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Gas transport properties of carbon molecular sieve membranes derived from metal containing sulfonated poly(phenylene oxide)
Desalination 193 (2006) 66–72
Gas transport properties of carbon molecular sieve membranes derived from metal containing sulfonated poly(phenylene oxide) Miki Yoshimune*, Ichiro Fujiwara, Hiroyuki Suda, Kenji Haraya Research Institute for Innovation in Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba 305-8565, Japan Tel./Fax: +81 (29) 861-4801; email: [email protected] Received 15 March 2005; accepted 13 April 2005
Abstract Hollow-fiber carbon molecular sieve (CMS) membranes derived from sulfonated poly(phenylene oxide) (SPPO) were studied. Mono-, di- and trivalent metal cations such as Na+, Mg2+, Al3+, Ag+, Cu2+ and Fe3+ were substituted to the proton of sulfonic acid group of SPPO by the ion-exchange method to investigate the effects on gas transport properties for SPPO CMS membranes. In the case of H-SPPO, CMS membranes exhibited higher performance than those of the polymeric precursor, and maximum permeability was obtained when pyrolyzed at 923 K. The introduction of metal cations into SPPO affected the structures of the resulting CMS membranes, of which the Ag- and Cu- form enhanced overall permeabilities. In addition, Fe-SPPO CMS membranes increased He/N2 and H2/N2 permselectivities compared to non-substituted CMS membranes prepared by the same pyrolysis procedure. The highest performance was attained by the Ag-SPPO CMS membrane pyrolyzed at 923 K, where O2 permeability was 178 Barrer and O2/N2 permselectivity was 8.7 at 298 K. Keywords: Gas permeation; Carbon molecular sieve membranes; Sulfonated poly(phenylene oxide); Metal cations; Ion-exchange method
1. Introduction Since carbon molecular sieve (CMS) membranes have shown that these materials successfully to compete with polymeric membranes due *Corresponding author.
to their superior permeability-selectivity combination and suitable performance for high temperature or corrosive environment, recent interest has been focused on a selection of new materials and an improvement in membrane preparation techniques in order to develop gas separation performance [1–3].
Presented at the International Congress on Membranes and Membrane Processes (ICOM), Seoul, Korea, 21–26 August 2005. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.
doi:10.1016/j.desal.2005.04.138
M. Yoshimune et al. / Desalination 193 (2006) 66–72
Organic–inorganic nanocomposite membranes have been the subject of growing interest as a promising technology; they are prepared by incorporation of inorganic materials such as zeolite, silica and others into a polymer matrix [4,5]. This technology is being applied for CMS membranes. Park et al. first prepared carbon membranes containing silica by the pyrolysis of copolyimides [6]. Another approach to improve the performance of CMS membranes is metal incorporation into CMS membranes. Metal cations can be considered to increase the polarity and/or to form interlayer spaces and/or to have an affinity toward target gases. Yoda et al. investigated CMS membranes derived from Pt- and Pddispersed polyimide film prepared by supercritical impregnation [7]. Kim et al. reported CMS membranes containing alkali metel ions using metal-substituted sulfonated polyimide [8]. Barsema et al. found that the incorporation of nano-sized Ag particles into a CMS matrix resulted in a selectivity increase of O2 over N2 with a substantial increase of permeability [9,10]. Recently, we succeeded in manufacturing CMS membranes derived from poly(phenylene oxide) (PPO) and modified PPO, which exhibited as high a performance as CMS membranes derived from other polymeric precursors [11,12]. PPO has been recognized as a membrane material with fairly high permeability with a high glass transition temperature among other polymers [13]. In addition, PPO is easily modified to provide various kinds of derivatives. It can be considered as another interesting object for the chemical modification of PPO. Matsuura and co-workers are dedicated to the development of polymeric PPO-based membranes [14], and they have investigated the effects of metal cations on gas separation properties for polymeric dense membranes of metal containing sulfonated PPO (SPPO) [15–18]. In this report the gas transport properties of CMS hollow-fiber membranes derived from SPPO containing various kinds of metal cations are studied. Metal
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cations were incorporated into polymeric SPPO membranes by substitution of the proton of a sulfonic acid group of SPPO by the ion-exchange method. We examined the effects of metal cations with various features, not only the alkali cations such as Na+, Mg2+ and Al3+ but also the transition metal ions such as Ag+, Cu2+ and Fe3+, which were used for substituted metal cations in this work. 2. Experimental 2.1. Preparation of SPPO Poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) was purchased from Aldrich Chemicals. Chlorosulfonic acid and chloroform were purchased from Wako Pure Chemical Industries as reagent grade. All chemicals were used without any purification. Sulfonation of PPO was carried out using chlorosulfonic acid in a chloroform solvent under nitrogen atmosphere according to the literature [19,20]. The synthetic scheme is shown in Fig. 1. The obtained SPPO was stored as Na-form (Na-SPPO) to prevent decomposition by hydrolysis. The degree of substitution (DS) was determined by the acid–base titration method. 2.2. Membrane manufacture The hollow-fiber polymeric membrane of NaSPPO was manufactured with the coagulation method. A 20 wt.% Na-SPPO solution in methanol was immersed into a saturated NaCl aqueous solution bath as a coagulating agent, which was stored into this solution until the metal substitution process that follows. The conversion into
Fig. 1. Synthesis of sulfonated poly(phenylene oxide).
