- Email: [email protected]
Counter diffusion self assembly synthesis of ordered mesoporous silica membranes
From Zeolites to Porous MOF Materials – the 40th Anniversary of International Zeolite Conference R. Xu, Z. Gao, J. Chen and W. Yan (Editors) © 2007 Elsevier B.V. All rights reserved.
363
Counter diffusion self assembly synthesis of orderedmesoporous silica membranes Shriya K. Seshadri a, Hatem M. Alsyouri b and Y. S. Lin a* a
Department of Chemical Engineering, Arizona State University, Tempe, AZ 85287, USA. Email: [email protected] b Industrial Chemistry Center, Royal Scientific Society, P.O. Box 1438, Amman 11941, Jordan
ABSTRACT We report the synthesis of ordered mesoporous silica membranes in macroporous supports by a novel acid catalyzed counter diffusion self assembly (CDSA) method. Effect of pore size, morphology and surface chemistry of the support on the formation of oriented mesoporous silica plugs has been studied. Hydrophobically modified small pore alumina supports (0.3Pm) show good quality silica membranes as sub-millimeter plugs. Preliminary observations on straight pored hydrophilic and hydrophobic track etch polycarbonate membranes supports (pore diameters 5 Pm and 8 Pm respectively) indicate the formation of ordered mesoporous silica plugs within the support pores, as seen by results of scanning electron microscopy and X-ray diffraction studies. Experimental results suggest that the pore size and morphology of the support play a vital role in the formation of these ordered mesoporous silica plugs, while surface chemistry seems affect the quality of the plugs formed. 1. INTRODUCTION Ordered mesoporous materials have been the focus of extensive research since they were first reported by the researchers of Mobil [1]. The unique, highly ordered pore structure, well defined pore diameter (2 to 30 nm), high surface area (1000m2/g) and different pore connectivities of these mesophases identify them as exceptional candidates for various applications such as facilitated separation, catalysis, sensors, low K dielectric material and bioreactors. To be able to harness the properties of these mesoporous materials, it is desired to synthesize these materials as membranes. One of the present challenges is to be able to synthesize these mesoporus membranes with the pore channel aligned perpendicular to the membrane surface. Numerous studies have been conducted on the synthesis of ordered mesoporous silica films on various dense and porous substrates. The most widely used methods are the casting and dip coating methods, both of which have been used successfully to make good quality membranes. However, the pores are either randomly aligned or aligned parallel to the surface of the substrates [2,3]. A combination of nonionic surfactants and polyamide coating has been shown to yield oriented mesoporous membranes, but the pores of these membranes run parallel to the surface [4]. Methods using magnetic fields [5] and shear flow [6] have been tried. Eutectic deposition of
364 SiO2–Fe2O3 system and chemical etching has also been shown to produce mesoporous silica [7] with perpendicular ordering. All these methods have yielded only partial vertical ordering. Mesoporous silica-anodic composite membranes have the desired vertical pore orientation, but still suffer from a number of drawbacks. SEM images show that these mesoporous silica plugs do not run the entire length of the support pores and there is a substantial gap between the plugs and the pores of the support [8]. Here we use a novel counter diffusion self assembly (CDSA) method for preparation of perpendicularly oriented mesoporous silica membranes. The CDSA method is an extension of the interfacial synthesis method for synthesizing ordered mesoporous silica fibers containing a large number of ordered, 3 nm pores aligned straight or helical across the fiber axis. In the interfacial method, the silica source is added as a thin layer on the surface of the liquid mixture of water, surfactant and acid. The silica source diffuses through the interface into the water phase where it undergoes hydrolysis and condensation around the surfactant micelles to form highly ordered mesoporous fibers under certain precisely tuned conditions [9, 10]. In the CDSA method, a porous support is placed at the interface separating the water phase and silica precursor phase. The precursors are expected to inter-diffuse through the support pores and the oil phase is expected condenses around the surfactant micelles on the support surface or with in the support pores and polymerizes to form silica plugs with hexagonally ordered pores within the support pores. The present study was focused on CDSA growth of short silica fibers (referred to as silica plugs) on supports of varying pore diameter (0.2 μm to 20 μm), morphologies (straight and tortuous pores) and surface chemistry (hydrophobic and hydrophilic). The structure and morphology of the silica plugs were studied using X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques. The various factors that effect the formation of ordered mesoporous plugs within the pores, to form a good quality membrane have been determined. 