1116
Ind. Eng. C h e m . Res. 1992,31, 1116-1122
Swelling and Thickness Effects on the Separation of Styrene and Ethylbenzene Based on Facilitated Transport through Ionomer Membranes Carl A. Koval,* Terry Spontarelli, Paul Thoen, and Richard D. Noblet Department of Chemistry and Biochemistry, Campus Box 215, University of Colorado, Boulder, Colorado 80309
The substitution of silver ion for sodium ion in hydrated ionomer membranes causes the transmembrane fluxes for styrene (STY) and ethylbenzene (EBZ) to increase by 2-3 orders of magnitude. These flux increases result from the reversible interaction of STY and EBZ with silver ion in the membrane (facilitated transport). Separation factors of STY/EBZ for thick membranes (approximately 25 and 175 km thick) have been previously reported to range from 8 to 36. Treatment of the 175-km membranes with glycerine at elevated temperatures causes a permanent expansion of the membrane structure (swelling), which increases diffusion coefficients by 1 order of magnitude. The heat-treated membranes have higher water contents resulting in lower silver ion concentrations and diminished STY/EBZ separation factors. Cast ionomer membranes of approximately 2.5 and 5 pm in thickness display permeate fluxes that are 2-100 times greater than those observed for the non-heat-treated commercial membranes while STY/EBZ separation factors are maintained near 10.
Introduction The transport of molecules across and within membranes is important to a wide variety of separation, analytical, and biological processes. The phenomena of facilitated transport can selectively enhance transmembrane fluxes for certain components of a mixture. Facilitated transport results from the reversible complexation of solutes with membrane bound complexing agents referred to as carriers. Depending on the selectivity of the complexation reaction and the relative solubilities of solutes in the membrane phase, a facilitated transport membrane (FTM) can provide highly selective separations. There have been several review articles describing various aspects of facilitated transport in membranes (Noble et al., 1989; Noble and Way, 1987; Way et al., 1982; Schultz, 1986; Kimura et al., 1979). Recently, we reported that transmembrane fluxes for 1-hexene and 1,Bhexadiene across hydrated Nafion (E. I. du Pont de Nemours & Co., Wilmington, DE) membranes are increased by factors of several hundred when silver ion is used as a carrier (Koval and Spontarelli, 1988). In these experiments, the aqueous membrane phase forms a semipermeable barrier between two organic phases. One of the organic phases contains olefin(s) (the feed stream), while the other does not (the sweep stream). When the ionomer membrane is in the Na+ form, the olefins cross the membrane barrier at different rates, depending on their solubilities and diffusion coefficients in the membrane phase. For olefins with similar molecular weights, this diffusive process results in poor selectivity and/or low fluxes in membranes which do not utilize facilitated transport. The reversible one-to-one complexation of olefins by aqueous Ag+ ion is a well-known reaction (Beverwijk et al., 1970). When the membranes are in the Ag+ form, olefins that form complexes with silver ion have an additional mechanism available for crossing the membrane and their transport is facilitated. There are examples in the literature of gas-phase olefin FTM separations (Hughes et al., 1986; Teramota et al.,
* To whom correspondence should be addressed.
Department of Chemical Engineering, Campus Box 424, University of Colorado, Boulder, CO 80309.
