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Ind. Eng. Chem. Res. 2005, 44, 3672-3680
Nitric Acid Dehydration Using Mixed Perfluorosulfonate and -carboxylate Ionomer Membranes Richard L. Ames,*,†,‡ J. Douglas Way,‡ Elizabeth A. Bluhm,† Daniel M. Knauss,§ Rajinder P. Singh,‡ and Jesse E. Hensley‡ Los Alamos National Laboratory, NMT-2, P.O. Box 1663, Los Alamos, New Mexico 87575, and Chemical Engineering and Chemistry and Geochemistry, Colorado School of Mines, 1500 Illinois Street, Golden, Colorado 80401
Investigations confirmed the feasibility and potential advantage of using mixed perfluorosulfonate/carboxylate membranes for nitric acid dehydration. Experimentation consisted of in situ generation of pure sulfonate and mixed sulfonate/carboxylate (as high as 71 mol % carboxylate) polymer films from a perfluorosulfonyl fluoride precursor membrane, the characterization of the material, and a study of the transport characteristics of these membranes. The carboxylate concentration was determined using direct Fourier transform infrared and X-ray fluorescence spectroscopy. Nitric acid dehydration transport tests confirmed that bulk fluxes decreased and the water separation factor dramatically increased as the carboxylate side-chain content increased to 53 mol %. Introduction Perfluorocarboxylate ionomer membranes are being considered at Los Alamos National Laboratory (LANL) for uses in nitric acid dehydration and low-level waste solution processing. LANL performs actinide processing in an aqueous system with nitric acid and hydrochloric acid based unit operations. As a result of the everincreasing cost of waste disposal and limited waste disposal resources, LANL facilities use reprocessed or recycled nitric acid in many systems not requiring acid of high purity. A nitric acid recycle system (NARS) has been designed around a distillation column where bottoms from the column are recycled to plant facilities and overheads are discarded as dilute aqueous waste. Increased reprocessing requirements have made it necessary to add an additional purification unit to the NARS that will eliminate reprocessing of dilute acid overheads. One proposed option uses a perfluoro ionomer membrane for acid dehydration because the perfluorosulfonate and composite perfluorosulfonate/carboxylate ionomer films have been investigated for the dehydration of a number of mixtures including acetic acid, methanol/water, and nitric acid.1-7 Nitric acid dehydration via pressure filtration and pervaporation has been investigated as a unit operation replacement for distillation in the generation of highly concentrated acid (beyond the nitric acid/water azeotrope).1,8 Sportsman and co-workers claimed that perfluorosulfonate ionomer membrane films would not only dehydrate concentrated acid feeds (distillation bottoms) but also dehydrate dilute waste streams (distillation overheads), while excluding ionic contaminants, and operate in either a pressurized or pervaporation environment.1,2 They conducted preliminary research on the use of perfluorosulfonate/carboxylate composite mem* To whom correspondence should be addressed. Tel.: 1-505606-0165. Fax: 1-505-665-1780. E-mail:
[email protected]. † Los Alamos National Laboratory. ‡ Chemical Engineering, Colorado School of Mines. § Chemistry and Geochemistry, Colorado School of Mines.
branes (Nafion 90209) for the dehydration of concentrated nitric acid (>10 wt %) and demonstrated that the polymer ionomer material offered improved water separation capabilities when compared to pure sulfonate ionomer films. Following the acid dehydration by Sportsman, an objective described herein was to produce thin mixed sulfonate/carboxylate membranes from commercially available films and demonstrate the films’ ability to dehydrate nitric acid. Perfluorocarboxylate and mixed carboxylate/sulfonate films can be synthesized from the same perfluoro precursor material as the sulfonate form.9-15 Tetrafluoroethylene (TFE) copolymerization forms the perfluoro precursor and the addition of a functionalized form of TFE allows the introduction of the side chain. Both of the sulfonate and carboxylate (Nafion) products are made from the perfluorosulfonyl fluoride precursor polymer film shown in Figure 1, where m is 0, 1, or 2, p is from 1 to 10, q is from 3 to 15, and M is usually a halogen (F in this instance) or hydrogen.9,10 For example, the precursor material shown in Figure 1 with a sulfonyl fluoride side chain
-O-(CF2)2-SO2F
(1)
can be oxidized to a carboxylate derivative:
-O-CF2-COOH
(2)
This reaction can be carried out at an elevated temperature (50-60 °C) or in the presence of a metal catalyst (salts of Fe, V, or Cu) at room temperature. Hydrolysis of the sulfonyl fluoride precursor generates a sulfinic acid or sulfonate form of the membrane:
-O-(CF2)2-SO3H
(3)
Materials with a mixture of both carboxylate and sulfonate pendant chains (eqs 2 and 3) can be generated by controlled exposure of the precursor to oxidizing and reducing reagents.
10.1021/ie0488391 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/16/2005
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Figure 1. Nafion precursor.
