Facile Synthesis of Mesoporous Silica Sublayer with Hierarchical Pore

Laurent Veyre , Werner E. Maas , Melanie Rosay , Ralph T. Weber , Chloé Thieuleux , Christophe Coperet , Anne Lesage , Paul Tordo , and Lyndon Em...
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Langmuir 2005, 21, 5859-5864

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Facile Synthesis of Mesoporous Silica Sublayer with Hierarchical Pore Structure on Ceramic Membrane Using Anionic Polyelectrolyte Taewook Kang,† Seogil Oh,† Honggon Kim,‡ and Jongheop Yi*,† School of Chemical and Biological Engineering, Seoul National University, San 56-1 Shilim-dong, Kwanak-gu, Seoul 151-742, Korea, and Korea Institute of Science and Technology, Seoul, Korea Received January 3, 2005. In Final Form: March 14, 2005 A facile method for introducing mesoporous silica sublayer onto the surface of a ceramic membrane for use in liquid-phase separation is described. To reduce the electrostatic repulsion between the mesoporous silica sol and the ceramic membrane in highly acidic conditions (pH < 2), thus facilitating the approach of hydrolyzed silica sol to the surface of the membrane, poly(sodium 4-styrenesulfonate) (Na+PSS-, denoted as PSS-) was used as an ionic linker. The use of PSS- led to a significant reduction in positive charge on the ceramic membrane, as confirmed by experimental titration data. Consistent with the titration results, the amount of mesoporous silica particles on the surface of the ceramic membrane was low, in the absence of PSS- treatment, whereas mesoporous silica sublayer with hierarchical pore structure was produced, when 1 wt % PSS- was used. The results show that mesoporous silica grows in the confined surface, eventually forming a multistacked surface architecture. The mesoporous silica sublayer contained uniform, ordered (P6mm) mesopores of ca. 7.5 nm from mesoporous silica as well as macropores (∼µm) from interparticle voids, as evidenced by transmission electron microscopy and scanning electron microscopy analyses. The morphologies of the supported mesoporous silica could be manipulated, thus permitting the generation of uniform needlelike forms or uniform spheroid particles by varying the concentration of PSS-.

Introduction Mesoporous materials with a well-defined pore geometry1 have attracted considerable attention due to their high surface area, pore volume, and uniform pore channel for use in a wide range of application areas such as catalysts,2-4 molecular recognition as in a chemical sensor,5 and separation processes.6-9 Among the wide variety of potential application areas, in particular, the development of functionalized mesoporous silica for liquid-phase separation application has attracted a great deal of * To whom correspondence should be addressed. Telephone: 82-2-880-7438. Fax: 82-2-885-6670. E-mail: [email protected]. † Seoul National University. ‡ Korea Institute of Science and Technology. (1) (a) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (b) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. Science 1998, 279, 548. (c) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. J. Am. Chem. Soc. 1998, 120, 6024. (2) (a) Kim, Y.; Kim, P.; Kim, C.; and Yi, J. J. Mater. Chem. 2003, 13, 2353. (b) Corriu, R. J. P.; Lancelle-Beltran, E.; Mehdi, A.; Reye, C.; Brandes, S.; Guilard, R. J. Mater. Chem. 2002, 12, 1355. (3) (a) Yang, R. T.; Pinnavaia, T. J.; Li, W.; Zhang, W. J. Catal. 1997, 172, 488. (b) Margolese, D.; Melero, J. A.; Christiansen, S. C.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 2000, 12, 2448. (c) Van Rhijn, W. M.; De Vos, D. E.; Sels, B. F.; Bossaert, W. D.; Jacobs, P. A. Chem. Commun. 1998, 317. (d) Van Rhijn, W. M.; De Vos, D. E.; Bossaert, W.; Bullen, J.; Wouters, B.; Jacobs, P. A. Stud. Surf. Sci. Catal. 1998, 117, 183. (4) (a) Cho, Y. S.; Park, J. C.; Lee, B.; Kim, Y.; Yi, J. Catal. Lett. 2002, 81, 89. (b) Mukhopadhyay, K.; Sarkar, B. R.; Chaudhari, R. V. J. Am. Chem. Soc. 2002, 124, 9692. (5) Zhou, H. S.; Sasabe, H.; Honma, I. J. Mater. Chem. 1998, 8, 515. (6) (a) Liu, M.; Hidajat, K.; Kawi, S.; Zhao, D. Y. Chem. Commun. 2000, 1145. (b) Hossain, K. Z.; Mercier, L. Adv. Mater. 2002, 14, 1053. (c) Bibby, A.; Mercier, L. Chem. Mater. 2002, 14, 1591 (7) (a) Dai, S.; Burleigh, M. C.; Shin, Y. S.; Morrow, C. C.; Barnes, C. E.; Xue, Z. L. Angew. Chem., Int. Ed. 1999, 38, 1235. (b) Dai, S.; Burleigh, M. C.; Ju, Y. H.; Gao, H. J.; Lin, J. S.; Pennycook, S. J.; Barnes, C. E.; Xue, Z. L. J. Am. Chem. Soc. 2000, 122, 992. (8) (a) Lim, M. H.; Stein, A. Chem. Mater. 1999, 11, 3285. (b) Lee, B.; Bao, L. L.; Im, H. J.; Dai, S.; Hagaman, E. W.; Lin, J. S. Langmuir 2003, 19, 4246. (c) Ho, K. Y.; Mckay, G.; Yeung, K. L. Langmuir 2003, 19, 3019.

