Carbon Dioxide Separation Using a High-toughness Ion Gel with a Tetra- armed Polymer Network

Abstract

A high-performance CO2 separation membrane consisting of ion gel has been prepared. The ion gel, tetra- armed poly(ethylene glycol) (Tetra-PEG) ion gel contains a large fraction of ionic liquid (94 wt%) and shows excellent CO2 permselectivity over the high temperature range up to 100 °C. We also demonstrate that the ion gel can absorb CO2 without solvent-seeping at the high pressure of 3 MPa.

Full text

Carbon Dioxide Separation Using a High-toughness Ion Gelwith a Tetra-armed Polymer Network
Kenta Fujii,*1TakashiMakino,*2KeiHashimoto,3Takamasa Sakai,4Mitsuhiro Kanakubo,2and Mitsuhiro Shibayama*3
1Graduate SchoolofScience and Engineering, YamaguchiUniversity, 2-16-1 Tokiwadai, Ube, Yamaguchi755-8611
2Research Center for Compact ChemicalSystem, NationalInstitute ofAdvanced IndustrialScience and Technology,
4-2-1 Nigatake, Miyagino-ku, Sendai,Miyagi983-8551
3Institute for Solid State Physics, The University ofTokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581
4Department ofBioengineering, SchoolofEngineering, The University ofTokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656
(E-mail:k-fujii@yamaguchi-u.ac.jp, makino.t@aist.go.jp, sibayama@issp.u-tokyo.ac.jp)
Ahigh-performance CO2separation membrane consisting
ofan ion gelhas been prepared. The tetra-armed poly(ethylene
glycol)ion gelcontains a large fraction ofionicliquid (94 wt %)
and shows excellent CO2permselectivity over a wide temper-
ature range, up to 100 °C. We also demonstrate that the ion gel
can absorb CO2without solvent seeping at a high pressure of
3 MPa.
Room-temperature ionicliquids (ILs) have been widely
applied to electrochemical, synthetic, and separation processes
as green solvents due to theirunique properties such as
nonvolatility, nonflammability, good thermalstability, and high
ionic conductivity. ILs are recognized as designablesolvents,
and their structuraldiversity enables us to controltheirsolvent
properties, for example, the miscibility with various chemicals
such as metalions, synthesized and biologicalpolymers, and
acidic gases. In the last decade, CO2absorption and separation
technologies using ILs have attracted much attention, because
CO2ishighlysolublein ILs relative to other neutralgases like
N2,H
2, and CH4.Alarge number ofinvestigations have been
performed until now to improve the CO2absorption properties
since the first report on the high solubility ofCO2in the
dialkylimidazolium-based IL.1A membrane separation process
generally requires smaller operationalenergy, lower running
cost, and smaller equipment footprint compared with an
absorption process. The supported IL membranes (SILMs),
i.e., polymericorinorganic porous materialsfilled with ILs,
show comparableorhigher permeabilities and selectivities of
CO2than conventionalpolymeric membranes.2SILMs have
ahigher long-term stability than supported membranes with
organicsolvents, because ofnonvolatility and high thermal
stability ofILs. However, SILMs cannot hold ILs under
pressurized conditions, which is a serious disadvantage for the
gas separation membrane. Polymeric ILs, i.e., self-crosslinking
ILs were also applied as CO2separation membranes.3However,
it was pointed out that their separation performances are inferior
to those ofthe SILMs due to the limited CO2diffusion in their
rigidpolymer matrix. Therefore, ion gelswith a low polymer
content and/or a high fraction of“free”IL content are more
favorable materialsfor CO2separation membrane.
