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Mixed-Matrix Membrane Hollow Fibers of Cu3(BTC)2 MOF and Polyimide for Gas Separation and Adsorption Jun Hu, Hongpei Cai, Huiqing Ren, Yongming Wei, Zhengliang Xu, Honglai Liu,* and Ying Hu State Key Laboratory of Chemical Engineering and Department of Chemistry, East China UniVersity of Science and Technology, Shanghai 200237, China

Metal-organic framework (MOF) crystals of Cu3(BTC)2 with a high surface area (1396 m2 · g-1) were synthesized and mixed with polyimide (PI) to prepare mixed-matrix membranes (MMMs) for gas separations. The PI-Cu3(BTC)2 blend was successfully spun into MMM hollow fiber by the dry/wet-spinning method. SEM images of the fiber cross sections revealed significant plastic deformation of the polymer matrix owing to the strong affinity between Cu3(BTC)2 and PI. The H2 permeance and the selectivity of H2 with respect to other gases such as N2, CO2, O2, and CH4 both increased markedly with increased Cu3(BTC)2 loading. At a loading of 6 wt % Cu3(BTC)2, the permeance of H2 increased by 45%, and the ideal selectivity increased by a factor of 2-3 compared to the corresponding values for pure PI. 1. Introduction Modern membrane engineering is an important way to implement the process intensification strategy by innovative design aimed at low costs, low energy consumption, and easy operation. Most commercial-scale membranes are polymer-based spiral-wound- or hollow-fiber-type species. As polymer-based gas separations have seemingly reached their limit in terms of the permeability/selectivity tradeoff as reported by Robeson,1 new materials and procedures for membrane fabrication based on new concepts are being investigated to improve membrane performance. Among various novel materials, mixed-matrix membranes (MMMs), first reported by Zimmerman in 1998,2 have received considerable attention because of the dramatic improvements over their pristine state in terms of thermal stabilities, mechanical properties, and other special features as a result of the introduction of small fractions of inorganic additives. Nanoporous materials, such as carbon molecular sieves,3-7 silica zeolites,8-16 inorganic oxides,17-21 and carbon nanotubes and fullerenes,22,23 have been incorporated into polymers to prepare MMMs. Hayakawa et al.24 reported that a composite membrane composed of plasma polymers and a porous support exhibited an oxygen/nitrogen separation factor of RO2/N2 ) 3.7 and an oxygen permeance of pO2 ) 10-6 cm3 (STP)/(cm2 · s · cmHg) (where STP indicates standard temperature and pressure). Ismaila et al.25 reported that MMMs of polyethersulfone (PES) modified with 20 wt % zeolite 4A yielded significant selectivity enhancements of 7.26 and 46.28 for gas mixtures of O2/N2 and CO2/CH4, respectively. Marais et al.26 studied the barrier effect of montmorillonite on the transport mechanisms of small molecules through polyamide 12/montmorillonite nanocomposites. More recently, progress has been made in the use of metal-organic frameworks (MOFs) in MMMs for gas separation. MOFs represent a class of porous materials that exhibit the properties of ordered structures, high thermal stability, extra high porosity, and adjustable chemical functionality that are extremely advantageous for gas adsorption. Currently, hundreds of crystalline MOFs are available.27,28 Zhang et al.29 reported that the CH4/N2 selectivity of matrimid membrane increased from 0.95 to 1.7 upon addition of Cu-BPY-HFS (copper-4,4′* To whom correspondence should be addressed. E-mail: hlliu@ ecust.edu.cn. Tel. and Fax: +86-21-64252921.

