazolium palmitate [Bmim[n-C16H33COO, and 1-butyl-3- methylimidazolium stearate [Bmim]n-C18H35COO at T = (303.15, 313.15, 323.15, and 333.15) K. The incorporation of the cyano functional group substitution on the alkyl chain results in a remarkable decrease of the interactions of hydrocarbons compared with the 1,3-dialkylimidazolium ILs [174, 175].
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To design IL-based processes, knowledge of the solubilities of gases in ILs is needed. However, only limited information about these properties is available in the literature. CO2 has been used in many previous studies of solubility in ionic liquids, because it is of interest for a variety of ILs applications. Therefore, it was a convenient solute choice for many studies [176]. Several cations and anions were studied at different temperatures and pressure also several binary systems for these compounds were investigated. Table 2-4 present examples of cations, anions, functionalized cations and functionalized anions that have been studied for CO2
capture.
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Table 2-4 Studied anions and cations for CO2 capture
Cation Anion Reference Cation Anion Reference
[Emim] [NTf2] [177] [N113 10] [NTf2] [159]
[Emim] [OTf] [178] [N111 10] [NTf2] [159]
[Emim] [dca] [178] [N2226 [NTf2] [159]
[Emim] [BF4] [163] [Hmpy] [NTf2] [173]
[Emim] [B(CN)4] [161] [Bmpy] [NTf2] [143]
[Emim] [Et2PO4] [152] [Hmpy] [NTf2] [143]
[C3mim] [BF4] [145] [P666 14] [NTf2] [153]
[Bmim] [NTf2] [177] [P666 14] [OTf] [153]
[Bmim] [SCN] [179] [P666 14] [PF6] [153]
[Bmim] [PF6] [12] [P4443] [AA] [172]
[Bmim] Acetate [180] [HEF] Formate [166]
[Bmim] [NO3] [82] [HEA] Lactate [166]
[Bmim] [DCA] [82] [THEA] Acetate [166]
[Bmim] [OTf] [82] [THEA] Lactate [166]
[Bmim] [TFA] [82] [HEA] Formate [166]
[Bmim] [Bu2PO4] [152] [HEA] Acetate [166]
[Bmim] [C7F15CO2] [82] [Hemim] [NTf2] [166]
[Bmim] Palmitate [174] [Hemim] [OTf] [166]
[Bmim] Stearate [174] [Hemim] [PF6] [166]
[Hmim] [BF4] [145] [C6H4F9mim] [NTf2] [8]
[Hmim] [PF6] [4] [C8H4F13mim] [NTf2] [8]
[Hmim] [NTf2] [143] [TMGH] L [173]
[Omim] [BF4] [145] [CpMIm] [NTf2] [174]
[Omim] [NTf2] [142] [CpMMIm] [Dca] [174]
[Dmim] [NTf2] [147] [CpMIm] [NTf2] [174]
[Dmim] [MP] [179] [CpMMIm] [Dca] [174]
[N1111] [Gly] [151] [NH2Cnmim] [NTf2] [39]
[N1111] [Lys] [151] [NH2Cnmim] [SCN] [39]
[N2222] [Gly] [151] [NH2Cnmim] [TFA] [39]
[N2222] [Lys] [151] [NH2Cnmim] [CH3SO4] [39]
[N4111] [NTf2] [177] [Hmim] [eFAP] [143]
[N2224] [CH3COO] [181] [Hmim] [pFAP] [143]
[N4444] Doc [82] [Etoim] [NTf2] [179]
[N1888] [NTf2] [159]