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Modified Mesoporous Silica Gas Separation Membranes on Polymeric Hollow Fibers Kwang-Suk Jang, Hyung-Ju Kim, J. R. Johnson, Wun-gwi Kim, William J. Koros, Christopher W. Jones,* and Sankar Nair* School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332-0100, United States
bS Supporting Information KEYWORDS: mesoporous silica, membrane, gas separation, hollow fiber, CO2 capture
M
embrane-based separations are becoming increasingly relevant for a number of applications1 due to their low energy requirements, potentially low fabrication cost, and steady-state operation. Polymeric membranes are amenable to large-scale fabrication processes, for example in the form of hollow fibers. Polymeric hollow-fiber membrane modules typically have attractive, large surface area/volume ratios (>1000 m2/m3).2 However, polymeric membranes also have an intrinsic “upper bound” on their performance, reflecting a trade-off between their permeability and selectivity.3 Over the past decade, inorganic membranes have been shown to possess high permeability, tunable selectivity, and high thermal and chemical resistance.4 Their applications are yet limited by the difficulty of fabricating inorganic membranes on a technologically scalable, low-cost platform. For example, polymeric hollow fiber substrates cannot withstand the high temperatures often needed for postsynthesis processing of molecular sieving zeolite membranes, and usually cannot withstand the hydrothermal membrane synthesis conditions. In exceptional cases that require only mild synthesis conditions, the fabrication of zeolite membranes on tubular polymer supports has been demonstrated.4d Ordered mesoporous silica materials, prepared using surfactant templates, have uniform pore channels with a diameter range of 2 10 nm. The mesopores are advantageous for rapid diffusion of molecules, can be modified in a variety of ways,5 and have high chemical and thermal stability, thus allowing for specific separation applications over a range of molecular sizes (e.g., from small gas molecules to larger pharmaceutical or biological molecules6). Mesoporous silica membranes have been proposed as potential candidates for separation applications,7,8 and have been synthesized by hydrothermal methods on ceramic substrates.7 Here we report the technologically scalable fabrication of an inorganic membrane platform based upon mesoporous silica membranes on polymeric hollow fibers. These asymmetric mesoporous silica membranes are continuous over large areas, are defect-free, and have high gas flux. Furthermore, we modify the mesopores via the use of amine-containing polysilsesquioxane (POSS) molecules. The resulting modified membranes exhibit high permeability and selective behavior in separation of CO2 from various gas streams. Scheme 1 shows the membrane fabrication process, which is described in detail in the Supporting Information. MCM-48, a mesoporous silica with three-dimensional channels of 2.7 nm r 2011 American Chemical Society
Scheme 1. Mesoporous Silica/Torlon Hollow Fiber Membrane Fabrication
diameter, was selected for investigation. The process comprises the fabrication of a macroporous polymeric hollow fiber, followed by the formation of a thin mesoporous silica layer on the hollow fiber at room temperature by static immersion in an acidic silica/surfactant precursor solution. The initial mesoporous framework formed in this process is then fully condensed by supplying additional silica species via a TEOS (tetraethylorthosilicate) vapor treatment at 100 °C, and the mesopores are then activated by a room-temperature surfactant extraction step. The membranes are subsequently ready for surface modifications, such as impregnation with POSS molecules considered in this work. Initial experiments to grow MCM-48 films were carried out on flat, nonporous surfaces of the poly(etherimide), Ultem, coated on glass substrates. Figure S1 in the Supporting Information shows an SEM image and an X-ray diffraction (XRD) pattern of the thin (150 nm) MCM-48 coating. Nonporous substrates such as poly(imides), silica glass, metal oxides, mica, and graphite have also been used for producing coatings of thin films of other mesoporous silicas with 2D or 3D pore networks.9 It is hypothesized that the formation of the mesoporous silica film on the polymer substrate is initiated by the adsorption and micellar selfassembly of the surfactant molecules on the hydrophobic surface of the substrate. For subsequent fabrication of membranes on hollow fibers, we selected Torlon, a poly(amide imide), as the polymeric substrate material. Torlon is chemically resistant and withstands high pressures up to 2000 psi without plasticization.10 These characteristics, as well as the amenability of Torlon for Received: April 1, 2011 Revised: May 11, 2011 Published: May 23, 2011 3025
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Figure 2. EDS line scanning analysis of the mesoporous silica/Torlon hollow fiber membrane.
