- Email: [email protected]
Transport of alkali metal cations through monoazacrown ether-modified NafionTM 117 membrane
j o u r n a l of MEMBRANE SCIENCE ELSEVIER
Journal of Membrane Science 116 (1996) 243-251
Transport of alkali metal cations through monoazacrown ether-modified Nation 117 membrane Takashi Hayashita l, Jong Chan Lee 2, Richard A. Bartsch * Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX 79409-1061, USA
Received 16 October 1995; revised 8 January 1996; accepted 30 January 1996
Abstract Conditions have been developed for the surface modification of Nafion TM 117 cation-exchange membrane by conversion to the sulfonyl chloride form and coupling with secondary amines to form sulfonamide groups. Nation 117 membranes modified with diethylamine (13), dibutylamine (14), morpholine (15), bis(2-methoxyethyl)amine (16), and 8-aza-2,5,11,14tetraoxapentadecane (17), monoaza-12-crown-5 (18), monoaza-15-crown-5 (19), and monoaza-18-crown-6 (20) were prepared and their competitive, proton-coupled transport of alkali metal cations was compared with that of unmodified Nation 117 membrane. Alkali metal cation permeation of membranes 13-16 was found to be poor. However, membranes 17-19 which were produced by coupling with an acyclic or cyclic polyether amine having more than three oxygen atoms gave efficient alkali metal cation permeation which was comparable with that for unmodified Nation 117 membrane. The permeation selectivity order for these surface-modified Nation 117 membranes was Cs+> Rb+> K + > Na÷> Li + which differs from the ordering of K +> Rb ÷> Cs +> Na +> Li + observed for unmodified Nation 117 membrane. Keywords: Nafion membrane; Chemical modification; Alkali metal cation transport
1. Introduction For selective metal ion permeation through a membrane to take place, desired metal ion species have to be preferentially sorbed or distributed into the membrane phase in the presence of other metal ion species. Cartier-mediated transport of metal ions in a liquid membrane system can provide selective
* Corresponding author. Tel.: (806) 742-3069; Fax: (806) 7421289; E-mail: [email protected]. 1 Present Address: Department of Chemistry, Saga University, 1 Honjo, Saga 840, Japan. 2 Present Address: Department of Chemistry, College of Sciences, Chung-Ang University, Seoul 156-756, South Korea.
metal ion permeation [1,2]. Molecules initially developed as extractants for solvent extraction are frequently utilized as carriers for metal ion separations in liquid membrane systems [3]. However, the instability of liquid membrane systems and slow transport rates pose serious problems for their practical applications in industry. If polymeric ion-exchange membranes with good metal ion permeation selectivity could be developed, efficient and stable metal ion separation processes would result [4]. To obtain new synthetic polymeric ionomer membranes for use in metal ion separations, chemical modification of a perfluorinated ionomer membrane material was examined in this study. An ion-cluster model for Nation TM perfluorinated membranes has
0376-7388/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved Pll S0376-7388(96)00045-2
T. Hayashita et al. / Journal of Membrane Science 116 (1996) 243-251
244
Source phase
Membrane phase
Receiving phase
2,
Experimental
2.1. Apparatus
@ %---
o OX Metal recognition site
O
O
Ion exchange site
: Alkali metal cations : Crown ethers
Fig. 1. Alkali metal cation separation by a crown ether-modified Nation membrane.
been developed by Gierke et al. [5]. According to this model, the polymeric ions and absorbed electrolyte phase are separated from the fluorocarbon backbone as approximately spherical clusters which are connected by narrow channels about 10 ,~ in length. This structure significantly affects the permeation selectivity of alkali metal cations [6]. Monoazacrown ethers, which are well known ionophores for alkali metal cations in solution [7], have now been introduced onto the surface of Nation 117 membrane (Fig. 1). The objective was to provide barrier layers or channel gates on each side of the membrane through which the metal ion permeation would be controlled by the identity of the ionophore molecule. The unaltered interior of the Nation membrane would give efficient metal ion transport between the barrier layers. Methods and conditions have been explored for conversion of the sulfonic acid groups of Nation membrane into sulfonyl chloride functions followed by reactions with monoazacrown ethers and some other secondary amines to attach them to the membrane by sulfonamide linkages. Proton-coupled, competitive transport of alkali metal cations by the modified Nation membranes has been compared with that of unmodified Nation membrane to assess changes in the permeation selectivity and efficiency.
1H NMR spectra were taken with an IBM AF-200 nuclear magnetic resonance spectrometer. IR spectra were measured with a Perkin-Elmer model 1600 or Nicolet MS-X infrared spectrophotometer. Concentrations of alkali-metal cations in the aqueous phases were determined with a Dionex model 2000i ion chromatograph, pH measurements were made with a Fisher Scientific Accumet model 825MP pH meter and a Coming 7605 glass body combination electrode. Elemental analysis was performed by Desert Analytics Laboratory (Tucson, AZ) and Galbraith Laboratories (Knoxville, TN).
2.2. Reagents Deionized water was prepared by passing distilled water through three Barnstead D8922 combinations cartridges in series. Nation 117 membrane was obtained from Aldrich Chemical Copany, Inc. LiOH, LiC1, NaC1, and KC1 were purchased from Fisher Scientific. CsC1 (99%) and RbC1 (99%) were obtained from Morton Thiokol, Inc. (Alfa Products). Stock aqueous solutions of 0.50 M alkali metal chlorides (NaC1, KC1, RbC1, and CsC1), 0.50 M LiC1, and 0.50 M LiOH were stored in polyethylene bottles. Sample solutions were prepared by mixing and diluting these stock solutions. Other inorganic and organic compounds were reagent grade commercial products and were used as received. Monoaza12-crown-4 [8], monoaza-15-crown-5 [8], monoaza18-crown-6 [8] and 1,11-dihydroxy-3,9-dioxa-6-(Ntosylaza)undecane [9] were prepared by the reported procedures.
2.2.1. 2, 5,11,14- Tetraoxa- 8- (tosylaza)pentadecane Under nitrogen, 1.28 g (32.0 mmol) of Nail (60% dispersion in mineral oil) was washed with dry pentane to remove the protecting mineral oil and 20 ml of THF was added. A solution of 1,11-dihydroxy3,9-dioxa-6-(N-tosylaza)undecane (2.77 g, 8.0 mmol) dissolved in 20 ml of dry THF was added to the suspension. After stirring for 1 h at room temperature, a solution of iodomethane (4.54 g, 32.0 mmol) dissolved in 50 ml of THF was added slowly. After
T. Hayashita et al. / Journal of Membrane Science 116 (1996) 243-251
,1
24 h, the solvent was removed in vacuo and the residue was dissolved in dichloromethane (100 ml), washed with 1 N aqueous NaOH, then water and dried over magnesium sulfate. Evaporation of the solvent in vacuo gave 2.44 g (81%) of a yellow oil. IR (neat); 1340, 1158 (802); 1158 ( C - O ) cm -~. 1H NMR (CDC13): 8 2.42 (s, 3 H), 3.21-3.67 (m, 22 H), 7.27-7.33 (m, 2 H) 7.65-7.73 (m, 2 H). Anal. Calcd. for C IvH29NO68: C, 54.38; H, 7.78%. Found: C, 54.37; H, 7.59%. 2.2.2. 8-Aza-2,5,11,14-tetraoxapentadecane Under nitrogen at room temperature, Na2HPO 4 (1.87 g, 13.2 mmol) and 6% sodium amalgam (14.40 g) were added to a solution of 2,5,11,14-tetraoxa-8(tosylaza)pentadecane (2.27 g, 6.0 mmol) in 200 ml of anhydrous dioxane-methanol (1:1). The mixture was refluxed for 2 days. The precipitate was filtered and washed with methanol (50 ml). After evaporation of the combined filtrate and washing in vacuo, chloroform (100 ml) was added to the residue. The mixture was filtered and the filtrate was evaporated in vacuo. The crude product was chromatographed on silica gel with ethyl acetate and then methanol as eluents to give 0.80 g (60%) of yellow liquid. IR (neat): 3504 (NH), 1199 ( C - O ) cm - l . ~H NMR (CDC13): 6 2.18 (s, NH), 3.41 (s, 6 H), 3.52-3.67 (m, 16 H). Anal. Calcd. for C10H23NO4: C, 54.27; H, 10.47%. Found: C, 53.88; H, 10.25%. 2.3. General procedure for modification of Nation 117 membrane
245
Pump Receiving phase Source phase Clamp Sificone washer Glass washer Membrane Stirrer
Fig. 2. Glass transportcell utilized for metal ion transportstudies.
