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Article Cite This: ACS Omega 2018, 3, 7474−7482
Structural, Thermal, and Gas-Transport Properties of Fe3+ IonExchanged Nafion Membranes Mohsin Mukaddam,†,‡ Yingge Wang,‡ and Ingo Pinnau*,†,‡ Functional Polymer Membranes Group, ‡Advanced Membranes and Porous Materials Center, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia
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ABSTRACT: The physical and gas-transport properties of Fe3+ cross-linked Nafion membranes were examined. Wide-angle X-ray diffraction results revealed a lower crystallinity for Nafion Fe3+ but showed essentially no changes in the average chain spacing upon cation exchange of Nafion H+. Raman and Fourier transform infrared spectroscopy techniques qualitatively measured the strength of the ionic bond between the Fe3+ cations and sulfonate anions. Thermal gravimetric analysis indicated that the incorporation of Fe3+ adversely affected the thermal stability of Nafion due to the catalytic decomposition of perfluoroalkylether side chains. Gas sorption isotherms of Nafion Fe3+ measured at 35 °C up to 20 atm exhibited a linear sorption uptake for O2, N2, and CH4 following Henry’s law and slight concave behavior for CO2. Pure-gas permeation results showed reduced gas permeability but higher permselectivities compared to Nafion H+ with αN2/CH4 = 4.0, αCO2/CH4 = 35, and αHe/CH4 = 733 attributable to the strong physical cross-linking effect of Fe3+ that caused chain stiffening with enhanced size-sieving behavior. Gas mixture permeation experiments using 1:1 molar CO2/CH4 feed demonstrated reduced CO2 plasticization for Nafion Fe3+. At 10 atm CO2 partial pressure, CO2/CH4 selectivity decreased to 28 from the pure-gas value of 35, which was a significant improvement compared to the performance of a Nafion H+ membrane.
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aggregates that enable physical cross-linking.27,28 Consequently, cation exchange in ionomers can induce significant changes in their physical properties including enhancement in glass transition temperature,28−35 increase in Young’s modulus,32,34,35 and reduction in chain mobility27,28,32,33,36 that play a crucial role in the intrinsic gas separation performance of the material. For example, Park et al. reported pure-gas transport properties of sulfonated polysulfone membranes neutralized with several metal cations and showed that the cation-exchanged membranes improved the CO2/N2 selectivity moderately due to ionic cross-linking.20 However, the CO2 permeability was reduced by 63% from 5.4 to 2.0 Barrers due to reduction in the CO2 diffusion coefficient. Chen et al. reported the permeabilities for a series of gases including CO2 and CH4 in sulfonated polystyrene (PSS) membranes neutralized with Na+ and Mg2+ cations. PSS−Mg2+ and PSS−Na+ exhibited CO2/CH4 permselectivities of 65 and 49, respectively, which was substantially higher than the value of 18 in the acidic PPS−H+ form. Additionally, it was shown that the CO2 permeability in PSS−Mg2+ increased by only 9% between 1 and 7 atm, indicating that the membrane was less prone to CO2 plasticization.21 Rhim et al. investigated the CO2/CH4 separation properties in sulfonated poly(phenylene oxide) (SPPO) membranes cation-exchanged with several mono-, di-, and trivalent cations. The trivalent SPPO−Al3+
INTRODUCTION Polymeric membranes are of increasing interest and importance for industrial gas separation applications such as nitrogen production from air, olefin/paraffin separation, hydrogen recovery in petrochemical plants, and carbon dioxide removal from natural gas.1−6 The requirements for a preferred membrane material include high permeability, high selectivity, and good thermal and mechanical integrity at the operating conditions.7,8 Robeson’s pure-gas permeability/selectivity “upper-bound” curves identified an undesirable tradeoff: polymer membrane materials with high selectivity typically show low permeability and vice versa.9,10 In the area of CO2/ CH4 separation, removal of CO2 from natural gas or biogas is of particular commercial interest.1,4,11−13 The strong interaction and resulting high sorption uptake of CO2 in polymer membranes can result in swelling-induced plasticization. Therefore, mixed-gas CO2/CH4 selectivity is often significantly reduced compared to pure-gas values due to the dilation of the polymer matrix, which leads to high CH4 process losses, thus making membrane-based separation less attractive.14−19 Therefore, considerable research has been devoted to tailoring the structure/composition of the polymer to achieve improved performance. Ionomers, such as sulfonated polymers comprising strong acidic sulfonate groups, potentially provide a convenient and advantageous approach for tailoring material properties via metal complexations that can cause substantial improvements in gas separation properties.20−26 Ionomers are known for their complex morphology, in which sulfonic groups together with counterions form © 2018 American Chemical Society
Received: May 6, 2018 Accepted: June 26, 2018 Published: July 6, 2018 7474
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Figure 1. Structure of Nafion H+ and Fe3+ ion-exchanged Nafion.