Figure 1. Surface and cross-section SEM images of (a, b) bare Torlon hollow fiber supports; (c, d) mesoporous silica/Torlon hollow fiber membranes; and (e, f) remaining silica layers after dissolving out the Torlon supports, respectively. The mesoporous silica layer in (e) is freestanding, whereas the layer in (f) is adhered to an epoxy resin which was used to mount the fiber before polymer dissolution.
producing hollow fibers of controlled porosity, are expected to be important for separations applications. Macroporous Torlon hollow fiber supports were fabricated by a dry-jet/wet-quench method.10,11 The outer and inner diameters of the support fibers were about 220 and 100 160 μm respectively. The fibers do not possess skin layers, and have open pores of ∼100 nm size at the outer surface (Figure 1a,b). The CO2 and N2 permeances of the support Torlon hollow fibers were measured to be 50 000 GPU and 54 000 GPU respectively (1 GPU = 1 10 6 cm3 (STP) cm 2 s 1 cmHg 1). Images c and d in Figure 1 and Figure S2 in the Supporting Information show surface and cross-sectional SEM images of the mesoporous silica/Torlon hollow fiber membranes after completion of the membrane formation, TEOS vapor treatment, and surfactant extraction steps. Continuous and uniform silica layers are formed in a facile and highly reproducible manner. We used hollow fibers 30 cm in length, and there appears no significant limit on the length of the fibers that can be used. The apparent thickness of the mesoporous silica layer is 1.6 μm. The morphology of the silica layer is further revealed by dissolving the Torlon fiber with a strong solvent, N,N-dimethylformamide (images e
and f in Figure 1 and Figure S3 in the Supporting Information). The SEM images suggest that in addition to the thin silica layer formed above the Torlon fiber surface, the silica layer extends inside the pores of the fiber. The composition of the mesoporous silica/Torlon layer was also confirmed by energy-dispersive X-ray spectroscopy (Figure 2). Figure S4 in the Supporting Information shows the low-angle XRD pattern of the mesoporous silica/Torlon hollow fiber membrane. Although an intense diffraction signal was difficult to obtain due to the curved surface of the membrane, the formation of mesoporous silica is clearly indicated. We examined the pore structure of the membrane coating by transmission electron microscopy (TEM) after dissolving out the Torlon support fiber (see Figure S5 in the Supporting Information). A mesoporous structure containing worm-like channels with a diameter of about 2 nm was observed. It has been reported that MCM-48 is formed by a transformation from either a worm-like or 2D hexagonal structure.12 Wormlike mesoporous materials such as KIT-1 also have continuous channels, and possess a number of interesting properties different from ordered mesoporous silicas.13 On the basis of our combined structural characterizations, and permeation data (described below), we conclude that the membrane morphology can be identified as a mesoporous silica with a disordered, wormlike network of continuous channels. We speculate that the composite (polymer/silica) nature of the membrane layer may prevent the formation of continuous domains of highly ordered MCM-48. Table 1 summarizes pure-component CO2 and N2 gas permeation properties of the membranes at sequential stages of processing. The results support the formation of a continuous, defect-free mesoporous silica coating on the hollow fiber, and the activation of the mesopores after surfactant extraction leading to a high permeability. The membrane has nonzero permeability even before surfactant extraction, as gas molecules permeate through the low-density surfactant phase in the mesopores. CO2 and N2 permeances of the mesoporous silica/Torlon hollow fiber after surfactant extraction were measured at 35 °C and in a feed pressure range of 10 50 psig. These results are also shown in Figure S6 in the Supporting Information. The CO2 permeability of the asymmetric mesoporous silica hollow fiber membrane is calculated to be 7000 barrer (1 barrer = 1 10 10 cm3 (STP) cm cm 2 s 1 cmHg 1), a value significantly higher than that reported for MCM-48 membranes grown hydrothermally on ceramic disks (4600 barrer).7b The open mesopores can be filled or modified with a variety of functional groups to tailor the selective properties of the membrane. As an example of the modification of the mesopores, we chose to incorporate heptaisobutyl-2-aminoethyl 3026
