Exploring the effect of fluorinated anions on the CO2/N2 separation of supported ionic liquid membranes

Article abstract of DOI:10.1039/c7cp06297d

The CO₂ and N₂ permeation properties of ionic liquids (ILs) based on the 1-ethyl-3-methylimidazolium cation ([C₂mim]⁺) and different fluorinated anions, namely 2,2,2-trifluoromethylsulfonyl-N-cyanoamide ([TFSAM]^-), bis(fluorosulfonyl) imide ([FSI]^-), nonafluorobutanesulfonate ([C₄F₉SO₃]^-), tris(pentafluoroethyl)trifluorophosphate ([FAP]^-), and bis(pentafluoroethylsulfonyl)imide ([BETI]^-) anions, were measured using supported ionic liquid membranes (SILMs). The results show that pure ILs containing [TFSAM]^- and [FSI]^- anions present the highest CO₂ permeabilities, 753 and 843 Barrer, as well as the greatest CO₂/N₂ permselectivities of 43.9 and 46.1, respectively, with CO₂/N₂ separation performances on top of or above the Robeson 2008 upper bound. The re-design of the [TFSAM]^- anion by structural unfolding was investigated through the use of IL mixtures. The gas transport and CO₂/N₂ separation properties through a pure [C₂mim][TFSAM] SILM are compared to those of two different binary IL mixtures containing fluorinated and cyano-functionalized groups in the anions. Although the use of IL mixtures is a promising strategy to tailor gas permeation through SILMs, the pure [C₂mim][TFSAM] SILM displays higher CO₂ permeability, diffusivity and solubility than the selected IL mixtures. Nevertheless, both the prepared mixtures present CO₂ separation performances that are on top of or above the Robeson plot.

Full text of DOI:10.1039/c7cp06297d

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DOI: 10.1039/C7CP06297D.
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ARTICLE
Exploring the effect of fluorinated anions on the CO2/N2
separation of supported ionic liquid membranes
Andreia S.L. Gouveia,ab Liliana C. Tomé,ab Elena I. Lozinskaya,c Alexander S. Shaplov,cd Yakov S.
Vygodskiic and Isabel M. Marrucho*ab
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The CO2 and N2 permeation properties of ionic liquids (ILs) based on the 1-ethyl-3-methylimidazolium cation
([C2mim]+) and different fluorinated anions, namely 2,2,2-trifluoromethylsulfonyl-N-cyanoamide ([TFSAM]–),
bis(fluorosulfonyl) imide ([FSI]–), nonafluorobutanesulfonate ([C4F9SO3]–), tris(pentafluoroethyl)trifluorophosphate ([FAP]–),
and bis(pentafluoroethylsulfonyl)imide ([BETI]–) anions, were measured using supported ionic liquid membranes (SILMs).
The results show that pure ILs containing [TFSAM]– and [FSI]– anions present the highest CO2 permeabilities,753 and 843
Barrer, as well as the greatest CO2/N2 permselectivities of 43.9 and 46.1, respectively, ensuing CO2/N2 separation
performances on top or above the Robeson 2008 upper bound. The re-design of the [TFSAM]– anion by structural
unfolding was investigated through the use of IL mixtures. The gas transport and CO2/N2 separation properties through the
pure [C2mim][TFSAM] SILM are compared to those of two different binary IL mixtures containing fluorinated and cyano-
functionalized groups in the anions. Although the use of IL mixtures is a promising strategy to tailor gas permeation
through SILMs, the pure [C2mim][TFSAM] SILM displays higher CO2 permeability, diffusivity and solubility than those of the
selected IL mixtures. Nevertheless, both the prepared mixtures present CO2 separation performances that are on top or
above the Robeson plot.
www.rsc.org/
triazolium,12
thiazolium,13
pyridinium,14
cholinium,15
Introduction
ammonium,16 phosphonium,17 combined with halogens,
sulfonates, carboxylates, fluorinated or cyano-functionalized
anions, have been studied. Other works, mostly focusing on
imidazolium-based ILs, have also explored the effect of alkyl,18
fluoroalkyl,19 etoxyalkyl,20 and aminoalkyl21-functionalized
cations. Since IL anions have stronger influence on the CO2
separation performance of SILMs than IL cations,1 they
deserved from the start a closer look. The first studies on
SILMs made use of fluorinated anions such as the
bis(trifluoromethylsulfonyl)imide [NTf2]–, tetrafluoroborate
[BF4]–, and hexafluorophosphate [PF6]– and enabled to draw
conclusions about these anions CO2-phylic behaviour and high
CO2 permeabilities.22 More recently, low viscous ILs with
cyano-functionalized anions, as the tricyanomethanide
[C(CN)3]– and tetracyanoborate [B(CN)4]–,23-25 have been
recognized as better candidates for the development of
improved SILMs, because of their superior CO2 permeabilities
and permselectivities when compared to the most used [NTf2]–
anion. Task-specific ILs bearing amine groups, such as those
containing amino acid anions,26-28 have also been proposed to
prepare SILMs, since amine groups can chemically bond CO2
and act as carriers for CO2 facilitated transport through SILMs
at low pressures. However, the high viscosity of these task-
specific ILs is undoubtedly a key limitation, as CO2 diffusion is
strongly compromised.
The development of supported ionic liquid membranes
(SILMs) for CO2 separation has been widely investigated in
recent years mainly due to their easy preparation and
versatility.1-3 In contrast to traditional liquid membranes,
which are produced by impregnating a porous membrane
support with common organic solvents,4 SILMs use ionic
liquids (ILs) and thus benefit from negligible displacement of
the liquid phase from the membrane pores through
evaporation,5, 6 due to the low volatility of ILs.7 It should also
be emphasized that within the CO2 separation context, the
most important features of ILs are their high CO2 affinity over
light gases,8-10 as well as their inherent designer nature that
enables the tailoring of IL properties by proper selection of
cation and/or anion or via the addition of specific functional
groups.
Numerous works have investigated the effect of IL
chemical structure on the gas permeation properties of SILMs.
