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Transport of some nitrogen heterocyclic and aromatic compounds through metal ion containing Nafion ionomer membrane
journal of MEMBRANE SCIENCE
ELSEVIER
Journal of Membrane Science 125 (1997) 269-274
Transport of some nitrogen heterocyclic and aromatic compounds through metal ion containing Nation ionomer membrane Jayshree Ramkumar, B. Maiti *, T.S. Krishnamoorthy Analytical Chemistry Division, Bhabha Atomic Research Centre Mumbai-400 0085, India Received 26 March 1996; revised 1 July 1996; accepted 16 July 1996
Abstract The permeation of some nitrogen heterocyclic bases, such as 1,10-phenanthroline, 2,2'-bipyridyl and 2-aminopyridine, and several aromatic compounds, namely styrene, toluene, nitrobenzene, aniline and biphenyl, through Nation 117 ionomer membrane in H +, Na +, Cuz+ and Ag + form has been studied. The permeation fluxes of the organic solutes through the membrane were measured by determining the concentration of the compounds in the sweep solutions using HPLC or flow injection technique. The positive role of metal ions in enhancing the permeation flux has been attributed to the complex formation of the solutes with the metal ions in the membrane. The contribution of the molecular structure of the permeates and steric effects are also discussed. The separation of some binary mixtures under favourable conditions is also shown. Keywords: Nation; Heterocyclic bases; Separation of organic compounds; Facilitated transport; HPLC
1. Introduction Perfluorosulphonic acid membranes find many industrial applications due to their high chemical stability and mechanical strength. The permeation behaviour of gases [1], cations [2-4], organic acids [5] and amines [6] through these membranes have been reported. Bong To et al. [7] have studied the effect of protonation on the transport of some heterocyclic bases. The transport of cations have generally been studied under the influence of an electrical field where the permeation is due to the applied field but the mechanism of permeation of organic bases is likely to be different from that of cations and electrons.
* Corresponding author.
In an earlier paper [8], the authors had demonstrated the selective permeation of CU 2+ and UO 2+ through Nation membrane, using suitable complexing agents. The selectivity of a metal ion permeation and the rate of ion transport depended on the nature of the complexing agent added to the feed and receiving solutions. Based on this, the reverse behaviour, viz., enhanced permeation of organic compounds through the membrane due to complexation of these compounds with metal ions incorporated in the membrane can be expected to take place. This paper presents our studies on the role of some metal ions like Na +, Ag + and Cu 2+ incorporated in the ionomer membrane in bringing about permeation of some nitrogen heterocyclic bases and simple aromatic compounds. The studies on the transmembrane permeation of 1,10-phenanthroline, 2,2'-bipyridyl, 2-aminopyridine, aniline, nitrobenzene, styrene,
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J. Ramkumar et al. / Journal of Membrane Science 125 (1997) 269-274
toluene and biphenyl through Nation 117 membrane containing Cu 2+ or Ag + is presented in this paper. The effect of solvent polarity on the selectivity and the rate of permeation is also discussed in terms of steric factors and the complexing ability of the metal ions within the ion cluster of the ionomer membrane.
2. Experimental The heterocyclic bases used were obtained from Merck (Germany) or Fluka and biphenyl, toluene, nitrobenzene and styrene were from BDH, India. Aniline obtained from BDH was purified by steam distillation. Solutions of Cu 2÷ and Ag ÷ were prepared by dissolving the corresponding nitrates in dilute acid. All other reagents were of high analytical purity. Double distilled water and analytically pure solvents were used in feed and sweep solutions. Permeation studies were carded out in a ' U ' type cell described earlier [8]. Dry Nation 117 membrane obtained from Du Pont, USA, had a thickness of 0.178 mm and the effective area of permeation in the cell was 254 mm 2. After each experiment, the used membrane was regenerated by refluxing with 1:1 HNO 3 [1] for 1 h followed by repeated washing and soaking in deionized water. This membrane in H ÷ form could be converted into other ionic forms for reuse. For the transport experiments, about 10 -2 M solutions of a given organic compound in water or water-methanol mixture was added to the feed compartment, and the same solvent or solvent mixture was used in the sweep compartment. The solutions in both compartments were stirred magnetically. Since styrene, toluene, nitrobenzene, biphenyl, aniline, etc. have poor solubility in water, aqueous methanol was chosen as the solvent medium of permeation for these substances. The concentration of the permeated solute was measured by withdrawing a small volume (20 Ixl) of sweep solution and injecting into a DuPont 8800 series HPLC instrument equipped with a reversed phase Lichrosorb RP 18 (5 Ixm) prepacked Hibar Column (Merck, Germany) and a variable wavelength DuPont 862 UV-vis. spectrophotometfic detector operating at 254 nm. Double distilled water
or aqueous methanol after proper degassing served as a mobile phase. In case of permeation studies on mixtures of compounds, the solution containing the known concentration of the mixture was kept in the feed compartment and the concentration of the individual components that permeated into the sweep solution was determined from the peak area of each component after separation on the HPLC column and detection at 254 nm. The analytical separation of the components was carried out on a reversed phase HPLC column using pure methanol as the mobile phase for the separation of styrene and toluene, whereas a 70:30 methanol-water mixture was found suitable for separation of aniline and nitrobenzene. For the determination of the concentration of a single component in the pure form permeating into sweep solution, the HPLC system was converted into a single channel flow injection system by removing the column and joining the flow line with a tygon tubing. Separation factors or enrichment factors were calculated from these determinations.