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Table 1. Pure-Component Gas Permeation Properties of Mesoporous Silica/Torlon Hollow Fiber Membranes at Each Stage of Processinga membrane Torlon hollow fiber support
CO2 permeance (GPUa)
N2 permeance (GPU)
CO2/N2 permeance ratio
50000
54000
0.9
11 730
5.9 390
1.9 1.9
b
Mesoporous silica/Torlon after TEOS vapor treatment mesoporous silica/Torlon after extraction with ethanol for 2 h mesoporous silica/Torlon after extraction with ethanol for 24 h
1700
1900
0.9
mesoporous silica/Torlon after extraction with 0.05 N HCl/ethanol for 24 h
4400
3300
1.3
100
6.0
16.7
mesoporous silica/Torlon after POSS-infiltration
All measurements are made at 35 °C and a feed pressure of 50 psig unless otherwise noted. 1 psi = 6.89 kPa, and 1 GPU = 3.35 10 10 mol m Pa 1. b Measured at a feed pressure of 10 psig, because the flux through the macroporous support is too large to be measured at 50 psig. a
(3-aminopropyl) octasilsesquioxane (also known as aminoethylaminopropyl isobutyl-POSS, see Figure S7 in the Supporting Information), as a mesopore-filling molecule of ∼1 nm size containing amine groups to enhance selectivity for CO2 over gases such as N2 and CH4. The selective CO2 adsorption properties of amine groups are well-known, and amine-modified mesoporous silica materials have been widely studied as solid adsorbents for CO2 capture.14 A modification of a previously reported impregnation method8a was employed (see the Supporting Information) to infiltrate the POSS molecules into the mesoporous silica channels. The extent of infiltration of POSS molecules into the mesopores was estimated via N2 physisorption analysis and thermogravimetric analysis (TGA) on MCM48 particles treated with the same method (see Figures S8 and S9 and Table S1 in the Supporting Information). Considering the similar short-range pore structures and pore sizes of MCM-48 and the wormlike mesoporous silica membrane as characterized by XRD and TEM, we estimate that up to 62% of the mesopore volume could be infiltrated with POSS molecules via the infiltration process reported above. Pure-component gas permeation properties were measured at 35 °C and a feed pressure of 50 psig over 24 h. The measured CO2 permeance was 100 GPU, and the CO2/N2 and CO2/CH4 permeance ratios were 17 and 12 respectively. Mixed gas permeation from a 50:50 CO2/CH4 mixture was also measured at 35 °C and 100 psi total feed pressure. The measured CO2 permeance was 57 GPU, and the CO2/CH4 selectivity was 9. It is clear that the CO2 and N2 molecules permeate through nanopores filled partially with the POSS molecules. There is currently no quantitative mechanistic knowledge of gas permeation is such systems. However, based upon the above-measured permeation characteristics and the structure of the POSS molecules, we attribute the enhanced CO2 selectivity of the membrane to a combination of selective adsorption and steric (“molecular sieving”) effects caused by the presence of the aminoethylaminopropylisobutyl-POSS molecules. Although the permeance of the mesoporous membrane is expected to drop considerably after the POSS infiltration, the permeation results from this first study are quite promising and already comparable with the performance of commercial cellulose acetate hollow fiber membranes for CO2/CH4 separation.15 In conclusion, the fabrication of continuous mesoporous silica membranes on polymeric hollow fibers via a facile, low-temperature process has been demonstrated, thereby suggesting a technologically scalable platform for separations with inorganic membranes. The membranes are defect-free and have a high gas flux. We have also demonstrated an example of modification of the mesopores for enhancing the gas separation selectivity of the membrane.