pressure-dependent mixture performance using a 1:1 molar CO2/CH4 feed, are reported.
membrane showed the highest enhancement in CO2/CH4 selectivity from 11 in the protonated form to 20 in the cationexchanged polymer.25 Similar results were obtained by Khan et al., who reported an increase in the mixed-gas CO2/CH4 selectivity in an Al3+-neutralized sulfonated poly(ether ether ketone) membrane by approximately 62% to 19 compared with 11 in the protonated form.22 The enhanced selectivity was attributed to the strong cross-linking effect of trivalent cations that hindered the diffusivity of the larger-sized CH4 molecule more effectively that CO2. Interestingly, only a few studies reported the gas separation performance of cation-exchanged Nafion membranes. For example, Sakai et al. investigated gas transport properties of a K+ cation-exchanged Nafion membrane and showed that O2/ N2 selectivity increased from 2.2 to 7.4.37 Mohamed et al. investigated the free volume and gas permeation properties of Nafion Na+ and K+ cation-exchanged membranes.34 Their study demonstrated an increase in free volume coupled with a decrease in O2 permeability for the cation-exchanged Nafion membranes compared to the H+ form. This behavior was different from the general free volume concept, where polymers with higher free volume typically exhibit higher gas permeability.38,39 More recently, Fan et al. studied Nafion H+ neutralized with monovalent (Li+, Na+, K+) and divalent (Ca2+) cations to investigate the effect of ionic interaction on the gas-transport properties.40 Their study showed a higher gas permeability for cation-exchanged membranes compared to Nafion H+ and suggested that the high gas solubility of metalion-functionalized Nafion dominated gas permeation. Our group recently reported the effect of CO2 plasticization on CO2/CH4 separation performance in Nafion H+.41 Despite low CO2 solubility in Nafion H+ compared to conventional hydrocarbon polymers,42 binary mixed-gas experiments revealed that the strong affinity of CO2 to the sulfonate groups caused a large increase in segmental mobility, resulting in 40% reduction in the CO2/CH4 selectivity from 26 to 15 by increasing the CO2 partial pressure from 2 to 15 atm. This study investigated the physical modification of Nafion by exchanging the H+ ions with trivalent Fe3+ cations. It was anticipated that the cross-linking effect of Fe3+ could reduce the intersegmental mobility of Nafion, leading to improved resistance toward undesirable penetrant-induced plasticization effects under demanding CO2/CH4 gas mixture test conditions. The Fe3+ cation-exchanged membrane was characterized by wide-angle X-ray diffraction (WAXD), thermogravimetric analysis (TGA), Raman spectroscopy, and Fourier transform infrared spectroscopy (FT-IR). Pure-gas sorption, diffusion, and permeation properties of Nafion Fe3+, as well as
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RESULTS AND DISCUSSION Structural Properties of Fe3+ Ion-Exchanged Nafion. Nafion H+ films (NRE 211) with thickness of 25 μm and equivalent weight of 1100 g equiv−1 were purchased from Ion Power, Inc. Ion exchange was performed by soaking Nafion H+ films in an aqueous 0.05 molar Fe(NO3)3 solution, as described in more detail in Experimental Section. The structures of Nafion H+ and Fe3+ ion-exchanged Nafion are shown in Figure 1. The effect of cation exchange on the microstructure of Nafion Fe3+ was qualitatively assessed using the wide-angle Xray diffraction (WAXD) as illustrated in Figure 2. Similar to
Figure 2. Microstructure of Nafion Fe3+. The diffraction spectrum was corrected for background scattering and the crystalline and amorphous peaks were obtained by applying the Pearson VII distribution function.
