Brominated

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SEPARATIONS Nanocomposite Membranes for CO2 Separations: Silica/Brominated Poly(phenylene oxide) Xudong Hu, Hailin Cong, Youqing Shen, and Maciej Radosz* Soft Materials Laboratory, Department of Chemical and Petroleum Engineering, UniVersity of Wyoming, Laramie, Wyoming 82071

Silica-impregnated, brominated PPO [poly(phenylene oxide)] (BPPO) membranes exhibit enhanced CO2 permeability relative to pure BPPO membranes due to higher gas solubility and especially higher gas diffusivity. Among the three silica sizes (2, 10, and 30 nm) characterized in this work, the 10 nm silica is found to result in the highest gas permeability, about 5 times higher than that of the pure BPPO membranes. These permeability enhancements do not cause an appreciable loss of selectivity, which remains essentially unchanged, for example, about 21 for the CO2/N2 separation and about 15 for the CO2/CH4 separation. The permeability increases with increasing silica content, which, however, cannot exceed about 0.35 silica/BPPO weight ratio due to a substantial increase in brittleness. Introduction Gas membranes made of poly(2,6-dimethyl-1,4-phenylene oxide), PPO, are selective for CO2 relative to other common gases. Their CO2 permeability at room temperature was found to be about 60 barrer at a CO2/N2 selectivity of about 20 and a CO2/CH4 selectivity of about 15.1-5 One way to enhance the PPO permeability-selectivity characteristics is through chemical modification. For example, Chowdhury et al.6 found that membranes made of PPO brominated in the aromatic position (BPPO) had a CO2 permeability of about 100 barrer with a CO2/ N2 selectivity of about 23 and Story7 found a similar CO2 permeability with a CO2/CH4 selectivity of about 15. Another route to enhancing the membrane permeabilityselectivity characteristics is through impregnating it with nanoparticles. Recent studies have shown that such nanocomposite membranes have a higher gas permeability without losing much selectivity.8-15 Specifically, silica nanoparticles were found by Joly et al.16 to be especially effective in increasing the gas permeability of polyimide membranes; for example, 32 wt % silica increased the CO2 permeability from 1.8 to 2.8 barrer at the same CO2/N2 selectivity of about 20. Merkel et al.17 report that silica gel nanoparticles increased both the permeability and selectivity of glassy poly(4-methyl-2-pentyne) (PMP) membranes; for instance, 30 wt % silica increased the n-butane/ methane selectivity by a factor of 2 and the n-butane permeability by a factor of 3, relative to pure PMP. He et al.18 report similar findings for silica-poly(4-methyl-2-pentyne) nanocomposite membranes (permeability increased by a factor of 3-4), as do Kim et al.19 for a 27 wt % silica-poly(amide-6-b-ethylene oxide) membrane. However, it is not clear how the nanoparticle size and concentration can affect the gas permeability, selectivity, solubility, and diffusivity, for example, estimated on the basis of pure-component data alone.21 * To whom correspondence should be addressed. Tel.: (307) 7664926. Fax: (307) 766-6777. E-mail: [email protected].

The aim of this work therefore is to measure these transport properties as a function of silica size and concentration for BPPO membranes exposed to pure CO2, N2, and CH4. Materials and Methods The chemical structures of PPO and BPPO are as follows:

PPO is provided by Aldrich, with a molecular weight of about 20 000. The synthesis procedure for BPPO is based on the method of White et al.20 A reaction mixture contains 6 g of PPO and 50 mL of CHCl3. A solution of 5 mL of bromine diluted with 10 mL of chloroform is added dropwise to the mixture over a 30 min period. The mixture maintains a dark blood-red color throughout the bromination reaction. An argon purge is maintained to remove HBr as it is released from solution. After stirring at room temperature for 2 h, the polymer is precipitated out in 800 mL of mechanically stirred ethanol. The polymer is filtered and dried under vacuum at room temperature with a total yield of about 7 g. The 2 nm silicon dioxide (also called silica) is provided by Meliorum Technologies, Inc.; 10 nm silica is provided by Aldrich (Catalog No. 637246); and 30 nm silicon dioxide is provided by MTI Corp. Their properties obtained from the suppliers are given in Table 1. CO2, CH4, and N2 (ultrahigh purity) are purchased from US Airgas.