bipyridine hexafluorosilicate). Adams et al.30 reported that the pure-gas permeance and selectivity of 15% CuTPA-PVAc (copper tetrapropylammonium-polyvinyl acetate) MMMs show improvements over pure PVAc. Pereza et al.31 reported that when matrimid MMMs was loaded with 30% MOF-5, the permeance of the gases increased by 120%, whereas the ideal selectivity remained constant compared to that of pure matrimid membrane. However, MMMs are not exempt from challenges. For examples, the low affinity of the polymer for the inorganic additive might result in the formation of voids at the polymer-additive interface that degrades the performance of the membrane. Most studies on MMMs have dealt with the formation of flat dense films. To further expand the application of promising MMMs, a more effective membrane structure, namely, an asymmetric membrane, especially hollow fibers, should be explored. Koros et al.32 and Miller et al.33 focused on mixed-matrix hollow fibers for gas separations. Mixed-matrix hollow-fiber membranes with high permeance and high selectivity are always expected; nevertheless, the dense selective skin of asymmetric membranes is located in or near the inner, outer, or both skin surfaces, and changes in porosity and morphology from the dense selective skin to the substructure are urgently needed for an improved solution. In this work, we tried to incorporate MOFs into polymerbased hollow-fiber membranes to prepare MMMs and hence to improve its performance for gas separations. Cu3(BTC)2 was selected as the dispersed phase, because it is easy to synthesize and has a high storage capacity and a suitable pore size for natural gas separations.34 CO2 adsorption quantities of Cu3(BTC)2 can reach 10 mmol/g at 8 bar and 25 °C.35 In addition, polyimide (PI) was chosen as the continuous polymer matrix because of its appropriate glass temperature (Tg) of 285 °C, excellent properties, and wide applications in gas separation. The permeances and selectivities of gases such as N2, H2, CO2, and CH4 were investigated separately. 2. Experimental Section 2.1. Chemicals. Cu(NO3)2 · 6H2O, Na2SO4, ethanol, N,Ndimethylformamide (DMF), pyromellitic dianhydride (PMDA), and 4,4-oxydianiline (ODA) were A.P. reagents supplied by China National Medicines Corporation Ltd. Dimethylacetamide

10.1021/ie1014958  2010 American Chemical Society Published on Web 11/09/2010

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(DMAc) and 1,3,5-benzenetricarboxylic acid (H3BTC) were A.P. reagents supplied by Shanghai Ling-Feng Chemical Reagents Ltd.. The gases for permeance tests were H2, CH4, N2, and CO2. The purity of CO2 was 99.8%, and that of the others was 99.995%. All gases were used without further purification. 2.2. Synthesis of Cu3(BTC)2 · 3H2O. The sample was prepared by a modification of the synthesis described by Yaghi et al.36 in which H3BTC (1.0 g, 4.7 mmol) was dissolved in 30 mL of mixed ethanol/DMF solvent with a (volume) ratio of 1:1. Then, the H3BTC solution was added to 15 mL of an aqueous solution of Cu(NO3)2 · 6H2O (2.0 g, 6.9 mmol) in a capped glass vial with stirring over 10 min. The whole solution was heated to 85 °C and kept at this temperature for 8 h. The resulting blue precipitates were isolated by filtration, washed with methanol, and dried in a vacuum for 12 h. MOF Cu3(BTC)2 · 3H 2O powder was then obtained. 2.3. Preparation of MMMs. ODA and PMDA were ground and then dried and purified at 80 and 220 °C, respectively, for 4 h. DMAc was dried with Na2SO4 before use. Cu3(BTC)2 · 3H2O crystal was first ground into particles with an average diameter smaller than 500 nm with a ball mill and then dried at 120 °C for 6 h to eliminate the bonding H2O. Correspondingly, the color of the crystals changed from light blue to dark purple. A typical Cu3(BTC)2/poly(amic acid) composite preparation procedure was as follows: First, 17.62 g of ODA (88 mmol) was completely dissolved in 200 mL of DMAc, and then 19.20 g of PMDA (88 mmol) was added. After vigorous stirring at 4 °C for 12 h, a light, transparent poly(amic acid) solution was obtained. Then, 2.21 g of dried Cu3(BTC)2 powder was added to the poly(amic acid) matrix solution under vigorous stirring below 4 °C. After 6 h, a highly dispersed and viscous Cu3(BTC)2/poly(amic acid) mixed-matrix solution was obtained. PI-Cu3(BTC)2 hollow fibers were prepared by the dry/wetspinning method described elsewhere.37-39 The Cu3(BTC)2/ poly(amic acid) mixed-matrix solution was first added into a tank to defoam. Depending on the viscosity of the polymer solution, the spinning temperature was adjusted in the range of 30-45 °C, and then the mixed-matrix solution was extruded from the spinneret by N2 with a pressure of 0.1-0.3 MPa. The bore fluid was water at a flow rate of 0.1 mL · min-1. The nascent fibers entered a 0 °C water-ice coagulation bath immediately with a 1-1.5-cm air gap, where phase inversion occurred and the fiber structure was arrested. The fiber takeup speed was around 350 cm · min-1. The nascent fibers were stored in water for 24 h and washed in fresh methanol and hexane three times individually to remove residual solvent. Then, temperatureprogrammed imidation was conducted as follows 60 min