3. Results and discussion The permeation of organic solutes through the ionomer membrane can be described in terms of a two stage equilibrium process. The solutes in the feed solution diffuse first into the ion cluster of the membrane. The nitrogen containing compounds get solubilized in the membrane by dissolving in the polar solvent within the cluster due to protonation of the nitrogen when the membrane is kept in H ÷ form. As the solute concentration within the cluster builds up, it diffuses out of the membrane into the sweep compartment by deprotonation. The fall in the solute concentration in the membrane is compensated by further diffusion of the solutes from the feed solution. The heterocycles exit into the sweep solution as neutral molecules leaving the cation concentration unchanged in the membrane phase. If the protons in the membrane are replaced by Cu 2+ or Ag ÷ the permeation process would have a similar mechanism because the metal ions form complexes with the permeating compounds and the permeation flux is expected to increase. The overall transport process
J. Ramkumar et a l . / Journal of Membrane Science 125 (1997) 269-274
271
Table 1 Flux values ( J ) of nitrogen heterocycles through Nation 117 in H +, Na + and Cu 2+ forms in aqueous medium Compound
log K H+ [ 11 ]
1,10-phenanthroline 2,2'-bipyridyl 2-aminopyridine
log Kcu2+ [ 11]
4.93 4.43 6.80
Flux values (mol s - 1 cm - 2)
7.40 6.33 1.71
can be described as facilitated transport process and can be shown as S + X-Nation ~ S • • • X-Nation (feed compartment)
X-Nafion + S
(1)
(sweep compartment)
where S is the organic solute and X represents a proton or a metal ion. The selectivity of permeation through facilitated transport membrane (FI~M) would thus be primarily dependent on the factors influencing the cartier-permeate complexation reaction equilibrium as shown above. The relative solubility of a solute S in the membrane phase would be an impor-
JNa +
JH +
Jcu
1.37 × 10-10 9.0 × 10-10 3.13 X 10 .9
5.7 × 10-10 4.3 X 10- 9 7.16 X 10 -9
3.48 × 10 ~0 1.46 X 10 8 4.75 X 10-9
+
tant criterion for selective permeation and separation from other organic solutes present in a mixture. Table 1 gives the permeation flux ( J ) (an average value calculated from 3 to 4 points in the dynamic region before complete equilibrium is reached) for nitrogen heterocycles when different cation incorporated membranes were used. Since these heterocyclic bases under study have sufficient solubility in water, it was chosen as solvent medium for permeation studies. Jn + values for the nitrogen heterocycles are always higher than JNa ÷ showing that the protonation of the bases favours a higher solubilization of the heterocycles in the membrane, leading to a higher diffusion rate as per Eq. (1). JNa + values are gov-
X
I
E
0 x
d © CD
d o
o
o O
i
50
100
150
200
250
300
Time /rain Fig. 1. Permeation of styrene through Nation membrane in Na + (a), H + (b), Ag + (c) form and that of toluene through the membrane in Ag ÷ form (d). The concentration of the permeates in the feed compartment was 1.5 × 10 -2 M and the medium of permeation was methanol-water (70:30).
J. Ramkumar et a l . / Journal of Membrane Science 125 (1997) 269-274
272
erned by the molecular size a n d / o r steric factor and follow the order 2-aminopyridine > 2,2'-bipyridyl > 1,10-phenanthroline. It is seen that the presence of fused rings in 1,10-phenanthroline results in higher rigidity and hence a lower permeation flux. Aminopyridine has more than threefold higher J value compared to the bigger bipyridyl molecule. Cu 2+ containing membrane shows a higher permeation of bases than the membrane neutralized with Na +. 1,10-Phenanthroline, however, showed the lowest Jcu2+ value though logK for Cu 2+phenanthroline complex is the highest for the three bases. This can be attributed to the slower diffusibility of the bulky and rigid Cu2+-phenanthroline complex compared to less bulky and more flexible bipyridyl and aminopyridine complexes. It is also seen that Jcu.,+ for 1,10-phenanthroline is lower than JH ÷. There are two ring nitrogens available for protonation and solubilization, but when Cu 2÷ forms a bis complex (1:2) with this base, the resulting complex is much bulkier than the protonated base. The ligand with the fused rings makes the complex molecules less favourable for diffusion into the sweep solutions
and leads to a lower J value compared to bipyridyl and aminopyridine. The influence of metal ions in the membrane on the permeation of different organic compounds is shown in Figs. 1 and 2 where the concentration profiles of the solutes in the sweep solution have been plotted as a function of time. In the presence of a metal ion (e.g., Ag + or Cu 2+) capable of forming complex with the permeate, the concentration increases rapidly initially and then tends towards the equilibrium after a fraction of solute permeated into the sweep compartment. Fig. 1 shows the permeation curves for styrene and toluene. The average flux for styrene through Ag + containing membrane was about five times more than the one obtained for the membrane in H + form. Since Ag + forms a weaker complex with toluene (log K = 0.44) [9] as compared to styrene (log K = 1.27) [9], the permeation of toluene was lower. The effect of complexation of Cu 2+ ion with the heterocyclic bases is reflected in their permeation studies using Cu 2+ containing membrane. The permeation curves for nitrogen heterocycles through Nation is shown in Fig. 2.