1986),including the use of an ion-exchange membrane as a support (LeBlanc et al., 1980). Recently, the use of zeolite-filled silicone rubber membranes for olefin separations was also reported (Jia et al., 1991). Our research to date has focused on liquid olefins. Initially, we demonstrated that the rates of transport of 1-hexene and 1,shexadiene through thin (ca.30-pm) Nafion membranes can be increased by a factor of several hundred by substituting silver ion for sodium ion (Koval and Spontarelli, 1988). In a subsequent study, the separation of equimolar mixtures of styrene and ethylbenzene by facilitated transport through Nafiion membranes was studied under a variety of experimental conditions (Koval et al., 1989). Although separation factors (styrene permeability/ethylbenzene permeability) of up to 36 were achieved, the styrene transport fluxes were only on the order of 10-lomol cm-2 s-l. The reason for the low fluxes is probably a combination of the low solubility of organics in the aqueous membrane phase and the low mobility of species in the Nafion environment. The use of a 50% ethanol/water membrane solvent resulted in a flux increase of almost 2 orders of magnitude; however, the separation factor was reduced to 10. The increase in solubility of the permeates in the 50% ethanol/water membrane resulted in large diffusional fluxes which degraded the separating power of the membrane. In this study, two approaches for improving the productivity of ionomer membranes were examined. One approach involved a technique for permanently swelling the membrane and thereby changing its microstructure. The microstructure of water-swollen Nafion membranes has been studied by small-angle X-ray scattering (SAXS) and is thought to consist of ionic cluster regions 40-50 A in diameter connected by 10-A-wide channels and surrounded by a fluorocarbon polymer phase (Gierke and Hsu, 1982). Diffusion of permeates and complexes must occur through the ionic regions which contain most of the absorbed water. The diameter of a Ag+-STY or -EBZ complex can be estimated from a table of bond lengths and is on the same order of magnitude as the channel size of the membrane. The low mobility of species within Nafion membranes could be due to the small size of the cluster-channel network as well as the attraction between the fixed sulfonate exchange sites and the silver cation.
0888-5885/92/2631-1116$03.00/00 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 4,1992 1117 Heaney and Glugla have performed SAXS and conductivity studies on Ndion membranes that had been heated in glycerine (Heaney, 1988). The details of the glycerine treatment, as well as the interpretation of the SAXS data, were later reported by Heaney and Pellegrino (1989).They also measured increased facilitated transport for C02 using ethylenediamine as a carrier. Further evidence for enhanced facilitated transport for COPand H2S was also reported by Pellegrino et al. (1988).The authors concluded that the glycerine treatment resulted in a permanent increase in the dimensions of the cluster-channel network in Nation. We were interested in determining how this change in morphology would effect the membrane separation of STY and EBZ. In addition to STY and EBZ transport data for the glycerine-treated membranes, an analysis of time-dependent fluxes is used to calculate diffusion coefficients for the Ag+-styrene complex. There have been recent studies on the mechanism of facilitated transport in fixed site carrier membranes (Noble, 1990, 1991a,b). These studies have indicated that there is a mobility associated with interaction of the solute and bound carrier. This mobility gives rise to an "effective" diffusion coefficient. This analysis may also be applicable to facilitated transport in ion-exchange membranes since the complexing agent is electrostatically bound to the membrane. The mathematical treatment of facilitated transport using this model does not change; however, the physical interpretation of the diffusion coefficient does change. A second approach to improved productivity is the use of thinner membranes. Since thin Nafion membranes are not commercially available, a commercially available casting solution was used to prepare membranes with measured thicknesses of 2.5 and 5.0 pm. These cast membranes yield significantly larger fluxes and slightly diminished selectivities.
Experimental Methods Commercial Membrane Materials. The ionomer membrane material used in most experiments was Nafion 117 which has an equivalent weight of 1100 and a dry thickness of 175 pm. When spectroscopic measurements required a shorter path length (due to high absorbances), an experimental Ndion membrane was used, NE 111, provided by Dr. Louis L. Burton (Experimental Station, Central Research and Development, E. I. du Pont de Nemours & Co., Wilmington, DE). This membrane also has an equivalent weight of 1100 but a dry thickness of only 30 pm. The membranes as received in the H+ form were converted to the Na+ form by gentle stirring in a 1 M solution of sodium hydroxide for 24 h. Conversion to the Ag+ form was accomplished by treating the Na+ form membrane with a 1 M solution of silver nitrate. The Ag+ in Nafion reacts