2. EXPERIMENTAL 2.1. Supports To study the effect of the pore size, shape and surface chemistry of the support on formation of mesoporous silica plugs, various supports were used having a size range of 0.2 μm-20 μm have been studied. Both tortuous and straight pore supports having different surface chemistries have been studied. Macroporous Į-alumina disks 20mm in diameter and 2 mm in thickness were prepared by pressing calcined alumina powder of different particles sizes (coded as A15 and A16 powder, Alcoa, TX) as reported earlier [12, 13].These discs have tortuous pores and a average pore diameter of 0.35 μm. Other tortuous pores supports studied where Borosilicate fine (BSF), Borosilicate medium (BSM) and quartz supports, which were obtained commercially (Chemglass, NJ).Their pore diameter was experimentally determined to be of 3.08 μm, 5.83 μm and 20.29 μm respectively. Surface modification of Į-alumina supports was conducted using octadecyltrichlorosilane (C18H37SiCl3) (coded as C18) and hexyltrichlorosilane (C6H13SiCl3) (coded as C6) (Gelest, PA) [9] to make them hydrophobic. The straight pore supports studies were nuclepore track-etched polycarbonate (hydrophobic and hydrophilic) membranes (Whatman Inc., NJ). The hydrophobic membranes have a diameter of 13 mm, are about 10μm thickness, and have a pore diameter of 8 μm. The hydrophilic membranes have are 25 mm in diameter, about 10 μm thick and have a pore diameter of 5 μm. They have a porosity of <15%.
365 2.2. Growth and Characterization of Mesoporous Membranes Mesoporous silica was grown on the support disk by the CDSA approach under the optimum conditions that were identified for growth of silica fibers [9]. Tetrabutylorthosilicate (C16H36O4Si coded as TBOS) and cetyltrimethylammonium bromide (C19H42NBr coded as CTAB) were used as the silica source and surfactant in presence of distilled water and HCl, at a typical molar ratio of 1: 0.025: 2.29: 100 respectively. We have three different synthesis conditions using the basic CDSA idea. Here we call these methods A, B and C. Methods A, B and C differ with respect to the height of TBOS used, as will be elucidated shortly. A Teflon holder was specially fabricated to hold the track etch polycarbonate membranes. This consisted of a cap where the polycarbonate support was placed and a tube that was screwed into it to hold the support in place. A schematic of the CDSA growth method and the experimental set up is shown in Fig. 1. Silicon source
Holder
Support Acidified Water + Surfactant
Acidified Water + Surfactant
Fig.1. (a)Schematic illustration of the counter diffusion self assembly (CDSA) growth of silica membrane on porous support, (b) enlarged view of the interfacial interaction locations(c) schematic of the set up showing H, height of TBOS and h, thickness of membrane
2.2.1 Method A: The holder, with the support, was placed at the surface of the aqueous solution of H2O: CTAB: HCl, with one surface of the support facing the solution. Then, TBOS was added inside the tube on the top of the other surface of the support. The height of the TBOS column is denoted by the letter H and the thickness of the membrane support is denoted by the letter h. Here H/h > 1 (typically about 10000). This was the synthesis method used to grow mesoporous silica in all the tortuous supports. 2.2.2 Method B: Numerous studies have indicated that evaporation has a very defining effect on the formation of specific micelle structures [14]. In the evaporation-induced self-assembly of silica– surfactant nanocomposites at substrate surfaces, it has been proposed that preferential evaporation of alcohol byproduct from the surface induces the formation of surfactant micelles and further organization of the mesophase. This idea was used to develop method B. In this method, the holder was tilted over so that the tube was in the water phase to hold the polycarbonate support in place. TBOS was added to the top surface of the support as a thin layer (H/h greater than 1, typically about 10 times).