interest, due to easy-tunable surface functionality and uniform pore structure. However, the application of these mesoporous materials to industrial separation process still remains a challenge. From the point of view of industrial feasibility, a combination of mesoporous materials with a conventional support would be of great help in making mesoporous materials more feasible in industrial separations. For example, conventional ceramic membrane has excellent thermal, chemical, and mechanical stabilities, but their pore size distribution is wide. This wide distribution in pore size is unfavorable for processes that involve nanofiltration, ultrafiltration, pervaporation, and the separation of small objects, such as heavy metal ions, hazardous anionic complexes, and persistent organic pollutants from an aqueous medium, respectively. In general, separation performances such as selectivity for the target and permeability through a membrane are strongly dependent on the surface functionality of membranes, its pore size and distribution, and the pore volume fraction.10 Therefore, ordered mesoporous silica featuring a uniform, monodispersed pore size, high surface area, pore volume, and the easy introduction of a surface functionality represents a promising separation layer for producing a high-performance membrane. In other words, the membrane support does not impose mass-transfer limitations but offers the required mechanical strength, and the mesoporous silica sublayer provides the unique separation characteristics of the membrane support. Several groups have reported on the synthesis of a mesoporous silica film on a variety of interfaces.11-14 The (9) (a) Kang, T.; Park, Y.; Choi, K.; Lee, J. S., Yi, J. J. Mater. Chem. 2004, 14, 1043. (b) Kang, T.; Park, Y.; Yi, J. Ind. Eng. Chem. Res. 2004, 43, 1478. (c) Kim, Y.; Kim, C.; Choi, I.; Rengaraj, S.; Yi, J. Environ. Sci. Technol. 2004, 38, 924. (10) Burggraaf, A. J.; Cot, L. Fundamentals of Inorganic Membrane Science and Technology; Elsevier Science: New York, 1996; Vol. 4, p 21.

