Zwitterionic silver complexes as carriers for facilitated-transport composite membranes

Article abstract of DOI:10.1002/anie.200353632

Carry on: Zwitterionic silver complexes were prepared from silver salts and zwitterionic-type sulfopropyl imidazolium compounds (see scheme), and their performance as carriers for facilitated-transport composite membranes was investigated for the separati

Full text of DOI:10.1002/anie.200353632

Angewandte
Chemie
been considered a promising alternative to low-temperature
distillation to separate alkenes from their corresponding
alkane.[1–5] However,ion-exchange membranes often require
costly and complicated preparation steps,and exhibit alkene/
alkane selectivity only when the feed mixture is humidi-
fied.[6–7] Therefore,the exploration of new membrane mate-
rial to overcome the disadvantages of ion-exchange mem-
branes still remains a challenging problem.
Recently,ionic liquids have attracted increasing interest
as environmentally benign media in many organic trans-
formations,as electrolytes for batteries and capacitors,and in
extraction,adsorption,nanoparticle formation,and separa-
tion processes.[8–11] Accordingly,various types of ionic liquids
have been developed for specific purposes.[12–16] Among ionic
liquids reported,we are particularly interested in zwitter-
ionic-type ionic liquids that contain covalently bound anionic
sulfonate group,which can be used as a fixed sites to bind
[3]
silver ions,such as in sulfonated ionomer membranes.
Herein,we report on the synthesis and characterization of
zwitterionic-type silver complexes,and their performance as
carriers for facilitated transport membranes in separating
isoprene from a mixture with n-pentane. Isoprene/n-pentane
was chosen as the feed mixture because isoprene is tradition-
ally separated from C5 raffinate by using an energy-intensive
extractive distillation owing to azeotrope formation with n-
pentane.[17]
The zwitterionic silver complexes can be easily prepared
from silver salts and zwitterionic-type imidazolium com-
pounds that contain a covalently bound sulfonate group as
shown in Equation (1).[14]
Ionic Liquids
Zwitterionic Silver Complexes as Carriers for
Facilitated-Transport Composite Membranes**
Hyunjoo Lee, Dong Bum Kim, Seoung-Hyun Kim,
Hoon Sik Kim,* Sung Joon Kim, Dai Ki Choi,
Yong Soo Kang, and Jongok Won
Silver-based facilitated transport through ion-exchange mem-
branes such as perfluorosulfonated ionomer membrane has
Zwitterionic silver complexes, 2 were prepared in high
yields by the reaction of 1 with a silver salt. Treatment of 1a
with AgBF4 in THF for 3 h at a room temperature produced
2a–BF4 as a white solid. Elemental analysis of 2a–BF4
indicated that the complex is a 1:1 addition product of 1a
and AgBF4. To confirm this,we attempted a single-crystal X-
ray diffraction study of 2a–BF4,but failed to obtain a suitable
crystal for the X-ray analysis. Instead,crystals of 2c–BF4 were
grown in methanol and analyzed by X-ray crystallography
(Figure 1).[18]
To our surprise,the X-ray study reveals that 2c–BF4 is an
1:2 addition product in which the silver ion is coordinated by
two sulfopropyl imidazolium ions. It is not certain at the
moment why the complex 2c–BF4 exists as a 1:2 adduct
instead of 1:1,but the bulkiness and/or the electron-with-
drawing effect of the phenyl group on the imidazolium moiety
is likely to be responsible for the formation of the 1:2 adduct.