A
broad diversity of cations, such as imidazolium,11
a.Centro de Química Estrutural, Departamento de Engenharia Química, Instituto
Superior Técnico, Universidade de Lisboa, Avenida Rovisco Pais, 1049-001 Lisboa,
Portugal.
b.Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de
Lisboa, Av. da República, 2780-157 Oeiras, Portugal.
c. A.N. Nesmeyanov Institute of Organoelement Compounds Russian academy of
sciences (INEOS RAS), Vavilov St., 28, 119991 Moscow, Russia.
d.Luxembourg Institute of Science and Technology (LIST), 5 avenue des Hauts-
Fourneaux, L-4362, Esch-sur-Alzette, Luxembourg.
In an effort to improve the CO2 permeability and
permselectivity properties of SILMs, our recent studies
explored the use of IL mixtures by fixing the [C2mim]+ cation
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and researching on different anion chemical structures. Additionally, this work investigates the impact on gas transport
Initially, SILMs based on IL mixtures combining anions with through SILMs of using a pure IL versus a structurally similar IL
different CO2 solubility behaviours were investigated: mixture as liquid phase. Inspired by the fact that the [TFSAM]–
thiocyanate
([SCN]–),
dicyanamide
([N(CN)2]–)
and anion has an unusual asymmetric chemical structure, which
bis(trifluoromethylsulfonyl)imide ([NTf2]–) that present combines both fluorinated and cyano functionalities, the re-
physical solubility; acetate ([Ac]–) and lactate ([Lac]–), which design of the chemical structure of the pure [C2mim][TFSAM]
additionally have chemical solubility.29 Afterwards, we focused IL through the use of IL mixtures is explored. For that purpose,
on IL mixtures based on sulfate ([CH3SO4]–) and cyano- different pairs of ILs, based on the [C2mim]+ cation and anions
functionalized anions ([SCN]–, [N(CN)2]–, [C(CN)3]– and [B(CN)4]– containing fluorinated or cyano functionalities, were selected
).30 Moreover, we studied IL mixtures containing [C(CN)3]– and and their gas permeation properties compared to those of the
different amino acid anions, so that one IL component pure [C2mim][TFSAM] SILM. One of the IL mixtures contains
maintains the low viscosity, while the other provides the [NTf2]– and [N(CN)2]– anions, which gas permeation properties
desired chemical characteristics for the active transport of were previously determined,29 whereas the other IL mixture is
CO2.31 The overall results of these studies showed that mixing based on [OTf]– and [SCN]– anions and its gas transport
anions with specific chemical features allows variations in IL properties are here reported for the first time.
viscosity and molar volume that significantly impact the gas
transport through SILMs, and thus tailored permeabilities and
permselectivities can be achieved.29-31
Results and Discussion
In the present work, the gas permeation properties and
Gas permeation through SILMs having fluorinated anions
CO2/N2 separation performance of SILMs prepared with pure
ILs bearing the [C2mim]+ cation and different less conventional
The structures of the pure ILs bearing fluorinated anions
fluorinated anions, namely [TFSAM]–, [FSI]–, [C4F9SO3]–, [FAP]–
are depicted in Fig. 1.
and [BETI]–, were evaluated and the effect of the fluorinated
moieties in the IL anion was discussed. Despite the fact that
several SILMs with common fluorinated anions have already
been reported,1-3 the gas permeation properties of SILMs
containing fluorinated anions, such as those selected herein,
have still not been properly studied and discussed. Only three
studies have reported SILMs that made use of [C4F9SO3]–,
[FAP]– and [BETI]– anions. Pereiro et al.32 conducted single gas
permeation experiments through the [C2C1py][C4F9SO3] SILM,
at 294 K and 75 KPa, using CO2, N2, O2, hydrocarbon gases
(CH4, C2H6, C3H8, C3H6) and perfluorocarbon gases (CF4, C2F6,
C3F8). Scovazzo et al.22 determined ideal/mixed CO2/CH4 and
CO2/N2 permselectivities in [C4mim][BETI] SILM at 303 K and
200 kPa, while Althuluth et al.33 reported ideal/mixed CO2/CH4
permselectivities in [C2mim][FAP] SILM at 313 K and 700 kPa.
Nevertheless, the obtained results cannot be directly
compared due to the different measurement conditions, as
Fig. 1 Chemical structures of the pure ionic liquids (ILs) used in this work to
well as the use of diverse IL cation structures.
prepare SILMs.
Table 1 Water contents and thermophysical properties of the pure ILs.
IL sample
wt% H2O
0.02
M (g mol-1)
284.26
η (mPa s)a
23.7
ρ (g cm-3)b
1.3518
Vm (cm3 mol-1)
210.3
[C2mim][TFSAM]
[C2mim][FSI]
[C2mim][NTf2]c
0.09
291.30
22.4
1.4480
201.2
0.02
391.31
39.1
1.5240
256.8
[C2mim][FAP]d
[C2mim][BETI]
[C2mim][C4F9SO3]
-
556.17
491.33
410.26
76.4
85.5
1.7081
1.5989
1.5420
325.6
307.3
266.1
0.02
0.08
109.7
a Viscosity (η) and density (ρ) measured at 293 K, excepting for [C2mim][C4F9SO3] IL (303 K).
b Molar volume (Vm) obtained for 293 K, excepting for [C2mim][C4F9SO3] IL (303 K).
