J. Phys. Chem. 1994,98, 1262-1269
1262
Unexpectedly Large Selectivities for Olefin Separations Utilizing Silver Ion in Ion-Exchange Membranes P.M. Thoen, R. D. Noble,? and C. A. Koval' Department of Chemistry and Biochemistry, Campus Box 21 5, University of Colorado, Boulder, Colorado 80309 Received: September 8, 1993; In Final Form: November 22, 1993"
Perfluorosulfonate ionomer membranes (Nafion) that have been ion-exchanged with silver( I) ion exhibit unexpectedly large selectivities for the separation of certain unsaturated hydrocarbon mixtures, such as styrene from ethylbenzene and linear C5-Clo dienes from monoenes. Transmembrane fluxes increase linearly with the reciprocal of membrane thickness with no loss of selectivity for membrane thicknesses between 40 and 2.5 pm, The large selectivities are due to competitive absorption of the hydrocarbons by the membranes. This effect cannot be predicted from singlecomponent experiments or known complexation constants between the hydrocarbon and aqueous Ag( I). The unexpectedly large separation factors for the diene/monoene mixtures can be explained semiquantitatively by invoking the complexation of dienes to two Ag(1) ions.
Introduction Due to their extensive use in the polymer industry and as solvents, there is a continuing need for better separating processes for alkenes and other unsaturated organic compounds. Recent studies by the U.S. Department of Energy and others indicate that membrane-based separation processes for a variety of compounds will become economically viable, provided that membrane performance can be improved.'J One method for improving membrane performance is via carrier-mediated or facilitated transport.= A carrier provides an additional pathway for a permeate to cross the membrane, thereby increasing the fluxes and separation factors relative to the permeates that are not facilitated by the carrier. Liquid membranes and other ion-exchange membranes containing Ag+exhibit large (>loo) separation factors (fluxof A/flux of B, corrected for driving force) for alkenes with respect to alkanes due to reversible alkene-Ag+ complexation reaction^.^-^ Thus the transport of the alkene is facilitated by the Ag+, while the transport of the alkane is only by Fickian diffusion. For the separation of alkenes and aromatics, however, each component ofthe feedsolutioncan react with theAg+ (competitive transport). Since the equilibrium constants describing the complexation of Ag+ with many alkenes and aromatics in aqueous solution are similar,I0Ji large separation factors due to facilitated transport through water-swollen ionomer membranes are not anticipated.I2 Nafion,I3 a perfluorosulfonate ionomer, has been used for a wide variety of separation applications including fuel cells,'k17 selective electrodes,18-22and chlor-alkali cells.23 Numerous studieshave been performed in an effort to determine the transport mechanism of molecules and charge through these membranes.2b32 Recently, our groups have demonstrated that incorporation of Ag+ into Nafion results in dramatic enhancement of transmembrane fluxes of olefins and aromatics (factors of 102-103). In certain cases, e.g., styrene/ethylbenzene, large separation factors were observed for mixtures of solutes even though the solutes have comparable complexation affinities for Ag(I).33-36 In this paper, we demonstrate that unexpectedly large selectivities are observed using Ag+-Nafion membranes for the separation of other olefin and aromatic bicomponent mixtures. These selectivities are much larger than predicted from theory and from measurements involving single-component feed solutions.
* To whom correspondence should be addressed.
t Department of Chemical Engineering, Campus Box 424, University of Colorado, Boulder, CO 80309. Abstract published in Advance ACS Abstracts, January 1, 1994. @
0022-365419412098-1262304.50/0
Extraction of the hydrocarbons from the Ag+-Nafion membranes reveals that complexation by Ag+ is far more favorable in waterswollen Nafion than in free solution, and that this enhanced complexation results in competitive absorption. Applications of facilitated transport membranes (FTMs) can be limited by two effects. First, as the concentrations of reactive solutes in the feed mixture increases, the carrier can become saturated. This results in diminished selectivity and often severely limits the concentration range and type of separation processes in which FTM's will be practical. While the results presented herein do show a decreasing separation factor with increasing feed concentration, selectivities greater than 10 are maintained for a feed solution consisting of a 50/50% mixture of styrene and ethylbenzene (no feed solvent). This surprising result indicates that the effective operating range of Ag+-Nafion membranes can be greatly extended, and these membranes could be utilized in pervaporation or perstraction processes. The second possible limiting factor of FTMs is related to the kinetics of the complexation reaction. Normally, the productivity of a membrane is inversely proportional to the membrane thickness (L);therefore, it is cost-effective to use the thinnest membrane that can be produced without defects. Unfortunately, when the kinetics of the complexation reaction in FTMs are not fast with respect to the diffusion of free solutes, a diminution of separation factor is observed. By casting membranes ranging from 2.5 to 40 l m in thickness, we demonstrate that Ag+-Nafion membranes show no signs of a kinetic limitation, Le., productivity (transmembrane fluxes) increases as 1/L with no loss of selectivity.