10.1021/ie061055y CCC: $37.00 © 2007 American Chemical Society Published on Web 02/07/2007

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Table 1. Silica Properties from the Suppliers

supplier name manufacturing method surface area (m2/g) size deviation (nm) purity (%) size determination method surface functional group

average size ) 2 nm

average size = 10 nm

average size ) 30 nm

Meliorum Technologies silicon dioxide nanopowder chemical reduction from silicon chloride

Aldrich silica nanopowder sol-gel 600 (5 >99.5 transmission electron microscope (TEM) hydroxyl

MTI Corp. silicon dioxide nanopowder sol-gel >200

(0.74 >99.0 photocorrelation spectroscopy aminopropyltriethoxysilane

Table 2. Surface Area and Zeta-Potential Measured in This Work for Silica Samples Listed in Table 1 (m2/g)

surface area zeta-potential (mV)

2 nm

10 nm

30 nm

68 -40.3

614 -42.0

549 -39.6

The surface areas of the samples listed in Table 1 are determined in this work from a Micomeritics Tristar BET instrument. The zeta-potentials, a measure of the surface charge density, are determined from a Malven ZetaSizer. These data are compiled in Table 2. The BPPO membranes are prepared according to the method of Hu et al.21 The silica/BPPO membranes are prepared as follows: ∼0.3 g of BPPO and an appropriate amount of silica are mixed with 5 mL of chloroform in a sealed glass, which results in a 5.5 wt % BPPO in chloroform solution. The solution is vigorously stirred for 15 min to disperse the silica uniformly. For each silica sample (2, 10, and 30 nm), three weight ratios of silica/BPPO are used in this work: 10/100, 20/100, and 30/ 100. The solution is then cast onto a clean and dry glass at room temperature and left open to dry. The resulting membrane is peeled off and stored in a desiccator for testing. The thickness of the membrane is about 50 µm. The silica/BPPO membrane morphology is characterized using a JEOL JSM-5800LV scanning electron microscope on samples sputtered with gold. The membranes are tested in a constant-volume, variablepressure apparatus described in detail by Hu et al.,21 who also define in detail the diffusivity, permeability, and solubility. In brief, diffusivity is estimated from the time lag and membrane thickness. The permeability is estimated from the slope of the

Figure 1. SEM images of silica/BPPO (0.2 weight ratio) membranes.

>99.0 TEM hydroxyl

linear pressure increase with time for the downstream side of the membrane. The solubility is estimated as the ratio of permeability to diffusivity. The downstream side pressure is about 0.1 Torr upon charging the gas; then it increases with time, first nonlinearly and then linearly. Its range lies between 0.1 and 10 Torr. All such permeation experiments are performed at a feed pressure of 10 psig and 22 °C for three pure gases, CO2, CH4, and N2, and for all BPPO and silica/BPPO membranes. The gas transport properties (solubility, diffusivity, permeability, and permselectivity) are defined by Hu et al.21 For the pure gas permeation experiments reported in this work, the selectivity is an ideal selectivity, also referred to as permselectivity, and estimated as the ratio of the permeabilities of pure gases. Results and Discussion In contrast to common polymeric membranes, where the solvent is evaporated slowly, BPPO and silica/BPPO membranes tend to crack upon slow solvent evaporation, for example, when the solution film is partly covered. The film cracks toward the end of the slow drying process. We find, however, that if the membranes are completely exposed to air during the drying process, without any cover, they are crack-free (dense and smooth), except for high BPPO concentrations in solvent (>10%). In general, the silica particles exhibit some degree of clustering, as illustrated with SEM images for silica/BPPO (0.2 weight ratio) membranes in Figure 1. The white sections in these SEM images represent silica, and the black sections represent BPPO. The silica clusters are from

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Figure 2. Gas solubility in silica/BPPO membranes. Figure 3. Gas diffusivity in silica/BPPO membranes.

about a few hundred nanometers to 1 µm in diameter. These SEM images also suggest that the cluster size does not seem to depend on the silica size; i.e., the clusters are similar in size regardless of the nominal silica diameter. The gas solubility in the silica/BPPO membranes depends on the silica size, and it tends to increase with increasing silica content, as shown in Figure 2. For example, the 10 and 30 nm silica can nearly double the gas solubility. Perhaps one way to explain it is on the basis of the sorption capacity. For example, the CO2 sorption is about 25 cm3 (STP)/cm3 in pure 10 nm silica and about 12 cm3 (STP)/cm3 in pure BPPO, measured in support of this work in a magnetic suspension balance at the same temperature and pressure conditions.22 However, there can be other factors: for example, the solid nanoparticles dispersed in the polymer matrix create an interface that can play a role in enhancing the solubility relative to pure BPPO. Figure 3 illustrates that the diffusivity can increase by as much as a factor of ∼3 for all three gases, compared to the pure BPPO membranes. This is because the nanoparticles stretch the BPPO chains and create more free volume that enhances the gas diffusion.12,13,17 In general, therefore, the higher the silica content, the higher the diffusivity enhancement. However, the