with reducing agents and is subject to photoreduction. Therefore, after a membrane was exchanged to the Ag+ form, it was stored in 1 M AgNO, in the dark. All experiments involving membranes containing Ag+ were performed in a way as to minimize exposure to light. Gravimetric Measurements. In order for concentrations of species within the membranes to be calculated, the water contents of the Na+ and Ag+ Nafions were determined. Samples of both cationic forms were prepared, and the water-swollen membranes were weighed at 100% humidity. Since the flux measurements described below require the membranes to be in contact with water-saturated isooctane, the fully hydrated membranes were placed into water-saturated isooctane for 24 h and weighed again. The membranes were then placed in a vacuum desiccator
for 24 h to obtain the dry mass. Gravimetric measurements were also used to determine the concentration of silver carrier. A membrane was exchanged to Na+ and dried for 24 h in a vacuum desiccator. After the dry mass was measured, the membrane was exchanged to the Ag+ form and dried again. Assuming that the difference in dry mass of the two cationic forms is due only to a one-for-one exchange of Na+ for Ag+,the number of moles of silver ion can be calculated from the difference in atomic weights. Extraction of STY and EBZ from Membranes. Certain calculations require the concentration of the styrene and ethylbenzene in the aqueous membrane phase. This was determined by extraction of the STY and EBZ from the membranes with isooctane. Samples of the Na+ form membrane were prepared and dried in a vacuum desiccator for 24 h to obtain their dry mass. After the membranes were hydrated, they were placed in a 1 M solution of each of the permeates under study. These solutions contained the same species at the same concentrations as described under the Flux Measurement heading of this section. The membranes were stirred for approximately 24 h. They were then washed with isooctane to remove the STY and EBZ from the surface and placed into a measured amount of organic solvent for the extraction. After these mixtures were stirred for 48 h, the concentration of STY and EBZ in each of the solutions was determined by gas chromatography. The organic phases contacting the membranes were saturated with water to minimize water loss from the membranes. On the basis of the water content measurements described above and the amounts of permeates extracted from the membranes, concentrations of styrene and ethylbenzene were calculated. For verification that STY and EBZ were quantitatively extracted, UV spectroscopy was used to examine the membrane that had been exposed to styrene (Arnm = 246 nm, t = 1.4 X lo4 L mol-' cm-'). Using a membrane that had not been exposed to styrene as a blank, spectra were recorded for the Na+ form membrane before and after the extraction. The spectrophotometer used was a HewlettPackard 8451A diode array model. The completeness of the extraction was tested for styrene by examining the membrane before and after extraction with UV spectroscopy. The absorbance at 246 nm decreases from 1.4 to 0.089,indicating that styrene is quantitatively removed from the Na+ form membrane after 48 h of stirring in water-saturated isooctane. It is important that the organic solvent used for the extraction be water-saturated, since experiments performed with dry solvent showed little change in the UV spectrum. This was probably due to closing of the water-filled channels at the surface of the membrane, trapping the styrene in the membrane. The validity of the above procedure can be verified by using different concentrations of permeates in the contacting solutions. For Na+ form membranes, the measured concentrations of permeates in the membranes varies linearly with their concentrations in the contacting solutions. Glycerine Heat Treatment. After the membranes were exchanged to the Na+ form, they were dried in a vacuum desiccator for 24 h. They were then saturated with glycerine and slowly heated at 1-2 OC/min. When the desired temperature was reached, the membranes were allowed to cool to room temperature and were washed with distilled water to remove the glycerine. Examination of the membranes with infrared spectroscopy (IBM IR/30 series FTIR spectrometer), showed that glycerine removal was complete. The mass and dimensions of the water-
1118 Ind. Eng. Chem. Res., Vol. 31, No. 4, 1992 Table I. Transeort Data for Styrene (STY) and Ethylbenzene (EBZ) through Heat-Treated Nafion 117 Nat flux" Ag+ fluxa Ag+ time intercept, s treatment temp, O C STY EBZ STY EBZ STY EBZ none 1.1 x 10-11 3.3 x 10-12 1.1 x 104 6.0 x lo-" 7700 7500 1.7 X lo4 180 7.9 x 10-11 3.1 X lo-" 2.1 x 10-10 2400 3100 1.0 x 10-10 2.6 X lo4 2.5 X 1900 1700 210 2.2 x 10-10 a
Unita of mol cm-2
Table 11. Facilitation and Separation Factors for Styrene (STY) and Ethylbenzene (EBZ) through Heat-Treated Nafion 117 facilitation separation membrane thickness, factoP factorb cluster Pm STY EBZ Nat Ag+ mass ?& water [Agt], M treatment temp, "C d i m , nm 18 3.3 18 11 6.2 4-5 200 100 none 6.8 2.5 8.1 37 1.6 6-7 240 22 180 270 12 2.5 2.2 10 42 1.4 210 8-10 a
Ratio of flux for the Ag+ to the Na+ exchanged membrane.