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metal substituted SPPO (M-SPPO: Mn+ = Na+, Mg2+, Al3+, Ag+, Cu2+ and Fe3+) was performed by soaking SPPO in a H-form (H-SPPO) membrane into each metal nitrate solution (1 mol l-1) for 2 h. The H-SPPO membrane was obtained by soaking a Na-SPPO membrane into an aqueous solution of HCl (1 mol l!1) for 24 h. In case of the Ag substitution, the preparation was conducted in the absence of light, and the membrane was stored in the dark. For uniform ion exchanging, the solutions were circulated into the membranes. These polymeric precursor membranes of M-SPPO were dried at room temperature after washing with deionized water to remove excess metal ions. A carbonization process was carried out using a quartz tube furnace under vacuum. The polymeric precursor membranes of M-SPPO were at first preoxidized in air at 553 K for 45 min to yield optimum properties. Finally, the carbonized M-SPPO membranes were obtained by pyrolysis at 923–1273 K with a heating rate of 10 K min!1 for 2 h. 2.3. Characterization of M-SPPO The prepared M-SPPO membranes were characterized with FT-IR (Nicolet, Magna550). Thermogravimetric analysis was performed by means of Mac Science (Japan) TG-DGA 2000SA at a heating rate of 10 K min!1 under Ar atmosphere with a flow rate of 200 cm3 min!1. The microstructure of membranes was investigated by scanning electron microscopy (SEM) with a Hitachi S-2400. 2.4. Gas permeation measurement Pure gas permeability measurements for He, H2, CO2, O2 and N2 were conducted with a highvacuum time-lag apparatus at 298–363 K under the pressure difference of 1 atm. Both the feed and permeate sides of the membrane cell were evacuated (<10!5 Torr) prior to each measurement. Permeability coefficients are expressed in
Barrer, where 1 Barrer = 1×10!10 cm3 (STP) cm cm!2 s!1 cm Hg!1 = 3.35×10!16 kmol m m!2 s!1 kPa!1. Permselectivity was defined in this study as the ideal separation factor, which is the ratio of permeability of chosen gas over that of N2. 3. Results and discussion 3.1. Membrane formation and characterization The degree of substitution (DS) of SPPO in this study was determined as 40%. At higher DS of more than 50%, the SPPO membrane no longer maintained its structure because a part of the SPPO dissolved in water during the ion-exchange process. This phenomenon was described by Schauer et al. that the SPPO polymers with DS of about 20–40% were soluble in NMP but insoluble in water, whereas those with the DS higher than 85% were water soluble [21]. Fig. 2 shows the IR spectra of the original PPO (a) and Na-SPPO (b), which are in agreement with those reported [19,22]. In the spectrum of Na-SPPO (b), a new band appeared at 1070 cm!1, which is assigned to the S=O stretching vibration of the SO3! group. On the contrary, the intensity of the band at 830 cm!1 (out-of-plane vibration of 1,4-disubstituted benzene ring) considerably decreases, which reflects the substitution of an H atom in an aromatic ring by a sulfonic group. As can be seen in Fig. 2(c), the peak intensities for the Na-SPPO decreased due to thermal degradation after the pyrolysis at 1023 K. Fig. 3 presents the TGA analysis of H-, Naand Ag-SPPO. In the case of H-SPPO, four major weight losses of varying extent at around 273, 473, 650 and 700 K were observed corresponding to the removal of water vapor, the degradation of the sulfonic acid group, the degradation of the methyl group and polymer decomposition, respectively [15,16]. The metal containing MSPPO increased thermal stabilities in the following order: H+ < Ag+ < Na+. TGA spectra of Mgand Al-SPPO were similar to that of Na-SPPO
M. Yoshimune et al. / Desalination 193 (2006) 66–72
Fig. 2. FT–IR spectra of polymeric membranes for PPO (a) and Na-SPPO (b) together with CMS membrane for Na-SPPO pyrolyzed at 1023 K (c).