dichloromethane and then immersed in distilled water. 2.4. Hydrolysis of Nation sulfonyl chloride membrane A 2" × 2" piece of the Nation sulfonyl chloride membrane was immersed in 150 ml of 5% aqueous sodium hydroxide and refluxed for 24 h. The mixture was allowed to cool to room temperature, the liquid was decanted and the membrane was washed thoroughly with distilled water. The membrane was immersed in 5% hydrochloric acid for 1 h at room temperature, then washed several times with distilled water and dried in vacuo for 1 day. 2.5. Alkali metal cation permeation experiments
A 2" × 2" piece of 0.007-inch thick Nafion 117 membrane was refluxed in a mixture of phosphorus pentachloride/phosphorus oxychloride (1:2 w / w ) for 3 h. The liquid was poured off while hot and carbon tetrachloride (100 ml) was added. Following a brief reflux, the carbon tetrachloride was decanted. Twice more fresh carbon tetrachloride was added, refluxed and decanted. To the resulting Nation sulfonyl chloride membrane, the secondary amine (2.0 equivalents), triethylamine (1.0 equivalent) and 120 ml of dry DMF were added and the mixture was heated at 50°C for 48 h. The mixture was allowed to cool to room temperature and the modified Nation membrane was removed, washed thoroughly with
The alkali metal cation transport experiments were conducted in the cylindrical glass transport cell shown in Fig. 2 [4]. The membrane was sandwiched between two flat silicone rubber washers which were in turn sandwiched between a glass washer and a flange on the bottom of the glass tube. A clamp and screws made of polycarbonate (to prevent contamination by metal ions) were used to hold the membrane assembly. The effective membrane area was 1.1 cm 2. The membrane assembly and lower portion of the tube containing the receiving solution (0.10 M aqueous hydrochloric acid) were immersed in the aqueous source solution (1.0 mM in each of the five alkali
246
7". Hayashita et a l . / Journal of Membrane Science 116 (1996) 243-251
metal cations, at pH 11.0). The receiving solution was stirred by the effluent from a peristaltic pump (15.6 m l / m i n ) and the source solution was stirred with a magnetic stirrer (200 rpm). Periodically, samples of the receiving solution were removed, diluted, and the alkali metal cation concentrations were determined by ion chromatography. For each membrane the alkali metal cation transport experiment was repeated at least twice. Concentrations of the alkali metal cation in the receiving phase were found to be reproducible within _+5% of the stated values.
5 K+ .,dRb"
4
f
ee~
.E >
//..acs + ~ Na÷ LI+
3
| "~
2
O
0
3. Results and discussion
2
4
6
8
Time, h
3.1. Alkali metal cation permeation through Nation 117 membrane
Fig. 3. Proton-driven permeation of alkali metal cations through Nation 117 membrane: (zx) Li +, ( © ) Na +, ( n ) K +, ( & ) Rb +,
( e ) Cs +.
The structure of Nation 117 is shown in Scheme 1. Based upon literature precedent [10,11], it should be possible to convert this Nation perfluorosulfonic acid membrane into the sulfonyl chloride form 1 followed by reaction with a monoazacrown ether to give the ionophoric sulfonamide 2. To evaluate the influence of the different chemical treatments on Nation 117 membrane, alkali metal cation permeation experiments were conducted. Although metal ion permeation through perfluorosulfonate membranes usually utilizes large electrochemical potentials to drive the transport [12], the permeation system utilized in this study employs proton-coupled transport. The mechanism involves transport of alkali metal cations from an aqueous source solution into
3.2. Chemical modification of Nafion 117 membrane
-(CFzCFz)n-(CF2~F)mOCF2(~FOCF2CF2-SO3H CF3 PCIs-POCI3 RfSO3H
:RfSO3H Nation 117 membrane
.
RfS02Cl 1
RfSO2CI
+
H-N
an aqueous receiving solution accompanied by back transport of protons from the receiving phase to the source phase, which is known as Donnan dialysis [13]. Results for the permeation of alkali metal cations through unmodified Nation 117 membrane are shown in Fig. 3. The initial concentration of each alkali metal cation species in the source phase was 1.0 mM and the alkali metal cation concentrations in the receiving phase were recorded as a function of time. After 7 h, the permeation selectivity order was found to be K + > R b + > C s + > N a + > Li + with limited overall selectivity.
O
O4n
~
RISOE--N
O
-°4o 2
Scheme 1. Preparation of crown ether-modified Nation membranes.
To determine the optimum conditions for the sulfonyl chloride-forming step, a series of experiments was first conducted in which Nation membrane (acid form) was converted into the sulfonyl chloride form and then hydrolyzed back to the sulfonic acid form. The procedure involved refluxing 2" × 2" pieces of Nation 117 membrane in a solution of phosphorus pentachloride-phosphorus oxychloride (1:2 w / w ) for varying periods of time. The chlorinating solution was decanted while hot and carbon tetrachloride was added. Following a brief reflux, the carbon tetrachloride was decanted. This
247
T. Hayashita et al. / Journal of Membrane Science 116 (1996) 243-251
Table 1 Conversion of Nation 117 membrane to the sulfonyl chloride form followed by hydrolysis Membrane
Reflux with PC15-POC13
RfSO2C1 form
(h)
Weight change a (%)
Appearance
3 4 5
24 4 1
- 2.4 + 3.2 + 5.2
flexible with white color flexible with white color stiffened
6
0
a Relative to the weight of the original RfSO3H form.