silica size dependence does not seem to be monotonic: the diffusivity enhancement due to 10 nm silica is higher than that due to 30 nm silica, which in turn is higher than that due to 2 nm silica. Qualitatively, one would expect that, for a fixed weight ratio, the smaller the size, the higher the enhancement, because the smaller the particle, the higher the polymer/particle interfacial area and hence the looser the chains.23 It would be tempting to suggest that our data instead point to an optimum size that maximizes the diffusivity, below which the enhancement tends to decrease. However, we need to bear in mind that our 2 nm silica sample had a different surface chemistry and a much smaller surface area than the other two samples (Tables 1 and 2), which may have decreased its effectiveness relative to the other samples, independent of its size, probably due to strong self-association in its virgin state. (After this work was completed, we chemically modified the 2 nm silica surface by converting the self-associating primary amine groups to nonself-associating amine groups. We found that the surface area increased substantially, from 68 m2/g for the unmodified silica to 300 m2/g for the modified silica, which confirmed the selfassociation hypothesis.)

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Figure 4. Gas permeability in silica/BPPO membranes.

Figure 5. CO2/N2 and CO2/CH4 selectivity of silica/BPPO membranes.

Since the permeability is a product of the diffusivity and the solubility, it is not surprising that the permeability increases with increasing silica concentration, and that it exhibits a maximum for the 10 nm silica, as shown in Figure 4. For example, for a membrane that contains 0.3 weight ratio of silica/

Figure 6. Selectivity for CO2/N2 and CO2/CH4 separation versus CO2 permeability.

BPPO, the CO2 permeability reaches 523 barrer (about 5 times higher than that of the corresponding pure BPPO membrane)

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with a CO2/N2 selectivity of 21 and a CO2/CH4 selectivity of 15. Since the permeability seems to increase with increasing silica concentration, the question is how high it can be without affecting the membrane mechanical properties too much. In this work, the membranes remain relatively flexible up to about 0.3 silica/BPPO weight ratio. Above that, and specifically around 0.35, the membrane brittleness increases sharply. As shown in Figure 5, the silica size and its content do not affect the gas selectivity much. This suggests that the permeability enhancements shown in Figure 4 cannot be due to small pinhole-like defects at the silica/polymer interface, because such defects can cause a substantial loss of selectivity. Figure 6 presents the CO2/N2 selectivity and CO2/CH4 selectivity as a function of permeability for the BPPO and silica/ BPPO membranes characterized in this work. These are ideal selectivities, determined on the basis of the pure-component data alone, which can and likely will differ from the mixed-gas selectivities. The dashed line in Figure 6 is a so-called Robeson line24 that serves as a reference line for evaluating the performance of gas separation membranes. The filled circles show the 10 nm silica/BPPO membranes, which are close to the Robeson line for the CO2/CH4 separation and close to or above the Robeson line for the CO2/N2 separation. Therefore, the 10 nm silica/BPPO membranes at 0.3 weight ratio is an attractive candidate for CO2/N2 separation. Conclusion Silica-impregnated BPPO membranes are found to exhibit an enhanced CO2 permeability relative to pure BPPO membranes, due to enhanced gas solubility and especially enhanced gas diffusivity. Among the three silica sizes (2, 10, and 30 nm) characterized in this work, the 10 nm silica is found to result in the highest gas permeability, about 5 times higher than that of the pure BPPO membranes. The permeability enhancements due to silica do not cause an appreciable loss of selectivity, which remains essentially unchanged. The permeability increases with increasing silica content, which, however, cannot exceed about 0.35 (silica/BPPO weight ratio) due to a substantial increase in brittleness. Acknowledgment The Enhanced Oil Recovery Institute of the University of Wyoming provided funding for this work. Dr. Norbert G. Swoboda-Colberg at the Department of Geology & Geophysics of the University of Wyoming allowed us to use his SEM laboratory. Literature Cited (1) Koros, W. J.; Mahajan, R. Pushing the limits on possibilities for large scale gas separation: which strategies? J. Membr. Sci. 2000, 175, 181-196. (2) Chern, R. T.; Sheu, F. R.; Jia, L.; Stannet, V. T.; Hopfenberg, H. B. Transport of gases in unmodified and aryl-modified 2,6-dimethyl-1,4-poly(phenylene oxide). J. Membr. Sci. 1987, 35, 103-115. (3) Chowdhury, G.; Kruczek, B.; Matsuura, T. Polyphenylene oxide and modified polyphenylene oxide membranes; gas, Vapor, and liquid separation; Kluwer Academic Publishers: Norwell, MA, 2001; pp 105-146.