366 2.2.3. Method C: The precise placement of the support at the interface is crucial to the CDSA method, as we expect the support to be placed at the reaction interface so that the silica can condense around the ordered micelles within the pore of the support membrane. Method C was used both to facilitate evaporation of butanol and to ensure precise placement of the support. Here the support was soaked in TBOS; hence acting as a reservoir for TBOS (H/h = 1). It was then placed at the interface where it floated for the entire duration of the experiment. Once the experiment was set up, precursor and the water phase containing surfactant were allowed to inter-diffuse through the support for 7 to 14 days. After growth, the membrane was dried in air at room temperature. The deposited silica membranes were characterized by SEM (Philips, XL-30) to study the morphology and XRD (Bruker D8, CuKĮ) to study the phase structure. The membrane quality of the tortuous supports was also characterized by gas permeation studies using steady state and unsteady state permeation. 3. RESULTS AND DISCUSSION 3.1 CDSA growth in tortuous supports The first step in this work was to apply the concept of CDSA for growth of nanostructured silica membranes on tortuous supports. The small angle XRD patterns on both sides of all the supports showed an amorphous response for both the hydrophilic and hydrophobic modified supports. Some hydrophilic supports showed discontinuous silica films having hexagonal pore structure on the surface in contact with the water phase. These supports were seen to have defects of size up to 5Pm, as seen by SEM. Single gas permeance studies were conducted on both the hydrophilic and hydrophobic supports to check for the presence of any silica plugs within the pores of the membranes. As shown in Table 1, the as-synthesized CDSA silica membranes on alumina supports (A16 and A15) exhibited a small reduction in permeance (between 1.3-10 times) relative to the original support, implying insufficient growth inside the support pores. The as-synthesized silica membrane on the large pore supports (BSF, BSM and quartz) exhibited 1 to 3 fold reduction in permeance, which reflects deposition of more silica inside the support pores. However, the growth was insufficient to completely seal the larger pores. The final surfactant-free silica membrane had high permeance values (>> 10-6 mol/s.m2.Pa) as a result of the free or incompletely plugged macropores. There was considerable improvement of silica growth with the use of C6- and C18modified alumina supports. Nitrogen permeance of the as-synthesized membrane was low (~ 10-9 mol/s.m2.Pa, close to the lower limit of the permeation apparatus) indicating that the support pores are filled by grown silica (Table 1). The membranes, after surfactant removal, had a nitrogen permeance of 4.10-4.50u10-8 mol/s.m2.Pa, which is about 20 folds smaller than that of the supports. Permeance of various gases at different temperatures on the C18modified alumina supports, exhibit of Knudsen type permeance behavior, thus confirming the mesoporous structure of the silica membrane. The results indicate that the small pored hydrophobic supports inhibit the transport of silica precursor and the surfactant and their subsequent micelle arrangement with the supports pore is hindered. Hence, silica plugs cannot form in the support pores and amorphous silica deposition is seen on the TBOS side due the transfer of only water through the hydrophilic pores of the support. The larger pore supports were seen to have better plugging, showing that the facilitated transport is one of the key factors for the growth of silica membranes by the CDSA approach. The hydrophobic modification of the support showed a much improved
367 membrane, as there is improved transport of the silica precursor through the pores of the support [11]. From the above results, it followed that use of large pore supports was highly desired. Also, our goal is to obtain straight pores perpendicular to the support surface. It is expected that if the surfactant is introduced inside the pores of the support, they will align parallel to the pore wall and lead to mesopores with an orientation analogous to the shape of the pore. With these two ideas in mind, the CDSA growth was tried in straight pore macroporous polycarbonate track etch membranes. Table 1 Single gas permeance of CDSA silica membranes on ceramic supports at 170 kPa feed pressure and 298 K Membrane Permeance (mol/s.m2.Pa) Pore size Sample (μm) Support As-synthesized Surfactant-free D-A15a
0.35
8.04×10-7
2.01×10-7
7.76×10-7
BSF b
3.08
9.44×10-5
9.36×10-6
2.19×10-5
BSM b
5.83
1.78×10-4
5.19×10-6
-
-7
-4
b
Quartz
C18-DA15 a a
21.29 0.35
1.96×10 7.20×10-7
1.14×10 0.07×10-7
0.45×10-7
Nitrogen gas permeance ,b Helium gas permeance
3.2 CDSA growth in straight pore supports 3.2.1. CDSA growth by Method A: Both hydrophobic and hydrophilic polycarbonate membranes where used for this study. Fig. 2 shows SEM micrographs of hydrophobic (Fig. 2b) and hydrophilic (Fig. 2c) polycarbonate supports after CDSA growth of silica plugs, and a fresh polycarbonate support is shown for comparison. As shown, silica plugs filled all the pores of both supports. With the CDSA method, we were able to grow plugs that ran the entire length of the support pore. The morphology is typical of MCM-41 fibers, as shown in Fig. 2d. However, the XRD studies showed no ordered peaks. It is not clear whether this is due to the small amount of silica (as the supports have less than 15% porosity)which was insufficient to deflect X-rays, or due the disordered nature of the silica plugs. The SEM micrographs also revealed that there is slight gap between the silica plug and the support pore. The gaps were most likely formed due to shrinkage upon drying of the plugs. It is seen that the gap between the plug and the pore is about 0.5μm in the case of the hydrophobic supports and about 1μm in the case of hydrophilic support. This is in keeping with our expectation that the hydrophobic support will give better quality membranes.