o
I=_,
4
o
0 X o
6
e
~
cO (.D
d o
o
o
(~
T 50
1 O0
150
200
250
500
350
Time /rain Fig. 2. Permeation of 2,2'-bipyridyl through Nation membrane in Na + (a), H ÷ (b), Cu 2+ (C) form and that of 2-aminopyridine through the membrane in Na + (d), H + (e) and Cu 2+ (f) form. The concentration of the permeates in the feed compartment was 1.5 X 10 -2 M and the medium of permeation was water.
273
J. Ramkumar et a l . / Journal of Membrane Science 125 (1997) 269-274
Table 2 Permeation fluxes for organic solutes under different conditions of permeation Solute
Conc. in feed compartment (M)
X + ion
Medium CH3OH/H20
Transmembrane flux (mol s- 1 cm- z)
Styrene
1.5 × 10 - 2 1.5 × 10 -2 1.5 X 10 - 2 1.5 X 10 -2
H÷ H÷ Ag ÷ Na ÷
70:30 80:20 70:30 70:30
3.6 × 10 -9 5.6 × 10 -9 4.05 X 10 8 1.7 X 10 -9
Toluene
1.53 X 10 -2
Ag ÷
70:30
6.3 X 10 9
Bipyridyl
1.5 x 10 -2 1.5 X 10 2 1.47 X 10 -2
H÷ Cu 2+ Cu 2+
Water Water 85:15
4.95 x 10 - 9 1.05 X 10 -8 2.2 X 10 - 9
Biphenyl
1.47 × 10 -2
Cu 2÷
85:15
5.2 X 10- ~o
Aniline
1.59 X 10 -2
Cu 2+
90:10
1.72 X 10-8
Nitrobenzene
1.65 X 10 2
Cu 2+
90:10
8.73 X
T h e s e c o n c e n t r a t i o n vs. t i m e p r o f i l e s s h o w t h a t t h e r e is a t e n d e n c y to r e a c h a n e a r s a t u r a t i o n i n p e r m e a t i o n after 2 - 3 h. F r o m Eq. (1), it c a n b e s e e n t h a t in the f e e d side t h e f o r m a t i o n o f c a r r i e r - p e r m e a t e c o m p l e x p r o m o t e s t h e a b s o r p t i o n o f solutes into the m e m b r a n e . T h i s c o m p l e x in t h e m e m b r a n e p h a s e h a s to d i s s o c i a t e a n d r e l e a s e the solute into t h e s w e e p s o l u t i o n , f o r t r a n s p o r t t h r o u g h the m e m b r a n e d u e to F r M . S i n c e it is f o r m a t i o n o f t h e c a r r i e r - p e r m e a t e c o m p l e x o n o n e side a n d d i s s o c i a t i o n o f the s a m e o n t h e other, a n e q u i l i b r i u m h a s to b e r e a c h e d a f t e r s o m e time. T h e c o n c e n t r a t i o n at e q u i l i b r i u m will b e d e c i d e d b y the f o r m a t i o n / d i s s o c i a t i o n c o n s t a n t ( w h e r e o t h e r v a r i a b l e s like a r e a a n d t h i c k n e s s
10 - 9
o f t h e m e m b r a n e are c o n s t a n t ) . A t this stage, o n l y a v e r y s m a l l q u a n t i t y o f the solute w o u l d p e r m e a t e d u e to n a t u r a l d i f f u s i o n alone. It w a s also o b s e r v e d t h a t when the sweep solution was periodically removed and replaced with fresh solvents, the permeation was continuous. T h e r e are o t h e r factors like the p o l a r i t y o f the s o l v e n t m e d i u m i n f l u e n c i n g t h e p e r m e a t i o n o f the h y d r o p h o b i c o r g a n i c solutes. T a b l e 2 s u m m a r i z e s the flux v a l u e s f o r d i f f e r e n t o r g a n i c solutes at d i f f e r e n t s o l v e n t c o m p o s i t i o n s a n d the m e m b r a n e in H ÷, N a ÷, A g ÷ or C u 2÷ f o r m . S t y r e n e a n d t o l u e n e b e i n g insolu b l e in water, t h e p e r m e a t i o n w a s s t u d i e d in a q u e o u s methanol medium. Since the permeation of organic
Table 3 Permeation of organic solutes through X-Nation from mixtures Mixture No.
X ion
Compounds
Initial Conc. in feed sol. (M 1- l )
Conc. in sweep sol. after 3 h (M 1- 1)
1
Ag ÷
Styrene Toluene
1.50 X 10 2 1.53 x 10 -2
1.01 × 10 -3 Not detected
2
Ag ÷
Styrene Toluene
1.50 × 10- 2 6.0 x 10 -1
7.6 X 10 4
Cu 2+
Aniline Nitrobenzene
1.59 x 10 2 1.65 X 10 -2
1.4 X lO -4 1.2 x lO -4
1.12
Cu 2+
Aniline Nitrobenzene
1.59 X 10 -2 1.65 X 10 - 3
1.60 X 10 - 4 1.02 X 10 -4
1.5
4
Ratio of Conc.