c The values of [C2mim][NTf2] were taken from Tomé et al.29
d The density and viscosity values of [C2mim][FAP] were taken from Neves et al.52
2 | J. Name., 2012, 00, 1-3
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The water content (wt %), molar mass (M), viscosity (η), [FSI]– < [TFSAM]– < [NTf2]– < [FAP]– < [BETI]– < [C4F9SO3]–, it can be
density (ρ) and molar volume (Vm) values of the pure ILs concluded that these experimental data are in agreement with the
used as liquid phases in the studied SILMs are summarized in general trend usually observed in literature, where ILs with high
14, 15, 23, 40
Table 1. The thermophysical properties of the conventional viscosities yield SILMs with low gas permeabilities.3,
[C2mim][NTf2] IL are also included for comparison.29 From However, [C2mim][FAP] is the only exception since it presents a
Table 1, it can be observed that the IL containing the [C4F9SO3]– different behaviour (Table 2): despite its high viscosity (76.4 mPa s),
anion shows the highest viscosity, while [C2mim][FSI] presents it also exhibits high CO2 permeability (624 Barrer), higher than
the lowest viscosity. The IL viscosity values can be organized those of the [C2mim][NTf2] IL (39.1 mPa s and 589 Barrer). Notice
following the IL anion order: [C4F9SO3]– > [BETI]– > [FAP]– > that [FAP]– anion has the most different chemical structure among
[NTf2]– > [TFSAM]– > [FSI]–. A slightly different trend was all the IL anions studied in this work, consisting of a phosphorus
observed for molar volumes, with the IL comprising the [FAP]– atom surrounded by fluorine atoms, without sulfonyl functional
anion showing the highest molar volume, while [C2mim][FSI] groups (Fig. 1). Moreover, taking
exhibits the lowest molar volume. These data will be used permeabilities obtained through SILMs immobilized with the
ahead in the understanding of the gas permeation results. remaining ILs, it can be seen that higher CO2 permeabilities are
a closer look at the gas
The experimental gas permeability (P) values obtained through achieved for ILs with anions bearing a smaller number of fluorine
the prepared SILMs having ILs with fluorinated anions, measured at elements, such as [TFSAM]– and [FSI] – anions (Table 2).
293 K with a trans-membrane pressure differential of 100 kPa, are
shown in Table 2. To the best of our knowledge, the CO2/N2 accounts for gas permeability (Eq. 1). Typically, the higher the gas
separation properties of [C2mim][TFSAM], [C2mim][FSI], diffusivity, the faster is the gas flux through the SILM. The
Gas diffusivity (D) is a mass transfer property that directly
[C2mim][FAP], [C2mim][BETI] and [C2mim][C4F9SO3] SILMs are here experimental CO2 and N2 diffusivities values obtained through the
reported for the first time, while those of the [C2mim][NTf2] SILM prepared SILMs are presented in Table 3. The CO2 diffusivities of the
were previously determined using the same experimental SILMs with fluorinated anions can be ordered as follows: [FSI]–
>
conditions.29 It is important to mention that in order to attain stable [TFSAM]– > [FAP]– > [NTf2]– > [BETI]– > [C4F9SO3]–, which fully
SILMs, both hydrophilic and hydrophobic supports were used corresponds to IL anion order observed for CO2 permeabilities
according to the ILs hydrophobicity, and the results are compared in (Table 2). As for N2 diffusivities the subsequent order is attained:
this section, irrespective of the support used.
([FAP]– > [FSI]– > [TFSAM]– > [BETI]– > [NTf2]– > [C4F9SO3]–), which is
nearly the same IL anion order observed for N2 permeabilities, but
different from that obtained for CO2 diffusivities and permeabilities.
The relationship between IL viscosity and gas diffusion is in fact
the basis of the dependence of permeability from viscosity that we
have showed above. A number of works have reported the
Table 2 Gas permeabilities (P)a through the prepared SILMs of pure ILs.b
SILM sample
P CO2
P N2
α CO2/N2
[C2mim][TFSAM]
[C2mim][FSI]
[C2mim][NTf2]c
[C2mim][FAP]
[C2mim][BETI]
[C2mim][C4F9SO3]
753 ± 0.2
843 ± 0.5
589 ± 1.0
624 ± 0.4
437 ± 1.9
32 ± 0.2
17 ± 0.1
18 ± 0.2
17 ± 0.1
24 ± 0.1
18 ± 0.1
6 ± 0.1
43.9 ± 0.1
46.1 ± 0.5
35.5 ± 0.3
26.0 ± 0.1
24.8 ± 0.2
5.5 ± 0.1
inversely proportional relationship between gas diffusivity and IL
16, 17, 41
viscosity.3,
Following this line of thought, the relationship
between experimental CO2 diffusivities and IL viscosity for the
studied SILMs is depicted in Fig. 2.
a Barrer (1 Barrer = 10–10 cm3(STP)cm cm–2 s–1 cmHg–1
)
b
The listed uncertainties represent the standard deviations, based on
three experiments.
c Values taken from Tomé et al.29
From Table 2, the same trend in gas permeability is valid for all
the studied SILMs: P CO2 >> P N2, as expected. Regarding the
influence of the fluorinated-based anions, SILMs having the [FSI]–,
[TFSAM]– and [FAP]– anions present higher CO2 permeabilities of
843, 753 and 624 Barrer, respectively, than the SILM containing the
[NTf2]– anion, which is well-known for its high CO2 permeability (589
Barrer).29 It should be noted that in spite of the similar structures of
[NTf2]– and [FSI]– anions, in which the difference consists in two
extra –CF3 groups in the [NTf2]– anion structure (Fig. 1), the CO2
permeability through [C2mim][FSI] SILM is ~1.5 times higher than of
[C2mim][NTf2] SILM. Generally, CO2 permeabilities through the
studied SILMs are found to decrease in the following IL anion order:
[FSI]– > [TFSAM]– > [FAP]– > [NTf2]– > [BETI]– > [C4F9SO3]– (Table 2).
Considering the IL anion viscosity trend, obtained at 293 K (Table 1),
Fig. 2 Relationship between the CO2 diffusivities determined through the
SILMs prepared with pure ILs and respective IL viscosity measured at 293 K.
Error bars represent standard deviations based on three experimental
replicas.
In agreement to what was previously observed in literature for
21-23
other SILMs,4,
the CO2 diffusivity through the SILMs having
fluorinated anions decreases as the IL viscosity increases. The SILMs
with the lowest CO2 diffusivities are [C2mim][C4F9SO3] (55·10−12 m2
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s−1) and [C2mim][BETI] (167·10−12 m2 s−1), which also have the proposed models showed that CO2 solubility increases with
lowest CO2 permeabilities (437 and 32 Barrer), depict the highest increasing IL molecular weight, molar volume and free volume.10
viscosities (109.7 and 85.5 mPa). Similarly to what is Taking into consideration the gas solubility results obtained in this
aforementioned for CO2 permeabilities, again a deviant behaviour work (Table 4), as well as the range of molar volume (from 201.2 up
can be observed for [C2mim][FAP] SILM, since its CO2 diffusivity is in to 325.6 cm3 mol−1) and molecular weight (from 284.3 up to 556.2 g
between those of [C2mim][NTf2] and [C2mim][TFSAM] (Table 3), but mol−1) of the ILs used, divergences from the abovementioned
its viscosity values (76.4 mPa s) are higher than those of trends can be found for the studied SILMs having fluorinated
[C2mim][NTf2] (39.1 mPa s) and [C2mim][TFSAM] (23.7 mPa s) (Fig. anions. For example, the [C2mim][NTf2] SILM shows the highest CO2
−3
2).