Experimental Section The membranes used in the competitivetransport experiments were Nafion 111,which has a dry thickness of 25 pm. Membranes used for the thickness study were cast in our lab from a solution of 1% Nafion in DMSO. Moore and Martin have found that membranes cast from DMSO at high temperature have excellent physical and mechanical proper tie^.^',^^ The membranes made in our lab were prepared from a 5% solution of Nafion obtained from Solution Technology, using the following procedure: While the precise composition of this mixture is proprietary information, major components are low molecular weight alcohols and approximately 10% water. A small amount of the 5% solution is poured into a glass casting dish and the solvent is pulled off under a slight vacuum at room temperature. The resulting membrane is washed thoroughly in DI water in order to rinse away any excess ions that may have been present in the 5% Nafion 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 1263
Olefin Separations in Ion-Exchange Membranes
1 .
Feed Side: 1.5-Hexadiene 8 I-Hexene in water saturated isooctane.
Nation Mwnbran@
JI,bHexadle
\
Sweep Side: water saturated isooctane \ \
I /
Figure 1. Facilitated transport of 1J-hexadiene and 1-hexene through the Ag+ form of a Nafion membrane.
solution. The membrane is soaked overnight in a 1 M solution of NaOH in order to convert it from the H+ to the Na+ form (membranes cast in the H+ form a t high temperature appear burned). After being rinsed in DI water again, the Na+-form membrane is placed in the appropriate amount of DMSO to make a 1% Nafion solution by weight. This solution is then refluxed for 4 h to completely dissolve the membrane. Finally, the solution is poured into a glass casting dish which is then placed into a vacuum oven at 170 "C and 500 Torr to drive off the solvent. The resulting membranes are easily floated off the casting dishes by soaking in warm water. An Ames Model 135 G F bench comparator was used to measure the thicknesses of membranes 10 pm or thicker. As expected the thickness of a membrane is directly proportional to the amount of casting solution used. Therefore the thicknesses of the thinner membranes ( < l o pm) could be determined by the amount of casting solution used. The ion-exchange procedures used to convert these membranes to the Ag+ or Na+ form and the procedures used to measure transmembrane fluxes have been previously r e p ~ r t e d . ~For ~-~~ the membrane-transport experiments, the feed side solutions consisted of the solutes of interest in water-saturated isooctane, while the sweep side solutions consisted only of water-saturated isooctane (Figure 1). Flux values were obtained from plots of solute concentration in the sweep vs time which remained linear for hours. The reproducibility for individual flux values obtained from experiments on different membrane samples was approximately 30% relative standard deviation (rsd). However, separation factors (ratios of fluxes for two solutes) varied by only 10% rsd. The concentrationsof solutesin the membrane were determined by equilibrating pieces of Nafion with the feed solution for 48 h, rinsing them briefly in pure solvent to remove solute from the surface and thoroughlyextracting the solutesfrom the membranes with pure solvent. The extract was then analyzed using gas chromatography. Solute concentrations in the Nafion were calculated by assuming that hydrated Nafion contains 15% water by mass and that the solutes reside in the water-containing regions of the membrane. Nafion 117 membranes, which have a dry thickness of 180 pm, were used for these sets of experiments. These thicker membranes were used in order to minimize the effect of solutes adsorbed on the membrane surface that were not removed during rinsing and also to minimize the loss of solute from within the membrane during the rinsing stage. Fits to the single component absorption data (Figure 8) for 1,5-hexadieneand 1-hexene were performed using a linear least squares regression program (sigma plot). Calculations involving the bicomponent absorption data (Figure 9) were performed using a spreadsheet program.
Resulh Calculated and Ideal Separation Factors. For membrane separations, the separation factor for two components (A and B) in the feed mixture, SAB,is defined as the ratio of permeabilities or driving-force-corrected transmembrane fluxes:
One parameter used to describe facilitated transport is the facilitation factor, F, which is defined as the ratio of the solute flux with the carrier present to the solute diffusion flux at the same driving force. For FTMs that are operating in the reaction equilibrium regime with no mass transfer resistance a t the membrane interfaces, Dindi et al.39have shown that the facilitation factors for solutes A and B can be expressed as
where CYAis known as the mobility ratio for solute A and is defined as cyA = DA,[Cl'I'/DAIAIO
(3)
in which DACand DAare the diffusion coefficients for complexed and uncomplexed A, respectively, [C]Tis the total concentration of the carrier in the membrane, and [AI0 is the concentration of A in the membrane at the membrane-feed interface. KA is the dimensionless equilibrium constant for A and equals K+[A]o. CYBand KBare defined in the same manner. For facilitation factors much larger than 1
and the calculated separation factor,
q k , can be expressed as
Equation 5 can be simplified by assuming that D A = DB,DAC= DBC,and [AI0 = [Blo. Since the pairs of alkenes investigated in this study and their Ag+ complexes are nearly identical in size, the assumption of nearly equal diffusion coefficients is justified if the diffusion processes are Fickian. The assumption that the two solutes have equal physical solubilities in the membrane, Le., that [AI0 = [Blo, is also a reasonable assumption for the solute pairs considered and can be verified measuring solubilities in the Na+-form of the membrane. These assumptions result in a simplified equation for the separation factor
Equation 6 provides an estimate for the separation factor which would be expected if the Ag+-Nafion membrane were acting like a macroporous support with a concentrated solution of Ag+SO3-R in the pores. Separation factors are also often calculated from the results of transport experiments involving single-component feed streams using eq 1. These values are referred to as "ideal" separation factors, Szp', in that they assume the presence of each solute in the membrane will not influence the solubility of other components in the feed mixture and vice versa. For many systems involving organic molecules in polymer membranes, measured separation factors are often less than the ideal value due to cooperative solubility. For the case of hydrophobic solutes partitioning into the ionic, aqueous regions of Nafion, cooperative solubility and values of SAB< S:;' would be anticipated. Competitive Absorption and Large Solubilities of Olefm and Aromatic Mixtures. For 25-pm Nafion 111 membranes, we previously reported styrene/ethylbenzene separation factors of
Thoen et al.