368
(a)
(c)
(b)
(d)
silica plug
Fig. 2. SEM of polycarbonate track etch supports showing complete plugging of the support by Method A. (a) Top view of fresh track etch membrane before CDSA growth. (b) Top view of hydrophobic membrane after CDSA growth, showing complete plugging (c) Top view of hydrophilic membrane after CDSA growth, showing complete plugging, (d) Cross sectional view of membrane showing silica plug running through the length of the pore of the membrane
3.2.2 CDSA growth by Method B: A thin layer of ordered mesoporous silica was deposited on the surface of the support in contact with the water phase, as was confirmed by small angle XRD. SEM micrographs show that only about 10% of pores are filled with the silica plugs for the hydrophobic membrane support, but there was no plugging in the case of the hydrophilic membranes. The deposition of particles on the support and not within the support pores is probably due to poor alignment of the polycarbonate membrane support at the surface of the water phase during synthesis. Since the height of the water phase is adjusted to the height at which the polycarbonate support is held, it is possible that the excess water phase was added. This moves the reaction interface above the point where the support is held. Consequently, the reaction will take place, not on the surface of the polycarbonate support, but some distance above the surface of the support. This would lead to particles being deposited on the surface of the polycarbonate support and not within the pores of the support. 3.2.3. CDSA growth by Method C: It is observed that the membranes synthesized from Method C have a thin film on the surface of the polycarbonate support on the TBOS side. The SEM micrographs reveal that 80-90% of the pores are plugged in the case of hydrophobic supports, as seen in Fig. 3a. The presence of mesoporous silica is verified by small angle XRD data shown in Fig. 3a. The hydrophilic support exhibits heavy particulate deposition and no plugs, as seen in Fig. 3b. The plugs in the hydrophobic support seem to be well formed and uniform. The absence of plugging of all the pores of the support can be due to incomplete saturation of pores of the support with TBOS when being placed at the interface. Additionally, these plugs seem to be attached to the thin
369 film on top of the support. The accidental removal of the thin film during preparation of sample for SEM studies might have also resulted in the removal of these plugs.
(a)
(b)
Fig. 3. SEM and XRD of polycarbonate track etch supports showing membranes after CDSA growth by Method C. (a) Hydrophobic support showing plugging, (b) hydrophilic support showing particulate deposition
It is seen that the CDSA growth method yields good quality silica plugs in the case of hydrophobic supports by methods A and C. The lack of plugs by method B could be due to miss alignment of the polycarbonate support at the interface. Method A also yielded good quality silica plugs in the hydrophilic polycarbonate membrane support. 4. CONCLUSION The counter diffusion self assembly (CDSA) approach is effective in synthesizing mesoporous silica membranes. CDSA growth did not give a high quality membrane on tortuous hydrophilic alumina supports of 0.35 Pm pore diameter, due to inhibited transfer of the hydrophobic precursors through the pores. The modification of the support with hydrophobic organic groups improved the quality of the membrane. Gas permeance of the good quality silica membranes is of Knudsen type (0.45u10-7 mol N2/s.m2.Pa) confirming the mesoporous structure of the silica membrane. Preliminary results on straight pore polycarbonate track etch membranes have shown that good quality membranes can be synthesized with the CDSA growth approach. The key factor for membrane formation is the facilitated transport of precursors. Macroporous straight pore supports facilitate diffusion of the hydrophobic silica precursor and the amphiphilic surfactant leading to formation of ordered mesoporus structure with in support pores. The surface chemistry of the polycarbonate supports does not play a significant role in the formation of the plugs as long as the support pore is large enough to allow precursor transport. The structure of the silica plugs formed is influenced by the morphology of the pore. ACKNOWLEDGEMENT The authors are grateful to the Petroleum Research Fund, administrated by the American Chemical Society, for the support on this project (grant 42086-AC5).