Recently, some research groups reported the CO2separation
performances ofion gelswith relativelylow polymer con-
tents.2b,4 Lodge et al.4a reported an ion gelcontaining 15 wt %
ABA-type triblock copolymer and l-ethyl-3-methylimidazolium
bis(trifluoromethanesulfonyl)amide, [C2mIm+][TFSA¹]. The
ion gelpossesses a CO2permselectivity comparable to that of
the [C2mIm+][TFSA¹]-based SILM, although itmelts below
80 °C. They also reported a high-toughness ion gelwith 10 wt %
triblock copolymer, which was prepared by crosslinking
reactions in the IL.5The electricalconductivity is about 2/3
that ofthe neat IL because the ionic mobility is obstructed by the
nonconductive part in theirion gel. Kamioetal.4d reported a
CO2N2separation membrane using poly(vinylpyrrolidone)-
based ion gels containing 5070 wt %amino acid IL, and the ion
gelshows a compression breaking stress of1 MPa at 70 wt %IL
content. They demonstrated that both CO2permeability and
selectivity are significantlyimproved with increasing IL content.
In our recent study, we have proposed high-toughness ion gels
with low polymer concentration, i.e., tetra-armed poly(ethylene
glycol) (Tetra-PEG) ion gels.6The Tetra-PEG ion gels can be
prepared by AB-type cross-end coupling oftwo symmetrical
Tetra-PEGs in typicalaprotic ILs. Electricalconductivity
measurements for the Tetra-PEG ion gelshow that the
diffusivity ofionic species in the gelisalmost comparableto
that in the neat IL. It also shows high mechanicalproperties even
with much lower polymer contents, for example, the maximum
breaking strength by compression reaches 18 MPa (83.5%strain)
at 6 wt %polymer content. The excellent mechanicalproperties
originate from a homogeneous polymer network strucuture.6
In addition, the Tetra-PEG [C2mIm+][TFSA¹]gelis thermally
stable up to 300 °C. It is thus expected that the Tetra-PEG ion gel
is a promising materialfor CO2separation membrane. In this
work, we report the CO2separation performance ofthe Tetra-
PEG ion gelmembrane over wide temperature range, up to
100 °C. Furthermore, we also made a new trialto directly
evaluate the CO2absorption capacities ofthe Tetra-PEG ion gel
at pressures up to 3 MPa.
As reported in our previous work,6a the Tetra-PEG ion
gelcan be prepared by mixing two Tetra-PEG prepolymers,
i.e., tetraamine-terminated and tetra-N-hydroxysuccinimide-
terminated PEGs (Tetra-PEG-NH2and Tetra-PEG-NHS), in
aprotic[C
2mIm+][TFSA¹] IL. The gelation time in[C
2mIm+]-
[TFSA¹] was less than 60 s, which was too short to prepare
an ion gelmembrane by a conventionalmethod described in
Figure S1. Here, note that the gelation time ofthe Tetra-PEG gel
system strongly depends on the pH or concentration ofH+in
the solutions.7In the hydrogelsystem, it has been established
that (1) Tetra-PEG gelation follows simple second-order reaction
kinetics (¹d[NH2]/dt=kgel[NH2][NHS], kgel: reaction rate
constant) and (2) the gelation time isdirectlyrelated to the acid
base reaction ofthe reactive NH2end group within Tetra-PEG-
NH2(Tetra-PEG-NH2+H+Tetra-PEG-NH3+).7a,7b Thus, the
concentration ofthe NH2end group, [NH2] decreases with a
lower pH (=higher [H+]), resulting inalonger gelation time.