our previous study42 using Nafion H+, the spectrum showed a semicrystalline behavior for Nafion Fe3+ membranes that was represented by (i) a crystalline peak at 2θ = 17.7° and (ii) two amorphous halo peaks at 2θ = 16.9 and 40.1°. The average interchain spacings (d) were determined from Bragg’s equation (d = λ/2 sin θ) and are summarized in Table 1. Apparently, the interchain spacing in Nafion was only marginally affected by cation exchange, consistent with previously reported WAXD 7475
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toward a lower degradation temperature in the Fe3+ ionexchanged membrane corresponded to a decrease in thermal stability. A similar trend was observed for an Al3+ ionexchanged Nafion membrane in which the decrease in the thermal stability was attributed to the breaking of the perfluoroalkylether bonds catalyzed by aluminum oxides that act as Lewis acids.46,48 Above ∼400 °C, the final decomposition of the poly(tetrafluoroethylene) backbone began and continued until the polymer mass decomposed to 100% in Nafion H+ and 97.9% in Nafion Fe3+ at 600 °C. TGA was also used to quantify the amount of water lost from each membrane type by measuring the kinetics of the dehydration process at 80 °C, as illustrated in Figure 4. A period of ∼300 min was required to reach an essentially dry state in both membrane types. This result indicated that the Nafion films dried under vacuum for 2 days at 80 °C did not contain a significant amount of water when used for the gas sorption and permeation tests. The Raman and FT-IR spectra in Nafion H+ and Nafion Fe3+ membranes are presented in Figure 5a,b, respectively. The spectra provide information on the changes in the membrane microstructure (chain packing, ordering, bond strength) by measuring the shift and broadening of the spectra maxima.49 The maxima observed at 1057 (FT-IR) and 1062 (Raman) for Nafion H+ corresponded to the symmetric S−O stretching vibrations of the sulfonate group. The assignment of this band has been reported in previous work.40,50−52 As shown, the maxima shifted toward a higher frequency when the protons were exchanged with Fe3+ cations. The shift in frequency is a qualitative indication of the bond strength between the cation and the sulfonate anion. The strength of the ionic interactions influences the flexibility of the polymer chains,27 which has a significant effect on the gas transport properties, as discussed below. Pure-Gas-Transport Properties of Nafion Fe3+ Membrane. The permeabilities of pure gas He, H2, N2, O2, CH4, and CO2 and gas-pair selectivities of Nafion Fe3+ membranes are summarized in Table 2. The permeabilities of Nafion H+ membranes were determined in our previous study under the same conditions and are reported here for comparison.42 The permeabilities of the Fe3+ ion-exchanged membrane followed the order He > H2 > CO2 > O2 > N2 > CH4, indicative of its strong size-dependent permeation properties, qualitatively similar to the permeability behavior of Nafion H+.42 The Fe3+ ion-exchanged membrane demonstrated a
Table 1. Interchain Spacing and Percentage of Crystallinity for Nafion H+ and Fe3+ Ion-Exchanged Nafion Obtained from WAXD Nafion type
dc (Å)
da1 (Å)
da2 (Å)
crystallinity (%)
+
5.0 5.0
5.25 5.30
2.3 2.3
11.4 6.3
H Fe3+
results for Nafion and other cation-exchanged polymers.40,43 Based on this result, it is very difficult to draw any clear conclusions about the effects of interchain spacing on the gastransport properties. The only noticeable difference was the decrease in the crystallinity from 11.4% in Nafion H+ to 6.3% in Nafion Fe3+, which was calculated by applying the Pearson VII distribution function.42 Such a change in crystallinity upon cation exchange has been previously reported and influences the gas-transport properties as the crystallites act as impermeable regions in the polymer.23,44 The effects of cation exchange on the thermal properties of Nafion were analyzed by thermogravimetric analysis (TGA). The TGA