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Fig. 3. TGA curves for H-SPPO, Na-SPPO and Ag-SPPO.
Fig. 4. Scanning electron micrographs of cross sections for the polymeric Ag-SPPO membrane (A) and carbonized Ag-SPPO membrane (B, C).
and Cu- and Fe-SPPO gave analogous spectra to that of Ag-SPPO, indicating that the feature of metal cations may affect the pyrolysis behaviors of M-SPPO. The micrographs of cross sections for AgSPPO membranes before and after pyrolysis are given in Fig. 4. The prepared membranes had symmetric structures without defects, which remained after the heat treatment. The diameter of the hollow fiber was reduced from 460 µm to 400 µm by the shrinkage of the membranes due
to pyrolysis, where its thickness was largely unchanged. 3.2. Gas permeation properties of M-SPPO Fig. 5 shows the dependence of pyrolysis temperature on gas transport properties for H-SPPO together with its polymeric precursor. The obtained H-SPPO CMS membranes pyrolyzed at 923–1273 K exhibited superior performance to that of the polymeric precursor in both perme-
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Fig. 6. Relations between O2 permeability and O2/N2 permselectivity at 298 K for SPPO CMS membranes pyrolyzed at 923 K compared to other membranes [9,16].
Fig. 5. Influence of pyrolysis temperature on: (A) permeability; and (B) permselectivity for H-SPPO membranes compared with their polymeric precursor.
abilities and permselectivities, which is consistent with the trends in our previous work [11]. The gas permeability reached a maximum when pyrolyzed at 923 K, and then decreased at higher temperatures. On the other hand, the He/N2 and H2/N2 permselectivities were significantly increased at 1023 K, of which permselectivities were 318 and 567 at 363 K, respectively. The gas transport properties for M-SPPO CMS membranes pyrolyzed at 923 K are summarized in Table 1. The measurement temperature was fixed at 363 K because it was found that the permeability of the Na-SPPO CMS membrane was too small to measure at a lower temperature. The Na-SPPO CMS membrane yielded the highest permselectivity; however, a general trade-off
relationship between permeability and selectivity was observed for this membrane to give the lowest permeability. The permeabilities of the Mg- and Al-SPPO CMS membranes increased with decreasing the permselectivity, resulting in that those productivities were slightly increased compared with the Na-SPPO CMS membrane. These alkali cations are considered to cause a steric hindrance into the carbon matrix, and gas permselectivity increased and permeability decreased when substituted by a low-valence alkali cation. The difference among Na-, Mg- and AlSPPO CMS membranes was brought about mainly by the substitution amount with the proton of sulfonic acid groups of SPPO, which increases in the following order: Na+> Mg2+>Al3+, because di- and trivalent cations associate with more than one SO3! group. On the other hand, it is notable that the incorporation of transition metal ions into SPPO CMS membranes enhanced these performances significantly. The Fe-SPPO CMS membrane gained higher permselectivities in smaller gases such as He and H2, without the loss of those permea-
M. Yoshimune et al. / Desalination 193 (2006) 66–72
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Table 1 Effect of metal cations on gas transport properties for M-SPPO CMS membranes at 363 K Metal
H Na Mg Al Ag Cu Fe
Thickness (µm)
20 30 32.5 30 25 20 35
Permeability (Barrer)
Permselectivity
He
H2
CO2
O2
N2
He/N2
H2/N2 CO2/N2 O2/N2
669 75.5 354 310 731 566 626
1574 87.1 773 760 2090 1581 1074
320 4.6 1494 262 690 615 129
107 1.8 52 72 236 196 49
15 0.15 8.8 13 38 31 6.6
45 500 40 24 19 18 95
107 577 88 59 55 51 164
21.8 30.2 16.9 20.1 18.2 19.7 19.7
7.3 11.7 5.9 5.6 6.2 6.3 7.5
Note: Membranes were carbonized at 923 K for 2 h.
