Advanced Materials Research
ISSN: 1662-8985, Vols. 785-786, pp 827-831
doi:10.4028/www.scientific.net/AMR.785-786.827
© 2013 Trans Tech Publications, Switzerland
Online: 2013-09-04
CO2 Solubility and Physical Properties
of two containing fluorine functional ionic liquids
Lihua Fana, Yinghua Liangb and Yuhong Lic
College of Chemical Engineering, Hebei United University, Tangshan 063009, PR China
a
b
c
[email protected], [email protected], [email protected]
Keywords: Functional Ionic Liquid; CO2 Solubility; Density; Viscosity; Electrical Conductivity
Abstract. Two containing fluorine functional ionic liquids (ILs) (1-aminoethyl-3-methyl
imidazolium trifluoroacetic acid salt ([aemim][CF3COO]) and 1-aminoethyl-3-methyl imidazolium
trifluoromethane sulfonic acid salt ([aemim][TfO])) were synthesized and the samples were
characterized by means of FTIR, 1HNMR respectively. The densities, viscosities, and electrical
conductivities of the two samples were measured over the temperature range T = (303.15-333.15) K
at atmospheric pressure. The solubility of CO2 in the two samples were investigated at temperature
from 303.15 to 333.15 K and pressure from 0 to 7.14 MPa. Results showed that the maximum
solubility of CO2 in [aemim][TfO] was 0.759 (mole fraction) at 7.01 MPa and 323.15K, and the
which of [aemim][CF3COO] was 0.646 at 6.49 MPa and 313.15 K. The influence of the temperature
and CO2 partial pressure on the process of CO2 absorption was discussed. The CO2 solubilities in
different ILs were also compared. The Henry’s constants and infinite dilution partial molar volumes
for the ILs+ CO2 systems were obtained.
Introduction
In recent decades, the greenhouse effect and global warming have become a more serious problem.
CO2 is one of the main contributor of the greenhouse effect, which has a great impact on both human
living environment and ecological balance[1]. An increasing attention has been paid to ILs applied in
the CO2 capture, owing to their high thermal stability, negligible volatility and tunable properties. By
introducing functional groups which could enhance the CO2 absorption into the cation or anion, the
versatility consequently opens a large group of potential alternative absorbents[2]. Therefore, many
functional ILs which contain fluorine were synthesized to capture CO2, and the fluorine groups can
optimize the molecular structure of ILs to improve the CO2 solubility. Almantariotis[3] studied the
impact of fluorination of the cation on CO2 solubility through molecular dynamics simulation. They
found that the CO2 solubility was 20% higher in [C8H4F13mim][Tf2N] than in [C8H17mim][Tf2N], and
the anions ([Tf2N]-) has great affinity for CO2. The fluorine atoms can form complex hydrogen
network structure and strengthen the coulombic interactions, which can absorb CO2 molecules
effectively. In this paper, two containing fluorine functional ILs were synthesized through microwave,
ion exchange and neutralization method. The two samples were characterized and the physical
properties were measured. The CO2 solubilities in the two samples were determined at different
conditions, and the CO2 solubility data were correlated by K-K equation. The p-T-x relations for the
ILs+CO2 systems are compared with those of other containing fluorine ionic liquids.
Experimental
Two containing fluorine functional ILs (1-aminoethyl-3-methyl imidazolium trifluoroacetic acid salt
([aemim][CF3COO]) and 1-aminoethyl-3-methyl imidazolium trifluoromethane sulfonic acid salt
([aemim][TfO])) were synthesized via microwave, ion exchange and neutralization method. The
procedure is as follows: 8.211 g of 1-methylimidazole and 20.489 g of 2-bromo- ethylamine
hydrobromide were first dissolved in 20 ml ethanol. The solution was put into a microwave- oven
(300W, 45℃ , 15 s × 5), while the intermittent heating was adopted to prevent overheating. Upon
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completion, the liquid was mixed with KOH solution to protect the amine on the cation. After purified
by distillation and recrystalization, the product ([aemim][Br]) was obtained. Then [aemim][Br] was
transformed into [aemim][OH] by passing it through anion-exchange resin column. Slightly excess
trifluoroacetic acid (CF3COOH) or trifluoromethane sulfonic acid solution was added into the
[aemim] [OH] solution. After the neutralization reaction and purification, the two containing fluorine
functional ILs were obtained.
