5.3 E FFECT OF THE IL S CATIONS ON CO 2 SOLUBILITY
5.3.3 Phosphonium-based ILs
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solute−solvent size and shape asymmetries will generate important entropic and free volume contributions to the non ideality of the system with significant impact on the CO2 solubility on these systems [218].
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0 50 100 150 200 250 300
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
CO2
Time/(min)
20 bar 15 bar 10 bar 5 bar 1 bar
Fig 5-16 CO2 solubility in [P8,8,8,14]DOSS as a function of time at 298.15 K
0 50 100 150 200 250
0.0 0.1 0.2 0.3 0.4 0.5 0.6
CO2
Time/(min)
20 bar 15 bar 10 bar 5 bar 1 bar
Fig 5-17 CO2 solubility in [P8,8,8C6 P8,8,8]DOSS2 as a function of time at 298.15 K
0 50 100 150 200 250 300
0.0 0.1 0.2 0.3 0.4 0.5
CO2
Time/(min)
20 bar 15 bar 10 bar 5 bar 1 bar
Fig 5-18 CO2 solubility in [P8,8,8C10 P8,8,8]DOSS2 as a function of time at 298.15 K
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Although these ILs has different viscosities, the time required for the CO2 uptake to reach the equilibrium was comparable to [C2CNCnim]DOSS ILs. Moreover, no significant difference in the CO2 uptake time between the monocationic and dicationic ILs. The time required for these ILs to reach the equilibrium is greater than that for the ILs incorporating the amine functionality (180 min) [61] and also greater than that reported for non functionalized imidazolium-based ILs (90 to 180 min) [165, 168] but lower that that reported for [emim][NTf2] (1800-2400 hr) and for [bmim][NTf2] (2160 – 2880 min) [61].
The effect of the phosphonium cation type in the CO2 solubility in term of mol fraction for the ILs [P8,8,8,14]DOSS, [P6,6,6,14]DOSS, [P8,8,8C6P8,8,8]DOSS2 and [P8,8,8C10P8,8,8]DOSS2 is reported in Table 5-4 and presented in Fig 5-19.
Table 5-4 Experimental solubility data for CO2 in phosphonium-based ILs at 298 K Pressure
(bar)
CO2 (mol fraction) [P8,8,8,14]
DOSS
[P6,6,6,14] DOSS
[P8,8,8 C10 P8,8,8] DOSS2
[P8,8,8 C6P8,8,8] DOSS2
1±0.01 0.057215 0.048914 0.043483 0.045772
5±0.01 0.263374 0.225211 0.182001 0.210662
10±0.03 0.438105 0.384121 0.320463 0.340560
15±0.04 0.573422 0.510946 0.420511 0.458078
20±0.05 0.685009 0.599670 0.510930 0.556110
g CO2/g IL
1±0.01 0.00264 0.00244 0.00113 0.00123
5±0.01 0.01554 0.01377 0.00553 0.00685
10±0.03 0.03388 0.02955 0.01172 0.01326
15±0.04 0.05841 0.04951 0.01803 0.02170
20±0.05 0.09450 0.07098 0.02596 0.03216
In general, the monocationic-based phosphonium ILs shows higher CO2 solubility compared to the dicationic-based phosphonium ILs. For the monocationic-based phosphonium ILs, the CO2 solubility was higher in [P8,8,8,14]DOSS compared [P6,6,6,14]DOSS, which is due to the increases of the alkyl chain length. As expected the larger free volume originating from the longer alkyl chain of the cation makes the
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CO2 more soluble in IL with long alkyl chain. This trend is similar to that for the imidazolium-based ILs. However the dicationic-based phosphonium ILs has a grater free volume compare to the monocationic-based phosphonium ILs studied, the CO2 solubility in these ILs was lower. This result is a consequence of the extremely high viscosity of the dicationic-based phosphonium ILs compare to the monocationic- based phosphonium ILs (the viscosities of [P8,8,8,14]DOSS and [P6,6,6,14]DOSS is 1795 and 2051 mPa.s while for [P8,8,8C6P8,8,8]DOSS2 and [P8,8,8C10P8,8,8]DOSS2) is 16188 and 18767 mPa.s respectively. Moreover, the CO2 solubility in these ILs decreases with the increase of the spacer alkyl chain length.