30 min

60 min

30 min

30 min

30 min

30 min

30 min

30 min

30 min

30 min

30 min

200 °C f 200 °C f 240 °C f 240 °C f 260 °C f 260 °C f 280 °C f 280 °C f 30 min

30 min

dispersive X-ray spectroscopy (EDS) was performed using a JEOL JSM-6360-LV spectrometer. Transmission electron microscopy (TEM) was performed on a JEOL JEM-2010 microscope. The samples were dispersed in ethanol solution and supported on copper grids. Fourier transform infrared (FTIR) spectra were obtained on a Bruker-EQUNIOX-55 FTIR spectrometer with the KBr pellet technique. A Nanoscope III scanner (Ai-Jian nanotechnology Inc.) was used to study the surface morphology of the films using a tapping-mode atomic force microscope. 2.5. Gas Permeance of MMMs. Pure-gas permeance tests were carried out using the constant-volume method described elsewhere.40 Figure 1 presents a schematic diagram of the gas permeance testing apparatus. Containing three fibers with an effective length of about 10 cm, the hollow-fiber modules were assembled beforehand, and at least three modules were tested for each spinning condition. A feed-pressure difference ranging from 0.5 to 5 atm was applied to the shell side of the fibers. The pure-gas permeance, P/L, was calculated as P Q Q ) ) L A∆p nπDl∆p

(1)

where P/L is the permeance per unit thickness of the hollowfiber membrane [1 GPU ) 1 × 10-6 cm3 (STP)/(cm2 · s · cmHg)], P represents the gas permeability, L is the thickness of the dense layer (cm) determined from SEM images, Q is the permeated gas flux [cm3 (STP)/s] determined by a soap-film flowmeter, ∆p is the pressure difference between the feed side and the permeate side of the membrane (cmHg), A is the effective surface area (cm2) of the membrane, n is the number of fibers in the module, D is the outer diameter of the hollow fibers (cm) determined from SEM images, and l is the effective length of the hollow fibers (cm) measured by vernier caliper. The ideal selectivity of a hollow-fiber membrane is defined as the ratio of the permeations for two pure gases

30 min

10 °C f 80 °C f 80 °C f 160 °C f 160 °C f

30 min

Figure 1. Schematic diagram of the apparatus for gas permeance measurements.

30 min

275 min

285 °C f 285 °C f 300 °C f 300 °C f 285 °C f 10 °C

2.4. Characterization. Powder X-ray diffraction (XRD) patterns were recorded on a D/Max2550 VB/PC spectrometer using Cu KR radiation in the range of 0.6-8° (40 kV, 200 mA). The adsorptions of N2 and CO2 were measured at 77.4 and 303 K, respectively, on a Micrometrics ASAP-2020 sorptionmeter. The specific surface area and the pore size distribution were calculated according to the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods. Scanning electron microscopy (SEM) was performed on a JEOL JSM-6360-LV microscope. Samples were deposited on a sample holder with an adhesive carbon foil and sputtered with gold. Energy-

RA/B )

(P/L)A (P/L)B

(2)

3. Results and Discussion 3.1. Properties of Cu3(BTC)2 · 3H2O and Polyimides. Figure 2b shows the XRD pattern of MOF Cu3(BTC)2 · 3H2O, with sharp peaks distributed at 2θ ) 6.64°, 9.40°, 11.54°, 13.34°, and 18.94°. According to the Bragg equation, 2d sin θ ) nλ (where d is the interplanar spacing, θ is the diffraction angle, λ is the incident light wavelength, and n is the diffraction series; in this case, n ) 1), the interplanar spacing d is inversely proportional to sin θ. Because sin(6.640°):sin(9.40°):sin(11.54°): sin(13.34°):sin(18.94°) ) 1:2:3:2:8, the distribution of 1/d values accords with 1:2:3:2:8, which confirms that Cu3(BTC)2 is a highly ordered three-dimensional octahedral compound.

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Figure 2. XRD patterns of (a) PI, (b) Cu3(BTC)2, and (c) PI-Cu3(BTC)2 hollow-fiber samples. Figure 4. FTIR spectra of (a) PI, (b) Cu3(BTC)2, and (c) PI-Cu3(BTC)2 hollow fibers in the characteristic wavenumber ranges.

Figure 5. Sketch map of the possible structure of polyimide.