1264 The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 TABLE 1: Fluxes of Styrene and Ethylbenzene through Nafion 111 [feed side1 (M) membrane flux" separation factor form styrene ethylbnzn styrene ethylbnzn 0.82 0.00 0.50 2.9 0.50 0.00 21 1.6 0.38 0.50 0.10 3.5 0.22 16 0.50 0.50 6.6 0.25 13 0.50 1.o 7.3 0.17 11 0.50 2.0 14 0.16 8.9 0.50 5.0 0.041 0.50 0.0 j2.7 0.11 0.50 0.0 0.091 0.038 2.4 0.50 0.50 0
Units of mol/cm2 s x IC9.
TABLE 2: Concentrations of Styrene and Ethylbenzene in Nafion 117 [contacting concn in concn in phase]" Ag+-Nafionb (M) Na+-Nafionb (M) styrene ethylbzn styrene ethylbzn styrene ethylbzn 0.00 0.50 1.0 0.083 0.50 0.00 1.7 0.15 0.10 0.50 0.93 0.37 0.036 0.083 0.50 0.50 1.7 0.19 0.13 0.089 1.o 0.50 2.1 0.13 0.21 0.076 2.0 0.50 2.5 0.10 0.32 0.060 5.0 0.50 3.1 0.073 0.52 0.052
3.0 i2.5
- 2.0
0.0' l
0.0
1.5
2c l2
Styrene Flux
- 1.0 fic
[styrene] in Nafion
-
3
. . . . l . . . . l . . . . l . . . . l . . . . l . 0.50 ~ . . i
1.0
2.0
4.0
3.0
5.0
6.0
[Styrene]in Solutions (M) Figure 2. Styrene fluxes through (filled circles, left axis) and concentrationsof styrene in (opencircles, right axis) Ag+ form Nafion membranes vs the concentration of styrene in the feed or contacting solution. Solid
lines represent smooth curves through the data.
-
1.2
Ethylbenzene Flux [Ethylbenzene]in Nafion
2
1 .o 0.80
jO.60
18 and 33 for bicomponent feed solutions containing 1.O and 0.1 M of each solute, respectively. The literature values for the
cE
E
j.
v) yl
Contacting solutions are made up of H20-saturated isooctane and the indicated amountsof styrene and ethylbenzene. Concentrationsbased on 15%water uptake by Nafionandassuming thestyreneandethylbenzene are contained in the water-containing regions of the membranes.
equilibrium constants for Ag+ complexation in aqueous solution is 18 M-I for styrene and 2.7 M-1 for ethylbenzene. On the basis of these values, would be 6.7. Subsequent experiments revealed that if membrane fluxes are determined using a single solute in the feed solution, the ideal separation factor would be only 3.5. The bicomponent results indicated that competitive absorption into the membrane was occurring. Table 1 illustrates the magnitude of this competitive effect through a series of experiments in which the concentration of ethylbenzene in the feed is held constant at 0.5 M while the concentration of styrene is varied from 0.0 to 5.0 M. As the amount of styrene in the feed solutions increases, the flux of ethylbenzene is suppressed and rapidly approaches its nonfacilitated value. The amount of styrene flux increase diminishes as the styrene concentration in solution exceeds 1.0 M. This reduction indicates the onset of carrier saturation and the flux increase is primarily due to Fickian diffusion. Very similar trends are observed for the solutes' concentrations in the membrane. Table 2 lists the concentrations of styrene and ethylbenzene inside both Ag+ and Na+ form Nafion membranes and reveals that the presence of styrene effectively excludes ethylbenzene from the membrane. As expected there is a high correlation between the flux of a solute and its concentration in the membrane. The fluxes and concentrations of styrene and ethylbenzene verses the amount of styrene in the feed and contacting solutions are displayed in Figures 2 and 3. A few results for Na+-exchanged membranes are also reported in Table 1. These results illustrate that the high membrane fluxes, concentrations and separation factors are due to the presence of Ag+. Competitive absorption effects are not observed in Na+ form membranes. The results and analysis described above indicated that the complexation equilibria between Ag+ and olefins in Nafion could be much different than in aqueous solution. Therefore separations involving alkenes with smaller differences in equilibrium constants
4
5
jo.20
0.0
1.0
2.0
3.0
4.0
5.0
0.0 6.0
3
[Styrene]in Solutions (M)
Figure 3. Ethylbenzene fluxes through (filled circles, left axis) and concentrations of ethylbenzene in (open circles, right axis) Ag+ form
Nafion membranesvsthe concentration of styrene in the feed or contacting solution. Solid lines represent smooth curves through the data. TABLE 3: Fluxes of 1J-Hexadiene and 1-Hexene through Ag+ Nafion 111 feed side concn (M) flux X lo9 (mol/(cm2s) separation 1.5-hexadiene 1-hexene 1,Shexadiene 1-hexene factor 0.00 0.50 4.0 4.3 0.00 17 4.3 0.50 1.o 70 0.01 0.50 1.4 0.10 0.50 5.8 0.49 59 0.50 0.50 19 0.44 43 2.0 0.50 22 0.20 28 5.0 0.50 24 0.10 24 with Ag+ were explored. The competitive effect between 1,shexadiene (KW= 1850) and 1-hexene (Kq= 860) is summarized in Table 3, which reveals that an extremely small amount of l$hexadiene in the feed solution drastically suppresses the flux of 1-hexene. The presence of only 0.01 M 1,5-hexadiene in feed solutions containing 0.5 M 1-hexene produces a 4-fold reduction in the fluxof 1-hexene. Other experiments reveal that theamount of 1-hexene in the feed solution has little or no effect on the flux of 1,5-hexadiene or its concentration in themembrane. Although p aAB l c ' is 2.1 for this separation and single-component experiments indicate that S:;' is 4.3, an equimolar, bicomponent experiment results in a separation factor of 43! The concentrations of the two hexenes in the membrane are displayed in Table 4. Singlecomponent experiments indicate that the two permeates are able