The strength of the interactions cannot be solely responsible for the solubility of CO2 in ionic liquids as reported in some studies [157, 218, 220] and also the free volume. In addition, the ambient pressure molar volumes increase with increasing chain length, this effect diminishes with higher amounts of dissolved CO2 and the molar volumes of each IL becomes very similar [147]. At these pressures, the IL consists of greater than 70% mole of CO2 and thus the properties are better correlated with CO2 than the ambient pressure IL properties. The solubilities of CO2 in the present ILs increased rapidly to about 0.3 to 0.4 in mole fraction for pressures up to 10 bar, but towards higher pressures, the increasing rates slowed down and the solubility finally leveled off. The fast solubility increase in the low-pressure range may be due to the Henry‘s sorption in the inter-ion space. The cations of ILs are normally bulkier than the corresponding anions and provide inter-ion spaces where CO2 can squeeze in. As the openings between cation and anion of the IL are large, CO2 molecules can easily enter into that space. However, these openings are continuously filled with CO2 molecules with increasing pressure. Eventually, the spaces are filled up, leaving no more room for CO2 molecules to get in. To let more CO2 molecules enter into the IL, it is necessary for the inter-ion space to be expanded.
As this expansion requires energy, only small amounts of CO2 can penetrate into the IL. This is why ILs show that particular leveling off solubility behavior
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Fig 5-19 Solubility of CO2 in phosphonium-based ILs at 298.15 K as a function of pressure
The influence of the IL cation on the CO2 solubility is studied by comparing the solubility into ILs formed with DOSS anion and different cation structures. The CO2 solubility results showed that the [C2CNDim]DOSS, [C2CNHeim]DOSS and [P8,8,8,14]DOSS ILs have a higher solubility capacity among the studied imidazolium- based nitrile functionalized ILs, imidazolium-based dual functionalized ILs and phosphonium-based ILs. The effect of the cation type in the CO2 solubility in terms of mol fraction for these three ILs is presented in Fig 5-20. The [C2CNDim]DOSS IL shows the highest CO2 solubility followed by [C2CNHeim]DOSS while the [P8,8,8,14]DOSS IL shows the lowest value. The results show that the CO2 solubility capacity is greater for the imidazolium-based ILs than phosphonium-based ILs.
Moreover, the CO2 solubility was higher when the nitrile-imidazolium ILs incorporated with the decyl alkyl chain compared to the hydroxyl functional group.
The greater CO2 solubility in the [C2CNDim]DOSS IL may be due to the weaker inter-ion interaction forces between the cation and anion (large cation size) which contribute to providing larger free volume. It plays a significant role in attaining high values of CO2 solubility. In addition, the weak interaction forces between the cation and anion increases the van der Waals-type interactions between the gas and the liquid. The high CO2 solubility in the [C2CNHeim]DOSS compared to [P8,8,8,14]DOSS
0 5 10 15 20
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
CO2
P(bar)
[P8,8,8,14]DOSS [P6,6,6,14]DOSS [P8,8,8C
10P
8,8,8]DOSS
2
[P8,8,8C6P8,8,8]DOSS2
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IL due to the presence of the hydroxyl group which increases the ability to attract the electron-poor carbon atom of CO2 towards electronegative atoms (oxygen) [12].
Fig 5-20 Solubility of CO2 in [C2CNDim]DOSS, [C2CNHeim]DOSS and [P8,8,8,14]DOSS ILs at 298.15 K as a function of pressure
The most striking result from this analysis is that, in spite of the major differences in the chemical nature of the solvents investigated and of the interactions of their molecules in pure state, the non ideality of the absorption of CO2 in solution in the composition range studied is remarkably lower than could be anticipated and, in most cases, is essentially driven by entropic effects. Moreover, as has been noticed before by other authors, this analysis on the non ideality of these systems also stresses that there is no direct relationship between the stability of the EDA complex formed between the CO2 and a given solvent and the non ideality of CO2 on this solvent.
These two observations combined suggest that, by increasing the size difference between the CO2 and the solvent, incorporating of functional groups the solubility must increase as the entropic contribution to the solution non ideality increases [218].
Seki et al. [220] in a recent work show that, although the interactions of CO2 with BF4 and PF6 anion-based ILs are stronger than those with the NTf2, the solubility of CO2 on these ILs is larger than in the former, and thus the interactions alone are not enough to provide a full explanation for the CO2 sorption. They recognize that ―the
0 5 10 15 20
0.0 0.2 0.4 0.6 0.8
CO2
P(bar)
[C2CN Dim]DOSS [C2CN Heim]DOSS [P8,8,8,14]DOSS
157
strong Lewis acid−base interactions between the ILs and the dissolved CO2 is not the only effect on the solubility of CO2‖.