brief refluxing with fresh carbon tetrachloride was repeated twice more and then the membrane piece was dried in vacuo for 1 day. To rehydrolyze the sulfonyl chloride form, the membrane piece was refluxed for 24 h in 5% aqueous NaOH solution. After cooling, the liquid was decanted and the membrane was washed twice with distilled water. The membrane piece was immersed in 5% aqueous HC1 for 1 h at room temperature to convert the sodium sulfonate form into the sulfonic acid form. The membrane was washed several times with distilled water and dried in vacuo for 1 day. Results of these chemical treatments are recorded in Table 1. Alkali metal cation permeations for membranes 3, 4 and 5 are compared with that for untreated Nation membrane and for Nation membrane which was refluxed in 5 % N a O H for 24 h and then acidified with 5% HC1 (membrane 6) in Table 2. Each membrane was soaked in methanol for 12 h and then in deionized water for 12 h prior to evaluation. Compared with the original Nation membrane, membranes which had been refluxed in phosphorus pen-
tachloride-phosphorus oxychloride for 24 h (membrane 3) and 4 h (membrane 4) followed by hydrolysis gave very low alkali metal cation permeations. On the other hand for membrane 5 which was refluxed in the chlorinating solution for 1 h and then hydrolyzed, good metal ion permeation was observed and the transport selectivity order was the same as that found for the unmodified Nation membrane. Membrane 6, which was subjected to the hydrolysis procedure without chlorination, gave good metal cation permeation and a similar selectivity to that of unmodified Nation membrane. Thus the low metal ion permeations noted for membranes 3 and 4 appear to be attributable to the extended refluxing in the chlorinating mixture. The hydrolysis conditions utilized in this study are rather mild. If sulfonyl chloride formation took place deep within the membrane during extended reflux with the chlorinating solution, it might be difficult to regenerate the membrane in the sulfonic acid form. From these experiments, it was deduced that a short period of reflux with phosphorus t r i c h l o r i d e - p h o s p h o r u s o x y c h l o r i d e would be the best. Next the covalent attachment the ionophore monoaza-15-crown-5 to the Nation sulfonyl chloride membrane was examined. Pieces of Nation 117 membrane were refluxed with phosphorus pentachloride-phosphorus oxychloride (1:2, w / w ) for periods of 1 - 3 h followed by washing with carbon tetrachloride as described above and were dried for 1 day in vacuo. In all cases, there was a 3 - 5 % weight increase (Table 3). The sulfonyl chloride form was then heated with 2 equivalents of monoaza-15-crown 5 and 1 equivalent of triethylamine in DMF. (When the coupling with monoaza-15-crown-5 was conducted in refluxing THF, the resulting membrane
Table 2 Alkali metal cation transport across unmodified Nation 117 membrane and modified membranes 3-6 Permeation time (h)
Concentration in receiving phase (mmol/1) Li +
Na +
K+
Rb +
Cs +
7 24
3.2 0.0
3.8 0.1
4.4 0.1
4.2 0.0
3.9 0.0
4
15
0.0
0.3
0.2
0.0
0.0
5
15 15
4.7 5.9
6.3 7.7
6.5
6.5
6.4
8.4
8.5
8.3
Membrane UnmodifiedN~on 3
6
248
T. Hayashita et al. / Journal of Membrane Science 116 (1996) 243-251
Table 3 Chemical modification of Nafion 117 membrane with monoaza-15-crown-5 Membrane
7 8 9 10 11
Reflux with
RfSO2C1form
PC15-POCI 3 (h)
Weight change a (%)
Appearance
Time (h)
Temperature
Weight change c (%)
Appearance
1 2 3 3 3
+ 5.0 + 3.2 + 4.8 + 4.1 + 3.5
stiffened stiffened flexible flexible flexible
12 20 24 48 48 ~
reflux reflux reflux 50°C 50°C
- 24.0 - 8.2 - 7.2 + 6.2 - 6.4
swollen swollen badly distorted d d
Coupling b
Coupling product
a Relative to the weight of the original RfSO3H form. b With 2.0 equivalents of monoaza-15-crown-5 and 1.0 equivalent of triethylamine. c Relative to the weight of the RfSO2C1 form. d Swollen, but returned nearly to the original size after drying. e No monoaza-15-crown-5 or triethylamine present.
contained bubbles and appeared to be heterogeneous. Such heterogeneity was not apparent w h e n the coupling solvent was D M F . ) M e m b r a n e s 7, 8 and 9. which w e r e f o r m e d by coupling at reflux, were found to be swollen and r e m a i n e d so e v e n after drying. W h e n the temperature for c o u p l i n g was reduced to 50°C, m e m b r a n e 10 was p r o d u c e d w h i c h was initially swollen, but returned nearly to its original size after drying. Contrary to m e m b r a n e s 7, 8 and 9, a significant w e i g h t increase was o b s e r v e d for coupling with m o n o a z a - 1 5 - c r o w n - 5 to produce m e m b r a n e 10. For m e m b r a n e 11, the sulfonyl chloride f o r m was p r o d u c e d as before and was heated in D M F in the absence of m o n o a z a - 1 5 - c r o w n - 5 and triethylamine. A swollen m e m b r a n e was p r o d u c e d w h i c h returned nearly to the original size after drying. H o w e v e r a significant weight loss was noted instead o f the weight gain w h i c h was found with the coupled m e m brane 10. In another control experiment, a piece of N a t i o n
117 m e m b r a n e was swollen in D M F at 50°C for 4 h. Rinsing o f the m e m b r a n e with distilled water g a v e m e m b r a n e 12. Membranes 7-12 were washed with d i c h l o r o m e t h a n e and dried. Prior to evaluation of its alkali metal cation transport behavior, each m e m brane was i m m e r s e d in m e t h a n o l for 12 h and then in deionized water for 12 h. The alkali metal cation p e r m e a t i o n behaviors of m e m b r a n e s 7, 8, 10 and 12 are c o m p a r e d with that for u n m o d i f i e d N a t i o n 117 m e m b r a n e in Table 4. The m o d i f i e d N a f i o n m e m b r a n e s 7, 8 and 10 all g a v e alkali metal cation p e r m e a t i o n efficiencies similar to that of the u n m o d i f i e d N a t i o n 117 m e m b r a n e . The swollen m e m b r a n e s 7 and 8 were found to exhibit nearly the s a m e p e r m e a t i o n efficiency as did the better-formed m e m b r a n e 10. The permeation selectivity order for m e m b r a n e s 7, 8 and 10 is C s + > R b + > K + > N a + > Li ÷ w h i c h differs f r o m the ordering o f K ÷ > Rb ÷ > Cs ÷ > N a ÷ > Li + o b s e r v e d for the u n m o d i f i e d N a t i o n m e m b r a n e . M e m b r a n e 12
Table 4 Alkali metal cation transport across Nation 117 membrane and modified membranes 7, 8, 10, and 12 Membrane
Permeation time (h)
Concentration in receiving phase (mmol/1) Li +
Na +
K+
Rb ÷
Cs +
UnmodifiedNafion 7
7 7
3.2 2.1 2.2 2.0 4.0
3.8 2.8 3.0 2.8 5.5
4.4 3.7 3.9 4.3 6.9
4.2 3.9 4.1 4.6 6.6
3.9 4.0 4.4 5.0 6.7
8
7
10 12
7 7
T. Hayashita et al. / Journal of Membrane Science 116 (1996) 243-251
249
Et3N/DMF
exhibited somewhat higher alkali metal cation permeation than the unmodified Nation membrane, but the permeation selectivity order was the same for the two membranes. The difference in alkali metal cation transport selectivity orders for membrane 12 compared with membranes 7, 8 and 10 reveals that simple swelling in D M F does not cause the change in permeation selectivity order which is observed for the monoaza-15-crown-5-modified Nation membranes. Thus we have been successful in altering the metal ion permeation of Nafion 117 membrane by covalent attachment of monoaza-15-crown-5. Based upon this result, a coupling reaction of the Nation sulfonyl chloride membrane obtained from a 3-h chlorination reaction with a secondary amine for 48 h at 50°C was chosen as the optimum reaction conditions.
RIS02CI + H - N ~ I
H - N
,
RtSO~rNX.j~..~
14
=
RfSO~N
15
,"
RfSO~N /~.
/--k H-N
13
48 h, 50"=C RfSO~'-N
O
X.__/
/~,/OCH3
H-N X~OCH3
/0 /~.,-OCH3
16
OCH3
17
#o~ H-N
~o%
O
3.3. Effect o f secondary amine coupling agents upon alkali metal cation permeation
H
3
Under the optimized coupling conditions determined above, Nation 117 membrane was modified with monoazacrown ethers of different ring sizes. Membranes modified with diethylamine, dibutylamine, morpholine, bis(2-methoxyethyl)amine, and 8-aza-2,5,11,14-tetraoxapentadecane were also prepared to compare their permeation properties with those obtained for the monoazacrown ether-modified membranes (Scheme 2). After the coupling reactions, the resultant m e m b r a n e s were washed with dichloromethane and then immersed in distilled wa-
co o% H-N O oJ
•
R~O~N
~
R,so~-.N
/x
18
O
,,_oj
J
19
/-'-x
)
CO o% R~O~N O 20 03
Scheme 2. Structures of modified Nation membranes. ter. Membranes 1 3 - 2 0 had good physical appearance (transparent yellowish films). The modified membranes were evaluated for alkali metal cation permeation by the previously described transport experiment. The results are recorded in Table 5.