(4) Ghosal, K.; Chern, R. T. Aryl-nitration of poly(phenylene oxide) (PPO) and polysulfone, structural characterization and gas permeability. J. Membr. Sci. 1992, 72, 91-97. (5) Hamad, F.; Khulbe, K. C.; Matsuura, T. Comparison of gas separation performance and morphology of homogeneous and composite PPO membranes. J. Membr. Sci. 2005, 256, 29-37. (6) Chowdhury, G.; Vujosevic, R.; Matsuura, T.; Laverty, B. Effects of polymer molecular weight and chemical modification on the gas transport properties of poly(2,6-dimethyl-1,4-phenylene oxide). J. Appl. Polym. Sci. 2000, 77, 1137-1143. (7) Story, B. J.; Koros, W. J. Sorption and transport of CO2 and CH4 in chemically modified poly(phenylene oxide). J. Membr. Sci. 1992, 67, 191210. (8) Cornelius, C. J.; Marand, E. Hybrid silica-polyimide composite membranes: gas transport properties. J. Membr. Sci. 2002, 202 (1), 97118. (9) Doucoure, A.; Guizard, C.; Durand, J.; Berjoan, R.; Cot, L. Plasma polymerization of fluorinated monomers on mesoporous silica membranes and application to gas permeation. J. Membr. Sci. 1996, 117 (1), 143-150. (10) Kim, J. H.; Lee, Y. M. Gas permeation properties of poly(amide6-b-ethylene oxide)-silica hybrid membranes. J. Membr. Sci. 2001, 193 (2), 209-225. (11) Moaddeb, M.; Koros, W. J. Gas transport properties of thin polymeric membranes in the presence of silicon dioxide particles. J. Membr. Sci. 1997, 125 (1), 143-163. (12) Patel, N. P.; Miller, A. C.; Spontak, R. J. Highly CO2-permeable and selective membrane derived from crosslinked poly(ethylene glycol) and its nanocomposites. AdV. Funct. Mater. 2004, 14 (7), 699-707. (13) Pinnau, I.; Freeman, B. D. Advanced materials for membrane separations. ACS Symp. Ser. 2004, 876, Chapter 16. (14) Polotskaya, G. A.; Agranova, S. A.; Antonova, T. A.; Elyashevich, G. K. Gas transport and structural features of sulfonated poly(phenylene oxide). J. Appl. Polym. Sci. 1997, 66, 1439-1443. (15) Smaihi, M.; Jermoumi, T.; Marignan, J.; Noble, R. D. Organicinorganic gas separation membranes: preparation and characterization. J. Membr. Sci. 1996, 116 (2), 211-220. (16) Joly, C.; Goizet, S.; Schrotter, J. C.; Sanchez, J.; Escoubes, M. Sol-gel polyimide-silica composite membrane: gas transport properties. J. Membr. Sci. 1997, 130, 63-74. (17) Merkel, T. C.; Freeman, B. D.; Spontak, R. J.; He, Z.; Pinnau, I.; Meakin, P.; Hill, A. J. Sorption, Transport, and Structural Evidence for Enhanced Free Volume in Poly(4-methyl-2-pentyne)/Fumed Silica Nanocomposite Membranes. Chem. Mater. 2003, 15, 109-123. (18) He, Z.; Pinnau, I.; Morisato, A. Nanostructured poly(4-methyl-2pentyne)/silica hybrid membranes for gas separation. Desalination 2002, 146, 11-15. (19) Kim, J. H.; Ha, S. Y.; Lee, Y. M. Gas permeation properties of poly(amide-6-b-ethylene oxide)-silica hybrid membranes. J. Membr. Sci. 2001, 193, 209-225. (20) White, C. M.; Strazisar, B. R.; Granite, E. J.; Joffman, J. S.; Pennline, H. W. J. Air Waste Manage. Assoc. 2003, 53, 645-715. (21) Hu, X.; Tang, J.; Blasig, A.; Shen, Y.; Radosz, M. CO2 permeability, diffusivity and solubility in polyethylene glycol grafted poly(ionic liquids) membranes and their CO2 selectivity relative to methane and nitrogen. J. Membr. Sci. 2006, 281, 130-138. (22) Blasig, A.; Tang, J.; Hu, X.; Shen, Y.; Radosz, M. Magnetic suspension balance study of carbon dioxide solubility in ammonium-based polymerized ionic liquids: poly(p-vinylbenzyltrimethyl ammonium tetrafluoroborate) and poly([2-(methacryloyloxy)ethyl] trimethyl ammonium tetrafluoroborate). Submitted for publication in Fluid Phase Equilib. (23) Merkel, T. C.; Freeman, B. D.; Spontak, R. J.; He, Z.; Pinnau, I.; Meakin, P.; Hill, A. J. Ultrapermeable, Reverse-Selective Nanocomposite Membranes. Science 2002, 296, 519-522. (24) Robeson, L. M. Correction of separation factor versus permeability for polymeric membranes. J. Membr. Sci. 1991, 62, 165-185.

ReceiVed for reView August 11, 2006 ReVised manuscript receiVed December 14, 2006 Accepted January 2, 2007 IE061055Y