The silver ion in 2c–BF4 is located between two sulfopropyl
imidazolium ions and is coordinated by the oxygen atoms of
two sulfonate groups. The distances between oxygen atoms of
[*] Dr. H. S. Kim
Division of Environment and Process Technology, KIST
39-1, Hawolgokdong, Seongbukgu, Seoul 136-791 (Korea)
Fax: (+82)2-958-5859
E-mail: khs@kist.re.kr
Dr. H. Lee, Dr. S.-H. Kim, Dr. S. J. Kim, Dr. D. K. Choi, Dr. Y. S. Kang
Korea Institute of Science and Technology
D. B. Kim, Prof. J. Won
Department of Chemistry
Sejong University
Seoul (Korea)
[**] We acknowledge financial support by a grant (BC1-108) from
Carbon Dioxide Reduction & Sequestration Research Center, one of
the 21st Century Frontier Programs funded by the Ministry of
Science and Technology of Korean government. We also thank
Korea Institute of Science & Technology and Kolon Petrochemical
Industry Co. for financial supports.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 3053 –3056
DOI: 10.1002/anie.200353632
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3053

Communications
Figure 1. Molecular structure of 2c–BF4. Ellipsoids are shown at 30%
probability levels. The unlabeled atoms are carbon atoms and hydro-
gen atoms have been omitted for clarity. Selected bond lengths [Š] and
angles [8]: Ag(1)-O(3) 2.314(5), Ag(1)-O(6) 2.369(6), S(1)-O(1)
1.447(6), S(1)-O(2) 1.443(5), S(1)-O(3) 1.465(5), S(1)-O(3)-Ag(1)
137.06(3), O(3)-Ag(1)-O(6) 94.46(2), S(2)-O(6)-Ag(1) 118.02(3).
À
À
the sulfonates and the silver ion,Ag(1) O(3) and Ag(1)
O(6),are 2.314 and 2.369 Š,respectively.
Melting points of the zwitterionic complexes were deter-
mined by differentiating scanning calorimetry (DSC). An
ionic liquid is defined as the ionic compound that has a
melting point lower than 1008C.[8] From this point of view,
zwitterionic complexes, 2a–BF4 and 2c–BF4 can be consid-
ered as ionic liquids.
Figure 2. Cross section SEM–WDS photograph of the composite mem-
brane containing 2a–BF4 A) before, and B) after 10 h of use.
Bar=10 mm
Zwitterionic silver complexes were tested as transport
carriers for the separation of isoprene from n-pentane
through composite membranes. The composite membrane
was prepared by casting an aqueous solution of a zwitterionic
silver complex onto a polyester microporous membrane
(47 mm,0.1 mL,Whatman) by using a coater,and then dried
under a vacuum.
SEM–WDS (WDS = wavelength-dispersive spectrome-
try) photographs in Figure 2 display the cross sections of the
prepared composite membrane containing 2a–BF4 before and
after transportation study. The white solid lines denote the
distribution of silver across the membrane. WDS images
clearly show that 2a–BF4 is immobilized in the porous
structure of the polyester membrane and distributed mostly
at the inlet and outlet of the pores.
A comparison of the WDS images before and after use of
the membranes indicates that the initial distribution of 2a–
BF4 remains almost unchanged during the separation process.
It is hard to figure out the state of the carriers immobilized in
the pores of the membrane. From the SEM images shown in
the Supporting Information,the carries seem to exist as a thin
film on the inlet surface of the polyester membrane. However,
in the pores of the membrane,the carriers are likely to be in a
well-dispersed solid state,which might transform into a
swollen gel film upon interaction with a feed mixture.
Figure 3 shows the selectivity of isoprene over n-pentane
with time through composite membranes containing zwitter-
ionic silver complexes as transport carriers. The selectivity
was strongly dependent on the types of counterions of the
zwitterionic silver complexes in the membranes. In contrast to
polymer membranes that consist of polyvinyl alcohol and
silver salts in which the counterion effect was negligible,[19] the
zwitterionic complex prepared from AgNO3 showed much
higher selectivity than those from AgBF4 and AgClO4.
Figure 3. Change of selectivity of isoprene over n-pentane with time for
*
various membranes containing zwitterionic silver complexes; 2a–
~
&
*
NO3, 2a–ClO4, 2a–BF4, 2b–BF4, & 2c–BF4.
Investigation of the anion effect on isoprene transport is in
progress. The selectivity also varied significantly with the
change of alkyl group on the imidazolium cations. Of the
À
zwitterionic silver complexes that have the BF4 counterion,
2a–BF4 exhibits the highest selectivity and 2c–BF4 the lowest.