10.1021/la0500070 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/18/2005

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Figure 1. Schematic diagram of the procedure for preparing mesoporous silica sublayer with hierarchical pore structure on a ceramic membrane. (not drawn to scale).

formation of oriented films of layer-by-layer silicasurfactant nanocomposites has been reported using charge matching of the silica/surfactant. In such a case, a cationic surfactant, alkyltrimethylammonium bromide [CnH2n+1N(CH3)3+, CnTAB, where n denotes the carbon number in the alkyl chain] was used to produce layered silica-surfactant nanocomposite films. Another study of the binding of an inorganic molecular sieve, a zeolite, to a support has also been reported. This study has been carried out using various organic linkers (e.g., aminefunctionalized organic molecules or charged macromolecules were used as linkers) between the zeolite and the support, such as glass. Other studies of mesoporous films on hydrophobic supports such as mica or graphite have also been carried out. Even the free-standing formation of a mesoporous silica film at an air/water interface has been reported. The major focus of these studies was the synthesis of a mesoporous silica film on a nonporous support. However, considering the separation purpose of mesoporous silica sublayer in a flow system, mesoporous silica sublayer on a porous support would be expected to be more desirable than that on a nonporous support. To introduce mesoporous separation layer on a porous support, it should be considered that mesoporous silica tends to penetrate into and block the support pore instead of forming a separation layer on the support. Recently, Kim et al.15 successfully synthesized a continuous mesoporous silica thin film on a porous inorganic membrane by a dipcoating method using poly(vinyl alcohol) (PVA) as a porefilling substance and reported that the water-soluble PVA reduced the support surface roughness and prevented the coating solution from infiltrating into the support pores simultaneously. They also reported that the resulting ordered mesoporous sublayer possessed very promising properties that included a high porosity and a narrow pore size distribution suitable for gas separation and that (11) Ryoo, R.; Ko, C.; Cho, S.; Kim, J. J. Phys. Chem. B 1997, 101, 10610. (12) (a) Ogawa, M. J. Am. Chem. Soc. 1994, 116, 7941. (b) Ogawa, M. Langmuir 1995, 11, 4639. (c) Ogawa, M. Langmuir 1997, 13, 1853. (13) (a) Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G. A. Nature 1996, 379, 703. (b) Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. J. Mater. Chem. 1997, 7, 1285. (c) Yang, H.; Coombs, N.; Dag, O ¨ .; Sokolov, I.; Ozin, G. A. J. Mater. Chem. 1997, 7, 1755. (14) (a) Lee, G.; Lee, Y.; Yoon, K. B. J. Am. Chem. Soc. 2001, 123, 9769. (b) Chun, Y. S.; Ha, K.; Lee, Y. J.; Lee, J. S.; Kim, H. S.; Park, Y. S.; Yoon, K. B. Chem. Commun. 2002, 1846. (c) Lee, J. S.; Lee, Y. J.; Lee, E.; Park, Y. S.; Yoon, K. B. Science 2003, 301, 818. (15) (a) Kim, Y. S.; Yang, S. M. Adv. Mater. 2002, 14, 1078. (b) Kim, Y. S.; Kusakabe, K.; Yang, S. M. Chem. Mater. 2003, 15, 612.

the membranes also displayed very high permeance and selectivity for gas separation. Departing from the dip-coating method including spincasting, we considered the possibility of the facile preparation of mesoporous silica sublayer on the surface of a membrane, the pore structure of which might be suitable for liquid-phase separations. Because mesoporous silica solutions are typically highly acidic, in such a system, the surface charge of both the ceramic membrane and mesoporous silica sol would be expected to turn out positive. It should also be noted that the large pore size of the membrane (600 nm) would be expected to facilitate the infiltration of mesoporous silica sol into the inside of the support. Therefore, to deposit successfully a mesoporous silica sublayer on the surface of a ceramic membrane in one batch, it would be necessary to reduce the extent of electrostatic repulsion between the mesoporous silica sol and the surface of the membrane and prevent the infiltration of mesoporous silica sol efficiently. In this present study, with experimental evidence, we propose a method for the facile preparation of a ceramic membrane functionalized with mesoporous silica sublayer that contains a hierarchical pore structure (mesopore from mesoporous silica, macropore from interparticle void) for use in liquid-phase separation. The overall preparation procedures are illustrated in Figure 1. This novel strategy demonstrates that a ceramic membrane with ordered mesoporous silica sublayer can be prepared in one batch, whereas other methods require at least two steps (e.g., synthesis of mesoporous materials + deposition on the surface of the membrane by dip-coating or spin-casting). Because we adopted solution components from the preparation of SBA-15 (i.e., pH < 2), it would be expected that the silica source is first hydrolyzed at low pH to form a protonated species16 and that the surface of the ceramic membrane will also carry a positive charge. This electrostatic repulsion between the silica/surfactant composite and the membrane hinders the deposition of mesoporous silica. To reduce this long-range repulsion, an anionic polyelectrolyte (PSS-) was used as an ionic linker between the membrane surface and the mesoporous silica. We also expected that PSS- would act as a pore-filling substance to prevent the infiltration of mesoporous silica sol simultaneously. In addition to this, the time of membrane immersion into mesoporous silica solution was controlled, (16) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56, and references therein.