The two samples were characterized by means of FTIR, 1HNMR respectively, and the physical
properties were also determined. Excess water in the ILs were removed by vacuum drying at 353 K
for approximately 48 h just prior to measurements. The water content of [aemim][CF3COO] and
[aemim][TfO] was 123 × 10-6 and
145 × 10-6 mass fractions respectively, as determined by
Karl-Fischer titration. The purity of the samples was measured by the perchloric acid standard
titration, and the purity of [aemim][CF3COO] and [aemim][TfO] was 98.6% and 97.8% respectively.
The densities were measured using a pycnometer. The viscosities were determined with a NDJ-7
rotating cylinder viscometer. The electrical conductivities were determined using a DDS-307A digital
conductivity instrument. The expanded uncertainties for the densities, viscosities and electrical
conductivities are less than ±0.1%, ±1.5% and ±1.5%, respectively .The CO2 solubility measurement
was based on an isochoric saturation technique and was similar to the reported previously [4]. The CO2
solubility data were correlated using Krichevsky–Kasarnovsky equation with a multiparametric
nonlinear regression method[5].
Result and discussion
The FTIR spectra data of two samples is listed in Table 1. Because of the strong moisture absorption
in the air, the samples must be dried under vacuum before characterized. From the spectra data of
[aemim][CF3COO] in Table 1, it can be seen that there are absorption bands at 3456, 1685 and 1295
cm-1, which can be assigned to N-H stretching frequency, carbonyl stretching frequency and C-F
bending frequency respectively. The absorption band appears between 3000 and 3100 cm-1, which
explains that the [CF3COO]- is a kind of strong coordination anion and can form strong hydrogen
bond with other anions. From the spectra data of [aemim][TfO] in Table 1, three absorption bands
centered at 3159, 3113 and 2995 cm-1 can be associated with C-H stretching frequency of unsaturated
and saturated. The absorption band at 1253 cm-1 can be associated with C-F stretching frequency. The
absorption bands obtained at 1375 and 1172 cm-1 represents O=S=O stretching frequency[6].
Table 1 The FTIR data of ILs
，
FTIR υmax /cm-1
3456(m),3147(m),3098(m),2973(m),1685(m),1509(s),1430(m),1295(m),1205(m),
1135(m), 838(s),798(s),745(s),723(s)
[aemim][TfO]
3478(m),3159(m),3113(m),2995(m),1583(s),1459(s),1375(m),1253(m),1172(s),
1030(m),840(s),766(s),640(s),577(s)
m-stretching frequency; s-bending frequency.
ILs
[aemim][CF3COO]
The 1HNMR data of two samples is listed in Table 2. D2O was used as solvent, so an absorption
peak around 4.7 ppm appeared, which is the residual hydrogen value of D2O. The peaks appear from
7.3 to 7.6 ppm, which can be attributed to the chemical shifts of imidazole ring protons. Another
peaks at 3.5 ppm correspond to the N-H of alkyl imidazolium cation. The samples have identical
cations, and there are no hydrogen bond on the anions, so the chemical shifts of which show
approximately the same values. The data in Table 2 illustrates the changes in electronic environment
of various protons of different ILs, which can be beneficial to understand exactly the intermolecular
interactions[7].
Advanced Materials Research Vols. 785-786
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Table 2 The 1HNMR data of ILs
ILs
[aemim][CF3COO]
chemical shift(ppm)
3.335~3.431(m,2H,NCH2-CH2NH2),3.442~3.479(m,2H,NCH2-CH2NH2),
3.492~3.531(t,2H,CH2-NH2),3.815(s,3H,N-CH3),7.327(d,1H,C=CHNCH3),
7.518(d,1H,C=CHNCH2),8.558(s,1H,NCHN)
[aemim][TfO]
3.355~3.442(m,2H,NCH2-CH2NH2),3.457~3.479(m,2H,NCH2-CH2NH2),
3.498~3.591(t,2H,CH2-NH2),3.862(s,3H,N-CH3),7.327(d,1H,C=CHNCH3),
7.542(d,1H,C=CHNCH2),8.572(s,1H,NCHN)
s-single; d-double; t-triplet; m-multiplet.
The physical properties of the two samples were measured at different temperatures, including the
density(ρ), the viscosity(η), the electrical conductivity(σ), and the data were listed in Table 3.
From Table 3, it can be seen that the density decreases slightly with increasing temperature, which
has a close relationship with the structure of ILs. ILs usually consist of large bulky asymmetric cation
and inorganic anion, then the relative quality is big[8]. As temperature goes up, the free volume of IL is
increasing, which reduces the electrostatic attraction between ions, then the density decreases
consequently. With a constant cation, the density usually increases with a rise in the bulkiness of the
anion, so the density value of [aemim][TfO] is larger than that of [aemim][CF3COO].