In an aprotic IL system, the addition ofH+sources is required
Received: August 25, 2014 | Accepted: September 30, 2014 | Web Released: October 9, 2014 CL-140795
Chem. Lett. 2015, 44, 17–19 | doi:10.1246/cl.140795 © 2015 The Chemical Society of Japan | 17

for gelation control, because there isnodissociative H+within
either the cation or the anion in typicalaprotic ILs. Therefore,
we focused on protic ILs as “nonvolatile”H+sources. In this
work, 1-ethylimidazolium bis(trifluoromethanesulfonyl)amide
([C2ImH+][TFSA¹]), an analogous proticILofaprotic
[C2mIm+][TFSA¹], was added into Tetra-PEG-NH2/[C2mIm+]-
[TFSA¹] and Tetra-PEG-NHS/[C2mIm+][TFSA¹]solutions
with 6 wt %prepolymer content. The molecular weight was
20000 for both prepolymers. After mixing them, the gelation
reaction was investigated by rheologicalmeasurements to
estimate the gelation time. Figure 1 shows typicalresults of
the storage (G¤) and loss (G¤¤) moduli for the Tetra-PEG gelation
in[C
2mIm+][TFSA¹]with varying [C2ImH+][TFSA¹] concen-
tration. Both G¤and G¤¤ graduallyincreased with reaction time
for all the systems, and the intersection ofthe G¤and G¤¤ profiles,
corresponding to the gelation point, was clearly observed. The
gelation times were 775, 2660, and 62375 s for 4.6, 5.0, and
6.0 mM [C2ImH+][TFSA¹]solutions, respectively. Figure S2
shows the gelation time plotted against the concentration of
[C2ImH+][TFSA¹]. It was deduced from thisfigure that we can
successfully controlthe gelation time as a function ofproticIL
concentration, enabling easy preparation ofgas separation
membranes containing a large amount (94 wt %)ofIL. We
point out here that acidbase reactions ofthe terminalNH2
group within Tetra-PEG in aprotic[C
2mIm+][TFSA¹]signifi-
cantlyaffect the gelation time, which has been investigated by
potentiometrictitration in our recent work.8The mechanical
properties ofthe Tetra-PEG ion gelobtained inthis work were
evaluated by a stretching test, which is shown inFigure S3
(stretching stresselongation curve). The maximum breaking
elongation, max was over 4.0, which islarger than that ofthe
high-toughness ion gels reported recently.5,6a The breaking stress
was 24 kPa at =4.0, and the elastic modulus was estimated to
be 3.6 kPa.
The Tetra-PEG ion gelmembrane used for the CO2
separation test inthis work was prepared as illustrated in
Figure S1. The 6 wt %Tetra-PEG-NH2and Tetra-PEG-NHS in
[C2mIm+][TFSA¹]solutions were mixed and stirred for minutes
to obtain a homogeneous mixture. The concentration of
[C2ImH+][TFSA¹]in each solution was 6.0 mM. The mixture
was cast on a Teflon plate with 1 mm-thick silicon spacers, and
then it was pressed by another Teflon plate at room temperature
for 24 h inaN
2atmosphere. The gas permeability ofthe 6 wt %
Tetra-PEG ion gel(94 wt %IL) was investigated in the temper-
ature range from 25 to 100 °C. Detailsofthe gas separation
measurement are described in the Supporting Information (SI).
Figure 2a shows the CO2permeability, PCO2,for the ion gel
membrane with varying temperature, together with those for the
SILM ofporous hydrophilicTeflon filmfilled with [C2mIm+]-
[TFSA¹]. The CO2permeability for the Tetra-PEG ion gelwas
877 barrer at 25 °C, and the value is ca. 1.4 times larger than that
for the SILM. Thismight be ascribed to the difference in the
cross-sectionalarea where gas molecules come into contact with
the IL or a change in the thickness ofthe ion gelduring the
measurement. The CO2permeability monotonicallyincreased
with rising temperature, indicating the increase in the CO2
diffusion in the IL medium. Figure 2b shows the CO2N2
selectivity estimated from the permeabilities ofCO2and N2,
PCO2=PN2. The selectivity for the Tetra-PEG ion geldecreased
with increasing temperature, and the values at each temperature
were almost similar to those for the SILM. According to a
solution-diffusion transport mechanism, the idealselectivity
between two different gases (i,j)mainly depends on solubilities
(S) and diffusion coefficients (D), i.e., Pi/Pj=(Si/Sj)(Di/Dj).
The result obtained here implies that both solubilities and
diffusion coefficients ofCO2and N2in the Tetra-PEG ion gel
are almost the same as those in the neat IL. We thus concluded
that the supporting polymer, Tetra-PEG, does not hinder the gas
separation performance at the temperatures examined because of
the same selectivity in both the ion geland SILM systems.