Table 5 Alkali metal cation transport across Nation 117 membraneand modified membranes 13-20 Membrane
Permeation time (h)
Concentrationin receivingphase (mmol/l) Li +
Na+
K+
Rb+
Cs +
UnmodifiedNafion 13
7 7
14
7
15 16 17
7 7 7
18
7
19 20
7 7
3.2 0.0 0.0 0.0 0.1 2.0 0.5 1.4 0.5
3.8 0.0 0.0 0.0 0.2 3.0 1.4 2.5 0.9
4.4 0.1 0.0 0.2 0.5 4.1 3.0 4.3 1.5
4.2 0.1 0.0 0.0 0.6 4.2 3.7 4.6 1.6
3.9 0.0 0.0 0.0 0.8 4.6 4.3 5.1 2.0
250
T. Hayashita et al. / Journal of Membrane Science 116 (1996) 243-251
6 Cs*
i
5
K+ Na* ~
1
,,
~
18
Ion diameter,A
Fig. 4. Metal ion concentration in the receiving phase after permeation for 7 hours vs. the ionic diameter of the alkali metal cations. Membranes: (r,) unmodified Nation 117, (©) 16, (11) 17, (A) 18, (0) 19 and (Ill) 20.
When diethylamine, dibutylamine and morpholine were coupled with the Nafion sulfonyl chloride form (membranes 13, 14, and 15), the alkali metal cation permeation was very low. However when oxygen atoms were present in the secondary amine unit, permeation efficiency improved markedly. The membranes produced by coupling with an acyclic polyether amine (17) or monoazacrown ethers (18-
20) having more than three ether oxygens gave efficient alkali metal cation permeation which is comparable with that observed for the modified Nafion 117 membrane. Membrane 16, which was prepared by coupling with an acyclic amine containing two ether oxygens, exhibited intermediate permeation behavior. These results suggests that the hydrophilicity of the channel site in the membrane plays an important role in the metal ion permeation. The permeation selectivities are compared in Fig. 4 in which the alkali metal cation concentrations in the receiving solution after permeation for 7 h are plotted against the metal ion diameter. The permeation selectivity orders for the monoazacrown ether-modified Nation membranes 17, 18 and 19 are C s + > R b + > K + > N a + > L i ÷ which differs from the ordering of K + > Rb + > Cs + > Na + > Li ÷ observed for unmodified Nation 117 membrane. In addition, the overall range of selectivities for the modified Nation membranes is greater than that noted for Nation itself. Introduction of monoazacrown ethers and secondary amines at or near the two surfaces of the Nation membrane would provide a neutral barrier layer for metal ion permeation. The present results strongly suggest that differences in the polarity and hydration degrees of these barrier layers influence the alkali metal cation transport selectivity.
0.10
0.08 o
,-
0.06
o .o
,,¢
0.04
,A\ //B-
0.02
0.00
|
i
1300
1200
r
1100
i
1000
W a v e n u m b e r , c m "1
Fig. 5. ATR-IR spectra of unmodified and modified Nation membranes. Membranes: (A) unmodified Nation 117, (B) 18 and (C) 19. TM
T. Hayashita et al. / Journal of Membrane Science 116 (1996) 243-251 3.4. FT-IR analysis o f crown ether-modified Nation membrane To further confirm the presence of monoazacrown ether units on the surface of the modified Nation membrane, FF-IR spectra of Nation 117, monoaza12-crown-4-modified Nation (18) and monoaza-15crown-5-modified Nation (19) membranes were determined by attenuated total reflectance (ATR) analysis. Fig. 5 shows the ATR F r - I R spectra for the three membranes. For the monoazacrown ether-modified membranes, there are significant decreases in the strength of the 1057 cm-1 absorption due to the symmetric S = O stretching vibration for the sulfonic acid group [10]. This would be expected if sulfonamide groups were present on the surface of the monoazacrown ether-modified Nation membranes and once again verifies covalent attachment of monoazacrown ethers onto the surface of the modified Nation membranes.
4. Conclusions In this study, reaction conditions for conversion of sulfonic acid groups in Nation 117 membrane to sulfonyl chloride functions followed by coupling with secondary amines have been determined. For proton-coupled transport, alkali metal cation permeation is effective when the modified Nation membranes were derived from acyclic or cyclic secondary amines which contain three or more ethereal oxygens. The altered permeation selectivities of the modified Nafion membranes reveal that we have been successful in changing the metal ion transport behavior of the ion-exchange membrane. Since the monoazacrown ether-modified Nation membranes exhibit good permeation characteristics, it appears that the ionophore molecules have been introduced at or near the two surfaces of the membrane to provide barrier layers which influence the permeation selectivity for alkali metal cations.
251
Acknowledgements This research was supported by a grant from the Texas Higher Education Coordinating Board - Advanced Research Program and a contract from The Dow Chemical Company.
References [1] T. Hayashita, R.A. Bartsch, T. Kurosawa and M. Igawa, Selective concentration of lead(II) chloride complex with liquid anion-exchangemembranes, Anal. Chem., 63 (1991) 1024. [2] R.A. Bartsch, W.A. Charewicz, S.I. Kang, and W. Walkowiak, Proton-coupledtransport of alkali metal cations across liquid membranes by ionizable crown ethers, ACS Symp. Ser., 347 (987) 86-97. [3] T. Araki and H. Tsukube (Eds.), Liquid Membranes, CRC Press, Boca Raton, FL, 1990. [4] T. Hayashita, T. Kurosawa, S. Ohya, T. Komatsu and M. Igawa, Extractive permeation of zinc(II) and cadmium(II) chloride complexes through anion-exchangemembrane,Bull. Chem. Soc. Jpn., 63 (1990) 576. [5] T.D. Gierke and W.Y. Hsu, The cluster-networkmodel of ion clustering,ACS Syrup. Ser., 180 (1982) 283-307. [6] H. Yeager, Transport properties of perfluorosulfonatepolymer membranes,ACS Symp. Ser., 180 (1982) 41-63. [7] G.W. Gokel (Ed.), Crown Ethers and Cryptands, The Royal Society of Chemistry, Cambridge, 1991. [8] R.A. Schultz, B.D. White, D.M. Disbong, K.A. Arnold and G.W. Gokel, 12-, 15-, and 18-Membered-ringnitrogen-pivot lariat ethers: synthesis, properties, and sodium and ammonium cation binding properties, J. Am. Chem. Soc., 107 (1985) 6659. [9] P.L. Anelli, F. Montanari, S. Quici, G. Ciani and A. Sironi, Lipophilic cage ligands containing two tightly connected 1,7-dioxa-4,10-diazacyclodecanerings: synthesis and X-ray structure of a sodium perchlorate complex, J. Org. Chem., 53 (1988) 5292. [10] M.J. Covitch, S.R. Lowry, C.L. Gray and B. Blackford, in A.R. Cooper(Ed.), PolymerScienceand Technology,Plenum Press, New York, 1982, pp. 257-267. [11] S. Yokoyama,F. Muranakaand K. Kimoto,Chlorosulfonylation of perfluorocarboncation-exchangeresins for brine electrolytic cells, Jpn. Pat., 1977, 77,134,888. [12] D.S. Flett (Ed.), Ion Exchange Membranes,Society of Chemical Industry,London, 1983. [13] T. Ktari and B. Auclair, Simplifiedflux equation in Donnan dialysis, J. MembraneSci., 32 (1987) 251.