Ratio of the styrene to the ethylbenzene permeabilities.
swollen membranes were measured before and after the heat treatment to determine the degree of expansion. Flux Measurement. Flux measurements described in previous publications were performed using a vertically arranged transport cell that contained no air in the lower compartment (Koval and Spontarelli, 1988; Koval et al., 1989). This type of arrangement was used to minimize the pressure gradient across the membrane. However, assembly of the vertically arranged cell usually resulted in contamination of the membrane surfaces with the olefin feed solution. The amount of time required to assemble the cell was such that there was probably some water loss from the membrane as well. Experiments using a horizontally arranged cell have reproduced fluxes obtained with the vertical cell, indicating that a slight hydraulic pressure gradient has little effect on transport across Nafion membranes. Since the horizontally arranged cell can be assembled quickly and without contamination of the downstream compartment, it was used for all flux measurements described herein. The two compartments of the cell were separated by the membrane which was held in place with O-rings and a clamp. The surface area of the membranes exposed to the solution, measured from the O-ring joint diameter, was 1.8 cm2. Both compartments of the cell contained watersaturated isooctane and were mechanically stirred to provide efficient mass transport adjacent to the membrane. The cell was assembled by clamping the membrane between the two compartments and pipetting 20.00 cm3of solvent into one compartment and 20.00 cm3 of the STY and EBZ solution into the other compartment. After the cell was assembled, aliquots (1pL) of the solution were removed periodically with a syringe from the downstream compartment and analyzed for styrene and ethylbenzene by gas chromatography. Gas Chromatography. Gas chromatography was used to determine permeate concentrations for flux measurement and the extraction experiments. The gas chromatograph used was a Hewlett-Packard 5890 with flame ionization detection. Isooctane served as the organic solvent, and quantitative analysis was done by the method of internal standard using n-xylene as the standard. The temperature program of the GC oven was 60 "C for 5 min. Preparation of Cast Membranes. Cast membranes were prepared by procedures that are similar to previous reports (Moore and Martin, 1986,1988, Gebel et al., 1987). A 5% Nafion solution was obtained from Solution Technology Inc. The precise composition of this mixture is proprietary information, but major components are low molecular weight alcohols and approximately 10% water. Each membrane is cast on a hydrophobic microporous
Celgard 2500 (Hoechst Celanese Corp., Charlotte, NC) membrane in order to give the membrane mechanical strength. The Celgard membrane is clamped into a horizontal casting cell with a glass bottom and washed several times with methanol and water. An appropriate amount of the 5% Nafion solution is then diluted by a factor of 10 with methanol and then poured into the casting apparatus. The solvent is allowed to evaporate overnight, and the resulting membrane is then *cured" at 100 O C for 1h. Cast membrane thicknesses were determined with a micrometer in the dry form. Membranes measuring 25 pm thick were uniform over the surface to within &5%. Thinner membranes were made by scaling down the amount of Ndion in the casting solution.