370 REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
C. T. Kresge, M. E Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. S. H. Zhong, C. F. Li, Q. Li and X. F. Xiao, Sep. Purif. Technol., 32 (2003) 17. L. Huang, S. Kawi, K. Hidajat and S. C, Ng, Microporous Mesoporous Mater.,88 (2006) 254. H. Miyata,T. Noma, M. Watanabe and K Kuroda, Chem. Mater.,14, (2002),766 Y. Yamauchi, M. Sawada, T. Noma, H. Ito, S. Furumi, Y. Sakka and K. Kuroda, J. Mater. Chem., 15 (2005) 1137. H. W. Hillhouse, T. Okubo, J. W. van Egmond and M. Tsapatsis, Chem. Mater., 9 (1997) 1505. J. Otomo, S. Wang, H. Takahashi and H. Nagamoto, J. Membr. Sci.,279 (2006) 256. A. Yamaguchi, F. Uejo, T. Yoda, T. Uchida, T. Tanamura, T. Yamashita and N. Teramae, Nature Mater., 3 (2004) 337. H. M. Alsyouri and Y. S. Lin, Chem. Mater., 15 (2003) 2033. H. M. Alsyouri and J. Y. S. Lin, J. Phys. Chem. B, 109 (2005) 13623. H. M. Alsyouri , D. Li, Y. S. Lin, Z. Ye and S. P. Zhu, J. Membr. Sci., 282 ,( 2006) 266 H. M. Alsyouri, Ph. D. Dissertation, University of Cincinnati, Cincinnati, OH, 2004, 19. Y. S. Lin and A. J. Burggraaf, J. Membr. Sci., 79 (1993) 65. David Grosso, Florence Cagnol, Galo J. de A. A. Soler-Illia, Eduardo L. Crepaldi, Heinz Amenitsch, Aline Brunet-Bruneau, Alexi Bourgeois and Clement Sanchez, Adv. Funct. Mater., 14 (2004) 309.
363
Counter diffusion self assembly synthesis of orderedmesoporous silica membranes Shriya K. Seshadri a, Hatem M. Alsyouri b and Y. S. Lin a* a
Department of Chemical Engineering, Arizona State University, Tempe, AZ 85287, USA. Email: [email protected] b Industrial Chemistry Center, Royal Scientific Society, P.O. Box 1438, Amman 11941, Jordan
ABSTRACT We report the synthesis of ordered mesoporous silica membranes in macroporous supports by a novel acid catalyzed counter diffusion self assembly (CDSA) method. Effect of pore size, morphology and surface chemistry of the support on the formation of oriented mesoporous silica plugs has been studied. Hydrophobically modified small pore alumina supports (0.3Pm) show good quality silica membranes as sub-millimeter plugs. Preliminary observations on straight pored hydrophilic and hydrophobic track etch polycarbonate membranes supports (pore diameters 5 Pm and 8 Pm respectively) indicate the formation of ordered mesoporous silica plugs within the support pores, as seen by results of scanning electron microscopy and X-ray diffraction studies. Experimental results suggest that the pore size and morphology of the support play a vital role in the formation of these ordered mesoporous silica plugs, while surface chemistry seems affect the quality of the plugs formed. 1. INTRODUCTION Ordered mesoporous materials have been the focus of extensive research since they were first reported by the researchers of Mobil [1]. The unique, highly ordered pore structure, well defined pore diameter (2 to 30 nm), high surface area (1000m2/g) and different pore connectivities of these mesophases identify them as exceptional candidates for various applications such as facilitated separation, catalysis, sensors, low K dielectric material and bioreactors. To be able to harness the properties of these mesoporous materials, it is desired to synthesize these materials as membranes. One of the present challenges is to be able to synthesize these mesoporus membranes with the pore channel aligned perpendicular to the membrane surface. Numerous studies have been conducted on the synthesis of ordered mesoporous silica films on various dense and porous substrates. The most widely used methods are the casting and dip coating methods, both of which have been used successfully to make good quality membranes. However, the pores are either randomly aligned or aligned parallel to the surface of the substrates [2,3]. A combination of nonionic surfactants and polyamide coating has been shown to yield oriented mesoporous membranes, but the pores of these membranes run parallel to the surface [4]. Methods using magnetic fields [5] and shear flow [6] have been tried. Eutectic deposition of