Journal of Membrane Science 116 (1996) 243-251
Transport of alkali metal cations through monoazacrown ether-modified Nation 117 membrane Takashi Hayashita l, Jong Chan Lee 2, Richard A. Bartsch * Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX 79409-1061, USA
Received 16 October 1995; revised 8 January 1996; accepted 30 January 1996
Abstract Conditions have been developed for the surface modification of Nafion TM 117 cation-exchange membrane by conversion to the sulfonyl chloride form and coupling with secondary amines to form sulfonamide groups. Nation 117 membranes modified with diethylamine (13), dibutylamine (14), morpholine (15), bis(2-methoxyethyl)amine (16), and 8-aza-2,5,11,14tetraoxapentadecane (17), monoaza-12-crown-5 (18), monoaza-15-crown-5 (19), and monoaza-18-crown-6 (20) were prepared and their competitive, proton-coupled transport of alkali metal cations was compared with that of unmodified Nation 117 membrane. Alkali metal cation permeation of membranes 13-16 was found to be poor. However, membranes 17-19 which were produced by coupling with an acyclic or cyclic polyether amine having more than three oxygen atoms gave efficient alkali metal cation permeation which was comparable with that for unmodified Nation 117 membrane. The permeation selectivity order for these surface-modified Nation 117 membranes was Cs+> Rb+> K + > Na÷> Li + which differs from the ordering of K +> Rb ÷> Cs +> Na +> Li + observed for unmodified Nation 117 membrane. Keywords: Nafion membrane; Chemical modification; Alkali metal cation transport
1. Introduction For selective metal ion permeation through a membrane to take place, desired metal ion species have to be preferentially sorbed or distributed into the membrane phase in the presence of other metal ion species. Cartier-mediated transport of metal ions in a liquid membrane system can provide selective
* Corresponding author. Tel.: (806) 742-3069; Fax: (806) 7421289; E-mail: [email protected]. 1 Present Address: Department of Chemistry, Saga University, 1 Honjo, Saga 840, Japan. 2 Present Address: Department of Chemistry, College of Sciences, Chung-Ang University, Seoul 156-756, South Korea.
metal ion permeation [1,2]. Molecules initially developed as extractants for solvent extraction are frequently utilized as carriers for metal ion separations in liquid membrane systems [3]. However, the instability of liquid membrane systems and slow transport rates pose serious problems for their practical applications in industry. If polymeric ion-exchange membranes with good metal ion permeation selectivity could be developed, efficient and stable metal ion separation processes would result [4]. To obtain new synthetic polymeric ionomer membranes for use in metal ion separations, chemical modification of a perfluorinated ionomer membrane material was examined in this study. An ion-cluster model for Nation TM perfluorinated membranes has
0376-7388/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved Pll S0376-7388(96)00045-2
T. Hayashita et al. / Journal of Membrane Science 116 (1996) 243-251
244
Source phase
Membrane phase
Receiving phase
2,
Experimental
2.1. Apparatus
@ %---
o OX Metal recognition site
O
O
Ion exchange site
: Alkali metal cations : Crown ethers
Fig. 1. Alkali metal cation separation by a crown ether-modified Nation membrane.
been developed by Gierke et al. [5]. According to this model, the polymeric ions and absorbed electrolyte phase are separated from the fluorocarbon backbone as approximately spherical clusters which are connected by narrow channels about 10 ,~ in length. This structure significantly affects the permeation selectivity of alkali metal cations [6]. Monoazacrown ethers, which are well known ionophores for alkali metal cations in solution [7], have now been introduced onto the surface of Nation 117 membrane (Fig. 1). The objective was to provide barrier layers or channel gates on each side of the membrane through which the metal ion permeation would be controlled by the identity of the ionophore molecule. The unaltered interior of the Nation membrane would give efficient metal ion transport between the barrier layers. Methods and conditions have been explored for conversion of the sulfonic acid groups of Nation membrane into sulfonyl chloride functions followed by reactions with monoazacrown ethers and some other secondary amines to attach them to the membrane by sulfonamide linkages. Proton-coupled, competitive transport of alkali metal cations by the modified Nation membranes has been compared with that of unmodified Nation membrane to assess changes in the permeation selectivity and efficiency.
1H NMR spectra were taken with an IBM AF-200 nuclear magnetic resonance spectrometer. IR spectra were measured with a Perkin-Elmer model 1600 or Nicolet MS-X infrared spectrophotometer. Concentrations of alkali-metal cations in the aqueous phases were determined with a Dionex model 2000i ion chromatograph, pH measurements were made with a Fisher Scientific Accumet model 825MP pH meter and a Coming 7605 glass body combination electrode. Elemental analysis was performed by Desert Analytics Laboratory (Tucson, AZ) and Galbraith Laboratories (Knoxville, TN).
2.2. Reagents Deionized water was prepared by passing distilled water through three Barnstead D8922 combinations cartridges in series. Nation 117 membrane was obtained from Aldrich Chemical Copany, Inc. LiOH, LiC1, NaC1, and KC1 were purchased from Fisher Scientific. CsC1 (99%) and RbC1 (99%) were obtained from Morton Thiokol, Inc. (Alfa Products). Stock aqueous solutions of 0.50 M alkali metal chlorides (NaC1, KC1, RbC1, and CsC1), 0.50 M LiC1, and 0.50 M LiOH were stored in polyethylene bottles. Sample solutions were prepared by mixing and diluting these stock solutions. Other inorganic and organic compounds were reagent grade commercial products and were used as received. Monoaza12-crown-4 [8], monoaza-15-crown-5 [8], monoaza18-crown-6 [8] and 1,11-dihydroxy-3,9-dioxa-6-(Ntosylaza)undecane [9] were prepared by the reported procedures.
2.2.1. 2, 5,11,14- Tetraoxa- 8- (tosylaza)pentadecane Under nitrogen, 1.28 g (32.0 mmol) of Nail (60% dispersion in mineral oil) was washed with dry pentane to remove the protecting mineral oil and 20 ml of THF was added. A solution of 1,11-dihydroxy3,9-dioxa-6-(N-tosylaza)undecane (2.77 g, 8.0 mmol) dissolved in 20 ml of dry THF was added to the suspension. After stirring for 1 h at room temperature, a solution of iodomethane (4.54 g, 32.0 mmol) dissolved in 50 ml of THF was added slowly. After
T. Hayashita et al. / Journal of Membrane Science 116 (1996) 243-251
,1
24 h, the solvent was removed in vacuo and the residue was dissolved in dichloromethane (100 ml), washed with 1 N aqueous NaOH, then water and dried over magnesium sulfate. Evaporation of the solvent in vacuo gave 2.44 g (81%) of a yellow oil. IR (neat); 1340, 1158 (802); 1158 ( C - O ) cm -~. 1H NMR (CDC13): 8 2.42 (s, 3 H), 3.21-3.67 (m, 22 H), 7.27-7.33 (m, 2 H) 7.65-7.73 (m, 2 H). Anal. Calcd. for C IvH29NO68: C, 54.38; H, 7.78%. Found: C, 54.37; H, 7.59%. 2.2.2. 8-Aza-2,5,11,14-tetraoxapentadecane Under nitrogen at room temperature, Na2HPO 4 (1.87 g, 13.2 mmol) and 6% sodium amalgam (14.40 g) were added to a solution of 2,5,11,14-tetraoxa-8(tosylaza)pentadecane (2.27 g, 6.0 mmol) in 200 ml of anhydrous dioxane-methanol (1:1). The mixture was refluxed for 2 days. The precipitate was filtered and washed with methanol (50 ml). After evaporation of the combined filtrate and washing in vacuo, chloroform (100 ml) was added to the residue. The mixture was filtered and the filtrate was evaporated in vacuo. The crude product was chromatographed on silica gel with ethyl acetate and then methanol as eluents to give 0.80 g (60%) of yellow liquid. IR (neat): 3504 (NH), 1199 ( C - O ) cm - l . ~H NMR (CDC13): 6 2.18 (s, NH), 3.41 (s, 6 H), 3.52-3.67 (m, 16 H). Anal. Calcd. for C10H23NO4: C, 54.27; H, 10.47%. Found: C, 53.88; H, 10.25%. 2.3. General procedure for modification of Nation 117 membrane
245
Pump Receiving phase Source phase Clamp Sificone washer Glass washer Membrane Stirrer
Fig. 2. Glass transportcell utilized for metal ion transportstudies.