bilities, compared to a non-substituted SPPO CMS membrane prepared by the same pyrolysis procedure. In addition, the introduction of the Ag+ ion (Ag-SPPO) enhanced the permeabilities for all gases tested, along with decreasing the permselectivities. However, the differences in CO2/N2 and O2/N2 permselectivities between the H-form and Ag-form are small, and overall yields were increased in the Ag-SPPO CMS membrane. There also was similar advancement in Cu-SPPO, suggesting that there is potential in the transition metal ions such as Ag+ and Cu2+ to improve the performance of M-SPPO CMS membranes. In order to compare the performances of other membranes in the references, a detailed study was conducted for the M-SPPO CMS membranes in H-form and Ag-form pyrolyzed at 923–1023 K. Fig. 6 shows the relationship between O2 permeability and O2/N2 permselectivity around 298 K. H- and Ag-SPPO CMS membranes prepared in this study had properties exceeding the upper bound of conventional polymeric membranes reported by Robeson [23], including reported polymeric M-SPPO membranes [16]. A comparison of the performance with the reference data, H- and Ag-SPPO CMS membranes have as high a performance as that of the Ag-containing
CMS membrane based on co-polyimide [9,10]. The highest performance was attained by the AgPPO CMS membrane pyrolyzed at 923 K, where O2 permeability was 178 Barrer and O2/N2 permselectivity was 8.7 at 298 K. Ag+ is considered to generate metallic Ag nanoparticles after the heat treatment, suggesting that these nanoparticles increase the interlayer spaces, as proposed by Barsema et al. [10]. A similar mechanism could be applicable in the case of M-SPPO CMS membranes substituted by transition metal ions where the generated metal or metal sulfide nanoparticles mainly act as the interlayer spacers to increase the free volume of carbon matrix. Consequently, a significant increase in permeability was observed in the Ag-SPPO CMS membrane. 4. Conclusions Metal-containing CMS membranes derived from SPPO can be successively prepared by ion exchange. The effect of metal cations on gas transport properties was studied. It was seen that metal cations were greatly dispersed into the carbon matrix, and gas separation performance varied with the feature and valency of the metal cations.
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We found that the incorporation of transition metal ions such as Ag+ and Cu2+ into SPPO CMS membranes significantly enhanced gas permeabilities without affecting the permselectivity, and these metal-containing CMS membranes could be more favorable for the separation of CO2 or O2 from larger molecules such as N2 or CH4. This result presents an interesting possibility for the advancement of membrane preparation techniques for gas separation. References [1] W.J. Koros and R. Mahajan, J. Membr. Sci., 175 (2000) 181–196. [2] H.P. Hsieh, Inorganic membranes for separation and reaction, in: Membrane Science and Technology, Series 3, Elsevier, Amsterdam, 1996. [3] A.F. Ismail and L.I.B. David, J. Membr. Sci., 193 (2001) 1–18. [4] M. Jia, K.V. Peinemann and R.D. Behling, J. Membr. Sci., 57 (1991) 289–296. [5] M. Moaddeb and W.J. Koros, J. Membr. Sci., 125 (1997) 143–163. [6] H.B. Park, I.Y. Suh and Y.M. Lee, Chem. Mat., 14 (2002) 3034–3046. [7] S. Yoda, A. Hasegawa, H. Suda, Y. Uchimaru, K. Haraya, T. Tsuji and K. Otake, Chem. Mat., 16 (2004) 2363–2368. [8] Y.K. Kim, H.B. Park and Y.M. Lee, J. Membr. Sci., 226 (2003) 145–158.