profiles of Nafion H+ and Nafion Fe3+ membranes are presented in Figure 3a,b as percentage weight loss and firstorder derivatives, respectively. Both Nafion types exhibited four decomposition stages between 100 and 600 °C. The first stage weight loss occurred between 100 and 280 °C. The gases evolved during this temperature range were mostly water with trace amounts of SO2 and CO2 as identified using FT-IR.45 The second stage weight loss occurred between 280 and 350 °C and according to Wilkie et al. was primarily due to the release of SO2 and CO2 gases with minimal amount of water.45 SO2 evolved due to the cleavage of C−S bonds, forming CF2, SO2, and OH radicals.45 Interestingly, the weight reduction (excluding water loss) observed in Nafion H+ was ∼7%, whereas the weight loss in the cation-exchanged Nafion Fe3+ was only 1%, indicating that the stronger interaction between Fe3+ cations and the sulfonate anions prevented the cleavage of the C−S bonds. The absence of the desulfonation process for Nafion cationexchanged membranes has been previously reported.40,45−48 The third stage weight loss occurred between 350 and 410 °C in Nafion Fe3+, with a sharp decomposition temperature peak at 400 °C, whereas in Nafion H+, the weight loss occurred between 350 and 450 °C, with the peak maxima at 423 °C. This weight loss has been attributed to the degradation of perfluoroalkylether side-chain groups of Nafion.46,48 The shift
Figure 3. TGA profile presented as (a) percentage weight loss and (b) first-order derivative of Nafion H+ (black) and Nafion Fe3+ (red) operated under N2 atmosphere at a heating rate of 3 °C min−1. 7476
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Figure 4. Thermogravimetric analysis (TGA) profile for (a) Nafion H+ and (b) Nafion Fe3+ films comparing the amount of water lost as a function of time upon drying at 80 °C under inert N2 gas.
Figure 5. (a) Raman spectra and (b) FT-IR spectra of Nafion H+ and Nafion Fe3+ films. The black dotted line is drawn through the center of Nafion H+ maxima for visualizing the relative frequency shift.
Table 2. Pure-Gas Permeability and Ideal Selectivity in Nafion H+ and Nafion Fe3+ Membranes at 2 atm and 35 °C permeability (Barrer)b
selectivity
Nafion type
He
H2
CO2
O2
N2
CH4
He/CH4
CO2/CH4
N2/CH4
H+a Fe3+
37 33
7.2 6.2
2.3 1.6
1.0 0.78
0.24 0.18
0.083 0.045
446 733
28 35
2.9 4.0
Previous work.42 b1 Barrer = 10−10 cm3 (STP) cm (cm−2 s−1 cmHg−1).
a
reduced permeability for all the gases compared to the H+ type. Increased chain stiffness due to interchain cross-linking could be a plausible cause to explain the reduced permeability in the Fe3+ ion-exchanged membrane. The significant 45% drop in CH4 permeability enhanced the N2/CH4 selectivity from 2.9 to 4.0, CO2/CH4 selectivity from 28 to 35, and He/CH4 selectivity from 446 to 733. The higher selectivity can be attributed to the strong cross-linking effect by the trivalent Fe3+ cations that stiffened the amorphous phase in Nafion and, thereby, hindered the diffusivity of the larger-sized CH4 molecule more effectively than that of the other gases. To better understand the effect of cation exchange on the gas-transport properties of Nafion, the diffusivity and solubility data of the Fe3+ ion-exchanged film are compared with the previously measured values of the Nafion H+ membrane in Table 3.42 The gas solubility coefficients in the Nafion Fe3+ membrane were measured using the gravimetric technique and the diffusivity coefficients were then deduced from the
Table 3. Summary of Gas Diffusion and Solubility Coefficients in Nafion H+ and Nafion Fe3+ Membranes Determined at 2 atm and 35 °C diffusion coefficient (10−8 cm2 s−1)
solubility coefficient (10−2 cm3 (STP) cm−3 cmHg−1)
Nafion type
O2
N2
CO2
CH4
O2
N2
CO2
CH4
H +a Fe3+
5.88 2.00
1.85 0.56
2.64 0.70
0.44 0.08
0.17 0.39
0.13 0.32
0.87 2.30
0.19 0.57
a Solubility (S) values were obtained barometrically from a previous study.42 The corresponding diffusion (D) values were determined using D = P/S.
