Ind. Eng. Chem. Res. 1988, 27, 2161-2164
of the various ferrous chelates to NO (Figure 5) required for obtaining the desired NO removal efficiency appear to be too large to be practical.
Acknowledgment We appreciate the support and encouragement of Charles Drummond, and Michael Perlsweig. This work was supported by the Assistant Secretary for Fossil Energy, U.S. Department of Energy, under Contract DE-ACO376SF00098 through the Pittsburgh Energy Technology Center, Pittsburgh, PA. Registry No. NO, 10102-43-9; SOz, 7446-09-5.
Literature Cited Chang, G. S. ACS Symp. Ser. 1986,319, 159. Chang, S. G.; Littlejohn, D.; Lin, N. H. ACS Symp. Ser. 1982,188, 127. Chang, S. G.; Littlejohn, D.; Lynn, S. Enuiron. Sci. Technol. 1983, 17, 649. Gilmour, A. D.; McAuley, A. J. Chem. SOC. A 1970, 1006. Harkness, J. B. L.; Doctor, R. D. Paper 2e presented at the AIChE Spring National Meeting in New Orleans, LA, April 6-10, 1986.
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Harvey, A. E., Jr.; Smart, J. A.; Amis, E. S. Anal. Chem. 1955,27, 26. Littlejohn, D.; Chang, S. G. Environ. Sci. Technol. 1984, 18, 305. Littlejohn, D.; Chang, S. G. Anal. Chem. 1986, 58, 158. Littlejohn, D.; Chang, S. G. Paper FUEL-108 presented a t the 194th ACS National Meeting in New Orleans, LA, Aug 30-Sept 4,1987. Liu, D. K.; Chang, S. G. Can. J. Chem. 1987a, 65, 770. Liu, D. K.; Chang, S. G. Paper FUEL-107 presented at the 194th ACS National Meeting in New Orleans, LA, Aug 30-Sept 4, 198713. Liu, D. K.; Frick, L. P.; Chang, S. G. Enuiron. Sci. Technol. 1988, 22, 219. Olson, C. K.; Binkley, F. J. Biol. Chem. 1950, 186, 731. Schubert, M. J. Am. Chem. SOC. 1932,54,4077. Silver, J.; Hamed, M. Y. Inorg. Chim. Acta 1983, 80, 115. Stadtherr, L. G.; Martin, R. B. Inorg. Chem. 1972, 11, 92. Tanaka, N.; Kolthoff, L. M.; Stricks, W. J. Am. Chem. SOC. 1955, 77, 1996. Taylor, J. E.; Yan, J. F.; Wang, J.-L. J. Am. Chem. SOC. 1966, 88, 1663. Tu, M. D.; Chang, S. G. AIChE Environ. Prog. 1987, 6, 51. Virtue, R. W.; Lewis, H. B. J. Biol. Chem. 1934, 104, 415. Windholz, M., Ed. Merck Index, 9th ed.; Merck Rahway, NJ, 1976. Received for review December 21, 1987 Reuised manuscript received March 29, 1988 Accepted June 29,1988
Gas Permeation in a Dry Nafion Membrane Jeffrey S. Chiou and Donald R. Paul* Department of Chemical Engineering, The University of Texas a t Austin, Austin, Texas 78712
A perfluorosulfonic acid polymer membrane (Nafion 117) was extensively dried under vacuum after which gas transport properties were measured at 35 "C for He, H2, 02,Ar, N2, CH4, and C 0 2 . This membrane in the dry state has high separation factors for He/CH4, He/H2, and N2/CH4 relative t o other polymeric membranes; however, this material is no better than many other polymers for C02/CH4 or 02/N2separation. T h e transport properties are compared with other information in the literature and discussed in terms of the complex phase structure of the ionomeric material. Polymers based on perfluorosulfonic acid structures have become important materials for membranes in electrochemical applications. The structure and chemistry of these materials have been reviewed recently by Sondheimer et al. (1986). The ionic groups apparently aggregate into clusters within the hydrophobic matrix and may interconnect into an effective network as discussed by Yeager (1982), Gierke and Hsu (1982), and Dotson and Woodward (1982), resulting in useful ion and water transport mechanisms. Because of this physical structure and chemical functionality, these membranes offer interesting opportunities for doping with additives that may lead to enhancement of the transport of one species relative to another. Two recent reports have suggested this approach for adapting such membranes for gas separation applications. Sakai et al. (1985,1986,1987) deposited silver into the structure of a Du Pont Nafion membrane to enhance the transport of oxygen relative to that of nitrogen, while Dunkley-Timmerman (1988) loaded a similar membrane with ethylenediamine to increase the transport of C 0 2 relative to other gases, e.g., CH4. The purpose of this paper is to examine gas transport in neat Nafion membranes in the essentially dry state to provide an assessment of the potential of the base material for gas separations and information needed to understand the benefits and mechanisms of transport in doped materials like those mentioned above.