Figure 2. Chemical conversion flow.9
Experimental Section Thin sulfonate and mixed sulfonate/carboxylate films could be made in situ from commercially available precursor films (Du Pont Nafion R1100 films, 25-µm thickness). Early literature describes procedures that use corrosive fluorine gas environments to complete oxidation from the precursor to a carboxylate form but do not mention aqueous techniques used to convert the perfluorosulfonate derivative to a sulfonyl fluoride form.10 More recently, Fujimura et al. published a procedure using common aqueous reagents (HCl, CCl4, HI, etc.), and Grot et al. demonstrated a number of aqueous procedures that produced a carboxylate and carboxylate/sulfonate mixed film from a perfluorosulfonyl fluoride precursor.9,11,15 A. Ionomer Film Generation. In general, two methods of synthesis were examined for the purpose of making thin perfluorocarboxylate/sulfonate ionomer films from Nafion perfluoro products. Initial experiments used a procedure developed by Fujimura et al. where Nafion 111 (perfluorosulfonate) was chemically converted in situ to the mixed polymer form.15 The Fujimura procedure was effective, yet the treatment was expensive in terms of resources and time. A second conversion technique, developed by Grot et al., was also successful in producing the desired product from the Nafion R1100 sulfonyl fluoride precursor and proved to be simple in comparison to the Fujimura procedure. The films synthesized using this process demonstrated that a mixed ionomer film could be deliberately prepared by incomplete sulfonyl fluoride reduction and hydrolysis of the remaining sulfonyl fluoride, followed by sulfinic acid oxidation to a carboxylate-terminated pendant chain. This “Grot” procedure was used as the baseline conversion process during these investigations and is illustrated in Figure 2. Sample designations refer to the conversion procedure used (Grot et al.), to the conversion run number, next to the particular ionomer film generated during the run (first letter designation), and to the section of the particular film tested.9 B. Film Characterization. A number of analytical techniques were used to characterize the films produced. For example, chemical titration was utilized for deter-
mining the ionic equivalent weight (EW) or ionexchange capacity (IEC), both Fourier transform infrared (FTIR) spectroscopy and X-ray fluorescence (XRF) were used to determine the pendant-chain composition, and small-angle X-ray scattering (SAXS) was used for ionomer backbone analysis. All films were examined in the acid (H+) counterion form unless stated otherwise. The perfluoro ionomers containing anion-terminated pendant chains have associated counterions that can be exchanged as desired. Measurement of the ionic pendantchain content of the ionic side chain can be described by EW, which is defined as the number of dry grams of polymer per mole of ionic pendant chain (g equiv mol-1 or g equiv-1), or by IEC in millimoles of ionic pendant exchange sites per gram of dry polymer (mmol g-1 or equiv g-1), where EW ) 1000/IEC. Perfluorosulfonic and -carboxylate ionomer IEC is dependent on the ratio of the pendant side chain to the TFE monomer. EWs were determined by titration of the materials following the pretreatment by boiling in 1.0 M nitric acid, followed by rinsing with Milli-Q water, and then finally soaking of the film in 0.01 M KOH. A back-titration with HNO3 determined the IEC. Experimentation was also conducted with ionomer films that are commercially available with a variety of EWs and thicknesses. For example, Du Pont markets the Nafion series of membranes, Nafion 111-117. The Du Pont numbering convention for these ionomer films is defined by the first two digits being the EW (100 × g equiv-1) and the third digit being the membrane’s thickness (1 × 0.001 in.). All experimentation and characterization described herein includes the results for the Nafion 111 film, which has similar thickness and chemical characteristics when compared to in situ conversion films. Direct transmittance FTIR spectroscopy was used to determine whether the membrane material contained a mixture of sulfonate and carboxylic side chains or a sulfonyl fluoride precursor material.14 The perfluorocarboxylate content for FTIR was determined from the ratio of the area under the carboxylate (1790 cm-1) and the sulfonate (1060 cm-1) peaks, each film was measured once (one data point for each film listed), and the curve-fitting error was determined to be less than 8%. Data were collected using a ThermoNicolet Nexus 670 FTIR instrument with spectra measured in the range of 650-4000 cm-1 at room temperature. XRF was utilized to characterize mixed ionomer membrane compositions.12 The X-ray excitation and detection were performed using an EDAX Eagle II XLP micro-XRF system equipped with a Rh target excitation source and a SiLi detector (EDAX, Mahwah, NJ). The X-ray source was equipped with a polycapillary focusing optic (X-ray Optical Systems, Albany, NY). SAXS analysis was used to determine the polymer backbone cluster morphology of our films. The SAXS system used a Kratky small-angle instrument with a Rigaku rotating-copper-anode X-ray generator having λ ) 0.154 18 nm. Perfluoro ionomer samples were hydrated and sealed in a polyethylene envelope in order to keep the film hydrated during analysis. Perfluoro ionomer film SAXS scans were adjusted by removing the baseline scan of the polyethylene envelope material, accounting for the film thickness, and normalizing the absolute intensity of the beam with scans collected with and without the sample before and after a given SAXS run.