dichloromethane and then immersed in distilled water. 2.4. Hydrolysis of Nation sulfonyl chloride membrane A 2" × 2" piece of the Nation sulfonyl chloride membrane was immersed in 150 ml of 5% aqueous sodium hydroxide and refluxed for 24 h. The mixture was allowed to cool to room temperature, the liquid was decanted and the membrane was washed thoroughly with distilled water. The membrane was immersed in 5% hydrochloric acid for 1 h at room temperature, then washed several times with distilled water and dried in vacuo for 1 day. 2.5. Alkali metal cation permeation experiments
A 2" × 2" piece of 0.007-inch thick Nafion 117 membrane was refluxed in a mixture of phosphorus pentachloride/phosphorus oxychloride (1:2 w / w ) for 3 h. The liquid was poured off while hot and carbon tetrachloride (100 ml) was added. Following a brief reflux, the carbon tetrachloride was decanted. Twice more fresh carbon tetrachloride was added, refluxed and decanted. To the resulting Nation sulfonyl chloride membrane, the secondary amine (2.0 equivalents), triethylamine (1.0 equivalent) and 120 ml of dry DMF were added and the mixture was heated at 50°C for 48 h. The mixture was allowed to cool to room temperature and the modified Nation membrane was removed, washed thoroughly with
The alkali metal cation transport experiments were conducted in the cylindrical glass transport cell shown in Fig. 2 [4]. The membrane was sandwiched between two flat silicone rubber washers which were in turn sandwiched between a glass washer and a flange on the bottom of the glass tube. A clamp and screws made of polycarbonate (to prevent contamination by metal ions) were used to hold the membrane assembly. The effective membrane area was 1.1 cm 2. The membrane assembly and lower portion of the tube containing the receiving solution (0.10 M aqueous hydrochloric acid) were immersed in the aqueous source solution (1.0 mM in each of the five alkali
246
7". Hayashita et a l . / Journal of Membrane Science 116 (1996) 243-251
metal cations, at pH 11.0). The receiving solution was stirred by the effluent from a peristaltic pump (15.6 m l / m i n ) and the source solution was stirred with a magnetic stirrer (200 rpm). Periodically, samples of the receiving solution were removed, diluted, and the alkali metal cation concentrations were determined by ion chromatography. For each membrane the alkali metal cation transport experiment was repeated at least twice. Concentrations of the alkali metal cation in the receiving phase were found to be reproducible within _+5% of the stated values.
5 K+ .,dRb"
4
f
ee~
.E >
//..acs + ~ Na÷ LI+
3
| "~
2
O
0
3. Results and discussion
2
4
6
8
Time, h
3.1. Alkali metal cation permeation through Nation 117 membrane
Fig. 3. Proton-driven permeation of alkali metal cations through Nation 117 membrane: (zx) Li +, ( © ) Na +, ( n ) K +, ( & ) Rb +,
( e ) Cs +.
The structure of Nation 117 is shown in Scheme 1. Based upon literature precedent [10,11], it should be possible to convert this Nation perfluorosulfonic acid membrane into the sulfonyl chloride form 1 followed by reaction with a monoazacrown ether to give the ionophoric sulfonamide 2. To evaluate the influence of the different chemical treatments on Nation 117 membrane, alkali metal cation permeation experiments were conducted. Although metal ion permeation through perfluorosulfonate membranes usually utilizes large electrochemical potentials to drive the transport [12], the permeation system utilized in this study employs proton-coupled transport. The mechanism involves transport of alkali metal cations from an aqueous source solution into
3.2. Chemical modification of Nafion 117 membrane
-(CFzCFz)n-(CF2~F)mOCF2(~FOCF2CF2-SO3H CF3 PCIs-POCI3 RfSO3H
:RfSO3H Nation 117 membrane
.
RfS02Cl 1
RfSO2CI
+
H-N
an aqueous receiving solution accompanied by back transport of protons from the receiving phase to the source phase, which is known as Donnan dialysis [13]. Results for the permeation of alkali metal cations through unmodified Nation 117 membrane are shown in Fig. 3. The initial concentration of each alkali metal cation species in the source phase was 1.0 mM and the alkali metal cation concentrations in the receiving phase were recorded as a function of time. After 7 h, the permeation selectivity order was found to be K + > R b + > C s + > N a + > Li + with limited overall selectivity.
O
O4n
~
RISOE--N
O
-°4o 2
Scheme 1. Preparation of crown ether-modified Nation membranes.
To determine the optimum conditions for the sulfonyl chloride-forming step, a series of experiments was first conducted in which Nation membrane (acid form) was converted into the sulfonyl chloride form and then hydrolyzed back to the sulfonic acid form. The procedure involved refluxing 2" × 2" pieces of Nation 117 membrane in a solution of phosphorus pentachloride-phosphorus oxychloride (1:2 w / w ) for varying periods of time. The chlorinating solution was decanted while hot and carbon tetrachloride was added. Following a brief reflux, the carbon tetrachloride was decanted. This
247
T. Hayashita et al. / Journal of Membrane Science 116 (1996) 243-251
Table 1 Conversion of Nation 117 membrane to the sulfonyl chloride form followed by hydrolysis Membrane
Reflux with PC15-POC13
RfSO2C1 form
(h)
Weight change a (%)
Appearance
3 4 5
24 4 1
- 2.4 + 3.2 + 5.2
flexible with white color flexible with white color stiffened
6
0
a Relative to the weight of the original RfSO3H form.