9.5
0.8 X l 0 - 4
274
J. Ramkumar et a l . / Journal of Membrane Science 125 (1997) 269-274
compounds are primarily governed by their solubilities in the membrane phase, the polarity of the permeating media is an important factor. The membrane swells considerably in polar solvents like water, methanol, etc. with the simultaneous swelling in the size of the ion clusters in the membrane, facilitating the easy accommodation of more solutes. A nonpolar solvent does not swell the membrane and is of no use for permeation studies. Nonpolar solvents saturated with water have sometimes been used [10] but the permeation rate would be affected due to the multistage transfer of solutes from the solvent to water to membrane in the feed side and a similar reversed process in the sweep side. Aqueous methanol was thus used for the permeation of solutes insoluble in water. The effect of polarity of the medium is reflected in the flux values given in Table 2. The heterocyclic bases have good solubility in water and their Cu 2÷ complexes formed in the membrane are ionic in nature. Addition of methanol reduced the polarity of the medium and the permeation flux for 2,2'-bipyridyl (Table 2). On the other hand, methanol was a favourable medium for the permeation of styrene and toluene and the flux value increased with the increase in methanol content. The flux values listed in the table are concentration dependant. The permeation flux increased with the increase in the concentration of solute in the feed compartment and vice versa (not shown in the table). The concentration-time profile discussed earlier (Figs. 1 and 2) suggest the possibility of achieving considerable degree of selectivity in the separation of mixtures. Compounds capable of forming stronger complex with the ion incorporated in Nation may lead to greater permeation than those with weaker ones. Higher the stability of the carrier-permeate complex, higher will be the absorption of the solute into membrane phase and hence higher the permeation. With this in view, experiments were carried out to study the relative permeation of solutes from mixtures. Table 3 summarises the observations for mixtures of styrene-toluene and aniline-nitrobenzene. When a mixture of solutes of comparable chemical structure was placed in the feed compartment the flux values were dependent on the relative concentrations of the permeates in the feed compartment. From a equimolar mixture of styrene and
toluene (1.5 × 10 - 2 M) in the feed solution, only styrene selectively permeated through Ag + containing membrane, but at high toluene concentration the ratio of styrene to toluene was nearly unity. Similarly, the concentration ratio of aniline to nitrobenzene was nearly unity when the permeation of an equimolar mixture of the compounds through Cu 2+ containing membrane was studied but a ten fold decrease in the nitrobenzene concentration increased the ratio. Thus, the use of metal ion containing membranes show possibilities for selective separations or purification from related compounds.
References [1] B. Maiti and S. Schlick, Oxygen permeation in perfluorinated ionomer based on the reaction with methyl viologen cation radical. An ESR and optical study, Chem. Mater., 4 (1992) 458. [2] A. Chapotot, G. Pourcelly and C. Gavach, Transport competition between monovalent and divalent cations through cation exchange membranes. Exchange isotherms and kinetic concepts, J. Membrane Sci., 96 (1994) 167. [3] A.T. Cherif, A. Elimdaoin and C. Gavach, Separation of Ag +. Zn 2+ and Cu 2+ ions by electrodialysis with a monovalent cation specific membrane and EDTA, J. Membrane Sci., 76 (1993) 39. [4] W.J. Blacdel and T.J. Hampert, Exchange equilibrium through ion-exchange membranes. Analytical applications, Anal. Chem., 38 (1966) 1305. [5] H.J. Chum, A.K. Hauser and D. Sopher, New uses of Nation membranes in electroorganic synthesis and in organic acids separation, J. Electrochem. Soc., 130(12) (1983) 2508. [6] P.G. Glugla and H. Dindi, Transport of aniline, chloroaniline, nitroamiline, and urea through perfluorosulfonated ionomer membranes, J. Membrane Sci.. 28 (1985) 21. [7] T.T. Bong To, R.D. Noble and C.A. Koval, Effects of protonation on the transport of hydrophobic nitrogen heterocycles through perfluorosulfonate ionomer membranes, J. Membrane Sci., 75 (1992) 293. [8] J. Ramkumar, K.S. Shrimal, B. Maiti and T.S. Krishnamoorthy, Selective permeation of Cu 2+ andUO~ + throughNafion ionomer membrane, J. Membrane Sci., 116 (1996) 31. [9] A.E. Martel and R.M. Smith, Critical Stability Constants, Vol. 3, Other Organic Ligands, Plenum Press, New York, 1977. [10] C.A. Koval, T. Spontarelli and R.D. Noble, Styrene/ethylbenzene separation using facilitated transport through perfluorosulfonate ionomer membranes, Ind. Eng. Chem. Res., 28 (1989) 1020. [11] A.E. Martel and R.M. Smith, Critical Stability Constants, Vol. 1, Amino Acids, Plenum Press, New York, 1974.