Figure 3. TEM image of as-synthesized Cu3(BTC)2.

The low-temperature nitrogen adsorption-desorption experiments on Cu3(BTC)2 showed a type I isotherm without any hysteresis loops. The BET surface area, the pore volume, and the pore diameter were found to be 1396 m2 · g-1, 0.72 cm3 · g-1, and 0.8 nm, respectively, which agree with the values reported by Johnson et. al.41 The adsorption capacity of pure CO2 gas at 1 atm and room temperature was found to be about 1.2 mmol · g-1. Figure 3 shows a high-resolution TEM image of Cu3(BTC)2. The intense energy of the high-resolution electron beam rapidly undermines the ordered structure of Cu3(BTC)2. However, a large number of parallel fine lines can be observed in the image, indicating that as-prepared Cu3(BTC)2 exhibits a regularity in its porous structure. Measured from the image, the pore size is about 7-8 Å, which is consistent with the theoretical prediction and the analyzed results of the N2 adsorption-desorption measurements. Figure 4 shows FTIR spectra of (a) PI, (b) Cu3(BTC)2, and (c) PI-Cu3(BTC)2 hollow-fiber samples. In Figure 4b, the band at around 1644 cm-1 is attributed to the HsOsH banding vibration, which indicates that Cu3(BTC)2 contains crystal water. The bands at 1557 cm-1 are attributed to CdC skeletal vibration of benzene groups The two bands at 723 and 764 cm-1 are attributed to metal Cu substitution on benzene groups, which can be regarded as the characteristic bands of Cu3(BTC)2, and the two weak bands at 1042 and 1110 cm-1 indicate CsOsCu stretching. The results provide clear evidence of the structure of MOF Cu3(BTC)2. Figure 5 shows a sketch map of the possible structure of PI. With the cyclic amid groups, PI could stack in a manner

allowing the carbonyl acceptor on one chain to interact with the nitrogen donor on another adjacent chain. Such chargetransfer complexes hold the chains together very tightly, not allowing them to move around very much. This is the reason why PI has good qualities of high strength and stability. FTIR spectroscopy is the most common method used to determine the imidization degree of PI; for instance, 1780 cm-1 is always selected as the wavenumber to determine the imidization degree of PI. In Figure 4a, the strong band at around 1721 cm-1 and the weak band at 1780 cm-1 are attributed to CdO symmetric and asymmetric stretch of imide groups, and the band at 1375 cm-1 is attributed to the CsN stretch of imide groups. Moreover, the weak band at 1600 cm-1 is the CdO stretch of amide groups, and that at 1500 cm-1 is CsN stretching of the CsNsH group. They are characteristic imide bands in polyimide. The bands at 1116 and 1239 cm-1 and the weak band at 1090 cm-1 are attributed to CsO asymmetric and symmetric stretches, which are characteristic of diphenyl ether. The XRD pattern of PI as shown in Figure 2a indicates that PI is an amorphous polymer. 3.2. Properties of MMMs. From a comparison among the XRD patterns of (a) Cu3(BTC)2, (b) PI, and (c) PI-Cu3(BTC)2 hollow-fiber samples as shown in Figure 2, the PI in the hollow fibers remains in the amorphous state, and Cu3(BTC)2 in the hollow fibers still maintains its intrinsic crystalline structure. This means that chemical reaction does not significantly occur in the mixture, so PI-Cu3(BTC)2 is just the physical mixture. The FTIR spectrum of PI-Cu3(BTC)2 hollow fibers shown in Figure 4c provides further evidence of the physical mixture. The strong bands at 1780 and 1720 cm-1 provide obvious evidence that the fibers have imide groups. These bands, combined with the lack of a band at 1650 cm-1, which is attributed to the CdO stretch of poly(amic acid), suggest that imidization occurred almost completely. Moreover, the other

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Figure 6. TEM image of part of a PI-Cu3(BTC)2 hollow-fiber membrane. The Cu3(BTC)2 content is 6 wt %.