brief refluxing with fresh carbon tetrachloride was repeated twice more and then the membrane piece was dried in vacuo for 1 day. To rehydrolyze the sulfonyl chloride form, the membrane piece was refluxed for 24 h in 5% aqueous NaOH solution. After cooling, the liquid was decanted and the membrane was washed twice with distilled water. The membrane piece was immersed in 5% aqueous HC1 for 1 h at room temperature to convert the sodium sulfonate form into the sulfonic acid form. The membrane was washed several times with distilled water and dried in vacuo for 1 day. Results of these chemical treatments are recorded in Table 1. Alkali metal cation permeations for membranes 3, 4 and 5 are compared with that for untreated Nation membrane and for Nation membrane which was refluxed in 5 % N a O H for 24 h and then acidified with 5% HC1 (membrane 6) in Table 2. Each membrane was soaked in methanol for 12 h and then in deionized water for 12 h prior to evaluation. Compared with the original Nation membrane, membranes which had been refluxed in phosphorus pen-
tachloride-phosphorus oxychloride for 24 h (membrane 3) and 4 h (membrane 4) followed by hydrolysis gave very low alkali metal cation permeations. On the other hand for membrane 5 which was refluxed in the chlorinating solution for 1 h and then hydrolyzed, good metal ion permeation was observed and the transport selectivity order was the same as that found for the unmodified Nation membrane. Membrane 6, which was subjected to the hydrolysis procedure without chlorination, gave good metal cation permeation and a similar selectivity to that of unmodified Nation membrane. Thus the low metal ion permeations noted for membranes 3 and 4 appear to be attributable to the extended refluxing in the chlorinating mixture. The hydrolysis conditions utilized in this study are rather mild. If sulfonyl chloride formation took place deep within the membrane during extended reflux with the chlorinating solution, it might be difficult to regenerate the membrane in the sulfonic acid form. From these experiments, it was deduced that a short period of reflux with phosphorus t r i c h l o r i d e - p h o s p h o r u s o x y c h l o r i d e would be the best. Next the covalent attachment the ionophore monoaza-15-crown-5 to the Nation sulfonyl chloride membrane was examined. Pieces of Nation 117 membrane were refluxed with phosphorus pentachloride-phosphorus oxychloride (1:2, w / w ) for periods of 1 - 3 h followed by washing with carbon tetrachloride as described above and were dried for 1 day in vacuo. In all cases, there was a 3 - 5 % weight increase (Table 3). The sulfonyl chloride form was then heated with 2 equivalents of monoaza-15-crown 5 and 1 equivalent of triethylamine in DMF. (When the coupling with monoaza-15-crown-5 was conducted in refluxing THF, the resulting membrane
Table 2 Alkali metal cation transport across unmodified Nation 117 membrane and modified membranes 3-6 Permeation time (h)
Concentration in receiving phase (mmol/1) Li +
Na +
K+
Rb +
Cs +
7 24
3.2 0.0
3.8 0.1
4.4 0.1
4.2 0.0
3.9 0.0
4
15
0.0
0.3
0.2
0.0
0.0
5
15 15
4.7 5.9
6.3 7.7
6.5
6.5
6.4
8.4
8.5
8.3
Membrane UnmodifiedN~on 3
6
248
T. Hayashita et al. / Journal of Membrane Science 116 (1996) 243-251
Table 3 Chemical modification of Nafion 117 membrane with monoaza-15-crown-5 Membrane
7 8 9 10 11
Reflux with
RfSO2C1form
PC15-POCI 3 (h)
Weight change a (%)
Appearance
Time (h)
Temperature
Weight change c (%)
Appearance
1 2 3 3 3
+ 5.0 + 3.2 + 4.8 + 4.1 + 3.5
stiffened stiffened flexible flexible flexible
12 20 24 48 48 ~
reflux reflux reflux 50°C 50°C
- 24.0 - 8.2 - 7.2 + 6.2 - 6.4
swollen swollen badly distorted d d
Coupling b
Coupling product
a Relative to the weight of the original RfSO3H form. b With 2.0 equivalents of monoaza-15-crown-5 and 1.0 equivalent of triethylamine. c Relative to the weight of the RfSO2C1 form. d Swollen, but returned nearly to the original size after drying. e No monoaza-15-crown-5 or triethylamine present.
contained bubbles and appeared to be heterogeneous. Such heterogeneity was not apparent w h e n the coupling solvent was D M F . ) M e m b r a n e s 7, 8 and 9. which w e r e f o r m e d by coupling at reflux, were found to be swollen and r e m a i n e d so e v e n after drying. W h e n the temperature for c o u p l i n g was reduced to 50°C, m e m b r a n e 10 was p r o d u c e d w h i c h was initially swollen, but returned nearly to its original size after drying. Contrary to m e m b r a n e s 7, 8 and 9, a significant w e i g h t increase was o b s e r v e d for coupling with m o n o a z a - 1 5 - c r o w n - 5 to produce m e m b r a n e 10. For m e m b r a n e 11, the sulfonyl chloride f o r m was p r o d u c e d as before and was heated in D M F in the absence of m o n o a z a - 1 5 - c r o w n - 5 and triethylamine. A swollen m e m b r a n e was p r o d u c e d w h i c h returned nearly to the original size after drying. H o w e v e r a significant weight loss was noted instead o f the weight gain w h i c h was found with the coupled m e m brane 10. In another control experiment, a piece of N a t i o n
117 m e m b r a n e was swollen in D M F at 50°C for 4 h. Rinsing o f the m e m b r a n e with distilled water g a v e m e m b r a n e 12. Membranes 7-12 were washed with d i c h l o r o m e t h a n e and dried. Prior to evaluation of its alkali metal cation transport behavior, each m e m brane was i m m e r s e d in m e t h a n o l for 12 h and then in deionized water for 12 h. The alkali metal cation p e r m e a t i o n behaviors of m e m b r a n e s 7, 8, 10 and 12 are c o m p a r e d with that for u n m o d i f i e d N a t i o n 117 m e m b r a n e in Table 4. The m o d i f i e d N a f i o n m e m b r a n e s 7, 8 and 10 all g a v e alkali metal cation p e r m e a t i o n efficiencies similar to that of the u n m o d i f i e d N a t i o n 117 m e m b r a n e . The swollen m e m b r a n e s 7 and 8 were found to exhibit nearly the s a m e p e r m e a t i o n efficiency as did the better-formed m e m b r a n e 10. The permeation selectivity order for m e m b r a n e s 7, 8 and 10 is C s + > R b + > K + > N a + > Li ÷ w h i c h differs f r o m the ordering o f K ÷ > Rb ÷ > Cs ÷ > N a ÷ > Li + o b s e r v e d for the u n m o d i f i e d N a t i o n m e m b r a n e . M e m b r a n e 12
Table 4 Alkali metal cation transport across Nation 117 membrane and modified membranes 7, 8, 10, and 12 Membrane
Permeation time (h)
Concentration in receiving phase (mmol/1) Li +
Na +
K+
Rb ÷
Cs +
UnmodifiedNafion 7
7 7
3.2 2.1 2.2 2.0 4.0
3.8 2.8 3.0 2.8 5.5
4.4 3.7 3.9 4.3 6.9
4.2 3.9 4.1 4.6 6.6
3.9 4.0 4.4 5.0 6.7
8
7
10 12
7 7
T. Hayashita et al. / Journal of Membrane Science 116 (1996) 243-251
249
Et3N/DMF
exhibited somewhat higher alkali metal cation permeation than the unmodified Nation membrane, but the permeation selectivity order was the same for the two membranes. The difference in alkali metal cation transport selectivity orders for membrane 12 compared with membranes 7, 8 and 10 reveals that simple swelling in D M F does not cause the change in permeation selectivity order which is observed for the monoaza-15-crown-5-modified Nation membranes. Thus we have been successful in altering the metal ion permeation of Nafion 117 membrane by covalent attachment of monoaza-15-crown-5. Based upon this result, a coupling reaction of the Nation sulfonyl chloride membrane obtained from a 3-h chlorination reaction with a secondary amine for 48 h at 50°C was chosen as the optimum reaction conditions.
RIS02CI + H - N ~ I
H - N
,
RtSO~rNX.j~..~
14
=
RfSO~N
15
,"
RfSO~N /~.
/--k H-N
13
48 h, 50"=C RfSO~'-N
O
X.__/
/~,/OCH3
H-N X~OCH3
/0 /~.,-OCH3
16
OCH3
17
#o~ H-N
~o%
O
3.3. Effect o f secondary amine coupling agents upon alkali metal cation permeation
H
3
Under the optimized coupling conditions determined above, Nation 117 membrane was modified with monoazacrown ethers of different ring sizes. Membranes modified with diethylamine, dibutylamine, morpholine, bis(2-methoxyethyl)amine, and 8-aza-2,5,11,14-tetraoxapentadecane were also prepared to compare their permeation properties with those obtained for the monoazacrown ether-modified membranes (Scheme 2). After the coupling reactions, the resultant m e m b r a n e s were washed with dichloromethane and then immersed in distilled wa-
co o% H-N O oJ
•
R~O~N
~
R,so~-.N
/x
18
O
,,_oj
J
19
/-'-x
)
CO o% R~O~N O 20 03
Scheme 2. Structures of modified Nation membranes. ter. Membranes 1 3 - 2 0 had good physical appearance (transparent yellowish films). The modified membranes were evaluated for alkali metal cation permeation by the previously described transport experiment. The results are recorded in Table 5.