Experimental Section The membrane used in this study was Nafion 117 obtained from Du Pont. It is a copolymer having the following chemical repeat units: --(CF2
CF2 )x(CF2 CF)y-
I
OCF2CFCF3
I
0 C F2 CF2SOsH
This polymer has an equivalent weight of 1100, or an ion-exchange capacity of 0.91 mequivlg. The thickness of the as-received membrane was 0.0185 cm. Because Nafion can absorb large amounts of water from the atmosphere, the membranes were first dried in a vacuum oven a t room temperature for 5 days before they were installed in the gas permeation cell. Further drying of installed membranes was done in the permeation cell by maintaining a vacuum at the measurement temperature, 35 "C, for 1week. No other treatments were imposed on the as-received samples. The gas permeation measurements were conducted by using a high-pressure permeation cell whose design and operation have been described in detail elsewhere (Koros et al., 1976). Steady-state permeability coefficients and N2,Ar, CHI, diffusion time lags were measured for H2, 02, and C 0 2 at 35 "C. The time lags for He were too small
0888-5885/88/2627-2161$01.50/0 0 1988 American Chemical Society
2162 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 30r
i
I
i
1 1 N a f i o n d r i e d a t room
Frequency. 1 1 hz
v
-
0
40
80
120
I60 200 T ("C!
240 283 320 36C
Figure 1. DSC thermograms for Nafion 117. (A) As-received membrane. (B) Membrane dried under vacuum at 25 O C for 5 days. (C) Membrane dried under vacuum at 25 "C for 66 days.
ti
I
i
1
I
I
-100
100
T(T)
200
Figure 2. Dynamic mechanical behavior for Nafion 117 membrane after drying for 66 days under vacuum.
to measure, so only permeability coefficients were obtained for this gas. The upstream pressures were varied from 1 to 10 atm for H, and O2and from 1 to 20 atm for other gases, while the downstream pressure was kept at essentially zero. The thermal properties of the as-received and dried membranes of Nafion were measured by using a PerkinElmer differential scanning calorimeter, DSC, and a Rheovibron viscoelastomer. The heating rates were 20 "C/min in the DSC and 1 "C/min for the Rheovibron. The DSC scans were made using samples contained in closed but unsealed pans.
H2
N o f i o n / 35°C
CL
Results and Discussion Thermal Properties. Figure 1 shows DSC thermograms, normalized for sample mass, of as-received (undried) and vacuum-dried Nafion specimens. A rather similar series of thermograms were reported by Kyu and Eisenberg (1982) who used progressively higher temperatures for removal of sorbed water. The erratic response beyond about 200 OC may reflect a combination of factors including removal of tenaciously sorbed water, melting of slight amounts of crystallinity, or thermal decomposition. Of greater relevance, however, are the apparent increases in heat capacity that occur in the range 40-120 "C. These might be interpreted as a glass transition that moves progressively to higher temperatures as water is removed. However, the possibility of water evaporation on heating (an endothermic process) for incompletely dried specimens cannot be ignored and seriously compromises the ability to interpret this region. The magnitude of this thermal process is significantly reduced after drying under vacuum for 5 days (curve B versus curve A), leaving only a small hump a t about 80 "C. Vacuum drying for a longer time, e.g., 66 days, essentially removes this hump (see curve C). The latter thermogram shows a heat capacity shift, at an onset temperature of about 120 "C, like that expected for a glass transition. Dynamic mechanical behavior for a sample subjected to prolonged vacuum drying is shown in Figure 2. There is a major loss peak a t about 125 "C that Kyu and Eisenberg (1982) have attributed to the glass transition for the ionic domains. The other major loss peak at low temperatures appears to be the same relaxation that Kyu and Eisenberg (1982) and Nakano and MacKnight (1984) attribute to local motions of CF2 groups in the main chain. A less pronounced peak occurs near room temperature that
5
0
2 c>
15
IO p2:otm)
Figure 3. Gas permeability coefficients versus upstream pressure for Nafion 117 at 35 "C.