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Figure 2. FE-SEM images of the surface of a modified ceramic membrane (a) without PSS- treatment, (b) treatment with 1 wt % PSS-, and (c) treatment with 2 wt % PSS-. The insert figures in b and c represent the simplified shape of a primary particle of supported mesoporous silica particles. (d) Representative TEM images of uniform mesopore channels in b and c. (e) TEM image of d at a different zone axis. The insert figure in e shows the schematic pore arrangement of supported mesoporous silica.

because if the size of the silica network reaches a size comparable to the membrane pore size, it would be difficult for the mesoporous silica sol to penetrate into the membrane pore. Experimental Section A triblock copolymer [Pluronic P123 (EO20PO70EO20, Mav ) 5800), BASF] was used as a templating agent, and tetraethoxysilane (TEOS, Aldrich) was used as received. Poly(sodium 4-styrenesulfonate) [-(CH2CH(C6H4SO3-Na+))n-, Mav ) 70 000, Na+PSS- denoted as PSS-, Aldrich] was used as an ionic linker. The ceramic membrane (Al2O3, 49.0%; SiO2, 48.0%; Coorstek) was used as received. The shape of the membrane was a disctype, with a radius of 5 cm and a thickness of 6 mm. The average pore size of this membrane was 600 nm, and the apparent porosity (the ratio of pore volume to total volume) was 30.0%. The overall preparation procedures are summarized schematically in Figure 1. In a typical synthesis in steps b to c, the membrane support was pretreated with a PSS- solution. A solution containing 1 wt % PSS- was prepared by dissolving PSS- into an acidic solution for which the pH was the same as the mesoporous silica solution. Then, the bare ceramic membrane was dipped into as-made 1 wt % of PSS- solution and allowed to remain there for 30 min. After treatment with PSS-, the ceramic membrane modified with PSS- (Figure 1c) was dried at room temperature (RT). Mesoporous silica sol was prepared as described in the literature.1b In the preparation of a mesoporous silica solution, 10 g of triblock copolymer, P123, was dissolved in water and the resulting solution was stirred at 35 °C for 1 h. HCl was then added as catalyst, and, finally, TEOS was added. The mole ratio of reagents was 175.1:0.017:15.49:1 H2O:P123: HCl:TEOS, respectively. After adding TEOS, the resulting silica sol solution was stirred vigorously at 35 °C for 1 h. The ceramic membrane, treated with PSS-, was then immersed in the mesoporous silica solution (the volume of the mesoporous silica solution for the immersion of the ceramic membrane can be controlled), followed by stirring for 20 h at 35 °C. The resulting membrane functionalized with mesoporous silica (Figure 1d) was dried for 1 day. It should be noted that there was no thermal aging step in this case. To remove the structure-directing