Table 5 Alkali metal cation transport across Nation 117 membraneand modified membranes 13-20 Membrane
Permeation time (h)
Concentrationin receivingphase (mmol/l) Li +
Na+
K+
Rb+
Cs +
UnmodifiedNafion 13
7 7
14
7
15 16 17
7 7 7
18
7
19 20
7 7
3.2 0.0 0.0 0.0 0.1 2.0 0.5 1.4 0.5
3.8 0.0 0.0 0.0 0.2 3.0 1.4 2.5 0.9
4.4 0.1 0.0 0.2 0.5 4.1 3.0 4.3 1.5
4.2 0.1 0.0 0.0 0.6 4.2 3.7 4.6 1.6
3.9 0.0 0.0 0.0 0.8 4.6 4.3 5.1 2.0
250
T. Hayashita et al. / Journal of Membrane Science 116 (1996) 243-251
6 Cs*
i
5
K+ Na* ~
1
,,
~
18
Ion diameter,A
Fig. 4. Metal ion concentration in the receiving phase after permeation for 7 hours vs. the ionic diameter of the alkali metal cations. Membranes: (r,) unmodified Nation 117, (©) 16, (11) 17, (A) 18, (0) 19 and (Ill) 20.
When diethylamine, dibutylamine and morpholine were coupled with the Nafion sulfonyl chloride form (membranes 13, 14, and 15), the alkali metal cation permeation was very low. However when oxygen atoms were present in the secondary amine unit, permeation efficiency improved markedly. The membranes produced by coupling with an acyclic polyether amine (17) or monoazacrown ethers (18-
20) having more than three ether oxygens gave efficient alkali metal cation permeation which is comparable with that observed for the modified Nafion 117 membrane. Membrane 16, which was prepared by coupling with an acyclic amine containing two ether oxygens, exhibited intermediate permeation behavior. These results suggests that the hydrophilicity of the channel site in the membrane plays an important role in the metal ion permeation. The permeation selectivities are compared in Fig. 4 in which the alkali metal cation concentrations in the receiving solution after permeation for 7 h are plotted against the metal ion diameter. The permeation selectivity orders for the monoazacrown ether-modified Nation membranes 17, 18 and 19 are C s + > R b + > K + > N a + > L i ÷ which differs from the ordering of K + > Rb + > Cs + > Na + > Li ÷ observed for unmodified Nation 117 membrane. In addition, the overall range of selectivities for the modified Nation membranes is greater than that noted for Nation itself. Introduction of monoazacrown ethers and secondary amines at or near the two surfaces of the Nation membrane would provide a neutral barrier layer for metal ion permeation. The present results strongly suggest that differences in the polarity and hydration degrees of these barrier layers influence the alkali metal cation transport selectivity.
0.10
0.08 o
,-
0.06
o .o
,,¢
0.04
,A\ //B-
0.02
0.00
|
i
1300
1200
r
1100
i
1000
W a v e n u m b e r , c m "1
Fig. 5. ATR-IR spectra of unmodified and modified Nation membranes. Membranes: (A) unmodified Nation 117, (B) 18 and (C) 19. TM
T. Hayashita et al. / Journal of Membrane Science 116 (1996) 243-251 3.4. FT-IR analysis o f crown ether-modified Nation membrane To further confirm the presence of monoazacrown ether units on the surface of the modified Nation membrane, FF-IR spectra of Nation 117, monoaza12-crown-4-modified Nation (18) and monoaza-15crown-5-modified Nation (19) membranes were determined by attenuated total reflectance (ATR) analysis. Fig. 5 shows the ATR F r - I R spectra for the three membranes. For the monoazacrown ether-modified membranes, there are significant decreases in the strength of the 1057 cm-1 absorption due to the symmetric S = O stretching vibration for the sulfonic acid group [10]. This would be expected if sulfonamide groups were present on the surface of the monoazacrown ether-modified Nation membranes and once again verifies covalent attachment of monoazacrown ethers onto the surface of the modified Nation membranes.
4. Conclusions In this study, reaction conditions for conversion of sulfonic acid groups in Nation 117 membrane to sulfonyl chloride functions followed by coupling with secondary amines have been determined. For proton-coupled transport, alkali metal cation permeation is effective when the modified Nation membranes were derived from acyclic or cyclic secondary amines which contain three or more ethereal oxygens. The altered permeation selectivities of the modified Nafion membranes reveal that we have been successful in changing the metal ion transport behavior of the ion-exchange membrane. Since the monoazacrown ether-modified Nation membranes exhibit good permeation characteristics, it appears that the ionophore molecules have been introduced at or near the two surfaces of the membrane to provide barrier layers which influence the permeation selectivity for alkali metal cations.
251
Acknowledgements This research was supported by a grant from the Texas Higher Education Coordinating Board - Advanced Research Program and a contract from The Dow Chemical Company.
References [1] T. Hayashita, R.A. Bartsch, T. Kurosawa and M. Igawa, Selective concentration of lead(II) chloride complex with liquid anion-exchangemembranes, Anal. Chem., 63 (1991) 1024. [2] R.A. Bartsch, W.A. Charewicz, S.I. Kang, and W. Walkowiak, Proton-coupledtransport of alkali metal cations across liquid membranes by ionizable crown ethers, ACS Symp. Ser., 347 (987) 86-97. [3] T. Araki and H. Tsukube (Eds.), Liquid Membranes, CRC Press, Boca Raton, FL, 1990. [4] T. Hayashita, T. Kurosawa, S. Ohya, T. Komatsu and M. Igawa, Extractive permeation of zinc(II) and cadmium(II) chloride complexes through anion-exchangemembrane,Bull. Chem. Soc. Jpn., 63 (1990) 576. [5] T.D. Gierke and W.Y. Hsu, The cluster-networkmodel of ion clustering,ACS Syrup. Ser., 180 (1982) 283-307. [6] H. Yeager, Transport properties of perfluorosulfonatepolymer membranes,ACS Symp. Ser., 180 (1982) 41-63. [7] G.W. Gokel (Ed.), Crown Ethers and Cryptands, The Royal Society of Chemistry, Cambridge, 1991. [8] R.A. Schultz, B.D. White, D.M. Disbong, K.A. Arnold and G.W. Gokel, 12-, 15-, and 18-Membered-ringnitrogen-pivot lariat ethers: synthesis, properties, and sodium and ammonium cation binding properties, J. Am. Chem. Soc., 107 (1985) 6659. [9] P.L. Anelli, F. Montanari, S. Quici, G. Ciani and A. Sironi, Lipophilic cage ligands containing two tightly connected 1,7-dioxa-4,10-diazacyclodecanerings: synthesis and X-ray structure of a sodium perchlorate complex, J. Org. Chem., 53 (1988) 5292. [10] M.J. Covitch, S.R. Lowry, C.L. Gray and B. Blackford, in A.R. Cooper(Ed.), PolymerScienceand Technology,Plenum Press, New York, 1982, pp. 257-267. [11] S. Yokoyama,F. Muranakaand K. Kimoto,Chlorosulfonylation of perfluorocarboncation-exchangeresins for brine electrolytic cells, Jpn. Pat., 1977, 77,134,888. [12] D.S. Flett (Ed.), Ion Exchange Membranes,Society of Chemical Industry,London, 1983. [13] T. Ktari and B. Auclair, Simplifiedflux equation in Donnan dialysis, J. MembraneSci., 32 (1987) 251.