x
a
I
1
I
I
i
0
5
IO
15
23
p,
1
(ofm)
Figure 4. C02gas permeability coefficient versus upstream pressure for Nafion 117 at 35 "C.
the above authors agree results from the glass transition of the fluorocarbon phase. Gas Permeation. Permeability coefficients for various gases in Nafion are plotted versus the upstream driving pressure in Figures 3 and 4. Except for COz, the gas
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2163 Table I. Sorption and Transport Properties of Essentially Dry Nafion 117 for Various Gases at 35 OC and 1-atm Upstream Pressure
gas He HZ 0 2
N2 Ar CH4 COZ
P, Barrep 40.9 9.30 1.08 0.260 0.490 0.102 2.43
D,, cmz/s b 9.58 X loT7 4.57 x 10-8 1.73 X lo4 2.06 X 2.79 x 10-9 1.68 X
Sa, (cm3 (STP))/(cm3 cmHd b 9.70 X lo4 2.39 x 10-3 1.50 X 2.38 X 10” 3.65 x 10-3 1.45 X
1 Barrer = (cm3 (STP) cm)/(cmz s cmHg). bTime lag too small for measurement.
permeabilities are independent of the driving pressure within the average experimental error, about 3%. For C02, the permeability coefficient increases by about 70% as the pressure goes from 1to 20 atm. Such trends are typical for C 0 2 in rubbery polymers (El-Hibri and Paul, 1986; Chiou et al., 1985) and rare for glassy polymers (Chiou and Paul, 1986, 1987). For many glassy polymers, C 0 2 permeability coefficients decrease slightly over this pressure range (Koros et al., 1976; Chiou and Paul, 1987). Time lags, 8, were used to calculate apparent diffusion coefficients defined as
D, = 12/68
(1)
where 1 is the membrane thickness. Apparent solubility coefficients were computed from
P = D,S, (2) Table I lists the permeability, apparent diffusion, and solubility coefficients determined at 1 atm for each gas. Sakai et al. (1985, 1986, 1987) have reported similar data for H2, 02,and N2that may be compared with these results although their membranes were given a boiling HC1 pretreatment not used here. Their permeability coefficients are significantly larger than those in Table I, especially for Nz where the discrepancy is more than a factor of 2. The apparent solubility coefficients they report are somewhat inconsistent in that the value given for N2 is about equal to or less than that for the much less condensible H2. Nafion is highly hygroscopic, and sorption of water can increase the permeability to gases by very substantial amounts. A likely explanation of the higher permeability coefficients found by Sakai et al. (1985, 1986, 1987) is that their materials contained residual water. We were very cautious about this effect. We repeatedly measured gas permeability coefficients over a period of 2 months to determine whether any change occurred for a membrane sample continuously held under vacuum (in the permeation cell). A decrease of about 10% was observed during the first 6 days with no significant change thereafter. This indicates that a membrane predried under vacuum for 5 days may still contain a small amount of water even after further drying for 1week under vacuum in the permeation cell (see Experimental Section). While the pretreatment used by Sakai et al. (1985, 1986, 1987) may have some effect on the gas permeability coefficients, differences in the extent of dryness seem likely to be a major factor in the comparison of the two results. Gas Separation. An important issue in the selection of materials for gas separation membranes is the trade-off often observed between intrinsic productivity and selectivity. The latter may be estimated as the ratio of pure gas permeability coefficients, i.e.,
Table 11. Comparison of Selectivity and Productivity of Dry Nafion 117 Membranes with Polycarbonate for Selected Gas Pairs Nafion 117 polycarbonate gas pair (1/2) PI, Barrer a; PI,Barrer CY; 2.43 23.8 6.0 23.3 4.15 1.48 5.13 OdN2 1.08 He/CH4 401 40.9 13.6 53 0.289 1.12 2.55 N2/CH4 0.26
for a particular gas pair while the absolute permeability of the faster gas, P I , is an indicator of the former. Numerous papers have dealt with this trade-off relation (Muruganandam and Paul, 1987; Barbari et al., 1988; Koros et al., 1988; Kim et al., 1988), and it is interesting to see how the essentially dry Nafion used here compares in this regard with other materials. Polycarbonate serves as a useful basis for comparison since for most gas pairs it falls on the trade-off line formed by standard polymers often considered for gas separation membranes (Koros et al., 1988). This comparison is made in Table I1 where data for polycarbonate was taken from a recent compilation of results from this laboratory by Muruganadam and Paul (1987). For C02/CH4and 02/N2pairs, the essentially dry Nafion is both less productive and selective than polycarbonate; hence, it falls below the trade-off line for standard polymers. Consequently, dry Nafion is not as attractive for these separations as many other polymers. For He/CH,, dry Nafion is both more productive and selective than polycarbonate by significant factors. In a recent survey of the literature