In order to obtain deeper insight into the CO2N2
permselectivity ofthe Tetra-PEG ion gelmentioned above, we
extended our study to CO2absorption properties under high-
pressure conditions. As far as we know, thisis the first report on
the gas absorption for ion gelunder high pressures. First, we
carried out volume expansion measurements for the Tetra-PEG
ion gelprepared in a capillary (1.4 mmº)byusing an optical
microscope. The details are described in the SI. The volume
10-1
100
101
102
103
G', G'' / Pa
1022 4 6
1032 4 6
1042 4 6
105
t / s
4.6 mM
5.0 mM
6.0 mM
G'
G''
Figure 1. Dynamic moduli,G¤(solidline) and G¤¤ (broken
line), for the gelation ofTetra-PEG in[C
2mIm+][TFSA¹]with
varying the concentration ofprotic IL, [C2ImH+][TFSA¹].
(a)
(b)
1000
500
0
PCO2/ Barrer
6 wt% Tetra-PEG ion gel
SILM (neat IL)
30
20
10
0
PCO2 / PN2
100500
T / °C
6 wt% Tetra-PEG ion gel
SILM (neat IL)
Figure 2. Temperature dependences of(a) the CO2perme-
ability, PCO2, and (b) the CO2N2selectivity, PCO2=PN2,for the
6wt%Tetra-PEG+[C2mIm+][TFSA¹]ion gel, together with
those for the corresponding SILM.
18 |Chem. Lett. 2015, 44, 17–19 | doi:10.1246/cl.140795 © 2015 The Chemical Society of Japan

expansion, V/V0, was defined as a change indiameter between
the ion gels absorbing CO2(d) and without CO2(d0), that is,
V/V0=(d/d0)3. We assumed that the ion gelisotropically
expands with the absorption ofCO2.Figure 3 shows the V/V0as
afunction ofCO2pressure at 298 K for the 6 wt %Tetra-PEG
ion gel. It was found that the Tetra-PEG ion gelexpands without
solvent seeping out ofthe geleven under the high pressures
examined. The V/V0for the ion gellinearlyincreased with
increasing pressure (the amount ofCO2absorbed), and itis
practically equalto that for the neat [C2mIm+][TFSA¹] reported
inref9. Figure 4 shows the pressure dependence ofthe CO2
solubility (molarity scale) at 298.15 K. The experimental
apparatus and procedure are described elsewhere.10 The Tetra-
PEG ion gelcan absorb CO2physically as much as the
corresponding neat IL.9These results suggest the following: (1)
Liquid-like CO2absorption occurs in the solid-state ion gel
containing 94 wt %[C2mIm+][TFSA¹]; (2) Tetra-PEG as a
support network does not interfere with CO2absorption due to
its low content in the ion gel. We thus propose that the Tetra-
PEG ion gelshows the maximum CO2physicalabsorption
among polymerIL composite materials, because IL physical
absorbents without polymers absorb the largest amount ofCO2.
Asimilar behavior was observed for the ionic conductivity in
our previous work; that is, the conductivity ofthe Tetra-PEG ion
gelwith low polymer contents was nearly equalto that ofthe
neat IL.6a
In conclusion, a high-toughness Tetra-PEG ion gelwas
prepared by controlling gelation time with the addition ofa
proticILtoafford an ion gelmembrane with an amount ofIL
(94 wt %)larger than any other polymerIL membrane reported
inliterature. We demonstrated that the Tetra-PEG ion gel
membrane shows a CO2N2permselectivity comparable to that
ofthe corresponding SILM even in the high-temperature range
without any degradation. This excellent performance is derived
from the good CO2absorption and diffusion properties, which
are almost the same as those for the neat IL. The Tetra-PEG ion
gelabsorbs CO2without solvent seeping under pressure
conditions up to 3 MPa. Again, Tetra-PEG allows one to prepare
ahigh-toughness and thermally stableion gelmembrane even
with very low polymer concentrations. A low polymer content in
ion gelsis the key to maximize the potentialofIL membranes.
The present study may open up new possibilities for gas
separation membrane under a wide range oftemperatures and
pressures. For practicaluse, however, membranes much thinner
than the present one are required. Such trialto prepare thinner
and tougher Tetra-PEG ion gelsisin progress by optimizing the
condition ofthe acidbase reaction in ILs.
This work has been financially supported by Grant-in-Aid
for Scientific Research from the Ministry ofEducation, Culture,
Sports, Science and Technology (Nos. 24750066 and 25248027)
and IketaniScience and Technology Foundation.