ELSEVIER
Journal of Membrane Science 125 (1997) 269-274
Transport of some nitrogen heterocyclic and aromatic compounds through metal ion containing Nation ionomer membrane Jayshree Ramkumar, B. Maiti *, T.S. Krishnamoorthy Analytical Chemistry Division, Bhabha Atomic Research Centre Mumbai-400 0085, India Received 26 March 1996; revised 1 July 1996; accepted 16 July 1996
Abstract The permeation of some nitrogen heterocyclic bases, such as 1,10-phenanthroline, 2,2'-bipyridyl and 2-aminopyridine, and several aromatic compounds, namely styrene, toluene, nitrobenzene, aniline and biphenyl, through Nation 117 ionomer membrane in H +, Na +, Cuz+ and Ag + form has been studied. The permeation fluxes of the organic solutes through the membrane were measured by determining the concentration of the compounds in the sweep solutions using HPLC or flow injection technique. The positive role of metal ions in enhancing the permeation flux has been attributed to the complex formation of the solutes with the metal ions in the membrane. The contribution of the molecular structure of the permeates and steric effects are also discussed. The separation of some binary mixtures under favourable conditions is also shown. Keywords: Nation; Heterocyclic bases; Separation of organic compounds; Facilitated transport; HPLC
1. Introduction Perfluorosulphonic acid membranes find many industrial applications due to their high chemical stability and mechanical strength. The permeation behaviour of gases [1], cations [2-4], organic acids [5] and amines [6] through these membranes have been reported. Bong To et al. [7] have studied the effect of protonation on the transport of some heterocyclic bases. The transport of cations have generally been studied under the influence of an electrical field where the permeation is due to the applied field but the mechanism of permeation of organic bases is likely to be different from that of cations and electrons.
* Corresponding author.
In an earlier paper [8], the authors had demonstrated the selective permeation of CU 2+ and UO 2+ through Nation membrane, using suitable complexing agents. The selectivity of a metal ion permeation and the rate of ion transport depended on the nature of the complexing agent added to the feed and receiving solutions. Based on this, the reverse behaviour, viz., enhanced permeation of organic compounds through the membrane due to complexation of these compounds with metal ions incorporated in the membrane can be expected to take place. This paper presents our studies on the role of some metal ions like Na +, Ag + and Cu 2+ incorporated in the ionomer membrane in bringing about permeation of some nitrogen heterocyclic bases and simple aromatic compounds. The studies on the transmembrane permeation of 1,10-phenanthroline, 2,2'-bipyridyl, 2-aminopyridine, aniline, nitrobenzene, styrene,
0376-7388/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S 0 3 7 6 - 7 3 8 8 ( 9 6 ) 0 0 2 2 5 - 6
270
J. Ramkumar et al. / Journal of Membrane Science 125 (1997) 269-274
toluene and biphenyl through Nation 117 membrane containing Cu 2+ or Ag + is presented in this paper. The effect of solvent polarity on the selectivity and the rate of permeation is also discussed in terms of steric factors and the complexing ability of the metal ions within the ion cluster of the ionomer membrane.
2. Experimental The heterocyclic bases used were obtained from Merck (Germany) or Fluka and biphenyl, toluene, nitrobenzene and styrene were from BDH, India. Aniline obtained from BDH was purified by steam distillation. Solutions of Cu 2÷ and Ag ÷ were prepared by dissolving the corresponding nitrates in dilute acid. All other reagents were of high analytical purity. Double distilled water and analytically pure solvents were used in feed and sweep solutions. Permeation studies were carded out in a ' U ' type cell described earlier [8]. Dry Nation 117 membrane obtained from Du Pont, USA, had a thickness of 0.178 mm and the effective area of permeation in the cell was 254 mm 2. After each experiment, the used membrane was regenerated by refluxing with 1:1 HNO 3 [1] for 1 h followed by repeated washing and soaking in deionized water. This membrane in H ÷ form could be converted into other ionic forms for reuse. For the transport experiments, about 10 -2 M solutions of a given organic compound in water or water-methanol mixture was added to the feed compartment, and the same solvent or solvent mixture was used in the sweep compartment. The solutions in both compartments were stirred magnetically. Since styrene, toluene, nitrobenzene, biphenyl, aniline, etc. have poor solubility in water, aqueous methanol was chosen as the solvent medium of permeation for these substances. The concentration of the permeated solute was measured by withdrawing a small volume (20 Ixl) of sweep solution and injecting into a DuPont 8800 series HPLC instrument equipped with a reversed phase Lichrosorb RP 18 (5 Ixm) prepacked Hibar Column (Merck, Germany) and a variable wavelength DuPont 862 UV-vis. spectrophotometfic detector operating at 254 nm. Double distilled water
or aqueous methanol after proper degassing served as a mobile phase. In case of permeation studies on mixtures of compounds, the solution containing the known concentration of the mixture was kept in the feed compartment and the concentration of the individual components that permeated into the sweep solution was determined from the peak area of each component after separation on the HPLC column and detection at 254 nm. The analytical separation of the components was carried out on a reversed phase HPLC column using pure methanol as the mobile phase for the separation of styrene and toluene, whereas a 70:30 methanol-water mixture was found suitable for separation of aniline and nitrobenzene. For the determination of the concentration of a single component in the pure form permeating into sweep solution, the HPLC system was converted into a single channel flow injection system by removing the column and joining the flow line with a tygon tubing. Separation factors or enrichment factors were calculated from these determinations.