relationship D = P/S. The gas solubilities followed the order of increasing condensability: N2 < O2 < CH4 < CO2. The Nafion Fe3+ ion-exchanged membrane demonstrated a ∼2−3fold higher gas solubility values compared to the Nafion H+ membrane. Similar solubility enhancements have been 7477
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Figure 6. Sorption isotherms in Nafion Fe3+ (red) measured gravimetrically and Nafion H+ (black) measured barometrically42 at 35 °C: (a) N2; (b) O2; (c) CH4; and (d) CO2.
reported for Nafion37,40 and other ionomers upon cation exchange.20,21 Fan et al. suggested that the high gas sorption in the cation-exchanged Nafion membranes resulted from the large cation size that acted as a spacer causing polymer chain expansion.40 Furthermore, the reduction in crystallinity in Nafion Fe3+ could have contributed to the enhanced gas solubility. As shown in Figures 6a−c, the sorption isotherms in Nafion Fe3+ were linear up to 20 atm, following Henry’s law. Both Nafion types showed similar qualitative solubility trends; however, the solubilities in Nafion Fe3+ were significantly higher than those in Nafion H+ across the entire pressure range. Interestingly, the isotherm for CO2 in Nafion Fe3+, as shown in Figure 6d, was slightly concave to the pressure axis, contrary to the linear CO2 isotherm for Nafion H+.42 Crosslinking in Nafion induced by trivalent Fe3+ ions effectively reduced the segmental mobility in the vicinity of ionic aggregations and possibly created rigid regions consisting of fixed holes similar to glassy polymers that provide additional sites for gas sorption. The observed increase in glass transition temperature reported in Nafion28−30,34,35 and other ionomers32,33 upon cation-exchange supports this hypothesis. Pure-gas diffusion (D) coefficients in Nafion Fe3+ calculated from the ratio of permeability and the directly measured gravimetric sorption data at 2 atm and 35 °C are shown in Table 4. The Nafion Fe3+ ion-exchanged membrane showed lower diffusivities compared to the Nafion H+ membrane. The extent of these differences is significant: the diffusivity of CH4 in Nafion Fe3+ was ∼6 times lower than that in Nafion H+.
Table 4. Gas Diffusion and Solubility Selectivity in Nafion H+ and Nafion Fe3+ Membranes at 2 atm and 35 °C diffusion selectivity
solubility selectivity
Nafion type
N2/CH4
CO2/CH4
N2/CH4
CO2/CH4
H+ Fe3+
4.2 7.0
6.0 8.8
0.68 0.56
4.6 4.0
Multivalent cations are known to physically cross-link the Nafion matrix by coordinating with sulfonate anions, resulting in enhanced chain rigidity and, therefore, improved size-sieving properties.21,22 The higher N2/CH4 and CO2/CH4 selectivity in Nafion Fe3+ was primarily due to its enhanced diffusivity selectivity, as shown in Table 4. Pressure-Dependant Pure- and Mixed-Gas Permeation Properties. Pure-gas permeabilities of CO2 and CH4 in Nafion H+ and Fe3+ membranes are compared in Figure 7 as a function of pressure at 35 °C. Across the entire pressure range investigated, Nafion Fe3+ demonstrated lower CO2 and CH4 permeabilities compared to Nafion H+. Figure 7a shows that the pure CO2 permeability increased with pressure for both Nafion types, which can be attributed to increased diffusivity. Presumably, quadrupolar CO2 molecules interacted favorably with the polar sulfonate groups of Nafion to increase its segmental chain mobility, causing membrane plasticization.27,41,53,54 The Fe3+ ion-exchanged membrane was less affected by CO2 plasticization, as indicated by the smaller relative increase in CO2 permeability compared to the Nafion H+ membrane. Our previous work showed that pure CH4 7478
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Figure 7. Pressure-dependent pure-gas permeability of (a) CO2 and (b) CH4 in Nafion H+ (black) and Nafion Fe3+ (red) at 35 °C.
Figure 8. Mixed-gas permeabilities of (a) CO2 and (b) CH4 in Nafion H+ (black) and Nafion Fe3+ (red) as a function of CO2 partial pressure at 35 °C.