characteristic bands of PI as shown in Figure 4a all appear in Figure 4c with similar relative intensities, showing that the matrix of the PI-Cu3(BTC)2 hollow fibers is the same as that of pure PI. Because only a small amount of Cu3(BTC)2 was added, the intensity of its characteristic bands should be weak. However, one can still find that the weak bands at 764 and 1110 cm-1 indicate that the intrinsic crystalline structure of Cu2(BTC)3 has been preserved in the PI-Cu3(BTC)2 hollow fibers. Figure 6 shows a TEM image of parts of a PI-Cu3(BTC)2 hollow-fiber membrane. As the distinguished absorption ability of electronic beams between polymer and Cu3(BTC)2, the black spots in this figure represent Cu3(BTC)2, whereas the large number of gray clumps represent the polymer. Most Cu3(BTC)2 particles are homogenously dispersed in PI with diameters of

about 20-50 nm, whereas only a few of the particles congregate together. This also shows that the Cu3(BTC)2 particles and PI are closely linked without void gaps, which is an important property for a good gas-separation MMM. Figure 7 shows the planar and three-dimensional atomic force microscopy (AFM) images of pure PI and PI-Cu3(BTC)2 spin-coated films. In Figure 7a, the PI film shows a good compact nature with a very smooth surface. Figure 7b provides clear evidence that the Cu3(BTC)2 particles are highly dispersed and immersed in the PI film without any interface voids. As the spin-coating speed is 3000 rotations/min and the rotation time is 30 s, the film is thin ( CO2 > O2 > N2 > CH4, which coincides with the sequence of gas size. The H2 permeance increased with increasing of Cu3(BTC)2 content. For the sample with a Cu3(BTC)2 content of 6 wt %, the H2 permeance was 1266 GPU, 1.5 times as large as that of pure polyimide. However, for the other gases, they go in the opposite direction. For example, the CO2 permeance of PI-6 wt % Cu3(BTC)2 hollow fiber is 37.23 GPU, less than one-half that of pure PI. As shown in Figure 12a, the gas selectivities of the pairs H2/CO2, H2/CH4, H2/N2, and H2/O2 also increased with increasing Cu3(BTC)2 content. The values of RH2/CH4, RH2/N2, RH2/CO2, and RH2/O2 for PI-6 wt % Cu3(BTC)2 hollow fiber were found to be 240, 163, 28, and 42, respectively, which are twice those of pure polyimide,

Table 1. Permeation/Selectivity of Various Gases through PI-Cu3(BTC)2 Composite Hollow Fibers at 1 MPa and 298 K

PI PI-3 wt % Cu3(BTC)2 PI-6 wt % Cu3(BTC)2

CO2 permeance (GPU)

H2 permeance (GPU)

H2/CO2 selectivity

H2/CH4 selectivity

H2/N2 selectivity

H2/O2 selectivity

87.6 64.9 37.2

876 934 1270

10.0 18.0 27.8

128 138 240

86.3 107 163

16.3 24.3 41.7

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Figure 11. (a,c,e) Gas permeances and (b,d,f) selectivities of hollow fibers of (a,b) pure polyimide, (c,d) PI-3 wt % Cu3(BTC)2, and (e,f) PI-6 wt % Cu3(BTC)2.

at 128, 86, 10, and 16, respectively. These results are extremely encouraging because both the permeance and selectivity of the

hollow fibers could be enhanced at the same time by the MMMs of PI and Cu3(BTC)2 hollow fibers.

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Figure 12. Change in the gas selectivities of (a) H2 and (b) CO2 with respect to other gases as a function of Cu3(BTC)2 content.