3. Results and discussion The permeation of organic solutes through the ionomer membrane can be described in terms of a two stage equilibrium process. The solutes in the feed solution diffuse first into the ion cluster of the membrane. The nitrogen containing compounds get solubilized in the membrane by dissolving in the polar solvent within the cluster due to protonation of the nitrogen when the membrane is kept in H ÷ form. As the solute concentration within the cluster builds up, it diffuses out of the membrane into the sweep compartment by deprotonation. The fall in the solute concentration in the membrane is compensated by further diffusion of the solutes from the feed solution. The heterocycles exit into the sweep solution as neutral molecules leaving the cation concentration unchanged in the membrane phase. If the protons in the membrane are replaced by Cu 2+ or Ag ÷ the permeation process would have a similar mechanism because the metal ions form complexes with the permeating compounds and the permeation flux is expected to increase. The overall transport process
J. Ramkumar et a l . / Journal of Membrane Science 125 (1997) 269-274
271
Table 1 Flux values ( J ) of nitrogen heterocycles through Nation 117 in H +, Na + and Cu 2+ forms in aqueous medium Compound
log K H+ [ 11 ]
1,10-phenanthroline 2,2'-bipyridyl 2-aminopyridine
log Kcu2+ [ 11]
4.93 4.43 6.80
Flux values (mol s - 1 cm - 2)
7.40 6.33 1.71
can be described as facilitated transport process and can be shown as S + X-Nation ~ S • • • X-Nation (feed compartment)
X-Nafion + S
(1)
(sweep compartment)
where S is the organic solute and X represents a proton or a metal ion. The selectivity of permeation through facilitated transport membrane (FI~M) would thus be primarily dependent on the factors influencing the cartier-permeate complexation reaction equilibrium as shown above. The relative solubility of a solute S in the membrane phase would be an impor-
JNa +
JH +
Jcu
1.37 × 10-10 9.0 × 10-10 3.13 X 10 .9
5.7 × 10-10 4.3 X 10- 9 7.16 X 10 -9
3.48 × 10 ~0 1.46 X 10 8 4.75 X 10-9
+
tant criterion for selective permeation and separation from other organic solutes present in a mixture. Table 1 gives the permeation flux ( J ) (an average value calculated from 3 to 4 points in the dynamic region before complete equilibrium is reached) for nitrogen heterocycles when different cation incorporated membranes were used. Since these heterocyclic bases under study have sufficient solubility in water, it was chosen as solvent medium for permeation studies. Jn + values for the nitrogen heterocycles are always higher than JNa ÷ showing that the protonation of the bases favours a higher solubilization of the heterocycles in the membrane, leading to a higher diffusion rate as per Eq. (1). JNa + values are gov-
X
I
E
0 x
d © CD
d o
o
o O
i
50
100
150
200
250
300
Time /rain Fig. 1. Permeation of styrene through Nation membrane in Na + (a), H + (b), Ag + (c) form and that of toluene through the membrane in Ag ÷ form (d). The concentration of the permeates in the feed compartment was 1.5 × 10 -2 M and the medium of permeation was methanol-water (70:30).
J. Ramkumar et a l . / Journal of Membrane Science 125 (1997) 269-274
272
erned by the molecular size a n d / o r steric factor and follow the order 2-aminopyridine > 2,2'-bipyridyl > 1,10-phenanthroline. It is seen that the presence of fused rings in 1,10-phenanthroline results in higher rigidity and hence a lower permeation flux. Aminopyridine has more than threefold higher J value compared to the bigger bipyridyl molecule. Cu 2+ containing membrane shows a higher permeation of bases than the membrane neutralized with Na +. 1,10-Phenanthroline, however, showed the lowest Jcu2+ value though logK for Cu 2+phenanthroline complex is the highest for the three bases. This can be attributed to the slower diffusibility of the bulky and rigid Cu2+-phenanthroline complex compared to less bulky and more flexible bipyridyl and aminopyridine complexes. It is also seen that Jcu.,+ for 1,10-phenanthroline is lower than JH ÷. There are two ring nitrogens available for protonation and solubilization, but when Cu 2÷ forms a bis complex (1:2) with this base, the resulting complex is much bulkier than the protonated base. The ligand with the fused rings makes the complex molecules less favourable for diffusion into the sweep solutions
and leads to a lower J value compared to bipyridyl and aminopyridine. The influence of metal ions in the membrane on the permeation of different organic compounds is shown in Figs. 1 and 2 where the concentration profiles of the solutes in the sweep solution have been plotted as a function of time. In the presence of a metal ion (e.g., Ag + or Cu 2+) capable of forming complex with the permeate, the concentration increases rapidly initially and then tends towards the equilibrium after a fraction of solute permeated into the sweep compartment. Fig. 1 shows the permeation curves for styrene and toluene. The average flux for styrene through Ag + containing membrane was about five times more than the one obtained for the membrane in H + form. Since Ag + forms a weaker complex with toluene (log K = 0.44) [9] as compared to styrene (log K = 1.27) [9], the permeation of toluene was lower. The effect of complexation of Cu 2+ ion with the heterocyclic bases is reflected in their permeation studies using Cu 2+ containing membrane. The permeation curves for nitrogen heterocycles through Nation is shown in Fig. 2.