Results Water Content of Commercial and Heat-Treated Membranes. The water content of fully-hydrated Nafion 117 that had not been subjected to the glycerine heat treatment was gravimetrically measured and found to be 13 mass 5% in both the Na+ and Ag+ cationic forms. In order to determine if significant water loss was occurring during flux measurements, the membranes were exposed to water-saturated isooctane, and the amount of water dropped to 11mass % . The membranes that were treated at 180 and 210 "C were found to contain 40 and 44 mass % water, respectively,when fully hydrated. After exposure to organic solvent, the respective mass percents were 37 and 42. While these results indicate that some water loss does occur, the amount lost was relatively small and thereby assumed to have a minor effect on permeability. The factors that control the water content of Ndion and related membrane materials are not completely understood. For all of the calculated concentrationslisted below, the mass percentages of water used were those measured after exposure to the organic solvent. Heat-Treated Membrane Flux Data. The effect of the glycerine heat treatment on the transport properties of Na+ and Ag+ exchanged Nafion 117 is shown in Figures 1 and 2, respectively. These figures show the change in concentration of styrene in the downstream compartment of the transport cell with time for membranes treated at 180 and 210 OC, as well as a non-treated membrane. Linear regions of the concentraton vs time plots are indicative of a constant flux of the permeates through the membranes, which can be calculated from the slope of each line. The calculated fluxes for both styrene and ethylbenzene through the three membranes are listed in Table I. For both cationic forms of the membrane, the glycerine heat treatment results in a flux increase. Facilitation factors (Ag+ form/Na+ form flux ratios) and separation factors
Ind. Eng. Chem. Res., Vol. 31, No. 4, 1992 1119 Table 111. Fluxes: Facilitation Factors: and Separation Factorsc for Styrene (STY)and Ethylbenzened(EBZ) through Ionomer Membranes of Varying Thickness styrene flux ethylbenzene flux facilitation factor separation factor thickness, pm Na+ &+ Na+ Ag+ STY EBZ Na+ &+ 2.5' 7.4 320 3.0 30 43 10 2.5 11 2.6 82 1.1 8.9 5.0' 32 8.1 2.4 9 0.064 38 0.025 2.1 25f 590 84 2.6 18 11 0.033 1758 0.11 0.60 100 18 3.3 18 a Units of mol cm-2 s-l X lolo. Ratio of the solute flux for the Ag+ to the Na+ exchanged membrane. Ratio of the styrene/ethylbenzene permeabilities. dFeed side concentrations of styrene and ethylbenzene for flux measurements were 1.0 M. 'Cast Nafion membranes. fNafiion NE 111. 8Nafion 117.
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Figure 3. Transport of styrene (squares) and ethylbenzene (circles) through cast Nafion membranes in the Na+ form. Shown are concentrations in the downstream compartment versus time for 5.0-pm (open symbols) and 2.5-pm (filled symbols) membranes.
t
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Figure 1. Transport of styrene through Na+ form Nafion 117. Shown are concentrations in the downstream compartment versus time for commercial (triangles)and heat-treated membranes, 180 OC (circles) and 210 "C (squares). 3
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Figure 2. Tr-ansport of styrene through Ag+ form Nafion 117. Shown are concentrations in the downstream compartment versus time for commercial (triangles) and heat-treated membranes, 180 "C (circles) and 210 OC (squares).
lo3(s) Figure 4. Transport of styrene (squares) and ethylbenzene (circles) through cast Nafion membranes in the Ag+ form. Shown are concentrations in the downstream compartment versus time for 5.0-pm (open symbols) and 2.5-pm (filled symbols) membranes.
(STY/EBZ flux ratios) calculated from the flux data in Table I are contained in Table 11. Also noticeable in Figure 2 is the change in the time intercept with membrane expansion. This transient period is the time required to reach a steady-state flux acroas the membrane and depends on both the membrane thickness and mobility of the permeating species. The time intercepts for styrene and ethylbenzene through Ag+ exchanged membranes are'given in Table I. Although the transient period occurs for both Na+ and Ag+ form membranes, the concentrations measured in the Na+ case are so low that
the flux curves contain much more random error than for Ag+ membranes. For this reason, it was possible to obtain reliable time intercepta for the silver-containing membranes only. A detailed analysis of the transient period for heat-treated membranes is contained in the Discussion. Cast Membrane Flux Data. Data for the transport of STY and EBZ through the 2.5- and 5.0-clm membranes in the Na+ and Ag+ forms are contained in Figures 3 and 4. Fluxes, facilitation factors, and separation factors relating to these experiments are summarized in Table III. Since the cast membranes are substantially thinner than
Time X l o 3 ( 8 )
Time X
1120 Ind. Eng. Chem. Res., Vol. 31, No. 4, 1992
the commercial membranes, steady-state fluxes are achieved much more rapidly (