Figure 9. (a) Pure-gas and (b) mixed-gas (1:1 molar feed ratio) CO2/CH4 selectivity in Nafion H+ (black)41 and Nafion Fe3+ (red) membranes as a function of CO2 partial pressure at 35 °C.
permeability of Nafion H+ decreased with increasing feed pressure, which was caused by a decrease in gas diffusivity most likely due to chain compression of the relatively flexible protonated ionomer.41 On the other hand, CH4 permeability of Nafion Fe3+ remained essentially constant over the pressure range of 2 to 15 atm (Figure 7b), which can be ascribed to its enhanced chain rigidity due to cross-linking. The pure-gas CO2 and CH4 permeation properties of both Nafion types indicated that CO2/CH4 selectivity increases with feed pressure. However, the opposite trend was observed when the polymers were tested under mixed-gas conditions, as discussed below.
Figure 8 compares the permeabilities of CO2 and CH4 in Nafion H+ and Nafion Fe3+ membranes tested with a 1:1 molar feed gas mixture as a function of pressure at 35 °C. These mixture experiments are important to assess the membrane material performance as CO2 tends to introduce nonideal effects such as plasticization and competitive sorption, typically resulting in different permeation behavior compared to puregas permeabilities. In comparison to Nafion H+, CO2 mixture permeabilities in Nafion Fe3+ were lower at all pressure points, as shown in Figure 8a. It is important to note that mixed-gas CO2 permeabilities in Nafion Fe3+ were lower than their pure7479
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EXPERIMENTAL SECTION Materials. Nafion NRE 211 films with the thickness of 25 μm and the equivalent weight of 1100 g equiv−1 were purchased from Ion Power, Inc., in the H+ form. Before the Fe3+ exchange, the as-received Nafion H+ membranes were first washed with distilled water to remove any surface impurities. Next, the films were immersed in an aqueous Fe(NO3)3 solution of 0.05 molar concentration. The exchange process was monitored using a pH meter until the pH value stabilized. To ensure complete ion exchange, the films were then stored in the solution for at least 10 days. The wet films were gently wiped with soft tissue paper to remove any excess solution from the surface. The films were air dried for 12 h by sandwiching between two filter papers and then dried under high vacuum at 80 °C for 2 days before testing. The film density of Nafion Fe3+ was 2.14 g cm−3 ± 1% using a massvolumetric technique by accurately measuring the weight, area, and thickness of dry samples. Polymer Characterization and Methods. Wide-angle Xray diffraction (WAXD) measurements were conducted on a Bruker D8 Advance diffractometer.42 The thermal stability of Nafion H+ and Fe3+ samples was investigated on a TA instrument 2950 thermogravimetric analyzer (TGA) in a nitrogen atmosphere. Fourier transform infrared spectroscopy (FT-IR) spectra were obtained with a Varian 670-IR spectrometer. Raman spectra of Nafion H+ and Nafion Fe3+ were collected on a LabRAM ARAMIS (Horiba Jobin Yvon, Inc.) instrument. Pure-Gas Solubility and Permeation Tests. The gas solubility of Nafion Fe3+ was measured using a Hiden Intelligent Gravimetric Analyzer (IGA-003, Hiden Isochema, U.K.) as previously described.41 Pure-gas permeabilities at 35 °C were determined with a custom-designed constant volume/ variable pressure test system.42 Mixed-Gas Permeation Tests. The mixed-gas permeation experiments were performed at 35 °C with a feed gas containing 50 vol % CH4/50 vol % CO2 at total pressures ranging from 4 to 30 atm following a method described by O’Brien et al.57 and applied in our previous work.41