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Ind. Eng. Chem. Res., Vol. 44, No. 10, 2005 Table 1. Mixed Ionomer Film General Characterization Data
Figure 3. Pervaporation transport system.
Thermal gravimetric analysis (TGA) was used to determine correlations between the membrane type and water evaporation temperature for the perfluorosulfonate and mixed films. A Seiko model TG/DTA 220 was used for the TGA. Analysis was conducted in a helium environment, and sample temperatures were increased (1 °C in 3 min) from room temperature to a maximum of 360 °C. C. Pervaporation Transport Experimentation. Figure 3 shows a schematic of the pervaporation system used to produce transport data. The system consisted of a 316 stainless steel feed reservoir fitted with stainless steel or Teflon hardware, a Fluid Metering, Inc. (FMI), feed pump and controller, a 1-in.-diameter Millipore membrane cell, a glass liquid nitrogen vapor trap, a Welch Duo Seal vacuum pump, a permeate-side pressure transducer, and a Grandville-Phillips model 275 analogue pressure gauge. The permeate-side pressure was maintained at 10 ( 2 Torr absolute, and the permeate was collected in the liquid nitrogen vapor trap. The feed solution loop was left at room temperature (approximately 24 °C). Nitrate ion concentrations in the collected permeate were measured using an Orion nitrate ion specific electrode model 9307 and a model 9002 reference electrode in conjunction with a model 290A multipurpose meter. Results and Discussion A. General Characterization. Perfluoro ionomer water sorption and IEC characteristics were used to inventory the membranes produced and are summarized in Table 1. On average, mixed ionomer film IECs were similar to those of the pure perfluorosulfonate material, where sulfonate and mixed film EWs were determined to be 0.91 and 1.06 equiv g-1, respectively. Water sorption measurement results showed that the average perfluorosulfonate/carboxylate polymer water content (9.3 wt %) was significantly lower than that for the unprocessed Nafion 111 material (23 wt %) and slightly higher than the as-received perfluorocarboxylate (Flemion) ionomer film (8.0 wt %). B. FTIR Characterization. FTIR spectroscopy analysis was useful in qualitatively and semiquantitatively determining the carboxylate-to-sulfonate ratio for the terminated pendant chains in mixed ionomer films. Using relative absorbance intensities for peaks indicating the presence of carboxylate- and sulfonate-terminated side chains and taking measurements immediately following the conversion process, it was determined
sample
IEC (equiv g-1)
water sorbed in H+ form (wt %)
Flemion Nafion 11119,44 Grot 2ab Grot 2bb Grot 2cb Grot 2db Grot 5ab Grot 5bb Grot 6ab Grot 7ab Grot 7bb Grot 8ab Grot 9ab Grot 9bb Grot 10ab Grot 10bb Grot 11ab Grot 11bb Grot 12ab Grot 12bb Grot film average
1.25a 0.91 0.84 ( 0.07 0.93 ( 0.06 1.05 ( 0.14 0.85 ( 0.08 0.97 ( 0.07 1.04 ( 0.10 0.80 ( 0.09 1.29 ( 0.11 1.32 ( 0.07 1.19 ( 0.07 0.85 ( 0.06 0.88 ( 0.08 1.08 ( 0.11 0.99 ( 0.07 0.98 ( 0.08 1.25 ( 0.08 1.34 ( 0.07 1.38 ( 0.07 1.06 ( 0.10
8.0 ( 0.1 23.0a 7.1 ( 0.3 7.6 ( 0.3 3.1 ( 0.6 4.7 ( 0.4 11.8 ( 0.3 10.2 ( 0.4 6.9 ( 0.4 7.9 ( 0.5 12.8 ( 0.3 11.5 ( 0.3 11.4 ( 0.2 12.1 ( 0.3 12.5 ( 0.5 8.1 ( 0.3 8.8 ( 0.3 9.8 ( 0.4 10.7 ( 0.3 9.7 ( 0.3 9.3 ( 1.1
a
Manufacturer EW data or water sorption data.