364 SiO2–Fe2O3 system and chemical etching has also been shown to produce mesoporous silica [7] with perpendicular ordering. All these methods have yielded only partial vertical ordering. Mesoporous silica-anodic composite membranes have the desired vertical pore orientation, but still suffer from a number of drawbacks. SEM images show that these mesoporous silica plugs do not run the entire length of the support pores and there is a substantial gap between the plugs and the pores of the support [8]. Here we use a novel counter diffusion self assembly (CDSA) method for preparation of perpendicularly oriented mesoporous silica membranes. The CDSA method is an extension of the interfacial synthesis method for synthesizing ordered mesoporous silica fibers containing a large number of ordered, 3 nm pores aligned straight or helical across the fiber axis. In the interfacial method, the silica source is added as a thin layer on the surface of the liquid mixture of water, surfactant and acid. The silica source diffuses through the interface into the water phase where it undergoes hydrolysis and condensation around the surfactant micelles to form highly ordered mesoporous fibers under certain precisely tuned conditions [9, 10]. In the CDSA method, a porous support is placed at the interface separating the water phase and silica precursor phase. The precursors are expected to inter-diffuse through the support pores and the oil phase is expected condenses around the surfactant micelles on the support surface or with in the support pores and polymerizes to form silica plugs with hexagonally ordered pores within the support pores. The present study was focused on CDSA growth of short silica fibers (referred to as silica plugs) on supports of varying pore diameter (0.2 μm to 20 μm), morphologies (straight and tortuous pores) and surface chemistry (hydrophobic and hydrophilic). The structure and morphology of the silica plugs were studied using X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques. The various factors that effect the formation of ordered mesoporous plugs within the pores, to form a good quality membrane have been determined. 2. EXPERIMENTAL 2.1. Supports To study the effect of the pore size, shape and surface chemistry of the support on formation of mesoporous silica plugs, various supports were used having a size range of 0.2 μm-20 μm have been studied. Both tortuous and straight pore supports having different surface chemistries have been studied. Macroporous Į-alumina disks 20mm in diameter and 2 mm in thickness were prepared by pressing calcined alumina powder of different particles sizes (coded as A15 and A16 powder, Alcoa, TX) as reported earlier [12, 13].These discs have tortuous pores and a average pore diameter of 0.35 μm. Other tortuous pores supports studied where Borosilicate fine (BSF), Borosilicate medium (BSM) and quartz supports, which were obtained commercially (Chemglass, NJ).Their pore diameter was experimentally determined to be of 3.08 μm, 5.83 μm and 20.29 μm respectively. Surface modification of Į-alumina supports was conducted using octadecyltrichlorosilane (C18H37SiCl3) (coded as C18) and hexyltrichlorosilane (C6H13SiCl3) (coded as C6) (Gelest, PA) [9] to make them hydrophobic. The straight pore supports studies were nuclepore track-etched polycarbonate (hydrophobic and hydrophilic) membranes (Whatman Inc., NJ). The hydrophobic membranes have a diameter of 13 mm, are about 10μm thickness, and have a pore diameter of 8 μm. The hydrophilic membranes have are 25 mm in diameter, about 10 μm thick and have a pore diameter of 5 μm. They have a porosity of <15%.