The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 1265
Olefin Separations in Ion-Exchange Membranes
8.0
TABLE 4 Concentrations of 1,5-Hexadiene and 1-Hexene in Ag+-Nafion 117 concn of contacting phasen (M) concn in Ag+-Nafionb (M) 1,s-hexadiene 1-hexene 1,s-hexadiene 1-hexene 2.1 0.50 2.0 0.50 0.01 0.50 0.13 1.5 0.10 0.50 1.1 0.43 0.50 0.50 1.9 0.17 2.0 0.50 2.5 0.11 Contacting solutions are made up of H20-saturated isooctane and the indicated amounts of 1,s-hexadieneand 1-hexene. Concentrations based on 15% water uptake by Nafion and assuming the hexenes are contained in the water-containing regions of the membranes.
I . . . I . . . . I . . . . , . . I
5.0
-
4.0:
-,
7.0 :
6,0 5
-
1,s-Hexadiene
o
-
1-Hexene
3.0 0.0
1.0
2.0
3.0
4.0
5.0
6.0
Time x lo'' (s) F i p e 6 . Fluxof 1-hexene (opencircles) and 1,Shexadiene(closedcircles) through Ag+form Nafion membranes with a feed solution that is initially 0.5 M in 1-hexene. At 2700 s, 1,s-hexadieneis added to the feed solution
making its concentration 0.5 M.
TABLE 5 Fluxes and Separation Factors of Dienes vs Moaoenes through Ag+-Nafion 111
0.5
0.0
1.5
1.0
2.5
2.0
[1,5-Hexadiene]in Solutions (M) Figure 4. 1,s-Hexadiene fluxes through (filled circles, left axis) and concentrations of 1,Shexadiene in (open circles, right axis) Ag+ form Nafion membranes vs the concentration of 1,s-hexadienein the feed or contacting solution. Solid lines represent smooth curves through the data. 5
0
1
"
'
~
~
'
'
~
'
"
~
"
' 3 5~
'
'
I-Hexene Flux
[I-Hexene] in Nalion
4.0 * '
'
feed concn (M) flux X io9 (mol/cm2) separation l,4-pentadiene 1-pentene 1,4-pentadiene 1-pentene factor 0.00 0.50 12 1.3 0.00 16 1.3 0.50 0.50 22 2.9 6.8 0.50 1,s-hexadiene 1-hexene 1,Shexadiene 1-hexene 0.00 0.50 4.0 4.3 17 0.50 0.00 4.3 0.50 19 0.44 43 0.50 1,7-octadiene 1-octene 1,7-octadiene 1-octene 0.00 0.50 1.2 2.3 2.7 0.50 0.00 2.3 ~ 2.6 0.039 0.50 0.50 67 1.9-decadiene 1-decene 1,g-decadiene 1-decene 0.00 0.50 0.20 3.3 0.50 0.00 0.65 3.3 0.50 0.50 0.69 0.0084 82
3.0 X
x
iL
-
- 1.4
2.0
-
._ 6 c
-0.70
1.0-
< 0.01
-0.0
'
'
.
"
0.5
"
'
1 .o
I '
"
I
1.5
'
'
'
I
2.0
E
s "
"0.0 2.5
[l ,5-Hexadiene] in Solutions (M)
Figure 5. 1-Hexene fluxes through (filled circles, left axis) and concentrationsof 1-hexenein (open circles, right axis) Ag+ form Nafion
membranesvs theconcentrationof 1,s-hexadienein the feedor contacting solution. Solid lines represent smooth curves through the data. to penetrate the membrane to the same degree. However, bicomponent experiments reveals that an extremely small amount of 1,Shexadiene effectively excludes 1-hexene from the membrane. The fluxes and membrane concentrations of the two hexenes as a function of the amount of 1,Shexadiene present in the feed and contacting solutions are depicted in Figures 4 and 5 . Figure 5 , which depicts the situation for 1-hexene, clearly shows that the selectivity observed for this separation results from competitive absorption. From the data given above, one could argue that 1,5-hexadiene is simply able to penetrate the membrane much quicker than
1-hexene; thereby occupying a majority of the carrier sites and excluding 1-hexene from the membrane. However, both transport and concentration experiments prove that this is not the case. A transport experiment was performed in which the feed solution initially contained only 1-hexene. After the flux of 1-hexene had reached steady state, an equal concentration of 1,Shexadiene was added to the feed solution. As shownin Figure 6, the 1-hexene flux is immediately suppressed upon the addition of 1,Shexadiene to the feed. In another experiment, a piece of Nafion was immersed in a contacting solution initially containing only 1-hexene. After 48 h, an equal amount of 1,Shexadiene was added to the contacting solution. After an additional 48 h, the two solutes were extracted from the membrane and their concentrationsdetermined. The concentrations of 1,Shexadiene and 1-hexene in the membrane from this experiment were the same as those from an experiment in which the contacting phase initially contains both solutes. Therefore 1,Shexadiene actually displaces 1-hexene from the membrane. These two experiments prove that the competitive effect is not a function of how quickly solutes partition into the membrane. Effect of SoluteStructure. The way in which a solute's structure affects its transport was investigated via experiments involving a series of diene/monoene pairs. Four pairs, ranging from C Sto Clo,were evaluated. As Table 5 shows, the diene dominates the corresponding monoene in every case. Also evident from the
Thoen et al.