whether an individual conversion run had generated the desired product. With this information and with a change in the form of a given film (acid, potassium, methyl ester, etc.), peak positions in the spectrum can be used to distinguish between perfluorosulfonate and -carboxylate forms or to determine if a mixture of sulfonate and carboxylate side chains are present. If the in situ conversion was incomplete, perfluorosulfonyl fluoride groups would be present and a peak would be observed at 608 cm-1. Figure 4 shows example spectra for Nafion 111 and for converted Grot 8ab ionomer films. Figure 4a shows that Nafion 111 contains only the sulfonated pendant chain, as indicated by the lack of a peak at 1790 cm-1. Figure 4b illustrates the presence of both the carboxylate and sulfonate side chains for the mixed ionomer film Grot 8ab. Parts a and b of Figure 5 show a series of four spectra for membranes with varying carboxylate content. These spectra clearly demonstrate an increase in the intensity of the 1790-cm-1 peak (and a subsequent decrease in the intensity of the sulfonate peaks at 1060 cm-1) as the carboxylate content in the membranes increases. For this example, the Grot films can be arranged by increasing the carboxylate content as 7ab < 12bb < 5ab < 6ab. C. Carboxylate Content (XRF Characterization). Relative amounts of carboxylate- and sulfonate-terminated pendant chains were determined quantitatively in the mixed ionomer films using XRF. When the ionomer material was chemically converted to the alkali salt form (potassium), both the cation and sulfur constituent contents were determined. The basis for calculating side-chain ratios included the assumptions that each potassium cation was associated with a single anion pendant chain and that potassium cations not associated with the sulfur were associated with a carboxylate-terminated side chain.12 A signal strength adjustment (CR ) 1.17) for potassium based on reference sulfonate membranes Grot 7ab and 7bb was necessary to correct intensity variations between sulfur and potassium in reference sulfonate materials. Figure 6 shows the XRF spectra for the dried Nafion 111 reference (Figure 6a) and dried mixed ionomer samples Grot 5ab
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Figure 4. Direct transmission FTIR of dried ionomer films: (a) pure sulfonate pendant side-chain film (Nafion 111); (b) mixed film (Grot 8ab).
(Figure 6b) and Grot 5bb (Figure 6c). The carboxylate pendant-chain content was calculated using eq 4.
[-COOK] (mol %) )
Ik - IsCR × 100 Ik
(4)
Figure 7 shows a gray-scale image of a 50-mm2 (∼8 × 6 mm) sample for mixed ionomer film Grot 5ab. Variation in the overall brightness within the image represents a concentration change of the particular element being measured. The brightness increases as the elemental concentration increases. The image in Figure 7 also illustrates the uniformity of the formation of sulfonate pendant side chains during the conversion process. The sulfur concentration increases in the lower right corner when compared to the upper left corner of the sample film. It was assumed that the pendant-chain content remained constant throughout the in situ conversion process. Therefore, a reduction in the sulfur content as indicated by XRF (i.e., reduction in sulfonate
pendant chains) describes the corresponding increase in carboxylic-terminated pendant chains. Table 2 summarizes the pendant-chain content for the mixed ionomer membranes. Results indicate that the procedure developed by Grot et al. can be successfully employed to synthesize films with carboxylate pendantchain concentrations varying from 3 to 71 mol %. It is important to note that the Grot conversion films always contained a significant quantity of the sulfonated pendant chain. Table 2 shows a comparison between the carboxylate pendant-chain contents determined by XRF and FTIR. Overall, the results are very similar when looking at general content trends, but a significant difference is seen when comparing the contents for individual films. Also worth noting is that as the carboxylate content increased it was expected that the IEC would decrease because of the reduction in ionization of the carboxylate relative to the sulfonate pendant chain (section G). However, no relationship was shown to exist between
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Figure 7. Visual image of Grot 5ab (17 mol % carboxylate) XRF spectra. Table 2. Carboxylate Content of Conversion Films As Determined by XRF and FTIR Figure 5. FTIR intensity comparison for dry sulfonate and mixed carboxylate/sulfonate films.
b
Figure 6. XRF spectra for reference and mixed ionomer films.
the IEC and carboxylate pendant-side-chain content (or sulfonate content) for the films investigated. The reason for these unexpected results was attributed to the supposition where titration acid and base reagent concentrations were sufficiently high that no statisti-
sample
carboxylate content by XRF (mol %)a
carboxylate content by FTIR (mol %)a,b
Flemion Nafion 11120,49 Grot 2ab Grot 2cb Grot 5ab Grot 5bb Grot 6ab Grot 7ab Grot 7bb Grot 8ab Grot 9ab Grot 9bb Grot 10ab Grot 10bb Grot 11ab Grot 11bb Grot 12ab Grot 12bb
∼100 0 23 15 17 ( 13 28 ( 6 41 ( 5 0 0 53 3.0 ( 3 13 32 ( 7 71 ( 2 0 5.0 3.4 10 ( 3
∼100 0 25 31 38 0 0 4 30 75 0 0 0 5
a Data not showing an error indicate a single measurement. Error in the curve-fitting FTIR data of less than 8%.