Table 3 Gas diffusivity (D) values through the prepared SILMs of pure ILs.
Gas diffusivity (x1012) (m2 s-1)
solubility (26·10−6 m3
m
Pa−1), but it does not have the highest
(STP)
IL molar volume and molecular weight (Table 1). Likewise, the
lowest CO2 solubility (4·10−6 m3 m−3 Pa−1) belongs to the
(STP)
[C2mim][C4F9SO3] SILM, although it does not present the lowest IL
molar volume and molecular weight (Table 1). The effect of
SILM sample
fluorination, either in the IL cation or anion, on CO2 solubility has
D CO2
D N2
45-47
been studied by different researchers.19,
Tagiuri et al.48
explored the effect of cation on the CO2 solubility of three different
ILs combining the [FSI]– anion. Moreover, Kroon et al.49 determined
the CO2 solubility in the [C2mim][FAP] IL by measuring the bubble
point pressures of the binary mixture of [C2mim][FAP] + CO2. The
results showed that the CO2 solubility in [C2mim][FAP] is higher
when compared to that of ILs having the same cation combined
with other fluorinated anions such as [NTf2]–, [BF4]– and [PF6]–, due
to the fact that [FAP]– has large size and it is highly fluorinated.
Albeit it has been recognized that introducing fluorination in the
cation and/or anion can effectively improve CO2 solubility,46 it was
recently reported after critical analysis that no special effect of the
[C2mim][TFSAM]
[C2mim][FSI]
258 ± 1.1
315 ± 4.4
172 ± 1.7
291 ± 4.5
353 ± 7.5
203 ± 2.5
[C2mim][NTf2]*
[C2mim][FAP]
214 ± 1.9
167 ± 2.1
55 ± 1.2
326 ± 0.2
216 ± 2.4
88 ± 2.0
[C2mim][BETI]
[C2mim][C4F9SO3]
*The gas diffusivity values through the [C2mim][NTf2] SILM were taken
from Tomé et al.29
The gas solubility (S) values calculated using Eq. 1 are listed in fluorination upon the CO2 solubility has been observed for both
Table 4. It can be seen that [C2mim][C4F9SO3] SILM presents the perfluorocarbon and heavily fluorinated ILs.50 In fact, the
lowest CO2 solubility (4·10−6 m3
m−3 Pa−1), while the previously introduction of fluorination in the anions of the ILs studied in this
(STP)
reported [C2mim][NTf2] SILM has the highest CO2 solubility (26·10−6 work does not significantly affect the obtained gas solubility values
m3 m−3 Pa−1). The CO2 solubility values of the remaining studied (Table 4), except for the case of [C2mim][C4F9SO3] IL that displays a
(STP)
SILMs having different fluorinated anions are very similar (ranging very low CO2 solubility.
from 20·10−6 to 22·10−6 m3
m−3 Pa−1) and can be ordered as
(STP)
follows: [TFSAM]– ≈ [FAP]– > [FSI]– ≈ [BETI]–. Regarding the N2
solubility, its values are always significantly lower (between Re-designing the [TFSAM]– anion by structural unfolding: effect on
0.39·10−6 and 0.61·10−6 m3
SILMs studied.
m
Pa−1) than those of CO2 for all the gas permeation
−3
(STP)
Taking into account that the [TFSAM]– anion has an
unconventional and asymmetric chemical structure, combining
both fluorinated and cyano functionalities, which have both been
recognized to be responsible for high CO2 separation performance,
we here explore the effect of structural unfolding of the pure
[C2mim][TFSAM] IL on gas permeation properties of SILMs using IL
mixtures. Thus, two equimolar IL mixtures were used for this
purpose: [C2mim][SCN][OTf] (Fig. 3), which is here studied for the
first time, and [C2mim][N(CN)2][NTf2], which gas permeation and
Table 4 Gas solubility (S) values through the prepared SILMs of pure ILs.
Gas solubility (x106) (m3(STP) m-3 Pa-1)
SILM sample
S CO2
S N2
[C2mim][TFSAM]
[C2mim][FSI]
22 ± 0.09
20 ± 0.29
26 ± 0.28
0.44 ± 0.01
0.39 ± 0.01
0.61 ± 0.01
thermophysical properties were previously determined by us29
.
[C2mim][NTf2]*
Both these mixtures have IL anions that show structural similarities
with the [TFSAM]– anion (Fig. 3). The composition description,
water content (wt%), molar mass (M), viscosity (η), density (ρ) and
molar volume (Vm) values of the pure ILs, [C2mim][TFSAM],
[C2mim][SCN], [C2mim][OTf], [C2mim][N(CN)2], [C2mim][NTf2], and
the selected IL mixtures are listed in Table 5, while their gas
permeability, diffusivity and solubility values are depicted in Figs.
4(a) – (c), respectively.
[C2mim][FAP]
22 ± 0.21
20 ± 0.17
4 ± 0.07
0.55 ± 0.001
0.61 ± 0.01
0.49 ± 0.002
[C2mim][BETI]
[C2mim][C4F9SO3]
*The gas solubility values through the [C2mim][NTf2] SILM were taken
from Tomé et al.29
Over the past few years, a number of correlations have been
Figs. 4(a) and (b) show that the pure [C2mim][TFSAM] SILM
proposed with the intent of understanding the relationships exhibits higher gas permeabilities and diffusivities than SILMs
between CO2 solubility and the intrinsic properties of ILs.42-44 The composed of both [C2mim][SCN][OTf] and [C2mim][N(CN)2][NTf2] IL
4 | J. Name., 2012, 00, 1-3
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mixtures. Moreover, the addition of 0.5 mole fraction of 210.3 cm3 mol−1
[C2mim][SCN] did not significantly affects CO2 permeability but weight 284.3 g mol−1). Conversely, the [C2mim][SCN][OTf] mixture
has lower CO2 solubility (15·10−6 m3 m−3 Pa−1) than that
, respectively) and the same molecular
(STP)
observed for the pure [C2mim][TFSAM] SILM, which is in
agreement with the lower molar volume (169.9 cm3 mol−1) and
molecular weight (214.7 cm3 mol−1) presented by this IL mixture
containing the [SCN]– and [OTf]– anions. Despite the fact that it is
possible to obtain similar viscosities and molar volumes of the pure
[C2mim][TFSAM] IL by mixing the [C2mim][N(CN)2] and
[C2mim][NTf2] ILs, improved gas permeabilities, diffusivities and
solubilities were obtained through the pure [C2mim][TFSAM] SILM.