Supporting Information is availableelectronically on J-STAGE.
References
1L.A.Blanchard, D. Hancu, E. J. Beckman, J. F. Brennecke, Nature
1999,399,28.
2 a) P. Scovazzo, J. Membr. Sci.2009,343, 199. b) J. E. Bara, T. K.
Carlisle, C. J. Gabriel, D. Camper, A. Finotello, D. L. Gin, R. D.
Noble, Ind. Eng. Chem. Res. 2009,48, 2739.
3 a) J. E. Bara, S. Lessmann, C. J. Gabriel, E. S. Hatakeyama, R. D.
Noble, D. L. Gin, Ind. Eng. Chem. Res. 2007,46, 5397.b)J.E.
Bara, C. J. Gabriel, E. S. Hatakeyama, T. K. Carlisle, S. Lessmann,
R. D. Noble, D. L. Gin, J. Membr. Sci.2008,321,3.
4 a) Y. Gu, T. P. Lodge, Macromolecules 2011,44, 1732.b)J.C.
Jansen, K. Friess, G. Clarizia, J. Schauer, P. Izák, Macromolecules
2011,44,39.c)P.Li, K. P. Pramoda, T.-S. Chung, Ind. Eng. Chem.
Res. 2011,50, 9344. d) S. Kasahara, E. Kamio, T. Ishigami,H.
Matsuyama, Chem. Commun. 2012,48, 6903. e) L. C. Tomé, D.
Mecerreyes, C. S. R. Freire, L. P. N. Rebelo, I. M. Marrucho,
J. Membr. Sci.2013,428, 260.
5 Y. Gu, S. Zhang, L. Martinetti, K. H. Lee, L. D. McIntosh, C. D.
Frisbie, T. P. Lodge, J. Am. Chem. Soc. 2013,135, 9652.
6 a) K. Fujii, H. Asai,T.Ueki, T. Sakai, S. Imaizumi, U. Chung, M.
Watanabe, M. Shibayama, Soft Matter 2012,8, 1756. b) H. Asai,K.
Fujii, T. Ueki, T. Sakai, U. Chung, M. Watanabe, Y.-S. Han, T.-H.
Kim, M. Shibayama, Macromolecules 2012,45, 3902.
7a)K.Nishi, K. Fujii,M.Chijiishi, Y. Katsumoto, U. Chung, T.
Sakai,M.Shibayama, Macromolecules 2012,45, 1031.b)K.Nishi,
K. Fujii, Y. Katsumoto, T. Sakai,M.Shibayama, Macromolecules
2014,47, 3274.
8 a) K. Fujii, K. Hashimoto, T. Sakai, Y. Umebayashi,M.Shibayama,
Chem. Lett. 2013,42, 1250. b) K. Hashimoto, K. Fujii,M.
Shibayama, J. Mol.Liq. 2013,188, 143.
9 T. Makino, M. Kanakubo, Y. Masuda, T. Umecky, A. Suzuki,Fluid
Phase Equilib. 2014,362, 300.
10 T. Makino, M. Kanakubo, T. Umecky, A. Suzuki,Fluid Phase
Equilib. 2013,357,64.
1.30
1.25
1.20
1.15
1.10
1.05
1.00
V
/
V
0
543210
p / MPa
6 wt% Tetra-PEG ion gel
neat [C2mIm+][TFSA-]
Figure 3. Vo lume expansion V/V0plotted against pressure p
for the 6 wt %Tetra-PEG ion gel+CO2(filled circles) and the
corresponding neat IL +CO2(open circles)9systems.
6
5
4
3
2
1
0
cCO2 / mol dm-3
543210
p / MPa
6 wt% Tetra-PEG ion gel
neat [C2mIm+][TFSA-]
Figure 4. CO2solubilities cCO2plotted against pressure pfor
the 6 wt %Tetra-PEG ion gel+CO2(filled circles) and the
corresponding neat IL +CO2(open circles)9systems.
Chem. Lett. 2015, 44, 17–19 | doi:10.1246/cl.140795 © 2015 The Chemical Society of Japan | 19