cally significant ionization difference was seen when comparing carboxylate and sulfonate materials. D. TGA and Ionomer Water Loss. Boyle et al. experimentally determined that water diffusivities in pure perfluorocarboxylate and -carboxylate/sulfonate composite materials were comparable to those in the pure sulfonated Nafion product at similar water contents but that the hydration limits for the pure carboxylate ionomer could be as low as 50% of the hydration limit of the contents for similar sulfonate films.10,16 Yeager et al. observed that, when comparing the hydration of exchange sites for similar carboxylate and sulfonate materials, the hydration limit was approximately 22% lower for the carboxylate.17 TGA of the hydrated Grot conversion procedure membranes was performed in an attempt to explain observed water content and transport differences between pure sulfonate and mixed sulfonate/carboxylate films. The goal was to determine a relationship between the carboxylate content and the temperature at which water was thermally removed from the films. Our results show that changes in the mass loss per change in temperature occur at two distinct temperatures for each of the ionomer films. The exact temperature at
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Figure 8. Differential TGA determination of inflection points for sulfonate film Grot 7ab-b. Table 3. TGA Summary of Perfluorosulfonate and Mixed Filmsa
sample Grot 7ab-b Grot 7bb-b average
carboxylate content (mol %)
absorbed water transition temp (°C)
Sulfonate Pendant Chain 0 40 0 40 0 40 ( 6
bonded water transition temp (°C) 115 110 113 ( 45
Mixed Sulfonate/Carboxylate Pendant Chain Grot 5ab-b 17 ( 13 125 Grot 8ab-c 53 65 115 Grot 9ab-a 3(3 57 120 Grot 10ab-b 32 ( 7 64 130 average 26 62 ( 13 123 ( 20 a
Data not showing an error indicate a single measurement.
which these inflections occurred was determined by plotting differential thermal analysis (DTA) data against temperature. Figure 8 illustrates the method used to determine inflection points from the TGA and DTA data for film Grot 7ab, and Table 3 lists the inflection point temperatures for all films tested. These data show that the lower inflection point was found to be at a significantly lower temperature for the pure sulfonate films. The pure perfluorosulfonate inflection was observed at 40 °C, whereas the mixed ionomer inflection point was observed at approximately 62 °C. The trend for location of the second inflection temperature on the DTA curve appears to be at a higher temperature for the mixed ionomer material; however, the inflection points for both types of membranes occur at statistically the same temperature. Absorbed and bonded water in the membrane reveal two distinct mass loss rates, where absorbed water evolves at the higher rate (up to the lower DTA inflection temperature) and the bonded water evolves at a slower rate (up to the higher DTA inflection temperature). TGA data indicate small percentages of carboxylate-terminated pendant chains retaining absorbed water at higher temperatures when compared to those films with only sulfonate-terminated pendant chains. This TGA/DTA information is consistent with previously published water content characterization data and with pure water flux information for both pure carboxylate and mixed films.10,16-18 In summary, materials containing carboxylate-terminated side chains show higher resistance to hydraulic flux, lower water content, and a trend for retaining water at higher temperatures consistent with increased bonding strengths between the hydroxyl oxygen and water
Figure 9. SAXS data from Grot samples.
hydrogen in the carboxyl materials when compared to pure sulfonate materials. E. SAXS in Mixed Ionomer Films. A series of Grot procedure synthesized polymer samples were analyzed by SAXS for determining morphological differences between perfluorosulfonate and mixed sulfonate/carboxylate films. Processed SAXS data for five samples revealed systematic differences between all SAXS curves, which are due to scattering associated with a welldefined shoulder above 1 nm-1 and characteristic of the hydrated sulfonate pendant-chain cluster.19 Model fits of spherical objects were used to simulate shoulders caused by scattering, and these yielded estimates for the average feature diameter (nm).20-22 An integrated SAXS was used to quantify scattering and is proportional to the volume fraction of the scattering objects. The sulfonate pendant-chain content and intensity of the cluster feature simultaneously increase. As the carboxylate content increases, a decrease in the intensity of the characteristic sulfonate cluster morphology shoulder occurs and films can then be arranged by increasing the shoulder intensity as 8ab < 6ab < 5ab < 12bb < Nafion 111. Figure 9 shows the difference in SAXS data for Grot 12bb (10 wt %) and Grot 8ab (53 wt %) membranes, with the characteristic shoulder being prevalent in the film containing the higher sulfonate concentration. SAXS characterization information shows that the mixed sulfonate/carboxylate films can be synthesized from the Nafion R1100 perfluorosulfonyl fluoride precursor using the proposed Grot procedure. F. Pervaporation. Pervaporation of a single-component liquid permeate through a membrane can be described by a simple three-step mechanism. First, on the feed surface, the membrane absorbs the penetrant, the penetrant diffuses through the membrane, and finally the penetrant evaporates as it exits the lowpressure side of the membrane. The driving force for the process is the difference between the vapor pressure of the water on the feed side of the membrane and the pressure (vacuum) on the permeate side of the membrane. For dilute aqueous solutions, a thermodynamic relationship gives the bulk flux as a function of the transmembrane pressure drop, as shown by eq 5.