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’ ASSOCIATED CONTENT
bS
Supporting Information. Experimental details, SEM and TEM images, XRD patterns, permeation data, N2 physisorption data, and TGA data. This material is available free of charge via the Internet at http://pubs.acs.org/.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (S.N.);
[email protected] (C.W.J.).
’ ACKNOWLEDGMENT This work was supported by the ConocoPhillips Company. K.-S. J. was partially supported by a National Research Foundation of Korea Grant from the Korean Government (NRF-2009352-D00052). The authors also acknowledge Dr. K. C. McCarley (ConocoPhillips) for useful discussions. ’ REFERENCES (1) (a) Lin, H.; Wagner, E. V.; Freeman, B. D.; Toy, L. G.; Gupta, R. P. Science 2006, 311, 639. (b) Bernardo, P.; Drioli, E.; Golemme, G. Ind. Eng. Chem. Res. 2009, 48, 4638. (c) Vankelecom, I. F. J. Chem. Rev. 2002, 102, 3779. (d) Striemer, C. C.; Gaborski, T. R.; McGrath, J. L.; Fauchet, P. M. Nature 2007, 445, 749. (2) (a) Baker, R. W. Membrane Technology and Applications, 2nd ed.; Wiley: Chichester, U.K., 2004. (b) Strathmann, H. AIChE J. 2001, 47, 1077. (3) (a) Robeson, L. M. J. Membr. Sci. 1991, 62, 165. (b) Robeson, L. M. J. Membr. Sci. 2008, 320, 390. (4) (a) Yoo, W. C.; Stoeger, J. A.; Lee, P.; Tsapatsis, M.; Stein, A. Angew. Chem., Int. Ed. 2010, 122, 8881. (b) Choi, J.; Jeong, H.-K.; Snyder, M. A.; Stoeger, J. A.; Masel, R. I.; Tsapatsis, M. Science 2009, 325, 590. (c) Kanezashi, M.; O’Brien-Abraham, J.; Lin, Y. S.; Suzuki, K. AIChE J. 2008, 54, 1478. (d) Ge, Q.; Wang, Z.; Yan, Y. J. Am. Chem. Soc. 2009, 131, 17056. (e) Xu, L.; Lee, H. K. Anal. Chem. 2007, 79, 5241. (5) (a) Hicks, J. C.; Jones, C. W. Langmuir 2006, 22, 2676. (b) Wight, A. P.; Davis, M. E. Chem. Rev. 2002, 102, 3589. (c) Melero, J. A.; Grieken, R.; van; Morales, G. Chem. Rev. 2006, 106, 3790. (d) Hodgkins, R. P.; Ahniyaz, A.; Parekh, K.; Belova, L. M.; Bergstr€om, L. Langmuir 2007, 23, 8838. (6) (a) Sen, T.; Sebastianelli, A.; Bruce, I. J. J. Am. Chem. Soc. 2006, 128, 7130. (b) Zhang, L.; Qiao, S.; Jin, Y.; Yang, H.; Budihartono, S.; Stahr, F.; Yan, Z.; Wang, X.; Hao, Z.; Lu, G. Q. Adv. Funct. Mater. 2008, 18, 3203. (c) Hartono, S. B.; Qiao, S. Z.; Jack, K.; Ladewig, B. P.; Hao, Z.; Lu, G. Q. Langmuir 2009, 25, 6413. (7) (a) Chew, T.-L.; Ahmad, A. L.; Bhatia, S. Adv. Colloid Interface Sci. 2010, 153, 43. (b) Kumar, P.; Ida, J.; Guliants, V. V. Microporous 3027
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