1266 The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 TABLE 6 Fluxes and Facilitation Factors of 0.5 M Id-Hexadiene through Ag+ and Na+ Form Nafion Membranes of Various Thicknesses thickness" Ag+ form fluxb Na+ form fluxb facilitation factor 2.5 99 0.38 26 1 5.0 44 0.17 259 10 43 0.13 331 20 22 0.090 244 15
40 0
0.049
5.0
I
I
f
I
I
4.0
306
Units of micrometers. * Units of mol/(cm2 s) x l e 9 . 15-Hxdne. K1 i19,K2 = 0
TABLE 7: Fluxes and Separation Factors of 0.5 M 1,5-Hexadiene and 0.5 M 1-Hexene through Ag+-Nafion Membranes of Various Thicknesses thickness' 1,5-hexadienefluxb 1-hexene fluxb separation factor 2.5 67 2.0 34 5.0 39 1.1 35 10 23 0.56 41 20 16 0.59 27 40 0
13
0.33
39
Units of micrometers. * Units of mol/(cm* s) x le9. 70.0
I
I
7
1
I
I
7
I
I
I
I
I
I
I
1
I
I
,
I
-
7
1,s-HexadieneFlux X lo9
30.0
0.0
0.0
0.1
0.2
4
0.3
0.4
0.5
l/Membrane Thickness (pm).'
Figure 7. Fluxes of 1,5-hexadiene (closed circles) and 1-hexene (open circles) through cast Nafion membranes ranging in thickness from 2.5 to 40 pm. The feed solution was 0.5 M in 1,5-hexadiene and 1-hexene.
table is the fact that the separation factors increase with the length of the solute pairs. The actual separation factor for 1,4pentadiene/ 1-pentene is 6.8, while the actual separation factor for 1,9-decadiene/l-decene is 82! By comparison of the concentrations and fluxes of the decenes to those of the hexenes, it is apparant that the size of the solute greatly affects its mobility in the membrane. While the concentrations of the decenes in Nafion are approximately half those of the hexenes, the decene fluxes are slower than those of the hexenes by a factor of 20. Effect of Membrane Thickness. A major challenge for most membrane systems is to increase the flux of solutes through the membrane while maintaining a high separation factor. The simplest way to increase the rate of solute transport is to use a thinner membrane. However, in many cases this results in a reduction in the separation factor. To investigate the productivity/ selectivity relationship for Ag+-Nafion, a set of membranes ranging from 2.5 to 40 wm was prepared and transmembrane fluxes of 1,s-hexadiene and 1-hexene were measured. Table 6 contains the 1,s-hexadiene fluxes for Ag+ and Na+ forms of the membranes. For all membrane thicknesses, the facilitation factors were several hundred, indicating that a facilitated transport pathway remains active. If the complexation kinetics for 1,shexadiene and 1-hexene are at equilibrium, the separation factor should also be invariant over the entire range of membrane thicknesses. Table 7 and Figure 7 contain data for transport of bicomponent feedsolutions of 1,5-hexadieneand 1-hexene through
".I
0.0
1.o
2.0
3.0
4.0
5.0
.
6.0
[Permeate] in Contacting Phase (M)
Figure8. Linear regression of single-componentisotherms for absorption of 1,5-hexadiene (crosses) and 1-hexene(squares) into Ag+ form Nafion membranes. See text for details.
the Ag+ forms of the membranes. The separation factor (13hexadiene flux/ 1-hexene flux) remains relatively constant averaging approximately 35 over the entire thickness range. The fact that the fluxes increase linearly with 1/Lis also an indication that the system is a t reaction equilibrium. AbsorptionIsotherms. The results acquired for binary mixtures of 1,s-hexadiene and 1-hexene led us to believe that singlecomponent absorption isotherms for these solutes would be quite different. However, Figure 8 reveals that the isotherm of 1,shexadiene is very similar to that of 1-hexene. Single-component experiments using 1.O M contacting phases resulted in membrane concentrations of 0.13 and 0.076 M for 1,5-hexadiene and 1-hexene, respectively. Since the concentration of Ag+ in the water-containing regions of the membrane is roughly 7 M, one would not expect to observe any competitive effects with a contacting phase containing both solutes at 1.O mM. However, even at these small concentrations, the concentration of 1-hexene in the membrane is suppressed by a factor of 3 to 0.024 M. The concentration of 1,s-hexadiene in the membrane is not affected by 1-hexene. The single-component absorption isotherms for styrene and ethylbenzene were also measured and arevery similar. These isotherms are also very similar. As with the hexenes, a bicomponent contacting phase containing 1.OmM concentrations of styrene and ethylbenzene results in competitive absorption causing a decrease in the ethylbenzene concentration.