Fig. 3 Chemical structures of the pure IL and IL mixtures studied in this
work.
decreased to almost half the N2 permeability compared to those of
the pure [C2mim][OTf] SILM. From Fig. 4(b), where the relationship
between CO2 diffusivity and IL viscosity is illustrated, it can also be
seen that both [C2mim][N(CN)2][NTf2] and [C2mim][SCN][OTf] IL
mixtures present slightly higher viscosities (Table 5) and lower CO2
diffusivities than those of the pure [C2mim][TFSAM] SILM. Also, the
presence of 0.5 mole fraction of [C2mim][SCN] leads to a decrease
in IL viscosity and an increase in CO2 diffusivity. The same behaviour
was found for [C2mim][N(CN)2][NTf2] with the addition of 0.5 mole
fraction of [C2mim][N(CN)2]. These results are in accordance with
the general trend observed in literature: the CO2 diffusivity
decreases with the increase in IL viscosity.4, 21-23
In what concerns gas solubility, from Fig. 4(c), it can be observed
that CO2 solubilities of both IL mixtures are in between of the
individual IL components. Moreover, the presence of 0.5 of mole
fraction of [C2mim][OTf] or [C2mim][NTf2] in the corresponding
mixtures, leads to an increase in the CO2 solubilities, probably due
to the fact that they present higher molar volume compared to the
pures [C2mim][SCN] or [C2mim][N(CN)2], respectively. Furthermore,
the [C2mim][N(CN)2][NTf2] mixture displays a CO2 solubility (20·10−6
Fig. 4 (a) Gas permeability, (b) CO2 diffusivity as a function of IL viscosity
and (c) CO2 solubility as a function of IL molar volume for the studied SILMs.
Error bars represent standard deviations based on three experimental
replicas. The gas permeability, diffusivity and solubility values of
[C2mim][N(CN)2][NTf2] and [C2mim][NTf2] were taken from Tomé et al.29
m3
m−3 Pa−1) closer to that of the pure [C2mim][TFSAM] SILM
(STP)
(22·10−6 m3
m−3 Pa−1), probably due to the fact that these IL
(STP)
phases have very close molar volumes (208.7 and
M
η a
ρ b
Vm
Composition description
IL sample
wt% H2O
(g mol-1)
(mPa s)
(g cm-3)
(cm3 mol-1)
(mole fraction)
[C2mim][TFSAM]
[C2mim][SCN]c
Pure
0.02
0.09
0.16
0.10
0.09
0.12
0.02
284.26
169.25
214.74
260.23
177.21
284.26
391.31
23.7
27.9
36.4
49.5
18.0
29.2
39.1
1.3518
1.1190
1.2638
1.3850
1.1060
1.3620
210.3
151.2
169.9
187.9
160.2
208.7
256.8
Pure
[C2mim][SCN][OTf]
[C2mim][OTf]
[C2mim][N(CN)2]c
[C2mim][N(CN)2][NTf2]c
[C2mim][NTf2]c
0.5 [C2mim][SCN] + 0.5 [C2mim][OTf]
Pure
Pure
0.5[C2mim][N(CN)2] + 0.5 [C2mim][NTf2]
Pure
1.5240
Table 5 Composition descriptions, water contents and thermophysical properties of the pure ILs and IL mixtures.
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PCCP
separation efficiency of the [C2mim][N(CN)2][NTf2] SILM is on top of
the upper bound, since it presents lower permselectivity (41.8),
despite its high CO2 permeability (589 Barrer) in comparison to that
of the [C2mim][SCN][OTf] SILM (428 Barrer). Actually, it is the
[C2mim][N(CN)2][NTf2] SILM that discloses the most similar CO2/N2
separation performance results to the pure synthesized
[C2mim][TFSAM] IL (Fig. 5).
Comparison of CO2/N2 separation performance
The gas permeabilities and CO2/N2 permselectivities of all the
studied SILMs are listed in Table 6. Amongst the pure SILMs with
fluorinated anions, the [C2mim][FSI] and [C2mim][TFSAM] SILMs not
only show the highest gas permeabilities (843 and 753 Barrer,
respectively), but also have the largest CO2/N2 permselectivities
(43.9 and 46.1, respectively). In contrast, the lowest gas
permeabilities and CO2/N2 permselectivity belong to the
[C2mim][C4F9SO3] SILM. The CO2/N2 permselectivities of the pure
SILMs decrease as the fluorinated chain increases in the IL anion:
[FSI]– > [TFSAM]– > [NTf2]– > [BETI]–, being the only exceptions the
[C2mim][FAP] and [C2mim][C4F9SO3] SILMs.
Concerning the effect of the structural unfolding of [TFSAM]–
anion, and as previously discussed, the pure [C2mim][TFSAM] SILM
presents higher gas permeabilities compared to those of its
structurally similar IL mixtures (Table 6). Nevertheless, the greatest
CO2/N2 permselectivity (57.6) was achieved for the SILM containing
SILM sample
P CO2
P N2
α CO2/N2
Fig.