J)
P(∆P) DS(∆P) ) l l
(5)
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Table 4. Mixed Film (H+ Form) Flux and Water Separation Data
run
carboxylate content (mol %)
0
Nafion 111 Grot 5ab Grot 6ab Grot 7ab Grot 7bb Grot 8ab Grot 10ab Grot 12bb
0 17 41 0 0 53 32 10
3.4 0.045 0.0023 0.83 1.5 0.21 0.022 0.63
a
flux (kg m-2 h-1) at a given nitric acid concn (%)a 1 2 1.7 0.046 0.003 1.1 1.5 0.030 0.0070 0.62
2.4 0.048 0.0020 0.84 1.3 0.030 0.0060 0.57
5 1.9 0.032 0.0015 0.61 1.3 0.027 0.0062 0.33
water separation (R) at a given nitric acid concn (%)b 1 2 5 2.2 94.9 11.3 4.2 3.3 274 77.3 23.8
26.9 376 32.5 2.6 1.9 666 176 11.8
21.3 469 103 3.1 1.7 796 433 3.3
Error in flux of less than 21%. b Error in water separation factors of less than 12%.
Figure 10. Total pervaporation flux through perfluoro ionomer membranes as a function of the nitric acid concentration in a feed solution.
Equation 4 is developed from Fick’s law by equating the chemical potentials of a component in the membrane at the feed and permeate interfaces. In a binary system, the performance is measured by the separation efficiency (R). A component separation efficiency for a binary solution is the ratio of the component concentration ratios in the permeate to the component concentration ratios in the feed
R)
[ci/cj]permeate [ci/cj]feed
)
[pi/pj]permeate [ci/cj]feed
(6)
where i and j are the two components in the solution.23 In general, an optimal membrane separation unit operation will have large permeate fluxes and large separation efficiencies. Flux and separation efficiencies can be adjusted as required through the manipulation of physical characteristics such as the membrane thickness (resistance). However, a tradeoff is usually realized because when the permeability (flux) increases, the separation efficiency generally decreases. Pure water and nitric acid dehydration pervaporation transport tests were conducted with films generated using the Grot chemical conversion process described above. Pervaporation feed solutions consisted of pure water and 1, 2, and 5% nitric acid. Both the total mass flux and the permeate nitrate concentration were measured. Table 4 lists the flux and water separation factor data for each of the membranes tested. Figure 10 illustrates the total flux as a function of the nitric acid concentration for the film transport data listed in Table 4. Flow dramatically decreases 3 orders of magnitude as the carboxylate content increases from 0 to 41 mol %. Two extremes can be seen in films Grot 7bb and Grot 6ab showing a very strong correlation between the
Figure 11. Pervaporation water separation factors in nitric acid for membranes as a function of the carboxylate content.
carboxylate content and a decrease in the total mass flux. The total flux is a strong function of the polymer carboxylate content but not a function of the acid concentration, as shown in Figure 10. An exception to the trend is seen in the data for film Grot 8ab (53 mol %), where the water flux is approximately 1 order of magnitude higher than that for Grot 6ab (41 mol %). Reasons for the discrepancy could include nonhomogeneous conversion of the particular film tested (pockets of sulfonate-terminated pendant chains) as well as contributions from experimental error and statistical error. The relationship between the water separation factor and the carboxylate concentration is detailed in Table 4 and shown graphically in Figure 11. Results indicate that the magnitudes of the water separation factors increase by 2 orders of magnitude as the carboxylate content increases from pure sulfonate (Grot 7ab and 7bb) to 53 mol % (Grot 8ab). Presented in Figure 12 are water separation characteristics in relation to the bulk solution flux for six Grot membranes and Nafion 111 at nitric acid feed concentrations of 1, 2, and 5 wt %. The plot shows that the water separation efficiency, which is strongly proportional to the carboxylate content, is inversely proportional to the flux for this set of mixed ionomer films. Figure 12 demonstrates a tradeoff in terms of application of this film to the dehydration of nitric acid: When high water separation factors are required, low transmembrane fluxes will be expected. Some variability is seen in the tradeoff summary curve in that films with higher carboxylate content (Grot 8ab) were shown to have a larger than expected water flux relative to films with 25% less carboxylate pendant-chain conversion
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difference between pKa values demonstrates the higher extent of proton dissociation and lower water hydrogenhydroxyl oxygen bonding strength for sulfonic acid. Summary and Conclusions
Figure 12. Flux-separation tradeoff for conversion films (1, 2, and 5 wt % nitric acid).