Discussion Concentrations of Olefins in Aqueous Salt Solutions and Hydrated Nafion Membranes. The absorption and mobility of hydrophobic organic molecules such as olefins and aromatics in hydrated Nafion membranes exhibit a number of interesting features. These features can be illustrated by comparing data obtained for hydrated Nafion to similar data for aqueous salt solutions. For the experiments reported here and e l ~ e w h e r e , ~ l - ~ ~ the water content for the Nafion membranes in either the Ag+ or Na+ form is 15 1% by mass. On the basis of the equivalent weight of dry Nafion (1 100 g/equiv) and assuming that all of the ion-exchange sites are hydrated, one can calculate that the concentration of ions in the water in hydrated Nafion is 6.7 M. The values of Kq used to calculate separation factors (eqs 5 and 6) pertain to 1.O M AgNO3 solutions. To demonstrate that these Kq values were appropriate for absorption of organics from isooctane solutions and that Na+ ions had little effect on this absorption, the concentrations of styrene and ethylbenzene in 1.O M NaN03 and 1.OM AgN03 equilibrated with the feed solutions in Table 2 were determined. For 1.OM NaN03, the solubilities of styrene and ethylbenzene for 0.5 M contacting isooctane phases were 5.2 X 10-4 and 3.1 X 10-4 M, respectively. These solubilities were not influenced by
*
Olefin Separations in Ion-Exchange Membranes the presence of a second component in the contacting phase. The concentration of styrene in the aqueous phase varied with its concentration in the isooctane as expected. For 1.OM AgNO3, the solubilities of styrene and ethylbenzene for 0.5 M contacting isooctane phases were 8.9 X and 9.1 X 1 V M, respectively. As with the NaNO3 solutions, there was no evidence for competitive absorption into these aqueous phases. Analysis of all of the data for these experiments yielded average values for Kq of 17 f 1 and 2 f 1 M-I for styrene and ethylbenzene, respectively, which are within experimental error of the literature values. These experiments indicate that large separation factors due to competitive absorption would not be observed for liquid membranes composed of aqueous silver salts. Data from X-ray scattering experiments indicate that the microstructure of Nafion consists of ionic clusters that are approximately 50 A in diameter connected by channels that are lOA MostofthewaterinhydratedNafioniscontained in this cluster/channel network. For hydrated, Na+ form Nafion, the concentration of olefins found in membranes equilibrated with 0.5 M contacting solutions are on the order of 0.1 M based on the water content of the membrane. These values are hundreds of times greater than what is observed for aqueous NaN03 solutions, indicating that most of the hydrophobic organics are probably located in the interfacial regions between the water in the cluster/channel network and the regions of the membrane consisting of dense, perfluoroethylene polymer. Despite the relatively large concentrations of olefins found in Na+-Nafion, competitive absorption effects are not observed and large separation factors for alkene/alkane or alkene isomer mixtures are not observed. Concentrations for alkenes and aromatics found in Ag+-Nafion exceed those found in the Na+ form by about 1 order of magnitude. These higher concentrations result in enhanced transmembrane fluxes and values of SAB that are much greater than . ':S Kinetics of the Ag(I)-Olefm Complexation Reactions. Since membrane separations are inherently rate processes, it is essential that the chemical reaction responsible for the high selectivity in an FTM must have facilekinetics. If this is not the case, relatively thick membranes must be used to achieve high selectivity. This limits the productivity of the membrane and adversely affects the cost effectiveness of the separation process. For membrane separations that do not rely on chemical reaction, such as the separation of 0 2 and N2, Martin and co-workers have demonstrated that polymer films as thin as 50 nm still exhibit high ~electivity.~~ In an earlier p ~ b l i c a t i o n ,diminished ~~ styrene/ ethylbenzene separation factors for 2.5- and 5.0-pm membranes prepared from Nafion casting solutions were reported. By using the DMSO casting procedure reported herein, this loss in selectivity for thin membranes is not observed. As indicated in Figure 7, Ag+-Nafion membranes as thin as 2.5 pm can be prepared with no loss in selectivity for olefin separations. Furthermore, transmembrane fluxes are inversely proportional to the membrane thickness. These data indicate the membrane is operating in the reaction equilibrium (diffusion-limited)regime. According to the facilitated transport model of Dindi et al.,39the reaction equilibrium regime is observed when the ratio of reaction time to diffusion time is CO.01. This ratio can be expressed as DAc/k-lL2,where kl is the rate constant for the reverse of the complexation reaction and L is the membrane thickness. In an earlier publication the diffusion coefficient for the complex formed between styrene and Ag+ in Nafion 117 was calculated from the transient period that occurs at the beginning of a flux experiment and found to be 1.3 X cm2 SKI. For reaction equilibrium to be in effect for a 5-pm membrane, k-1 would have to be greater than 0.05 SKI. This modest requirement for the kinetic facility of the complexation reaction is due to the low diffusion coefficient found for organic cations in Nafion and indicates that the onset