5 CO2/N2 separation performance of all the studied SILMs. The
[C2mim][TFSAM]
[C2mim][FSI]
753 ± 0.2
843 ± 0.5
624 ± 0.4
437 ± 1.9
32 ± 0.2
17 ± 0.1
18 ± 0.2
24 ± 0.1
18 ± 0.1
6 ± 0.1
43.9 ± 0.1
46.1 ± 0.5
26.0 ± 0.1
24.8 ± 0.2
5.5 ± 0.1
experimental error is within the data points. Data are plotted on a log-log
scale and the upper bound is adapted from Robeson.51 The literature data
previously reported for other SILMs (□)15, 17, 22, 24, 29, 30, 32, 53 is also illustrated
for comparison. The values of [C2mim][N(CN)2][NTf2] and [C2mim][NTf2]
SILMs were taken from Tomé et al.29
[C2mim][FAP]
[C2mim][BETI]
[C2mim][C4F9SO3]
[C2mim][SCN]c
263 ± 0.6
428 ± 0.5
479 ± 0.6
476 ± 0.8
589 ± 1.9
589 ± 1.0
5 ± 0.2
56.6 ± 1.9
57.6 ± 0.4
41.4 ± 0.3
67.8 ± 0.6
41.8 ± 0.7
35.5 ± 0.3
[C2mim][SCN][OTf]
[C2mim][OTf]
[C2mim][N(CN)2]c
[C2mim][N(CN)2][NTf2]c
[C2mim][NTf2]c
7 ± 0.1
12 ± 0.1
7 ± 0.1
14 ± 0.2
17 ± 0.1
the [C2mim][SCN][OTf] IL mixture.
Table 6 Single gas permeability (P)a and ideal permselectivities (α) of all the
studied SILMs.b
a Barrer (1 Barrer = 10–10 cm3(STP)cm cm–2 s–1 cmHg–1
)
IL mixtures, Fig. 5 clearly shows that the CO2/N2 separation
performance of the SILM immobilized with the [C2mim][SCN][OTf] IL
mixture surpasses the upper bound, primarily due to its higher
CO2/N2 permselectivity (57.6). On the other hand, the CO2/N2
b The listed uncertainties represent the standard deviations, based on three
experiments.
c Values taken from Tomé et al.29
With the purpose of comparing the performance results
obtained in this work to those reported in the literature for other
SILMs, Fig. 5 displays the Robeson plot for CO2/N2 separation,
where the CO2/N2 permselectivity is plotted against CO2
permeability and the solid black line represents the empirical 2008
upper bound for this gas pair.51 It can be seen that among the SILMs
immobilized with the pure IL having fluorinated anions, both the
[C2mim][TFSAM] and [C2mim][FSI] SILMs fall or exceed the Robeson
2008 upper bound, meaning that these two ILs are the most
promising candidates for CO2/N2 separation processes. Comparing
the results of the pure [C2mim][TFSAM] SILM with those of selected
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washed with small amount of water and dried over anhydrous
MgSO4. The magnesium sulfate was filtered off and
dichloromethane was stripped off under the reduced pressure. The
product was obtained as slightly yellow transparent fluid oil, which
was finally dried at 323K and 100 Pa for 12 h with a special flask
filled with P2O5 and introduced into the vacuum line. Yield: 5.61 g
(68%); Anal. Calcd. for C10H11N2F9SO3 (410.26), %: C, 29.28%; H,
2.70%; F, 41.68%; Found, %: C, 28.99%; H, 2.83%; F, 41.39%; 1H
NMR (300 MHz, DMSO-d6): 9.09 (s, 1H, Н2 (Im)), 7.77 (s, 1H, H4
(Im)), 7.68 (s, 1H, H5 (Im)), 4.22-4.17 (m, 2H, СН2СН3), 3.85 (s, 3H,
СН3), 1.43-1.39 (m, 3H, СН2СН3); 13C NMR (100.6 MHz, DMSO-d6):
Experimental
Materials
Lithium bis(pentafluoroethylsulfonyl)imide (Li(CF3CF2SO2)2N, 136.2, 123.5, 121.9, 117.1 (qt, -CF2-CF3, 1J
= 288 Hz, 2J =
LiBETI, 98%, Chameleon Reagent) and lithium nonafluoro-1- 34 Hz), 113.4 (tt, (-)O3S-CF2-, 1J = 288 Hz, 2J = 34 Hz), 110.4 (tp, (-
butanesulfonate (LiC4F9SO3, > 95%, TCI Chemicals) were used )O3S-CF2-CF2-, 1J = 266, 2J = 33 Hz), 109.8 (qq, -CF2-CF3, 1J = 268 Hz,
without purification. Reagent-grade dichloromethane, acetonitrile, 2J = 38 Hz), 44.1, 35.6, 14.9; 19F NMR (282.4 MHz, DMSO-d6): -80.8,
hexane and ethyl acetate were obtained from Aldrich or Merck and -114.9, -121.6, -125.9; IR (KBr pellet): 3156 (s, νС-Н), 3117 (s, νС-Н),
were dried by vacuum distillation over P2O5. N-methylimidazole 2993 (m, νС-Н), 1574 (s), 1462 (m), 1432 (w), 1393 (w), 1353 (s),
(98%, Aldrich) and bromoethane (98%, Acros) were distilled under 1261 (vs, νasSO2), 1236 (vs), 1214 (vs, νCF), 1170 (vs, νsSO2), 1134 (s,
inert atmosphere over CaH2.
νCF), 1057 (vs), 1119 (m), 1006 (m), 988 (w), 870 (m), 845 (m), 802
The
1-ethyl-3-methylimidazolium
bis(fluorosulfonyl)imide (m), 736 (m), 699 (m), 679 (w), 656 (s), 620 (s), 596 (m), 564 (m),
([C2mim][FSI], 99.5 wt%, Solvionic), 1-ethyl-3-methylimidazolium 532 (s) cm-1.
tris(pentafluoro-ethyl)trifluorophosphate ([C2mim][FAP], 98 wt%,
Merck) 1-ethyl-3-methylimidazolium thiocyanate ([C2mim][SCN]), 1-Ethyl-3-methylimidazolium bis(pentafluoroethylsulfonyl)imide
>
98
wt%,
IoLiTec)
and
1-ethyl-3-methylimidazolium ([C2mim][BETI]). The procedure previously described for
trifluoromethanesulfonate ([C2mim][OTf], ≥ 98 wt%, Aldrich) were [C2mim][C4F9SO3], was also used for the synthesis of [C2mim][BETI].
obtained from the specified suppliers. To reduce the content of After purification and drying, [C2mim][BETI] was obtained as
water and other volatile substances, the pure ILs were dried at colorless transparent fluid liquid. Yield: 82 %; Anal. Calcd. for
approximately 1 Pa and 318 K for at least 4 days.