surfactant and PSS-, the as-made membrane functionalized with mesoporous silica sublayer was calcined at 450 °C for 6 h (Figure 1e). The morphology of the supported mesoporous silica on the surface of the ceramic membrane was examined by scanning electron microscopy (SEM). SEM images were obtained using both a Philips XL-20 scanning electron microscope and JEOL JSM-6330F microscope. N2 adsorption/desorption measurements on the unsupported mesoporous silica were carried out using a Micromeritics ASAP 2010 analyzer, and the pore size distribution was calculated using the Barrett-Joyner-Halenda (BJH) model on the adsorption branch. To confirm the presence of mesopore channels in the supported mesoporous silica, transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2000EXII instrument. Small-angle X-ray scattering (SAXS) patterns of unsupported mesoporous silicas were collected on a Bruker GADDS diffractometer using Cu KR radiation at 40.0 kV and 45.0 mA.

Results and Discussion Figure 2 reveals that the deposition of mesoporous silica on the surface of a ceramic membrane was minimal in the absence of PSS- (Figure 2a). Both the bare ceramic membrane (Supporting Information) and the modified membrane without treatment with PSS- show a nearly identical surface morphology. Qualitatively, the surface morphology of the bare ceramic membranes did not change markedly in any obvious way when compared to a membrane that was not treated with PSS-. This can be attributed to the electrostatic repulsion between the surface of the ceramic membrane and the mesoporous silica sol under the given (highly acidic) conditions. It is well-known that ordered mesoporous silica, SBA-15, is prepared under strongly acidic conditions. Hence, in this condition, when the ceramic membrane is immersed in the synthesis solution of SBA-15 midway, the surface of the ceramic membrane becomes positively charged because the point of zero charges for both alumina (pHpzc ) ca. 8) and silica (pHpzc ) ca. 2) are higher than the pH of

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Figure 3. Titration curves for (a) bare ceramic membrane and ceramic membrane treated with (b) 0.1, (c) 0.5, and (d) 1 wt % of PSS- solution, respectively.

the mesoporous silica solution. Electrostatic repulsion between the surface of the ceramic membrane (C+) and the mesoporous silica sols (M+) are then generated, preventing them from approaching each other, which is unfavorable for bond formation between mesoporous silica and the membrane surface, when an assembly procedure is used. This long-range electrostatic repulsion without any charge-compensation layer prevents the deposition of significant amounts of mesoporous silica on the surface of the ceramic membrane. Therefore, to immobilize mesoporous silica on the surface of the ceramic membrane in the one-step procedure, the repulsive interactions between C+ and M+ should be decreased by the insertion of an anionic intermediate layer (I-). In this regard, as a means to increase the number of ionic linkages, an anionic polyelectrolyte was employed. Therefore, the self-assembly route for this case can be assumed to be C+/I-/M+. To confirm that the positive surface charge was reduced due to the effect of the PSS- pretreatment, titration experiments were carried out (Figure 3). Basically, we assume that, as the system pH decreases, the ceramic membrane without pretreatment with a PSS- solution acquires a positive surface charge, as mentioned above, and meanwhile, the ceramic membrane pretreated with a PSSsolution exhibits a lower positive surface charge than the bare ceramic membrane without PSS- treatment. Thus, from the point of view of an electrical double layer, it would be easily expected that the PSS- layer will approach and attach the surface of the membrane due to electrostatic interactions, thus effectively screening the positive surface charge on the membrane. The changes in the surface charge (Q) of the ceramic membrane by the deposition of PSS- were analyzed at different system pH values by titration with 0.1 M HNO3 and 0.1 M NaOH, respectively, and calculated using eq 1.17 The pH value at the point of zero charge (pHpzc) was then determined by plotting Q versus the system pH:

Q)

1 (C - CB - [H+] + [OH-]) w A

(1)

where Q is the surface charge (mol/(g of dry weight of sample)) and w is the dry weight of the sample in an aqueous system (g/L). CA and CB represent the concentration of added acid and base in the aqueous system (mol/L). [H+] and [OH-] are the concentrations of H+ and OH- (mol/L). Changes in the surface charge on the ceramic membrane (Figure 1b) and the PSS--modified ceramic membrane (17) (a) Netpradit, S.; Thiravetyan, P.; Towprayoon, S. Water Res. 2003, 37, 763. (b) Stumm, W.; Morgan, J. J. Aquatic Chemistry, 3rd ed.; John Wiley and Sons: New York, 1996; pp 534-540. (c) Kiefer, E.; Sigg, L.; Schosseler, P. Environ. Sci. Technol. 1997, 31, 759.