As shown in Figure 11, in the pressure range of 0-0.4 MPa, the permeances of the gases remained almost constant at a high level, although the permeances of CO2, O2, N2, and CH4 decreased with increasing Cu3(BTC)2 content in the PI-Cu3(BTC)2 MMMs. Meanwhile, the decreases of the permeances of the gases were different, which caused the selectivities RCO2/O2 and RCO2/N2 to decrease with increasing Cu3(BTC)2 content, as shown in Figure 12b. The relatively large pores of Cu3(BTC)2 and also the hydrophobic organic linkers contained in its framework make Cu3(BTC)2 absorb polymer chains more easily. This can be confirmed by the phenomenon of Cu3(BTC)2 immersed in PI film without any interface voids, as mentioned in regard to Figure 7. In fact, PI chains can block the pores of Cu3(BTC)2 to some degree; moreover, because of the relatively stiff chains of PI and the confinement effect of Cu3(BTC)2 particles, the rigidification of the PI chains can occur simultaneously in the interface of the particles. Both the Cu3(BTC)2 pore blockage and the rigidification of PI will cause a decrease of the permeance of gases. Consequently, the CO2, O2, N2, and CH4 gas permeances through the PI-Cu3(BTC)2 hollow fibers obviously decrease with increasing amount of Cu3(BTC)2. However, the blockage might narrow only a part of the pores of Cu3(BTC)2, which are still large enough for H2 molecules to move unobstructedly through them, so that the blockage significantly enhances the H2 selectivity. In contrast, for the other gases, the free paths of CO2, O2, N2, and CH4 are much larger than the pore diameter of Cu3(BTC)2, and Knudsen diffusion dominates their permeance; that is, the heavier the molecule, the slower is its diffusion. Among the gases, CO2 is the heaviest, indicating that its permeance and selectivity should be mostly decreased with increasing MOF content. 4. Conclusions Cu3(BTC)2 MOF crystals with a high surface area (1396 m2/ g) were synthesized and mixed with PI to prepare MMMs for gas separations. PI-Cu3(BTC)2 MMMs were successfully spun into hollow fibers by the dry/wet-spinning method and had smooth and impact-dense layers at the outside and inner surfaces, as well as a fingerlike-void layer and a spongelike supporting layer between them. The Cu3(BTC)2 particles were highly dispersed and faultlessly affinitive with the PI matrix in the fingerlike-void layer. The gas permeances in both PI and PI-Cu3(BTC)2 MMM hollow fibers remained at higher levels with the sequence H2 > CO2 > O2 > N2 > CH4. Both the H2 permeance and the selectivity of H2 with respect to other gases

were markedly enhanced as the Cu3(BTC)2 content in the PI-Cu3(BTC)2 MMMs increased. When the Cu3(BTC)2 content was 6 wt %, the H2 permeance was 1266 GPU, 1.5 times as large as that of pure PI. The selectivities RH2/CH4, RH2/N2, RH2/CO2, and RH2/O2 were found to be 240, 163, 28, and 42, which are twice that of pure PI, respectively. These results indicate that using MMMs of PI and MOFs might be an effective way to enhance the performance of H2 separation. With careful design and selection of the structures of PI and MOFs, a membrane with simultaneously high permeance and high selectivity could then be obtained. Acknowledgment This work was supported by the National Natural Science Foundation of China (20736002, 20776045), the National High Technology Research and Development Program of China (2008AA062302), the Program for Changjiang Scholars and Innovative Research Team in University of China (IRT0721), and the 111 Project of China (B08021). Supporting Information Available: Schematic diagram of the apparatus for spinning hollow fibers and SEM images of the PI morphology and of 3 wt % PI-Cu3(BTC)2. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Robeson, L. M. Correlation of separation factor versus permeability for polymeric membranes. J. Membr. Sci. 1991, 62, 165–170. (2) Zimmerman, C. M. Advanced gas separation membrane materials: Hyper rigid polymers and molecular sieve-polymer mixed matrices. Ph.D. Dissertation, The University of Texas at Austin, Austin, TX, 1998. (3) Vu, D. Q.; Koros, W. J.; Miller, S. J. Effect of condensable impurity in CO2/CH4 gas feeds on performance of mixed matrix membranes using carbon molecular sieves. J. Membr. Sci. 2003, 221, 223–239. (4) Vu, D. Q.; Koros, W. J.; Miller, S. J. Mixed matrix membranes using carbon molecular sieves. I. Preparation and experimental results. J. Membr. Sci. 2003, 221, 311–334. (5) Vu, D. Q.; Koros, W. J.; Miller, S. J. Mixed matrix membranes using carbon molecular sieves. II. Modeling permeation behavior. J. Membr. Sci. 2003, 211, 335–348. (6) Anson, M.; Marchese, J.; Garis, E.; Ochoa, N.; Pagliero, C. ABS copolymeractivated carbon mixed matrix membranes for CO2/CH4 separation. J. Membr. Sci. 2004, 243, 19–28. (7) Bertelle, S.; Gupta, T.; Roizard, D.; Vallieres, C.; Favre, E. Study of polymer-carbon mixed matrix membranes for CO2 separation from flue gas. Desalination 2006, 199, 401–402. (8) Jiang, L.; Chung, T.; Kulprathipanja, S. An investigation to revitalize the separation performance of hollow fibers with a thin mixed matrix composite skin for gas separation. J. Membr. Sci. 2006, 276, 113–125.

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ReceiVed for reView July 13, 2010 ReVised manuscript receiVed September 30, 2010 Accepted October 21, 2010 IE1014958