by Koros et al. (1988), the highest separation factor for He/CH4 found for any polymer was about 140. Since that review was made, Barbari et al. (1988) have reported a2* = 264, P, = 9.4 Barrer for a polyetherimide, and Min and Paul (1988) found that stereoisomers of poly(methy1 methacrylate) have spectacularly large separation factors in the range 1300-3800, although the absolute permeability coefficients for He are low (1.3-4.7 Barrer). Most polymers have higher permeability coefficients for CH, than for N2 (Koros et al., 1988); however, recently polymers (Kim et al., 1987) have been described with the reverse selectivity; i.e., N2 permeates more rapidly than CH,, which has some importance for natural gas applications. Polycarbonate passes these two gases at nearly the same rate as seen in Table 11. Interestingly, Nafion has a N2/CH4selectivity of 2.55 which is larger than that for any polymer included in a recent review (Koros et al., 1988) or the stereoisomers of poly(methy1 methacrylate). The gas pair He/Hz is usually not considered since there are no applications of this separation; however, dry Nafion gives a separation factor of 4.4 which is considerably higher than that for any other polymer we know of. Isotactic poly(methy1 methacrylate) has a separation factor for this pair of 2.91; however, its helium permeability coefficient is 30-fold less than that of Nafion. In summary, dry Nafion offers no productivity-selectivity advantage relative to other polymers for C02/CH4 or 0 2 / N 2separations; however, it appears to offer some attractive features for other gas pairs like He/CH4, N2/ CH,, or He/H2. The permeability coefficient can be factored into diffusion and solubility coefficients as seen in eq 2. Thus, the separation factor is the product of mobility and solubility selectivity terms
(D1/D2)(Si/S2) (4) The solubility ratios calculated from the data in Table I are very similar to those given by Murunganandam and Paul (1987) for polycarbonate with the maximum differazl =
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Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988
ence being 25% for N2/CH4. Similar comparisons cannot be made for cases involving He since its time lag was too short to measure; hence, P cannot be factored into D and S. Further investigations to learn the reasons for the advantageous mobility ratios, D,/D2,for certain gas pairs are in order. It will be important to learn whether such effects stem from the ionic or the fluorocarbon character of Nafion or from some combination of the two. Summary As mentioned a t the outset, Nafion-type membranes offer convenient structures for doping with agents that facilitate the transport of certain gas species relative to another. The results reported here show that the neat material in the dry state has some unique selectivity characteristics that could be useful for certain gas separations. These facts provide an incentive to learn more about the detailed mechanisms of gas transport in this class of complex materials. A small fraction of Nafion is a crystalline phase not expected to be permeable to gases. As mentioned earlier, the remaining amorphous material is believed to consist of a fluorocarbon phase having a glass transition temperature below ambient and clusters of ionic groups giving a phase with a glass transition above room temperature. There is some evidence that the two amorphous phases are co-continuous. A major question is the relative importance of each phase for transport of individual gas types. Sakai et al. (1985,1986, 1987) have given some evidence for involvement of the ionic phase. The pressure dependence of the C02 permeability shown in Figure 4 suggests, but does not prove, that the more rubbery fluorocarbon phase dominates in this case. Since gas sorption isotherms have different shapes for glassy and rubbery polymers (Chiou and Paul, 1966), detailed solubility measurements would provide useful insight about these questions as the relative amount and nature (e.g., degree of neutralization of acid groups and type of counterions) of the ionic phase are varied. A detailed examination of the effects of sorbed water on permeability and selectivity is also needed since most applications would involve feeds containing some moisture. Acknowledgment This research was supported by the Separations Research Program at The University of Texas at Austin and by the US Army Research Office. Nomenclature
D, = apparent diffusion coefficient in polymer 1 = film thickness P = permeability coefficient for gas in polymer S , = apparent solubility coefficient for gas in polymer Greek Symbols aZ1=
ideal separation factor for gas 1 relative to gas 2
6 = diffusion time lag
Subscript
i = refers to diffusion, solubility, or permeability coefficient of gas species i Registry No. Nafion 117, 66796-30-3; H2, 1333-74-0; 02, 7782-44-7; N4, 7727-37-9; Ar, 7440-37-1; CHI, 74-82-8; C02, 12438-9; He, 7440-59-7.