o
I=_,
4
o
0 X o
6
e
~
cO (.D
d o
o
o
(~
T 50
1 O0
150
200
250
500
350
Time /rain Fig. 2. Permeation of 2,2'-bipyridyl through Nation membrane in Na + (a), H ÷ (b), Cu 2+ (C) form and that of 2-aminopyridine through the membrane in Na + (d), H + (e) and Cu 2+ (f) form. The concentration of the permeates in the feed compartment was 1.5 X 10 -2 M and the medium of permeation was water.
273
J. Ramkumar et a l . / Journal of Membrane Science 125 (1997) 269-274
Table 2 Permeation fluxes for organic solutes under different conditions of permeation Solute
Conc. in feed compartment (M)
X + ion
Medium CH3OH/H20
Transmembrane flux (mol s- 1 cm- z)
Styrene
1.5 × 10 - 2 1.5 × 10 -2 1.5 X 10 - 2 1.5 X 10 -2
H÷ H÷ Ag ÷ Na ÷
70:30 80:20 70:30 70:30
3.6 × 10 -9 5.6 × 10 -9 4.05 X 10 8 1.7 X 10 -9
Toluene
1.53 X 10 -2
Ag ÷
70:30
6.3 X 10 9
Bipyridyl
1.5 x 10 -2 1.5 X 10 2 1.47 X 10 -2
H÷ Cu 2+ Cu 2+
Water Water 85:15
4.95 x 10 - 9 1.05 X 10 -8 2.2 X 10 - 9
Biphenyl
1.47 × 10 -2
Cu 2÷
85:15
5.2 X 10- ~o
Aniline
1.59 X 10 -2
Cu 2+
90:10
1.72 X 10-8
Nitrobenzene
1.65 X 10 2
Cu 2+
90:10
8.73 X
T h e s e c o n c e n t r a t i o n vs. t i m e p r o f i l e s s h o w t h a t t h e r e is a t e n d e n c y to r e a c h a n e a r s a t u r a t i o n i n p e r m e a t i o n after 2 - 3 h. F r o m Eq. (1), it c a n b e s e e n t h a t in the f e e d side t h e f o r m a t i o n o f c a r r i e r - p e r m e a t e c o m p l e x p r o m o t e s t h e a b s o r p t i o n o f solutes into the m e m b r a n e . T h i s c o m p l e x in t h e m e m b r a n e p h a s e h a s to d i s s o c i a t e a n d r e l e a s e the solute into t h e s w e e p s o l u t i o n , f o r t r a n s p o r t t h r o u g h the m e m b r a n e d u e to F r M . S i n c e it is f o r m a t i o n o f t h e c a r r i e r - p e r m e a t e c o m p l e x o n o n e side a n d d i s s o c i a t i o n o f the s a m e o n t h e other, a n e q u i l i b r i u m h a s to b e r e a c h e d a f t e r s o m e time. T h e c o n c e n t r a t i o n at e q u i l i b r i u m will b e d e c i d e d b y the f o r m a t i o n / d i s s o c i a t i o n c o n s t a n t ( w h e r e o t h e r v a r i a b l e s like a r e a a n d t h i c k n e s s
10 - 9
o f t h e m e m b r a n e are c o n s t a n t ) . A t this stage, o n l y a v e r y s m a l l q u a n t i t y o f the solute w o u l d p e r m e a t e d u e to n a t u r a l d i f f u s i o n alone. It w a s also o b s e r v e d t h a t when the sweep solution was periodically removed and replaced with fresh solvents, the permeation was continuous. T h e r e are o t h e r factors like the p o l a r i t y o f the s o l v e n t m e d i u m i n f l u e n c i n g t h e p e r m e a t i o n o f the h y d r o p h o b i c o r g a n i c solutes. T a b l e 2 s u m m a r i z e s the flux v a l u e s f o r d i f f e r e n t o r g a n i c solutes at d i f f e r e n t s o l v e n t c o m p o s i t i o n s a n d the m e m b r a n e in H ÷, N a ÷, A g ÷ or C u 2÷ f o r m . S t y r e n e a n d t o l u e n e b e i n g insolu b l e in water, t h e p e r m e a t i o n w a s s t u d i e d in a q u e o u s methanol medium. Since the permeation of organic
Table 3 Permeation of organic solutes through X-Nation from mixtures Mixture No.
X ion
Compounds
Initial Conc. in feed sol. (M 1- l )
Conc. in sweep sol. after 3 h (M 1- 1)
1
Ag ÷
Styrene Toluene
1.50 X 10 2 1.53 x 10 -2
1.01 × 10 -3 Not detected
2
Ag ÷
Styrene Toluene
1.50 × 10- 2 6.0 x 10 -1
7.6 X 10 4
Cu 2+
Aniline Nitrobenzene
1.59 x 10 2 1.65 X 10 -2
1.4 X lO -4 1.2 x lO -4
1.12
Cu 2+
Aniline Nitrobenzene
1.59 X 10 -2 1.65 X 10 - 3
1.60 X 10 - 4 1.02 X 10 -4
1.5
4
Ratio of Conc.