Figure 4. (a) N2 adsorption/desorption isotherm of unsupported mesoporous silica. The insert shows the pore size distribution. The pore size was calculated from the adsorption branch of the nitrogen adsorption/desorption isotherm using the BarrettJoyner-Halenda formula. (b) Small-angle X-ray scattering pattern of unsupported mesoporous silica.

(Figure 1c) are shown in Figure 3. The point of zero charge for the bare ceramic membrane was found to be around pH 4.5, whereas that for the PSS--modified ceramic membrane is about pH 2.5. As the quantity of PSS- in the solution increases, the positive surface of the ceramic membrane modified with PSS- is reduced. Consistent with this titration result, after treatment with 1 wt % PSS-, mesoporous silica particles were successfully deposited on the surface of the ceramic membrane (Figure 2b). Based on these findings, it can be concluded that the deposition of PSS- on a ceramic membrane can reduce the positive surface charge on the membrane, thus permitting the mesoporous silica sol to approach the surface of the membrane. The physical properties of mesoporous silica particles on the surface of the ceramic membrane such as pore size and the orderliness of pore arrangement can be estimated by an observation of the unsupported mesoporous silica in the synthetic solution. N2 adsorption/desorption isotherms (Figure 4) for the unsupported mesoporous silica show an irreversible type IV adsorption isotherm with an H1 hysteresis loop, as defined by IUPAC. The pore size distribution is narrow, in the range of 6.5-8.5 nm (centered at 7.5 nm). The narrow pressure range for capillary condensation (evaporation) of the unsupported mesoporous silica is also indicative of a narrow mesopore size distribution. The SAXS pattern (Figure 4) of the unsupported mesoporous silica shows a strong (100) reflection of 0.97 (2θ value). Two additional high-order peaks for both (110) and (200) planes with lower intensities with 2θ values of 1.5-1.9° are also observed. These characteristic peaks in the SAXS analysis are typical of a hexagonal pore arrangement (space group, P6mm). It is also evident from the TEM analyses that supported mesoporous silica contains uniform pore channels and a hexagonal pore symmetry. It should be noted that TEM analyses were