(Grot 10ab and Grot 6ab). Again, a reason for the discrepancy could include nonhomogeneous conversion of films tested (pockets of sulfonate-terminated pendant chains). Water flux and separation data reveal the flexibility of the material in that the carboxylate content can be manipulated in order to generate a film with a given water separation capability, provided that the materials flux is adequate for the application. A decrease in the total flux with the carboxylate content appears to complement the basic information developed by Boyle et al. and Yeager et al. in terms of a reduction in the overall water content and water diffusivities.10,16,17 Both the total flux and water separation data support speculation by Sportsman that the reduction of the total flux and an increase in the water separation factor (nitric acid solutions) are due to the presence of perfluorocarboxylate ionomer material.1,2 G. Water Flux in Perfluorosulfonate and Mixed Membranes. A number of studies have been conducted in an attempt to explain why the water flux in pure perfluorocarboxylate and mixed carboxylate/sulfonate membranes is much lower than that in similar pure perfluorosulfonate polymers. Boyle et al. determined that the water diffusivities in a pure perfluorocarboxylate and mixed carboxylate/sulfonate material were comparable to those in the pure sulfonated Nafion ionomer at a similar water content.10,16 The lower hydration of exchange sites (moles of water per moles of exchange site) in the carboxylate material is believed to be the predominant reason for the relatively lower water flux or permeability in the ionomer. Yeager attributed a reduction in the carboxylate film hydration to the increased hydrogen bond strength between the carboxylate hydroxyl oxygen and the water hydrogen: Inductive charge/bonding effects translate to higher hydrogen-to-oxygen bonding strength and less water content.17 The hydrogen-oxygen bonding strength for the sulfonate ionomer (sulfonate hydroxyl) was 62% of the pure water hydrogen-oxygen bonding strength, whereas the carboxylate film oxygen-hydrogen bonding strength was determined to be approximately 92% of that determined for pure water. When the acid/base equilibrium constant, or pKa, for triflic acid (trifluoromethanesulfonic acid) is compared with that for trifluoroacetic acid (trifluoromethanecarboxylic acid), these two acids contain the same trifluoromethane substituent and have pKa’s of -0.25 and -13.6 for carboxylic and sulfonic acids, respectively. A dramatic
In summary, results from this investigation demonstrated the viability of an in-house procedure by which perfluorosulfonate and mixed sulfonate/carboxylate films could be synthesized. The in situ conversion procedure used to synthesize the films from the Nafion R1100 sulfonyl fluoride precursor proved to be simple and could be completed in a relatively short period of time of approximately 2 days. In addition, our studies provided extensive characterization information for synthesized materials and produced transport data that showed that the nitric acid bulk flux decreases and water separation efficiencies increase as the carboxylate content increases in the mixed ionomer films. Synthesized ionomer materials were subject to general characterization that included ionomer EW and water sorption measurements. On average, the mixed film IEC compared well with that of perfluorosulfonate ionomer films, where sulfonate and mixed ionomer IECs were determined to be 0.91 and 1.06 equiv g-1, respectively. Immediately following the conversion process, relative FTIR absorbance peak intensities were used to qualitatively determine whether an individual run had generated the desired mixed ionomer perfluoro product. Relative amounts of carboxylate- and sulfonate-terminated pendant chains were determined quantitatively using XRF. In total, 16 films were studied using XRF, confirming a valid in situ conversion procedure for generating films with a range of carboxylate contents. XRF results indicated that the film carboxylate content ranged from that of pure sulfonate to approximately 71 mol %. A positive correlation was found between the carboxylate pendant-chain contents determined by XRF and the semiqualitative FTIR method. Most importantly, the pervaporation transport data show how the flux decreased by approximately 3 orders of magnitude and the water separation factor increased by as much as 2 orders of magnitude as the carboxylate side-chain content was increased from 0 (pure sulfonate) to 53 mol % carboxylate. Transport results show that transmembrane flux and separation efficiencies are a function of the ratio of the sulfonate to carboxylate contents in the mixed perfluoro ionomer. Acknowledgment Funding was provided by the Department of Energy (DOE) and the Plutonium Stabilization and Scrap Recovery Program at LANL. LANL is operated by the University of California under Contract No. W-7405ENG-36. The authors also acknowledge the support provided by Dr. George Havrilla and Matthew Stanton (XRF) of LANL and Dr. Donald Williamson (SAXS) of the CSM Physics Department. Nomenclature COOK ) carboxylate-terminated pendant chain CR ) flux XRF intensity correction factor ci,j ) aqueous component concentration (mol L-1) D ) diffusion coefficient (cm2 s-1) Ik ) XRF potassium intensity Is ) XRF sulfur intensity
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J ) bulk or component flux (kg m-2 h-1) l ) membrane thickness (cm) M ) ionomer counterion, precursor halogen or hydrogen m ) pendant-chain ether repeating unit n ) repeating polymer block unit P ) permeability (m3 m m-2 Pa-1) p ) perfluoro ionomer repeating unit pi,j ) partial pressure of individual constituents in a binary solution pKa) acid/base equilibrium constant q ) tetrafluoroethylene repeating unit S ) solubility of the solvent in membrane material (cm3 cm-3 cmHg-1) R ) transmembrane separation efficiency ∆P ) transmembrane pressure differential (psi or Pa) λ ) SAXS X-ray wavelength Acronyms CSM ) Colorado School of Mines DOE ) Department of Energy DTA ) differential thermal analysis EW ) ionic equivalent weight (g mol-1) FMI ) Fluid Metering, Inc. FTIR ) Fourier transform infrared spectroscopy Grot ) in situ chemical conversion process developed by Grot et al. (1982) IEC ) ion-exchange capacity (mmol g-1) LANL ) Los Alamos National Laboratory MCT ) mercury cadmium telluride NARS ) nitric acid recycle system NMT ) Nuclear Materials Technology SAXS ) small-angle X-ray scattering TFE ) tetrafluoroethylene TGA ) thermal gravimetric analysis XRF ) X-ray fluorescence spectroscopy
Literature Cited (1) Sportsman, K. S. The Dehydration of Nitric Acid through Nafion Ionomer Membranes Using High-pressure Membrane Processes. Ph.D. Dissertation, Colorado School of Mines, Golden, CO, 2002. (2) Sportsman, K. S.; Way, J. D.; Chen, W.-J.; Pez, G. P.; Laciak, D. V. The Dehydration of Nitric Acid Using Pervaporation and a Nafion Perfluorosulfonate/perfluorocarboxylate Bilayer Membrane. J. Membr. Sci. 2002, 203 (1), 155. (3) Pivovar, B. S.; Wang, Y.; Cussler, E. L. Pervaporation Membranes in Direct Methanol Fuel Cells. J. Membr. Sci. 1999, 154, 155. (4) Kusumocahyo, S. P.; Sudoh, M. Dehydration Acedic Acid by Pervaporation with Charged Membranes. J. Membr. Sci. 1999, 161, 77. (5) Xie, G.; Okada, T. Water Transport Behavior in Nafion 117 Membranes. J. Electrochem. Soc. 1995, 142 (9), 3057. (6) Bowen, W. R.; Mohammad, A. W. A Theoretical Basis for Specifying Nanofiltration MembranessDye/Salt/Water Streams. Desalination 1998, 117, 257.