Table 3 Density(ρ), viscosity(η) and conductivity(σ) of pure ILs at different temperatures
ILs
303.15
308.15
[aemim][CF3COO]
[aemim][TfO]
1.3366
1.5647
1.3313
1.5644
[aemim][CF3COO]
[aemim][TfO]
900
1600
770
1200
[aemim][CF3COO]
[aemim][TfO]
7.77
0.84
7.89
0.98
T(K)
313.15
318.15
ρ (g/cm3)
1.3252
1.3204
1.5639
1.5635
η (mPa·s)
580
420
900
710
σ (mS/cm)
8.25
8.66
1.07
1.16
323.15
328.15
333.15
1.3198
1.5607
1.3143
1.5605
1.3108
1.5593
350
520
270
400
230
290
9.04
1.26
9.44
1.40
9.91
1.55
The data in Table 3 presents clearly that a rise in temperature provides an attractive decline in the
viscosity. With a constant cation, the viscosity usually increases with a rise in the alkyl chain length of
anions[9]. [aemim][TfO] has a long and dispersed structure anion, which increases the space steric
hindrance and enhances the strength of van der Waals interactions, thus the viscosity value is high.
Noticeably, the viscosity of [aemim][TfO] is higher than that of [aemim][CF3COO] at the same
temperature.
Table 3 also shows that the conductivity increases with a rise in temperature. The possible reason
is that the kinetic energy of ions enhances and can overcome the coulombian force while temperature
increasing[10]. The conductivity of [aemim][CF3COO] is higher than that of [aemim][TfO], as the
former has lower viscosity at the same temperature. The viscosity has a great effect on the
conductivity, as the kinetic resistance increases and the migration ability of ions reduces when the
viscosity increases.
The data of CO2 solubility in the two samples were listed in Table 4. The CO2 solubility in terms
of mole fraction (x) is defined as number of moles of CO2 as portion of the total number of moles in
the solution.
Table 4 The data of CO2 solubility in the two samples
T=303.15K
P(MPa)
x
[aemim][CF3COO]
0.53
0.084
1.08
0.198
2.62
0.396
3.78
0.416
T=313.15K
P(MPa)
x
0.43
1.22
2.71
3.23
0.062
0.193
0.324
0.392
T=323.15K
P(MPa)
x
0.50
1.28
3.17
3.80
0.067
0.165
0.383
0.410
T=333.15K
P(MPa)
x
0.47
1.23
2.99
3.25
0.062
0.182
0.364
0.394
830
Current Trends in the Development of Industry
4.26
5.04
5.68
6.64
6.88
7.14
[aemim][TfO]
0.55
1.33
2.47
3.39
4.27
5.16
5.63
6.02
6.43
6.91
0.453
0.478
0.511
0.542
0.578
0.608
3.38
4.30
4.85
5.42
5.97
6.49
0.435
0.471
0.523
0.581
0.629
0.646
4.18
4.69
5.27
5.98
6.20
6.53
0.432
0.453
0.461
0.491
0.574
0.612
3.88
4.43
5.41
5.62
5.94
6.49
0.416
0.441
0.466
0.503
0.527
0.554
0.121
0.294
0.536
0.563
0.621
0.636
0.656
0.689
0.709
0.737
0.53
1.28
2.58
3.21
3.74
4.51
5.03
5.78
6.35
6.64
0.114
0.255
0.478
0.533
0.577
0.643
0.661
0.669
0.696
0.729
0.47
1.32
2.74
3.38
3.69
4.83
5.32
5.96
6.47
7.01
0.099
0.266
0.485
0.532
0.561
0.615
0.621
0.631
0.688
0.759
0.46
1.29
2.44
3.25
3.74
4.47
5.23
5.76
6.15
6.77
0.095
0.247
0.464
0.498
0.550
0.595
0.624
0.633
0.647
0.689
From Table 4, it can be noticed that the CO2 solubility in the samples increases with increasing
pressure and decreases with increasing temperature. The process involves chemical and physical
interactions. With a constant cation, anion plays an important role in the CO2 solubility[11]. Among the
two samples tested, [aemim][TfO] exhibits a higher absorption capacity. This can be attribute to the
amine groups in the cation, which can enhance the kinetic of chemical absorption. Additionally, the
fluorination of anion leads to form complex hydrogen network structure and strengthen the coulombic
interactions, which can be benefical to the physical absorption. In the measurement range, the
maximum CO2 solubility in [aemim][TfO] was 0.759 at 7.01 MPa and 323.15K, the which of
[aemim][CF3COO] was 0.646 at 6.49 MPa and 313.15 K.