The lower performance of phenyl-substituted imidazolium
silver complex, 2c–BF4 can be largely attributed to the fact
that the vacant site of silver ion is occupied by an additional
sulfopropyl imidazolium ion,thereby blocking the coordina-
tion of isoprene to the silver ion to a certain extent. The
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Chemie
filtered to remove unconverted AgNO3,and the solvent was removed
selectivities for isoprene over n-pentane increased rapidly
with time up to about 2 h and then remained nearly constant
throughout the experiments,thus demonstrating the stability
of zwitterionic silver complexes towards reduction of the
silver ions. In fact,the initial white color of the membrane was
retained even after a long period of use. This result is a clear
indication that the reduction of silver ions did not occur
during the separation process. Such a high stability of silver
ions in zwitterionic complexes is believed to stem from the
strong interaction of silver ion with the sulfonate groups and a
favorable electronic effect of the sulfopropyl imidazolium
ions.
under vacuum to give an air-stable white solid. Yield: 97%; mp:
119.98C; elemental analysis calcd (%): C 22.47,H 3.23,N 11.23;
found: C 22.63,H 3.21,N 11.20; 1H NMR (300 MHz,D 2O,25 8C): d =
2.17 (m,2H; CH 2),2.77 (t,2H; CH 2),3.74 (3,3H; CH 3),4.21 (t,2H;
CH2),7.29 (s,1H; CH),7.30 (s,1H; CH),8.61 ppm (s,1H; CH).
Other zwitterionic silver complexes were prepared in a similar
manner to that of 2a–BF4 or 2a–NO3.
2a–ClO4: Yield: 98%; mp: 103.48C; elemental analysis calcd
(%): C 20.43,H 2.94,N 6.81; found: C 20.62,H 3.01,N 6.78; 1H NMR
(300 MHz,D 2O,25 8C): d = 2.16 (m,2H; CH 2),2.77 (t,2H; CH 2),3.74
(3,3H; CH 3),4.21 (t,2H; CH 2),7.29 (s,1H; CH),7.36 (s,1H; CH),
8.60 ppm (s,1H; CH).
2b–BF4: Yield: 97%; mp: 104.08C; elemental analysis calcd (%):
The permeation fluxes measured were 8 ” 10À9 for 2a–
NO3, 9 ” 10À9 for 2a–BF4, 6 ” 10 À9 for 2a-ClO4, 9 ” 10 À9 for
2b–BF4,and 4 ” 10 À10 molcmÀ2 sÀ1 for 2c–BF4. As expected
from the reduced number of silver ions present in the
membrane, 2c–BF4 gave the lowest flux,but other zwitter-
ionic silver complexes exhibited similar permeation fluxes.
The initial fluxes were maintained throughout the separation
experiments.
C 27.24,H 4.11,N 6.35; found: C 27.34,H 4.10,N 6.41;
1H NMR
(300 MHz,D 2O,25 8C): d = 0.78 (t,3H; CH 3),1.18 (m,2H; CH 2),1.72
(m,2H; CH ),2.19 (m,2H; CH 2),2.79 (t,2H; CH 2),4.07 (t,2H;
2
CH2),4.24 (t,2H; CH 2),7.39 (s,1H; CH),7.41 (s,1H; CH),8.69 ppm
(s,1H; CH).
2c–BF4: Yield: 97%; mp: 77.48C; elemental analysis calcd (%):
C 39.63,H 3.88,N 7.70; found: C 39.80,H 3.92,N 7.68;
1H NMR
(300 MHz,D 2O,25 8C): d = 2.63 (m,2H; CH 2),2.86 (t,2H; CH 2),4.35
(t,2H; CH 2),7.50 (s,5H; C 6H5),7.60 (s,1H; CH),7.76 (s,1H; CH),
9.17 ppm (s,1H; CH).
The membranes containing zwitterionic silver complexes
were also effective for the separation of mixtures of 1-pentene
and n-pentane. When 2a–NO3 and 2a–BF4 were used as
carriers,selectivities of 93 and 47 were obtained,respectively.
The lower selectivities for 1-pentene compared with those for
isoprene can be attributed to the fact that the binding ability
of 1-pentene to silver ions is weaker than that of isoprene.
It should be mentioned here that this is the first attempt to
use ionic liquid-based silver complexes as carriers for the
facilitated transport membranes in separating unsaturated
compounds from their corresponding alkanes. More impor-
tantly,the membranes containing zwitterionic silver com-
plexes are easier to prepare,more stable,and more selective
for separating isoprene from n-pentane compared with
conventional ion-exchange membranes.
Single crystals of 2c–BF4 suitable for X-ray diffraction studies
were grown in methanol at À108C.