C10H11N3F10S2O4 (491.32), %: C, 24.45%; H, 2.26%; N, 8.55%; Found,
%: C, 24.50%; H, 2.03%; N, 8.49%; 1H NMR (300 MHz, DMSO-d6):
9.11 (s, 1H, Н2 (Im)), 7.76 (s, 1H, H4 (Im)), 7.68 (s, 1H, H5 (Im)),
IL synthesis and characterization
1-Ethyl-3-methylimidazolium bromide ([C2mim][Br]). [C2mim][Br] 4.22-4.16 (m, 2H, СН2СН3), 3.84 (s, 3H, СН3), 1.43-1.40 (m, 3H,
was synthesized by the reaction between N-methylimidazole and СН2СН3); 13C NMR (100.6 MHz, DMSO-d6): 136.7, 123.9, 122.4,
excess of bromoethane following the reference method.34 117.7 (qt, -CF2-CF3, 1J = 287 Hz, 2J = 34 Hz), 110.1 (tq, -CF2-CF3, 1J =
Spectroscopic data of the target compound were in accordance 293 Hz, 2J = 37 Hz), 44.6, 36.0, 15.4; 19F NMR (282.4 MHz, DMSO-
with those reported in the literature.
1-Ethyl-3-methylimidazolium 2,2,2-trifluoromethylsulfonyl-N- (w, νС-Н), 1574 (m), 1472 (w), 1432 (w), 1355 (vs, νasSO2), 1331 (vs,
cyanoamide ([C2mim][TFSAM]). [C2mim][TFSAM] was prepared by CF), 1223 (vs, νCF), 1172 (vs, νsSO2), 1087 (s, νCF), 978 (s), 824 (w),
d6): -78.8, -117.6; IR (KBr pellet): 3160 (m, νС-Н), 3124 (m, νС-Н), 2993
ν
ion exchange between [C2mim][Br] and KTFSAM, in an aqueous 775 (w), 755 (m), 742 (m), 701 (w), 644 (m), 616 (s), 536 (m), 525
medium in accordance with described procedure.35 Yield: 85%; (m) cm-1.
Anal. Calcd. for C8H11N4F3SО2 (284.26), %: N, 19.71%; C, 33.80%; H,
NMR spectra were recorded on AMX-400 spectrometer (Bruker)
3.90%; Found, %: N, 19.68%; C, 33.78%; H, 3.99%; 1H NMR (300 at 298 K in the indicated deuterated solvent and are listed in ppm.
MHz, DMSO-d6): 9.08 (s, 1H, Н2 (Im)), 7.72 (s, 1H, H4 (Im)), 7.64 (s, The signal corresponding to the residual protons of the deuterated
1H, H5 (Im)), 4.22-4.15 (m, 2H, СН2СН3), 3.84 (s, 3H, СН3), 1.42 (t, solvent was used as an internal standard for H and 13C NMR, while
1
3H, СН2СН3, JHH = 7.5 Hz); 13C NMR (100.6 MHz, DMSO-d6): 136.1, for 19F NMR the CHCl2F was used as an external standard. IR spectra
1
125.0-115.4 (q, JCF = 325 Hz), 123.4, 121.8, 44.1, 35.5, 14.8; 19F were acquired on a Nicolet Magna-750 Fourier IR-spectrometer
NMR (282.4 MHz, DMSO-d6): -77.8; IR (KBr pellet): 3158 (m, νС-Н), using KBr pellets (128 scans, resolution is 2 cm-1).
3117 (m, νС-Н), 2192 (vs, νС-N), 1573 (m), 1468 (w), 1333 (vs, νasSO2),
1236 (s), 1217 (vs, νCF), 1170 (vs, νsSO2), 1119 (s, νCF), 832 (s), 752 IL mixture preparation. The IL mixture, [C2mim][SCN][OTf],
(w), 639 (m), 595 (s), 479 (m) cm-1.
containing
0.5 mole fraction of both [C2mim][OTf] and
[C2mim][SCN], was prepared using an analytical high precision
1-Ethyl-3-methylimidazolium nonafluorobutanesulfonate balance with an uncertainty of ± 10−5 g by syringing known masses
([C2mim][C4F9SO3]). Lithium nonafluoro-1-butanesulfonate (8.00 g, of the IL components into a glass vial. Good mixing was assured by
0.026 mol) was dissolved in 20 mL of distilled water and added magnetic stirring for 30 min at 298 K. Then, the IL mixture was dried
dropwise to the solution of [C2mim][Br] (3.84 g, 0.020 mol) in 15 mL at roughly 1 Pa and 318 K for at least 4 days immediately prior to
of H2O at ambient temperature. The solution was stirred for 2 h at use. The water contents of all IL samples were determined by Karl
room temperature and then [C2mim][C4F9SO3] was extracted with Fischer titration using a 831 KF Coulometer (Metrohm).
dichloromethane (4 × 40 mL). The combined CH2Cl2 solution was
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PCCP
Density and viscosity determination. The density and viscosity linear portion of the pd vs. t curve back to the time axis, where the
measurements of the pure ILs and the [C2mim][SCN][OTf] IL mixture intercept is equal to θ.39
were performed at 293 K and atmospheric pressure using an SVM
3000 Anton Paar rotational Stabinger viscometer-densimeter,
where the standard uncertainty for the temperature is 0.02 K. The
repeatability of density and dynamic viscosity of this equipment is
0.0005 g·cm−3 and 0.35%, respectively. Triplicates of each sample
were performed to ensure accuracy and the reported results are
average values. The highest relative standard uncertainty registered
for the density and dynamic viscosity measurements was 1·10−4 and
0.03, respectively.