Literature Cited Barbari, T. A.; Koros, W. J.; Paul, D. R. ”Polymeric Membranes Based on Bisphenol-A for Gas Separations”. J . Membrane Sci. 1988, in press. Chiou, J. S.; Paul, D. R. “Sorption and Transport of COz in PVFz/ PMMA Blends”. J. Appl. Polym. Sci. 1986, 32, 2897. Chiou, J. S.; Paul, D. R. “Effects of C02 Exposure on Gas Transport Properties of Glassy Polymers”. J. Membrane Sci. 1987,32, 195. Chiou, J. S.; Barlow, J. W.; Paul, D. R. “Sorption and Transport of Gases in Miscible Poly(methy1 acrylate)/Poly(epichlorohydrin) Blends”. J. Appl. Polym. Sci. 1985, 30, 1173. Dotson, R. L.; Woodward, K. E. “Electrosynthesis with Perfluorinated Ionomer Membranes in Chlor-Alkali Cells”. ACS Symp. Ser. 1982, 180, 311. Dunkley-Timmerman, T. “Transport of Carbon Dioxide in Perfluorosulfonate Membranes Enhanced by Relative Humidity and Ethylenediamine Content”. Ph.D. Dissertation, The University of Texas at Austin, Austin, TX, 1988. El-Hibri, M. J.; Paul, D. R. “Gas Transport in Poly(viny1idene Fluoride): Effects of Uniaxial Drawing and Processing Temperature”. J . Appl. Polym. Sci. 1986, 31, 2533. Gierke, T. D.; Hsu, W. Y. “The Cluster-Network Model of Ion Clustering in Perfluorosulfonated Membranes”. ACS Symp. Ser. 1982, 180, 283. Kim, T. H.; Koros, W. J.; Husk, G. R. “Advanced Gas Separation Membrane Materials: Rigid Aromatic Polyimides”. Sep. Pur$ Methods 1988, in press. Kim, T. H.; Koros, W. J.; Husk, G. R.; O’Brian, K. C . “Reverse Selectivity of N2 over CH, in Aromatic Polyimides”. J . Appl. Polym. Sci. 1987, 34, 1767. Koros, W. J.; Paul, D. R.; Rocha, A. A. T O 2 Sorption and Transport in Polycarbonate”. J. Polym. Sci., Polym. Phys. Ed. 1976,14,687. Koros, W. J.; Fleming, G. K.; Jordan, S. M.; Kim, T. H.; Hoehn, H. H. “Polymeric Membrane Materials for Solution-Diffusion Based Permeation Separations”. Adu. Polym. Sci. 1988, in press. Kyu, T.; Eisenberg, A. “Mechanical Relaxations in Perfluorosulfonate Ionomer Membranes”. ACS Symp. Ser. 1982,180,79. Min, K. E.; Paul, D. R. “Effect of Tacticity on Permeation Properties of Poly(methy1 methacrylate)”. J. Polym. Sci., Part B: Polym. Phys. 1988,26, 1021. Muruganandam, N.; Paul, D. R. “Evaluation of Substituted Polycarbonates and a Blend with Polystyrene as Gas Separation Membranes”. J. Membrane Sci. 1987,34, 185. Nakano, Y.; MacKnight, W. J. “Dynamic Mechanical Properties of Perfluorocarboxylate Ionomers”. Macromolecules 1984,17,1585. Sakai, T.; Takenaka, H.; Torikai, E. “Gas Diffusion in the Dried and Hydrated Nafions”. J . Electrochem. SOC.1986, 133, 88. Sakai, T.; Takenaka, H.; Torikai, E. “Oxygen/Nitrogen Separation by a Nafion-Ag Microcomposite Membrane”. J. Membrane Sci. 1987, 31, 227. Sakai, T.; Takenaka, H.; Wakabayashi, N.; Kawami, Y.; Torikai, E. “Gas Permeation Properties of Solid Polymer Electrolyte (SPE) 1985, 132, 1328. Membranes”. J . Electrochem. SOC. Sondheimer, S. J.; Bunce, N. J.; Fyfe, C. A. “Structure and Chemistry of Nafion-H: A Fluorinated Sulfonic Acid Polymer”. J. Macromol. Sci.-Rev. Macromol. Chem. Phys. 1986, C26, 353. Yeager, H. L. “Transport Properties of Perfluorosulfonate Polymer Membranes”. ACS Symp Ser. 1982, 180, 41. Received for review May 13, 1988 Accepted August 15, 1988