Mesoporous Silica Sublayer on a Ceramic Membrane

conducted by grinding the surface of the ceramic membrane that had been functionalized with mesoporous silica sublayer. The morphology of the supported mesoporous silica sublayer in the case of treatment with 1 wt % PSS- is a uniform needlelike shape ca. 3.0 µm in length and 200 nm in width, as shown in Figure 2b (see insert figure). Figure 2b indicates that these needlelike particles are further interconnected with each other, forming a 3-D network. From the TEM analyses, each particle contains a uniform and ordered mesoporosity (Figure 2d,e). Figure 2b also shows that the pore structure of the mesoporous silica sublayer is quite hierarchical in that both mesopores (ca. 7.5 nm) from the supported mesoporous silica and macropores (∼µm) from interparticle space are both present. Generally, hierarchical pore structure of the separation layer in liquid-phase separation is important because this pore architecture can reduce mass-transfer limitations. This 3-D interconnected needlelike morphology of the supported mesoporous silica is much different in size and shape from that of unsupported silica, which is a wheatlike macrostructure consisting of many ropelike particles with relatively uniform sizes of 1 µm.1b It is especially noteworthy that, when compared to the shape of the unsupported mesoporous silica, both the width (∼300 nm) of the primary particles and the extent of aggregation of the primary particles for the supported mesoporous silica are much smaller. As the concentration of PSS- is increased from 1.0 to 2.0 wt %, uniform spheroidlike particles are observed (Figure 2c, insert figure). The transition in the morphology of the supported mesoporous silica (needle to spheroid) can be partially explained by a cosurfactant effect. It is well-known that the morphology of mesoporous silica is strongly dependent on the surface curvature energy at the interface of the inorganic silica and the organic block copolymer species.18 More highly polar cosolvents or cosurfactants generate a more curved structure. As the concentration of PSS- increases, free PSS- or the tail of the deposited PSS- may act as a cosurfactant or intermediate anionic layer between the inorganic precursor (I+) and the positively charged block copolymer (PEO20-PPO70-PEO20). Therefore, it is understandable that the shape of mesoporous silica particles would become more spheroid-shaped in the case of 2 wt % PSS-. It should be noted that the packing density and hierarchal pore structure of the deposited mesoporous silica particles of spheroid shape are comparable to those of needlike shape. It is also assumed that, if the size of free mesoporous silica network reaches a size comparable to the membrane pore size, it would be become difficult for the mesoporous silica sol to penetrate into the membrane pore inherently. When a ceramic membrane modified with PSS- (Figure 1c) is immersed in a mesoporous silica sol 5 h after the addition of TEOS, the morphology of the supported mesoporous silica is nearly identical to that of unsupported mesoporous silica. Figure 5a suggests that fully developed mesoporous silica particles are deposited on the membrane surface. The size of the deposited mesoporous silica is much larger than that of the membrane pore (∼600 nm). Therefore, only the control of the immersion time of the membrane in the mesoporous silica solution may prevent the infiltration of mesoporous silica particles into the membrane pore efficiently. In a recent paper by Ruthstein et al. on the formation of SBA-15 using EPR spectroscopy, (18) (a) Zhao, D.; Yang, P.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1999, 11, 1174. (b) Zhao, D.; Sun, J.; Li, Q.; Stucky, G. D. Chem. Mater. 2000, 12, 275.

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Figure 5. SEM images of the mesoporous silica sublayer on a ceramic membrane synthesized using membrane immersion times of (a) 5 and (b) 1 h after TEOS addition. Panel b is identical to Figure 2b. Bars correspond to 10 µm.

it was reported that, after a 20 min reaction, the EO chains have a different environment, which can be attributed to their being located either in the micellar corona or in the micropores, respectively.19 In addition, based on EPR spectroscopy experiments, they found that the hexagonal structure appears 2 h after the beginning of the selfassembly of the surfactant and TEOS. In other words, the assembly of the surfactant and the silica precursor is completed within a few hours. Hence, it can be inferred that the macrostructure of mesoporous silica already has been formed within 5 h. It is well-known that when a support has a rough surface and a coarse pore structure, a continuous sublayer with mesoporosity is not easily formed. However, a crack without mesoporous silica sublayer can be avoided by repeating the deposition of PSS- and subsequent dipping in mesoporous silica sol. To test the feasibility of using the as-made membrane with mesoporous silica sublayer in liquid-phase separations, an as-made membrane was employed in the separation of Cu2+ from an aqueous stream. We used the membrane system in conjunction with a stirred cell system (Supporting Information). This system was a batch type that used a nitrogen gas pump to pressurize the membrane chamber. The initial feed concentration of Cu2+ in the solution was 1 mM, and the Cu2+ solution was fixed at pH 4. Transport experiments were carried out at room temperature and under transmembrane pressures (TMP) of 3.0, 1.5 bar. Prior to use in the separation of Cu2+, the ceramic membrane with the mesoporous silica sublayer was further functionalized with (3-aminopropyl)triethoxysilane (APTES) by the well-known grafting method (denoted as CM-A) because of the complexation between Cu2+ and the amine functionality in APTES.20 For comparison, the bare membrane without mesoporous silica sublayer was also treated with APTES (CA). The change (19) (a) Ruthstein, S.; Frydman, V.; Kababya, S.; Landau, M.; Goldfarb, D. J. Phys. Chem. B 2003, 107, 1739. (b) Ruthstein, S.; Frydman, V.; Goldfarb, D. J. Phys. Chem. B 2004, 108, 9016. (20) Stein, A.; Melde, B. J.; Schroden, R. C. Adv. Mater. 2000, 12, 1403.