The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 1267 for a limitation due to reaction kinetics for this process might occur only for very thin membranes. A second piece of evidence indicating that equilibrium for the reaction-induced absorption of olefins is rapidly established with respect to diffusion across the membrane is the "spiking" experiment depicted in Figure 6. When 1,5-hexadiene is added to a feed solution initially containing only I-hexene, fluxes that reflect the competitive absorption effect are observed at the first data point following the addition. Factors Responsible for Competitive Absorption and Large Selectivities. One plausible explanation for the high selectivities observed for dienes/monoenes and styrene/ethylbenzene separations is that absorption of different organics into the membrane results in changesin freevolumeor water content. This possibility was explored by comparing the physical dimensions, mass, and infrared spectra for membranes that had been equilibrated with isooctane solutions containing the different organics at high concentrations. These experiments revealed no significant differences in the physical properties of the membrane itself due to adsorption or different organic molecules. While the competetive adsorption and large selectivities are not completely understood at this time, these effects appear to be related to the ability of dienes and, possibly, styrene to be complexed with more than one Ag+ ion. While 1:2complexation is usually not observed in dilute aqueous solutions of AgNO3, the formation of 1:2 olefin-Ag+ complexes increases with increasing Ag+ concentration.1° Since the Ag+ concentration in the watercontaining regions of Nafion is calculated to be almost 7 M, the dual complexation may become important within the membrane. The importance of this second complexation step on the amount of alkene absorbed into a Nafion membrane can be calculated from the following equilibria in which L represents an alkene that can complex to only one Ag+ (e.g., 1-hexeneor ethylbenzene and L-L represents a diene or styrene): L L-L Ag+
K
+ Ag'
+ LAg'
+ Ag'
+ L-LAg'
+ L-LAg'
KI
Kz
+ Ag'L-LAg'
The concentrations of L and L-L that would be found in an aqueous solution or Nafion membrane containing Ag+ that is in contact with a feed solution containing these alkenes can be calculated as
= [L-L]{l
+ K,[Agl+] + K1K2[Ag']2)
(9)
In the equations above, [LIT and [L-L]T represent the total concentrations of L and L-L in the Ag+-containing phase and [L] and [L-L] represent the physical solubilities. The latter quantities can readily be determined by absorption measurements using membranes containing a noncomplexing ion such as Na+. Assuming that the physical solubilities are the same for Na+and Ag+-Nafion is reasonable sinceother properties such as water content are nearly identical. There are several conditions that would have to be satisfied in order for the second complexation step available to L-L to result in competitive absorption. Clearly, competition results from a high fraction of the Ag+ ion being complexed. This will occur when K,K1, K2, [L], and [L-L] are relatively large. For diene/ monoene pairs with similar sizes and structures, values of K and
1268
The Journal of Physical Chemistry, Vol. 98, No. 4, 1994 3.0,.
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Figure 9. Calculated concentrations 1,5-hexadiene (top) and 1-hexene (bottom)absorbed into Ag+ form Nafion membranes from bicomponent contacting solutions. Experimental points are from Figures 4 (top) and 5 (bottom). See text for explanation of assumptions involved in various calculations.
K I as well as [L] and [L-L] will be similar, therefore, Land L-L will compete for the Ag+ ions equally well, and large selectivities would not be anticipated if only 1:1 complexation was significant. The concentration of Ag+L-LAg+ will become important if Kz is comparable to K and K 1 and when the [Ag+] is large, i.e., > 1 M (see eq 9). These conditions would not normally exist for simple aqueous solution of Ag+ salts because the physical solubilities of hydrophobic organic molecules are too low. As discussed above, the physical solubilities of organic molecules in Na+-Nafion membranes are quite high based on the observed water contents. For example, the observed partition coefficients, K p= [alkenein Nafion]/ [alkene in hydrocarbon contacting phase] for Na+-Nafion are 0.108 and 0.083 for 1,s-hexadiene and 1-hexene, respectively. By assuming that the physical solubilities for organics are identical in Na+- and Ag+-Nafion membranes, these partition coefficients can be used to calculate the total concentrations of these alkenes in Nafion based on the above mechanism by assuming various values of K , K1, K2, and the total concentration of accessible Ag+ ions in Nafion. Some results of such calculations are contained in Figures 8 and 9. The lines in Figure 8 are the result of writing the algebraic equations outlined above in a linear form and performing leastsquares regression against the experimental data. For the single component adsorption isotherm of 1-hexene into Ag+-Nafion, the best fit to the data (solid line) resulted in a value of K = 31 and a total Ag+ concentration of 4.0 M. The fact that the value of total Ag+ concentration obtained from the best fit is somewhat smaller than the value obtained from the ion-exchange site density and water content (6.7 M) may indicate that all of the Ag+ ions are not accessible to the olefins. If the single-component absorption data for 1,s-hexadiene is fitted using the same