2
l
D =
(4)
i
6θ
After Pi and Di were known, the gas solubility (Si) was calculated
using the relationship shown in Eq. (1). The ideal permeability
selectivity (or permselectivity), αi/j, was obtained by dividing the
permeability of the more permeable specie i to the permeability of
the less permeable specie j. The permselectivity can also be
expressed as the product of the diffusivity selectivity and the
solubility selectivity:
Gas permeation measurements. Porous hydrophobic
poly(vinylidene fluoride) (PVDF) membranes supplied by Millipore
Corporation (USA), with a pore size of 0.22 μm and average
thickness of 125 μm, were used to support the [C2mim][FSI],
[C2mim][FAP] and [C2mim][BETI]. Since the impregnation of the
remaining IL samples into hydrophobic PVDF resulted in unstable
SILMs, the other IL samples were supported in porous
hydrophilic poly(tetrafluoroethylene) (PTFE) membranes acquired
from Merck Millipore, with a pore size of 0.2 μm and an average
thickness of 65 μm. All the SILM configurations were prepared by
the vacuum method.29








P
D
D
S
i
i
i
α
=
=
×
(5)
i / j








P
S
j
j
j
Conclusions
In this work, ILs containing a common cation ([C2mim]+) and
different fluorinated anions ([TFSAM]–, [FSI]–, [C4F9SO3]–, [BETI]–,
[FAP]–) were synthesized and used as liquid phases to prepare
SILMs for flue gas separation (CO2/N2). The single CO2 and N2
permeation properties through the prepared SILMs were
determined. The viscosity and density of the IL phases were also
evaluated. The results showed that CO2 permeabilities and
diffusivities through the studied SILMs follow the same fluorinated
Ideal gas permeabilities and diffusivities through the prepared
SILMs were measured using a time-lag apparatus.36 First, each SILM
was degassed under vacuum inside the permeation cell during 12 h.
Then, CO2 and N2 permeation experiments were carried out at 293
K with a trans-membrane pressure differential of 100 kPa. All the
permeation data were measured at least in triplicate on a single
SILM sample. The highest relative standard uncertainty registered
for gas permeability measurements was 0.03. The permeation cell
and lines were evacuated until the pressure was below 0.1 kPa
before each run. No residual IL was found inside the permeation
cell at the end of the experiments. The thickness of the SILMs was
assumed to be equivalent to the membrane filter thickness.
Gas transport through the prepared SILMs was assumed to
follow a solution-diffusion mass transfer mechanism,37 where the
permeability (P) is related to diffusivity (D) and solubility (S) as
follows:
anion order: [FSI]– > [TFSAM]– > [FAP]– > [NTf2]– > [BETI]–
>
[C4F9SO3]–, which is inversely related to IL viscosity, with the only
outlier being [C2mim][FAP]. Conversely, the introduction of
fluorination in the IL anions did not significantly affect gas solubility,
except for the case of [C2mim][C4F9SO3] SILM displaying a very low
CO2 solubility. Among the pure SILMs, it is worth noting that the
best separation performances were achieved for [C2mim][TFSAM]
and [C2mim][FSI] SILMs that fall on top or surpassed the Robeson
2008 upper bound, with CO2 permeabilities of 753 and 843 Barrer
and CO2/N2 permselectivities of 43.9 and 46.1, respectively.
Furthermore, the effect of structural unfolding of the [TFSAM]–
anion on gas permeation properties of SILMs was investigated using
IL mixtures comprising both fluorinated and cyano functionalities in
(1) the anions. The pure [C2mim][TFSAM] IL provided a membrane with
improved CO2 permeabilities, diffusivities and solubilities compared
to those of the SILMs based on the selected [C2mim][SCN][OTf] and
[C2mim][N(CN)2][NTf2] IL mixtures. Overall, and despite that the
A the effective
P = D × S
The permeate flux of each gas (Ji) was determined
experimentally using Eq. (2),38 where Vp is the permeate volume,
∆pd the variation of downstream pressure,
[C2mim][SCN][OTf] SILM revealed better CO2/N2 separation
performance essentially due to its higher CO2/N2 permselectivity
(57.6), the [C2mim][N(CN)2][NTf2] IL mixture disclosed the most
similar results to the pure synthesized [C2mim][TFSAM] IL, not only
in terms of thermophysical properties, but also regarding gas
transport and CO2/N2 separation performance.
membrane surface area, t the experimental time, R the gas
constant and T the temperature.
p
V ꢀp
d
J =
(2)
i
AtRT
Ideal gas permeability (Pi) was then determined from the
steady-state gas flux (Ji), the membrane thickness (ℓ) and the trans-
membrane pressure difference (Δpi), as shown in Eq. (3).38
Acknowledgements
J
i
P =
Andreia S.L. Gouveia and Liliana C. Tomé are grateful to
(3)
i
ꢀp / l
i
FCT (Fundação para a Ciência e a Tecnologia) for their Doctoral
Gas diffusivity (Di) was determined according Eq. (4). The time- (SFRH/BD/116600/2016) and Post-doctoral research grants
lag parameter (θ) was calculated by extrapolating the slope of the (SFRH/BPD/101793/2014), respectively, Isabel M. Marrucho
acknowledges FCT/MCTES (Portugal) for a contract under
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Investigador FCT 2012. This work was partially supported by 25 S. M. Mahurin, J. S. Yeary, S. N. Baker, D.-e. Jiang, S. Dai and G.
FCT through the project PTDC/CTM-POL/2676/2014, R&D unit A. Baker, J. Membr. Sci., 2012, 401–402, 61-67.
UID/Multi/04551/2013 (GreenIT) and through the grant of 26 E. Kamio, T. Matsuki, S. Kasahara and H. Matsuyama, Sep. Sci.
President of the Russian Federation ‘‘For Young Outstanding
Professors” (project no. МD-2371.2017.3).
Technol., 2017, 52, 209-220.
27 S. Kasahara, E. Kamio, T. Ishigami and H. Matsuyama, J. Membr.
Sci., 2012, 415–416, 168-175.
28 S. Kasahara, E. Kamio, T. Ishigami and H. Matsuyama, Chem.
Commun., 2012, 48, 6903-6905.
Notes and references
29 L. C. Tomé, D. J. S. Patinha, C. S. R. Freire, L. P. N. Rebelo and I.
M. Marrucho, RSC Adv., 2013, 3, 12220-12229.
1
L. C. Tomé and I. M. Marrucho, Chem. Soc. Rev., 2016, 45, 2785-
2824.
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