[9] J.N. Barsema, N.F.A. van der Vegt, G.H. Koops, M. Wessling, Adv. Funct. Mat., 15 (2005) 69–75. [10] J.N. Barsema, J. Balster, V. Jordan, N.F.A. van der Vegt and M. Wessling, J. Membr. Sci., 219 (2003) 47–57. [11] M. Yoshimune, I. Fujiwara, H. Suda and K. Haraya, Chem. Lett., in press. [12] M. Yoshimune, K. Haraya and H. Suda, Jpn. Patent Application 2005-044852, 2005. [13] K. Haraya and S.-T. Hwang, J. Membr. Sci., 71 (1992) 13–27. [14] G. Chowdhury, B. Kruczek and T. Matsuura, Polyphenylene Oxide and Modified Polyphenylene Oxide Membranes: Gas, Vapor and Liquid Separation, Kluwer Academic, Boston, 2001. [15] B. Kruczek and T. Matsuura, J. Membr. Sci., 146 (1998) 263–275. [16] B. Kruczek and T. Matsuura, J. Membr. Sci., 167 (2000) 203–216. [17] F.A. Hamad, G. Chowdhury and T. Matsuura, J. Membr. Sci., 191 (2001) 71–83. [18] F.A. Hamad, G. Chowdhury and T. Matsuura, Desalination, 145 (2002) 365–370. [19] H. Fu, L. Jia and J. Xu, J. Appl. Polym. Sci., 51 (1994) 1399–1404. [20] R.Y.M. Huang and J.J. Kim, J. Appl. Polym. Sci., 29 (1984) 4017–4027. [21] J. Schauer, W. Albrecht, T. Weigel, V. Kdela and Z. Pientka, J. Appl. Polym. Sci., 81 (2001) 134–142. [22] B. Kosmala and J. Schauer, J. Appl. Polym. Sci., 85 (2002) 1118–1127. [23] L.M. Robeson, J. Membr. Sci., 62 (1991) 165–185.
Gas transport properties of carbon molecular sieve membranes derived from metal containing sulfonated poly(phenylene oxide) Miki Yoshimune*, Ichiro Fujiwara, Hiroyuki Suda, Kenji Haraya Research Institute for Innovation in Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba 305-8565, Japan Tel./Fax: +81 (29) 861-4801; email: [email protected] Received 15 March 2005; accepted 13 April 2005
Abstract Hollow-fiber carbon molecular sieve (CMS) membranes derived from sulfonated poly(phenylene oxide) (SPPO) were studied. Mono-, di- and trivalent metal cations such as Na+, Mg2+, Al3+, Ag+, Cu2+ and Fe3+ were substituted to the proton of sulfonic acid group of SPPO by the ion-exchange method to investigate the effects on gas transport properties for SPPO CMS membranes. In the case of H-SPPO, CMS membranes exhibited higher performance than those of the polymeric precursor, and maximum permeability was obtained when pyrolyzed at 923 K. The introduction of metal cations into SPPO affected the structures of the resulting CMS membranes, of which the Ag- and Cu- form enhanced overall permeabilities. In addition, Fe-SPPO CMS membranes increased He/N2 and H2/N2 permselectivities compared to non-substituted CMS membranes prepared by the same pyrolysis procedure. The highest performance was attained by the Ag-SPPO CMS membrane pyrolyzed at 923 K, where O2 permeability was 178 Barrer and O2/N2 permselectivity was 8.7 at 298 K. Keywords: Gas permeation; Carbon molecular sieve membranes; Sulfonated poly(phenylene oxide); Metal cations; Ion-exchange method
1. Introduction Since carbon molecular sieve (CMS) membranes have shown that these materials successfully to compete with polymeric membranes due *Corresponding author.
to their superior permeability-selectivity combination and suitable performance for high temperature or corrosive environment, recent interest has been focused on a selection of new materials and an improvement in membrane preparation techniques in order to develop gas separation performance [1–3].