Thoen et al. equations, but constraining to [&+IT = 4.0 M (dashed line), a poor fit is obtained with K1 = 19. Allowing both Kl and [Ag+]r to be fitted for the 1,s-hexadiene results in values of K I= 97 and [ A ~ + ]= T 2.9. While these values produce a good fit to the data (not shown), thevalues of K1 and [ A g + lare ~ not compatible with the values obtained for 1-hexene, Le., K1 is much larger than expected and [&+IT is much too low. The dash-dot line in Figure 8 represents a best fit using algebra that allows for 1,s-hexadiene to complex to two Ag+ ions and constrains [ A ~ + ]= T 4.0 M. The values of K I = 24 and K2 = 1.3 obtained are quite reasonable in comparison to the calculated value of K obtained for 1-hexene. While assumptions allowing 1,5-hexadiene to react with one or two Ag+ ions both provide good fits to the single component absorption data, the ability of these two assumptions to explain the competitive absorption data is quite different. This is illustrated in Figure 9 which depicts the calculated values of the total membrane concentrations for feed solutions that are 0.5 M in 1-hexene and also contain varying concentrations of 1,shexadiene (this is the data shown on the right-hand y axes in Figures 4 and 5). As is obvious for both the calculated 1,5hexadiene (top) and 1-hexene (bottom) concentrations, the calculations based on best-fit values for single binding to Ag+ (solid lines) do not result in a good match to the observed values. However, allowing dual complexation of the diene (dashed lines) results in calculations that approach the experimental data but underestimate the competitive effect. It is not possible to use the best fit for the single-component 1,s-hexadiene where K1 = 97 and [&+IT = 2.9, because in such calculations [Ag+] = 2.9, because in such calculations [ A ~ + ] T must be equal for the two alkenes. Simply increasing the value of K1 for 1,s-hexadiene while maintaining [&+IT = 4.0 M never produces a good fit for the competitive absorption experiments. Closer agreement between the calculated curves and the experimental values can be obtained by increasing the value of K2. For molecules with noninteracting, identical binding sites, it can be shown that K2 is equal to If one assumes that thisis thecase for 1,s-hexadieneand 1-hexeneandfurtherassumes that the first binding constant for the two molecules are equal, values of 24, 24, and 6 for K, K1, and K2, respectively, would be anticipated. A calculated curve based on these assumptions is also shown in Figure 9 (dashed lines) and comes closer to the observed competitive absorption effect. The model proposed above is also consistent with the observations in other aspects. First, the data in Table V indicate that the competitive effect increases as the distance between double bonds in thedienes increases. This is quite reasonable since larger dienes will experience reduced electrostatic and steric repulsions upon complexation with a second Ag+ ion. Second, it is always observed that the concentration of monoene in the feed solution never has any appreciable effect on the concentration of diene in the membrane. This is impossible to explain if only 1:l complexation is occurring for both species and t h e R s aresimilar, which would be expected and is observed for aqueous solutions of Ag+. Calculations similar to those depicted in Figure 9 but allowing the concentration of 1-hexene in the feed solution to vary show virtually no effect on the total 1,5-hexadiene concentration in the membrane. This is because most of the 1,shexadiene exists as Ag+L-LAg+. Potential Applications of Separations Utilizing Facilitated Transport in Ionomer Membranes. Although additional studies will be required in order to elucidate the physical processes that produce the large and unexpected selectivities in detail, this work has a number of interesting implications regarding the potential applications of FTM membrane-based separations. Two important requirements for applicability are the abilities to maintain high selectivities for thin membranes and at high feed concentrations. Both of these requirements are met for alkene transport in Ag+-Nafion.
Olefin Separations in Ion-Exchange Membranes If the simple model proposed above is the appropriate (albeit certainly incomplete) explanation for the competitive adsorption that is observed for these systems, it suggests that any difunctional/ monofunctional pair of molecules could be separated with high selectivity using the appropriate complexing agent and membrane. What is required is a membrane with regions containing high concentrations of the complexing agent. The distance between complexing sites in these regions could, in principle, be controlled. This would effect the values of K1 and K2 for a particular difunctional molecule which could be used to optimize the selectivity for a given molecular size or for a particular isomer. Equally important are the effects of membrane structure and morphology on the physical adsorption of the solutes to be separated. If the physical solubility is too low, complexation will not be extensive and competitive absorption will not be observed. If it is too high, normal Fickian diffusion will dominate over facilitated transport resulting in poor selectivity. Finally, physical absorption of the solutes must occur in the same regions of the membrane that contain the complexing agents or the reactions responsible for facilitated transport can not proceed. We believe that meeting all of these demanding requirements for a given separation can be best achieved through collaboration between chemists, chemical engineers, and polymer scientists.
Acknowledgment. The authors gratefully acknowledge the financial support from British Petroleum America (BP CEMRA Grant) for this research.
diffusion coefficient, cmz s-l facilitation factor; ratio of solute flux with carrier present to the solute diffusion flux a t the same driving force dimensionless equilibrium constant for complexation; equal to K,['41 chemical equilibrium constant for complexation, M-I alkene that can bind to only one Ag+ ion alkene that can bind to one or two Ag+ ions separation factor for solutes A and B; ratio of permeabilities; see eq 1 separation factor calculated from eq 5 separation factor calculated from eq 6 ideal separation factor obtained from experiments involving single-component feed streams mobility ratio; see eq 3
References and Notes (1) Humphrey, J. L.; et al. Separation Technologies-Advances and Priorities; US.DOE Report ID-12920-1, 151-163,1991. (2) Davis, J. C.; et al. Sep. Sci. Techno/., in press. (3) Way, J. D.;Noble, R. D.;Flynn, T. M.; Sloan, E. D.J. Membr. Sci. 1982,12,239-259.
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