Fig.1 shows the comparison of the CO2 solubility results obtained in our study and others from
literature[2, 12] at the same temperature. It can be seen that the CO2 solubility in functional ILs is
higher than in conventional ILs. The trend observed for the CO2 solubility in five different ILs is as
follows: [aemim][Lys]> [aemim] [TfO] >[C4py] [Tf2N]>[aemim] [CF3COO]>[bmim] [PF6].
Noticeably, [aemim][Lys] displays the highest CO2 absorption capacity. This may be due to
favourable interactions between CO2 and the amine substituents on the lysine anion. The second is
[aemim][TfO]. With a constant cation, [aemim][TfO] has a bulky asymmetric and three fluoroalkyl
anion, which can be benefit to the CO2 absorption process. [C4py][Tf2N] also includs many
fluoroalkyl substituents, and the anion ([Tf2N]-) has great affinity for CO2. Because of the limit of the
pyridine cation, the CO2 solubility in [C4py][Tf2N] is lower than in the two formers, but higher than in
[aemim][CF3COO]. [bmim][PF6] is a conventional ionic liquid, which depends only on physical
interactions in absorption process, so the solubility of CO2 in it is the lowest.
0.9
323.15K
0.8
Solubility(mol/mol)
0.7
0.6
0.5
0.4
0.3
[aemim][Lys]
[aemim][CF3COO]
0.2
[aemim][TfO]
[bmim][PF6]
0.1
[C4py][Tf2N]
0.0
0
1
2
3
4
5
6
7
8
Pressure(MPa)
Fig.1 Comparison of the CO2 solubility in different ionic liquids at the same temperature
Advanced Materials Research Vols. 785-786
831
Table 5 shows the associated value of the two samples using the K–K equation. The Henry’s
constants increase as the temperature goes up, which indicates the CO2 solubility in the samples
decreases with increasing temperature. At the same temperature, the Henry’s constant of
[aemim][TfO] is lower than that of [aemim][CF3COO], which is in accordance with the CO2
solubility in them. Worthwhile, the value of infinite dilution partical molar volumes is negative.
Table 5 Henry’s constants and partial molar volumes of CO2 in ILs under different temperature
[aemim][CF3COO]
T(K)
H(MPa)
303.15
313.15
323.15
333.15
6.396
8.188
8.466
8.735
[aemim][TfO]
∞
VCO
2
(m3/mol)
-0.4324
-0.6381
-0.5905
-0.6281
H(MPa)
4.969
5.366
5.414
5.609
∞
VCO
2
(m3/mol)
-0.4814
-0.0512
-0.0508
-0.0531
Conclusion
Two containing fluorine functional ionic liquids, [aemim][CF3COO] and [aemim][TfO], were
synthesized, and their densities, viscosities, and electrical conductivities were measured at
(303.15-333.15) K and atmospheric pressure. The structure of IL has an important influence on its
physical properties, especially that of the anion. The solubility of CO2 in the two samples were
investigated at temperature from 303.15 to 333.15 K and pressure from 0 to 7.14 MPa. The
solubilities were correlated using K–K equation, from which Henry’s constants and infinite dilution
partial volumes of CO2 in these ILs were obtained. The solubilities of CO2 in these ILs increase with
pressure increasing and decrease with temperature increasing. The solubilities of CO2 in
[aemim][TfO] are higher than those of [aemim][CF3COO]. The solubilities of CO2 are compared
with those of three other containing fluorine ionic liquids, and the solubility of CO2 in these ILs are in
sequence: [aemim][Lys]>[aemim][TfO]>[C4py][Tf2N]>[aemim][CF3COO]>[bmim][PF6]. The
anion structure of ILs plays an important role in the CO2 absorption process.
Acknowledgements
This work was financially supported by the Hebei Natural Science Foundation (No. B2008000373),
and the Research Project of science and technology department of Hebei Province (No. 10215115D).
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Current Trends in the Development of Industry
10.4028/www.scientific.net/AMR.785-786
CO2 Solubility and Physical Properties of Two Containing Fluorine Functional Ionic Liquids
10.4028/www.scientific.net/AMR.785-786.827
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