Presented at the International Congress on Membranes and Membrane Processes (ICOM), Seoul, Korea, 21–26 August 2005. 0011-9164/06/$– See front matter © 2006 Published by Elsevier B.V.
doi:10.1016/j.desal.2005.04.138
M. Yoshimune et al. / Desalination 193 (2006) 66–72
Organic–inorganic nanocomposite membranes have been the subject of growing interest as a promising technology; they are prepared by incorporation of inorganic materials such as zeolite, silica and others into a polymer matrix [4,5]. This technology is being applied for CMS membranes. Park et al. first prepared carbon membranes containing silica by the pyrolysis of copolyimides [6]. Another approach to improve the performance of CMS membranes is metal incorporation into CMS membranes. Metal cations can be considered to increase the polarity and/or to form interlayer spaces and/or to have an affinity toward target gases. Yoda et al. investigated CMS membranes derived from Pt- and Pddispersed polyimide film prepared by supercritical impregnation [7]. Kim et al. reported CMS membranes containing alkali metel ions using metal-substituted sulfonated polyimide [8]. Barsema et al. found that the incorporation of nano-sized Ag particles into a CMS matrix resulted in a selectivity increase of O2 over N2 with a substantial increase of permeability [9,10]. Recently, we succeeded in manufacturing CMS membranes derived from poly(phenylene oxide) (PPO) and modified PPO, which exhibited as high a performance as CMS membranes derived from other polymeric precursors [11,12]. PPO has been recognized as a membrane material with fairly high permeability with a high glass transition temperature among other polymers [13]. In addition, PPO is easily modified to provide various kinds of derivatives. It can be considered as another interesting object for the chemical modification of PPO. Matsuura and co-workers are dedicated to the development of polymeric PPO-based membranes [14], and they have investigated the effects of metal cations on gas separation properties for polymeric dense membranes of metal containing sulfonated PPO (SPPO) [15–18]. In this report the gas transport properties of CMS hollow-fiber membranes derived from SPPO containing various kinds of metal cations are studied. Metal
67
cations were incorporated into polymeric SPPO membranes by substitution of the proton of a sulfonic acid group of SPPO by the ion-exchange method. We examined the effects of metal cations with various features, not only the alkali cations such as Na+, Mg2+ and Al3+ but also the transition metal ions such as Ag+, Cu2+ and Fe3+, which were used for substituted metal cations in this work. 2. Experimental 2.1. Preparation of SPPO Poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) was purchased from Aldrich Chemicals. Chlorosulfonic acid and chloroform were purchased from Wako Pure Chemical Industries as reagent grade. All chemicals were used without any purification. Sulfonation of PPO was carried out using chlorosulfonic acid in a chloroform solvent under nitrogen atmosphere according to the literature [19,20]. The synthetic scheme is shown in Fig. 1. The obtained SPPO was stored as Na-form (Na-SPPO) to prevent decomposition by hydrolysis. The degree of substitution (DS) was determined by the acid–base titration method. 2.2. Membrane manufacture The hollow-fiber polymeric membrane of NaSPPO was manufactured with the coagulation method. A 20 wt.% Na-SPPO solution in methanol was immersed into a saturated NaCl aqueous solution bath as a coagulating agent, which was stored into this solution until the metal substitution process that follows. The conversion into
Fig. 1. Synthesis of sulfonated poly(phenylene oxide).
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metal substituted SPPO (M-SPPO: Mn+ = Na+, Mg2+, Al3+, Ag+, Cu2+ and Fe3+) was performed by soaking SPPO in a H-form (H-SPPO) membrane into each metal nitrate solution (1 mol l-1) for 2 h. The H-SPPO membrane was obtained by soaking a Na-SPPO membrane into an aqueous solution of HCl (1 mol l!1) for 24 h. In case of the Ag substitution, the preparation was conducted in the absence of light, and the membrane was stored in the dark. For uniform ion exchanging, the solutions were circulated into the membranes. These polymeric precursor membranes of M-SPPO were dried at room temperature after washing with deionized water to remove excess metal ions. A