Angewandte
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
[1] H. Jarvelin,J. R. Fair, Ind. Eng. Chem. Res. 1993, 32,2201 – 2207.
[2] S. L. Kohls,R. D. Noble,C. A. Koval, J. Membr. Sci. 1997, 125,
61 – 73.
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
[3] A. J. van Zyl,J. A. Kerres,W. Cui,M. Junginger, J. Membr. Sci.
1997, 137,173 – 185.
[4] W. Hu,K. Adachi,H. Matsumoto,A. Tanioka,
J. Chem. Soc.
Faraday Trans. 1998, 94,665 – 671.
[5] J. Y. Kim,S. U. Hong,H.-K. Kim,Y. S. Kang, J. Ind. Chem. Eng.
2002, 8,276 – 282.
[6] T. Yamaguchi,C. Baertsch,C. A. Koval,R. D. Noble,C. N.
Bowman, J. Membr. Sci. 1996, 117,151 – 161.
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.
Membr. Sci. 2003, 213,221 – 224.
[8] P. Wasserscheid,T. Welton, Ionic Liquidsin Syntheiss ,Wiley-
VCH,Weinheim, 2003.
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
2002, 114,2895 – 2897; Angew. Chem. Int. Ed. 2002, 41,2771 –
2773.
Angew. Chem. Int. Ed. 2004, 43, 3053 –3056
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3055