Performance of membranes: Composite membranes were pre-
pared by casting an aqueous solution of a zwitterionic silver complex
(2.0 mmol in 0.5 mL H2O) onto a polyester microporous membrane
support (0.1 mm,47 mm,Whatman Industries Inc.) by using a coater.
The coated membrane was dried in an oven at room temperature for
12 h under a stream of nitrogen,and then further dried in a vacuum
oven at 408C for 24 h. The separation of isoprene/n-pentane (50:50
wt.%) was performed at room temperature with the cast membranes
in a module.[20] Helium gas (10 mLminÀ1) was used as a sweep gas and
the flow rate of He was controlled by using a mass flow controller. A
mixture of isoprene/n-pentane (10 mL, v/v = 50:50) was charged into
a FEP tube attached to the lid of the module. The permeate was
collected at À208C in a trap filled with n-heptane and analyzed by gas
chromatography (Younglin,Model 600D equipped with an FID and a
unibead 2S 60/80 packed column).
Efforts to improve the performance of the membranes by
modifying the imidazolium moiety in the zwitterionic silver
complexes are in progress as well as the detailed investigation
of the effect of water on the performance and stability of the
membranes.
Received: December 29,2003 [Z53632]
Keywords: facilitated transport · ionic liquids · membranes ·
.
silver · zwitterions
Experimental Section
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Compounds 1a, 1b,and 1c were prepared by treating 1,3-propane
sultone with the corresponding 1-alkyl or 1-phenylimidazole,sim-
ilarly to the literature procedure employed for the preparation of 1-
ethyl-3-sulfopropyl imidazolium compound.[14] Isoprene and n-pen-
tane were purchased from Aldrich and distilled prior to use. All other
chemicals were obtained from Aldrich and used as received. Melting
points were measured by using a DSC (TA Instruments).
2a–BF4: A solution of AgBF4(11 mmol) in THF (30 mL) was
treated with 1a (10 mmol) at room temperature for 3 h. The solution
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was filtered,washed with THF to remove unconverted AgBF ,and
4
dried under a reduced pressure to give air stable white solid. Yield:
96%; mp 94.98C; elemental analysis calcd (%): C 21.08,H 3.03,
N 7.02; found: C 21.12,H 3.05,N 7.00; 1H NMR (300 MHz,D 2O,
258C): d = 2.17 (m,2H; CH 2),2.78 (t,2H; CH 2),3.75 (3,3H; CH 3),
[7] A. Sungpet,P. Prayoonyong,N. Noiboungam,S. Vongthavorn, J.
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4.22 (t,2H; CH ),7.30 (s,1H; CH),7.38 (s,1H; CH),8.61 ppm (s,
[9] R. D. Rogers,K. R. Seddon, Science 2003, 302,792 – 793.
2
1H; CH).
[10] L. C. Branco,J. G. Crespo,C. A. M. Afonso,
Angew. Chem.
2a–NO3: AgNO3 (11 mmol) was treated with 1a (10 mmol) in
MeOH (30 mL) at room temperature for 3 h. The solution was
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Communications
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[18] Diffraction measurements were made on as Enraf CAD-4
automated diffractometer with graphite-monochromated MoKa
radiation. Crystal data for 2c–BF4: C24H28BN4O6F4S2Ag, Mr =
¯
727.30,Triclinic,Space group P1, a = 9.152(2), b = 11.197(2), c =
14.124(3) Š, a = 95.03(3), b = 93.37(3), g = 98.04(3)8, V=
1423.9(5) Š3,
T= 293 K,
Z = 2,
Dc = 1.696 gmÀ3
,
m =
0.928 mmÀ1,5826 reflections measured,5586 unique ( Rint
=
0.0343), F2 refinement, R1 = 0.0748 (I > 2s(I), wR2 = 0.1940 (I >
2s(I). CCDC-226717 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from
the Cambridge Crystallographic Data Centre,12 Union Road,
Cambridge CB21EZ,UK; fax: ( + 44)1223-336-033; or deposit@
ccdc.cam.ac.uk).
[19] D. L. Bryant,R. D. Noble,C. A. Koval, J. Membr. Sci. 1997, 127,
161 – 170.
[20] The module was a 47 mm membrane holder purchased from
Millipore and modified.
3056
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