carbonization process was carried out using a quartz tube furnace under vacuum. The polymeric precursor membranes of M-SPPO were at first preoxidized in air at 553 K for 45 min to yield optimum properties. Finally, the carbonized M-SPPO membranes were obtained by pyrolysis at 923–1273 K with a heating rate of 10 K min!1 for 2 h. 2.3. Characterization of M-SPPO The prepared M-SPPO membranes were characterized with FT-IR (Nicolet, Magna550). Thermogravimetric analysis was performed by means of Mac Science (Japan) TG-DGA 2000SA at a heating rate of 10 K min!1 under Ar atmosphere with a flow rate of 200 cm3 min!1. The microstructure of membranes was investigated by scanning electron microscopy (SEM) with a Hitachi S-2400. 2.4. Gas permeation measurement Pure gas permeability measurements for He, H2, CO2, O2 and N2 were conducted with a highvacuum time-lag apparatus at 298–363 K under the pressure difference of 1 atm. Both the feed and permeate sides of the membrane cell were evacuated (<10!5 Torr) prior to each measurement. Permeability coefficients are expressed in
Barrer, where 1 Barrer = 1×10!10 cm3 (STP) cm cm!2 s!1 cm Hg!1 = 3.35×10!16 kmol m m!2 s!1 kPa!1. Permselectivity was defined in this study as the ideal separation factor, which is the ratio of permeability of chosen gas over that of N2. 3. Results and discussion 3.1. Membrane formation and characterization The degree of substitution (DS) of SPPO in this study was determined as 40%. At higher DS of more than 50%, the SPPO membrane no longer maintained its structure because a part of the SPPO dissolved in water during the ion-exchange process. This phenomenon was described by Schauer et al. that the SPPO polymers with DS of about 20–40% were soluble in NMP but insoluble in water, whereas those with the DS higher than 85% were water soluble [21]. Fig. 2 shows the IR spectra of the original PPO (a) and Na-SPPO (b), which are in agreement with those reported [19,22]. In the spectrum of Na-SPPO (b), a new band appeared at 1070 cm!1, which is assigned to the S=O stretching vibration of the SO3! group. On the contrary, the intensity of the band at 830 cm!1 (out-of-plane vibration of 1,4-disubstituted benzene ring) considerably decreases, which reflects the substitution of an H atom in an aromatic ring by a sulfonic group. As can be seen in Fig. 2(c), the peak intensities for the Na-SPPO decreased due to thermal degradation after the pyrolysis at 1023 K. Fig. 3 presents the TGA analysis of H-, Naand Ag-SPPO. In the case of H-SPPO, four major weight losses of varying extent at around 273, 473, 650 and 700 K were observed corresponding to the removal of water vapor, the degradation of the sulfonic acid group, the degradation of the methyl group and polymer decomposition, respectively [15,16]. The metal containing MSPPO increased thermal stabilities in the following order: H+ < Ag+ < Na+. TGA spectra of Mgand Al-SPPO were similar to that of Na-SPPO
M. Yoshimune et al. / Desalination 193 (2006) 66–72
Fig. 2. FT–IR spectra of polymeric membranes for PPO (a) and Na-SPPO (b) together with CMS membrane for Na-SPPO pyrolyzed at 1023 K (c).
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Fig. 3. TGA curves for H-SPPO, Na-SPPO and Ag-SPPO.
Fig. 4. Scanning electron micrographs of cross sections for the polymeric Ag-SPPO membrane (A) and carbonized Ag-SPPO membrane (B, C).
and Cu- and Fe-SPPO gave analogous spectra to that of Ag-SPPO, indicating that the feature of metal cations may affect the pyrolysis behaviors of M-SPPO. The micrographs of cross sections for AgSPPO membranes before and after pyrolysis are given in Fig. 4. The prepared membranes had symmetric structures without defects, which remained after the heat treatment. The diameter of the hollow fiber was reduced from 460 µm to 400 µm by the shrinkage of the membranes due
to pyrolysis, where its thickness was largely unchanged. 3.2. Gas permeation properties of M-SPPO Fig. 5 shows the dependence of pyrolysis temperature on gas transport properties for H-SPPO together with its polymeric precursor. The obtained H-SPPO CMS membranes pyrolyzed at 923–1273 K exhibited superior performance to that of the polymeric precursor in both perme-
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Fig. 6. Relations between O2 permeability and O2/N2 permselectivity at 298 K for SPPO CMS membranes pyrolyzed at 923 K compared to other membranes [9,16].