gas values most likely due to competitive sorption effects. As pressure increased from 2 to 15 atm, mixed-gas CO 2 permeability was only marginally affected for Nafion H+ and Nafion Fe3+. Mixed-gas CH4 permeabilities in Nafion Fe3+ were lower than their pure-gas values across the investigated pressure range. As discussed above, competitive sorption and polymer chain compression can explain this decrease. In comparison to Nafion H+, mixture CH4 permeabilities in Nafion Fe3+ were lower at all pressure points, as shown in Figure 8b. As the pressure increased from 2 to 15 atm, the mixed-gas CH4 permeability in Nafion Fe3+ increased by ∼30%, whereas that in Nafion H+ increased by 56%. The strong cross-linking in Nafion Fe3+ suppressed the CO2-induced increase in segmental mobility more effectively compared to Nafion H+, and consequently reduced plasticization. Figure 9 compares pure- and mixed-gas (1:1 molar feed ratio) CO2/CH4 selectivities in Nafion H+ and Nafion Fe3+ membranes. In the pure-gas experiment, Nafion Fe3+ showed a higher CO2/CH4 selectivity with increased feed pressure, similar to the trend previously observed for Nafion H+, as shown in Figure 9a.41 The increased pure-gas selectivity resulted from two effects: (i) increased CO2 permeability due to plasticization and (ii) reduction in CH4 permeability to chain compression.41 However, when tested under mixed-gas conditions, the dominant increase in CH4 permeability due to CO2-induced plasticization caused the mixed-gas CO2/CH4 selectivities to decrease for both Nafion types. In comparison, Nafion Fe3+ exhibited a higher selectivity than Nafion H+ over the entire pressure range. The mixture CO2/CH4 selectivity of Nafion Fe3+ dropped by only 19% from 32 at 2 atm to 26 at 15 atm CO2 partial pressure. At typical natural gas operating conditions (∼10 atm CO2 partial pressure),55,56 Nafion Fe3+ exhibited the mixed-gas CO2/CH4 selectivity of 28, which was 48% higher than the value of 19 obtained for Nafion H+. When compared to the most commonly used membrane material for natural gas treatment, cellulose triacetate, Nafion Fe3+ had a slightly higher CO2/CH4 selectivity (28 vs 25) but a lower CO2 permeability (1.1 vs 8 Barrer), which may limit its commercial potential for CO2/CH4 separations.
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AUTHOR INFORMATION
Corresponding Author
CONCLUSIONS The physical and gas-transport properties of Nafion complexed with trivalent Fe3+ cations were investigated. The physical cross-linking in Nafion Fe3+ resulted in high pure-gas N2/CH4 and CO2/CH4 permselectivity of 4.0 and 35, respectively, compared to the values of 2.9 and 28 in Nafion H+. This was attributed to the strong ionic interactions between Fe3+ cations and sulfonate anions that caused chain stiffening leading to enhanced diffusivity selectivities. High-pressure gas solubility measurements showed linear solubility isotherms for N2, O2, and CH4 and a slightly concave isotherm for CO2, potentially indicative of the existence of microvoids that created additional sorption sites. Consequently, the gas solubilities in Nafion Fe3+ were higher than those in Nafion H+. Binary mixed-gas experiments demonstrated that physical cross-linking via trivalent Fe3+ cations was an effective approach to reduce the high-solubility CO2-induced plasticization of Nafion. At a typical natural gas feed CO2 partial pressure of 10 atm, Nafion Fe3+ exhibited a CO2/CH4 selectivity of 28 compared to 19 in Nafion H+.
*E-mail:
[email protected]. ORCID
Ingo Pinnau: 0000-0003-3040-9088 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by funding from King Abdullah University of Science and Technology (KAUST). REFERENCES
(1) Baker, R. W.; Lokhandwala, K. Natural Gas Processing with Membranes: an Overview. Ind. Eng. Chem. Res. 2008, 47, 2109−2121. (2) Bernardo, P.; Drioli, E. Membrane Gas Separation Progresses for Process Intensification Strategy in the Petrochemical Industry. Pet. Chem. 2010, 50, 271−282. (3) Yampolskii, Y. Polymeric Gas Separation Membranes. Macromolecules 2012, 45, 3298−3311. (4) Baker, R. W.; Low, B. T. Gas Separation Membrane Materials: a Perspective. Macromolecules 2014, 47, 6999−7013.
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DOI: 10.1021/acsomega.8b00914 ACS Omega 2018, 3, 7474−7482
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DOI: 10.1021/acsomega.8b00914 ACS Omega 2018, 3, 7474−7482
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DOI: 10.1021/acsomega.8b00914 ACS Omega 2018, 3, 7474−7482