(7) Jiang, J. S.; Greenberg, D. B.; Fried, J. R. Pervaporation of Methanol from Triglyme Solution Using a Nafion Membrane: Transport Studies. J. Membr. Sci. 1997, 132, 255. (8) Sportsman, K. S. Pervaporation of Aqueous Nitric Acid with an Ionomer Membrane. Masters Thesis, Colorado School of Mines, Golden, CO, 1998. (9) Grot, W. G.; Molnar, C. J.; Resnick, P. R. Fluorinated Ion Exchange Polymer Containing Carboxylic Groups, Process for Making Same and Film and Membrane Thereof. U.S. Patent 4,544,458, Oct 1, 1985. (10) Boyle, N. G.; McBrierty, V. J.; Douglass, D. C.; Eisenberg, A. NMR Investigation of Molecular Motion in Nafion Membranes. Macromolecules 1983, 16 (1), 80. (11) Fujimura, M.; Hashimoto, T.; Kawai, H. Small-Angle X-ray Scattering Study of Perfluorinated Ionomer Membranes. 2. Models for Ionic Scattering Maximum. Macromolecules 1982, 15, 136. (12) Boyle, N. G.; Coey, M. D.; Meagher, A.; McBierty, V. J.; Nakano, Y.; MacKnight, W. J. Aqueous Phase in a Perfluorocarboxylate Membrane. Macromolecules 1984, 17, 1331. (13) Lim, S. K.; Galland, D.; Pineri, M.; Coey, J. M. D. Microstructure Studies of Perfluorocarboxylated Ionomer Membranes. J. Membr. Sci. 1987, 30, 171. (14) Perusich, S. A. Fourier Transform Infrared Spectroscopy of Perfluorocarboxylate Polymers. Macromolecules 2000, 33 (9), 3431. (15) Fujimura, M.; Hashimoto, T.; Kawai, H. Small-angle X-ray Scattering Study of Perfluorinated Ionomer Membranes. 1. Origin of Two Scattering Maxima. Macromolecules 1981, 14, 1309. (16) Boyle, N. G.; McBrierty, V. J.; Douglass, D. C. A Study of the Behavior of Water in Nafion Membranes. Macromolecules 1983, 16 (1), 75. (17) Yeager, H. L.; Twardiwski, Z.; Clarke, L. M. A Comparison of Perfluorinated Carboxylate and Sulfonate Ion Exchange Polymers, I. Diffusion and Water Sorption. Electrochem. Sci. Technol. 1982, 129 (2), 324. (18) Blatt, E.; Sasse, W. H. F.; Mau, A. W. H. Comparative Photophysical Studies on Flemion and Nafion. J. Phys. Chem. 1988, 92, 4151. (19) Moore, R. B.; Martin, C. R. Chemical and Morphological Properties of Solution-Cast Perfluorosulfonate Ionomers. Macromolecules 1988, 21, 1334. (20) Guinier, A. Ann. Phys. 1939, 12, 161. (21) Williamson, D. L. Nanostructure of R-SiH and Related Materials by Small-Angle X-ray Scattering. Mater. Res. Soc. Symp. Proc. 1995, 377, 251. (22) Feigin, L. A.; Svergun, D. I. Structure Analysis by SmallAngle X-ray and Neutron Scattering; Plenum Press: New York, 1987. (23) Wijmans, J. G.; Baker, R. W. A Simple Predictive Treatment of the Permeation Process in Pervaporation. J. Membr. Sci. 1993, 70, 101.
Received for review December 1, 2004 Revised manuscript received March 8, 2005 Accepted March 14, 2005 IE0488391