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cake layer (m-1), TMP is transmembrane pressure (mbar), and η is the viscosity of water to be filtered (10-3 Pa‚s). As shown in Figure 6, CM-A shows a high rejection rate of over 20% compared to CA. The permeability of CM-A is found to be slightly higher than that of CA. From these results, it appears that the membrane with mesoporous silica sublayer prepared via our proposed procedure can be used effectively in the liquid-phase separation of heavy metal ions. To address the performance of the membrane with mesoporous silica sublayer for the separation of heavy metal ions in detail, further studies are currently underway. Conclusions

Figure 6. (a) Rejection rate (3 bar) of 1.0 mM Cu2+ using ([) a bare ceramic membrane, (0) a membrane functionalized with APTES without mesoporous silica sublayer, and (2) the APTESfunctionalized ceramic membrane with mesoporous silica sublayer. The rejection rate was calculated by the following equation: R ) (C0 - C)/C0, where R is a rejection rate (%), C0 is the concentration of the feed solution (ppm), and C is the concentration of the permeate that passed through the membrane (ppm). The insert corresponds to permeation fluxes (upper figure) for (0) a membrane functionalized with APTES without mesoporous silica sublayer and (2) an APTES-functionalized ceramic membrane with mesoporous silica sublayer. (b) Rejection rate (1.5 bar) of 1.0 mM Cu2+ using (0) a membrane functionalized with APTES without mesoporous silica sublayer and (2) the APTES-functionalized ceramic membrane with mesoporous silica sublayer.

in the permeation flux can be evaluated by the change of resistance. These resistance changes can be evaluated using the resistance-in-sereis model:21

TMP J ) 3.6 × 108 η(RM + RC)

(2)

where J is the permeate flux (L‚m ‚h , LMH), RM is the membrane resistance (m-1), RC is the resistance of the -2

-1

A method for introducing mesoporous silica sublayer into the surface of a ceramic membrane using an anionic polyelectrolyte as an ionic linker is described. SEM images suggest that the deposition of PSS- induces a reduction in the positive surface charge of the ceramic membrane and facilitates the approach of hydrolyzed, protonated silica sol to the surface of the membrane. This drastic reduction in the positive surface charge on the ceramic membrane after treatment with PSS- is also evidenced by experimental titration data. From both TEM images and SEM analyses of supported mesoporous silica, the mesoporous silica sublayer contains uniform, ordered (P6mm) mesopores of ca. 7.5 nm as well as macropores (∼µm) from interparticle voids. It should be noted that supported mesoporous silica particles are closely linked with each other, thus generating 3-D cagelike structures. The morphology of the primary unit of supported mesoporous particles is found to be uniform needlelike, whereas a uniform spheroid shape is observed for increasing concentrations of PSS-. In addition, the findings herein also suggest that a ceramic membrane with hierarchical mesostructured silica sublayer can be efficiently used in the separation of heavy metal ions in an aqueous stream. Acknowledgment. We acknowledge the National Research Laboratory (NRL) of the Korean Science and Engineering Foundation (KOSEF) for financial support. Supporting Information Available: SEM image of the bare ceramic membrane and a schematic diagram of the stirred cell system (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. LA0500070 (21) (a) Cheryan M. Ultrafiltration and Microfiltration Handbook; Technomic Publishing: Lancaster, PA, 1998. (b) Blo¨cher, C.; Dorda, J.; Mavrov, V.; Chmiel, H.; Lazaridis, N. K.; Matis, K. A. Water Res. 2003, 37, 4018.