9.5
0.8 X l 0 - 4
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J. Ramkumar et a l . / Journal of Membrane Science 125 (1997) 269-274
compounds are primarily governed by their solubilities in the membrane phase, the polarity of the permeating media is an important factor. The membrane swells considerably in polar solvents like water, methanol, etc. with the simultaneous swelling in the size of the ion clusters in the membrane, facilitating the easy accommodation of more solutes. A nonpolar solvent does not swell the membrane and is of no use for permeation studies. Nonpolar solvents saturated with water have sometimes been used [10] but the permeation rate would be affected due to the multistage transfer of solutes from the solvent to water to membrane in the feed side and a similar reversed process in the sweep side. Aqueous methanol was thus used for the permeation of solutes insoluble in water. The effect of polarity of the medium is reflected in the flux values given in Table 2. The heterocyclic bases have good solubility in water and their Cu 2÷ complexes formed in the membrane are ionic in nature. Addition of methanol reduced the polarity of the medium and the permeation flux for 2,2'-bipyridyl (Table 2). On the other hand, methanol was a favourable medium for the permeation of styrene and toluene and the flux value increased with the increase in methanol content. The flux values listed in the table are concentration dependant. The permeation flux increased with the increase in the concentration of solute in the feed compartment and vice versa (not shown in the table). The concentration-time profile discussed earlier (Figs. 1 and 2) suggest the possibility of achieving considerable degree of selectivity in the separation of mixtures. Compounds capable of forming stronger complex with the ion incorporated in Nation may lead to greater permeation than those with weaker ones. Higher the stability of the carrier-permeate complex, higher will be the absorption of the solute into membrane phase and hence higher the permeation. With this in view, experiments were carried out to study the relative permeation of solutes from mixtures. Table 3 summarises the observations for mixtures of styrene-toluene and aniline-nitrobenzene. When a mixture of solutes of comparable chemical structure was placed in the feed compartment the flux values were dependent on the relative concentrations of the permeates in the feed compartment. From a equimolar mixture of styrene and
toluene (1.5 × 10 - 2 M) in the feed solution, only styrene selectively permeated through Ag + containing membrane, but at high toluene concentration the ratio of styrene to toluene was nearly unity. Similarly, the concentration ratio of aniline to nitrobenzene was nearly unity when the permeation of an equimolar mixture of the compounds through Cu 2+ containing membrane was studied but a ten fold decrease in the nitrobenzene concentration increased the ratio. Thus, the use of metal ion containing membranes show possibilities for selective separations or purification from related compounds.
References [1] B. Maiti and S. Schlick, Oxygen permeation in perfluorinated ionomer based on the reaction with methyl viologen cation radical. An ESR and optical study, Chem. Mater., 4 (1992) 458. [2] A. Chapotot, G. Pourcelly and C. Gavach, Transport competition between monovalent and divalent cations through cation exchange membranes. Exchange isotherms and kinetic concepts, J. Membrane Sci., 96 (1994) 167. [3] A.T. Cherif, A. Elimdaoin and C. Gavach, Separation of Ag +. Zn 2+ and Cu 2+ ions by electrodialysis with a monovalent cation specific membrane and EDTA, J. Membrane Sci., 76 (1993) 39. [4] W.J. Blacdel and T.J. Hampert, Exchange equilibrium through ion-exchange membranes. Analytical applications, Anal. Chem., 38 (1966) 1305. [5] H.J. Chum, A.K. Hauser and D. Sopher, New uses of Nation membranes in electroorganic synthesis and in organic acids separation, J. Electrochem. Soc., 130(12) (1983) 2508. [6] P.G. Glugla and H. Dindi, Transport of aniline, chloroaniline, nitroamiline, and urea through perfluorosulfonated ionomer membranes, J. Membrane Sci.. 28 (1985) 21. [7] T.T. Bong To, R.D. Noble and C.A. Koval, Effects of protonation on the transport of hydrophobic nitrogen heterocycles through perfluorosulfonate ionomer membranes, J. Membrane Sci., 75 (1992) 293. [8] J. Ramkumar, K.S. Shrimal, B. Maiti and T.S. Krishnamoorthy, Selective permeation of Cu 2+ andUO~ + throughNafion ionomer membrane, J. Membrane Sci., 116 (1996) 31. [9] A.E. Martel and R.M. Smith, Critical Stability Constants, Vol. 3, Other Organic Ligands, Plenum Press, New York, 1977. [10] C.A. Koval, T. Spontarelli and R.D. Noble, Styrene/ethylbenzene separation using facilitated transport through perfluorosulfonate ionomer membranes, Ind. Eng. Chem. Res., 28 (1989) 1020. [11] A.E. Martel and R.M. Smith, Critical Stability Constants, Vol. 1, Amino Acids, Plenum Press, New York, 1974.