Fig. 5. Influence of pyrolysis temperature on: (A) permeability; and (B) permselectivity for H-SPPO membranes compared with their polymeric precursor.
abilities and permselectivities, which is consistent with the trends in our previous work [11]. The gas permeability reached a maximum when pyrolyzed at 923 K, and then decreased at higher temperatures. On the other hand, the He/N2 and H2/N2 permselectivities were significantly increased at 1023 K, of which permselectivities were 318 and 567 at 363 K, respectively. The gas transport properties for M-SPPO CMS membranes pyrolyzed at 923 K are summarized in Table 1. The measurement temperature was fixed at 363 K because it was found that the permeability of the Na-SPPO CMS membrane was too small to measure at a lower temperature. The Na-SPPO CMS membrane yielded the highest permselectivity; however, a general trade-off
relationship between permeability and selectivity was observed for this membrane to give the lowest permeability. The permeabilities of the Mg- and Al-SPPO CMS membranes increased with decreasing the permselectivity, resulting in that those productivities were slightly increased compared with the Na-SPPO CMS membrane. These alkali cations are considered to cause a steric hindrance into the carbon matrix, and gas permselectivity increased and permeability decreased when substituted by a low-valence alkali cation. The difference among Na-, Mg- and AlSPPO CMS membranes was brought about mainly by the substitution amount with the proton of sulfonic acid groups of SPPO, which increases in the following order: Na+> Mg2+>Al3+, because di- and trivalent cations associate with more than one SO3! group. On the other hand, it is notable that the incorporation of transition metal ions into SPPO CMS membranes enhanced these performances significantly. The Fe-SPPO CMS membrane gained higher permselectivities in smaller gases such as He and H2, without the loss of those permea-
M. Yoshimune et al. / Desalination 193 (2006) 66–72
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Table 1 Effect of metal cations on gas transport properties for M-SPPO CMS membranes at 363 K Metal
H Na Mg Al Ag Cu Fe
Thickness (µm)
20 30 32.5 30 25 20 35
Permeability (Barrer)
Permselectivity
He
H2
CO2
O2
N2
He/N2
H2/N2 CO2/N2 O2/N2
669 75.5 354 310 731 566 626
1574 87.1 773 760 2090 1581 1074
320 4.6 1494 262 690 615 129
107 1.8 52 72 236 196 49
15 0.15 8.8 13 38 31 6.6
45 500 40 24 19 18 95
107 577 88 59 55 51 164
21.8 30.2 16.9 20.1 18.2 19.7 19.7
7.3 11.7 5.9 5.6 6.2 6.3 7.5
Note: Membranes were carbonized at 923 K for 2 h.
bilities, compared to a non-substituted SPPO CMS membrane prepared by the same pyrolysis procedure. In addition, the introduction of the Ag+ ion (Ag-SPPO) enhanced the permeabilities for all gases tested, along with decreasing the permselectivities. However, the differences in CO2/N2 and O2/N2 permselectivities between the H-form and Ag-form are small, and overall yields were increased in the Ag-SPPO CMS membrane. There also was similar advancement in Cu-SPPO, suggesting that there is potential in the transition metal ions such as Ag+ and Cu2+ to improve the performance of M-SPPO CMS membranes. In order to compare the performances of other membranes in the references, a detailed study was conducted for the M-SPPO CMS membranes in H-form and Ag-form pyrolyzed at 923–1023 K. Fig. 6 shows the relationship between O2 permeability and O2/N2 permselectivity around 298 K. H- and Ag-SPPO CMS membranes prepared in this study had properties exceeding the upper bound of conventional polymeric membranes reported by Robeson [23], including reported polymeric M-SPPO membranes [16]. A comparison of the performance with the reference data, H- and Ag-SPPO CMS membranes have as high a performance as that of the Ag-containing
CMS membrane based on co-polyimide [9,10]. The highest performance was attained by the AgPPO CMS membrane pyrolyzed at 923 K, where O2 permeability was 178 Barrer and O2/N2 permselectivity was 8.7 at 298 K. Ag+ is considered to generate metallic Ag nanoparticles after the heat treatment, suggesting that these nanoparticles increase the interlayer spaces, as proposed by Barsema et al. [10]. A similar mechanism could be applicable in the case of M-SPPO CMS membranes substituted by transition metal ions where the generated metal or metal sulfide nanoparticles mainly act as the interlayer spacers to increase the free volume of carbon matrix. Consequently, a significant increase in permeability was observed in the Ag-SPPO CMS membrane. 4. Conclusions Metal-containing CMS membranes derived from SPPO can be successively prepared by ion exchange. The effect of metal cations on gas transport properties was studied. It was seen that metal cations were greatly dispersed into the carbon matrix, and gas separation performance varied with the feature and valency of the metal cations.
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