365 2.2. Growth and Characterization of Mesoporous Membranes Mesoporous silica was grown on the support disk by the CDSA approach under the optimum conditions that were identified for growth of silica fibers [9]. Tetrabutylorthosilicate (C16H36O4Si coded as TBOS) and cetyltrimethylammonium bromide (C19H42NBr coded as CTAB) were used as the silica source and surfactant in presence of distilled water and HCl, at a typical molar ratio of 1: 0.025: 2.29: 100 respectively. We have three different synthesis conditions using the basic CDSA idea. Here we call these methods A, B and C. Methods A, B and C differ with respect to the height of TBOS used, as will be elucidated shortly. A Teflon holder was specially fabricated to hold the track etch polycarbonate membranes. This consisted of a cap where the polycarbonate support was placed and a tube that was screwed into it to hold the support in place. A schematic of the CDSA growth method and the experimental set up is shown in Fig. 1. Silicon source
Holder
Support Acidified Water + Surfactant
Acidified Water + Surfactant
Fig.1. (a)Schematic illustration of the counter diffusion self assembly (CDSA) growth of silica membrane on porous support, (b) enlarged view of the interfacial interaction locations(c) schematic of the set up showing H, height of TBOS and h, thickness of membrane
2.2.1 Method A: The holder, with the support, was placed at the surface of the aqueous solution of H2O: CTAB: HCl, with one surface of the support facing the solution. Then, TBOS was added inside the tube on the top of the other surface of the support. The height of the TBOS column is denoted by the letter H and the thickness of the membrane support is denoted by the letter h. Here H/h > 1 (typically about 10000). This was the synthesis method used to grow mesoporous silica in all the tortuous supports. 2.2.2 Method B: Numerous studies have indicated that evaporation has a very defining effect on the formation of specific micelle structures [14]. In the evaporation-induced self-assembly of silica– surfactant nanocomposites at substrate surfaces, it has been proposed that preferential evaporation of alcohol byproduct from the surface induces the formation of surfactant micelles and further organization of the mesophase. This idea was used to develop method B. In this method, the holder was tilted over so that the tube was in the water phase to hold the polycarbonate support in place. TBOS was added to the top surface of the support as a thin layer (H/h greater than 1, typically about 10 times).
366 2.2.3. Method C: The precise placement of the support at the interface is crucial to the CDSA method, as we expect the support to be placed at the reaction interface so that the silica can condense around the ordered micelles within the pore of the support membrane. Method C was used both to facilitate evaporation of butanol and to ensure precise placement of the support. Here the support was soaked in TBOS; hence acting as a reservoir for TBOS (H/h = 1). It was then placed at the interface where it floated for the entire duration of the experiment. Once the experiment was set up, precursor and the water phase containing surfactant were allowed to inter-diffuse through the support for 7 to 14 days. After growth, the membrane was dried in air at room temperature. The deposited silica membranes were characterized by SEM (Philips, XL-30) to study the morphology and XRD (Bruker D8, CuKĮ) to study the phase structure. The membrane quality of the tortuous supports was also characterized by gas permeation studies using steady state and unsteady state permeation. 3. RESULTS AND DISCUSSION 3.1 CDSA growth in tortuous supports The first step in this work was to apply the concept of CDSA for growth of nanostructured silica membranes on tortuous supports. The small angle XRD patterns on both sides of all the supports showed an amorphous response for both the hydrophilic and hydrophobic modified supports. Some hydrophilic supports showed discontinuous silica films having hexagonal pore structure on the surface in contact with the water phase. These supports were seen to have defects of size up to 5Pm, as seen by SEM. Single gas permeance studies were conducted on both the hydrophilic and hydrophobic supports to check for the presence of any silica plugs within the pores of the membranes. As shown in Table 1, the as-synthesized CDSA silica membranes on alumina supports (A16 and A15) exhibited a small reduction in permeance (between 1.3-10 times) relative to the original support, implying insufficient growth inside the support pores. The as-synthesized silica membrane on the large pore supports (BSF, BSM and quartz) exhibited 1 to 3 fold reduction in permeance, which reflects deposition of more silica inside the support pores. However, the growth was insufficient to completely seal the larger pores. The final surfactant-free silica membrane had high permeance values (>> 10-6 mol/s.m2.Pa) as a result of the free or incompletely plugged macropores. There was considerable improvement of silica growth with the use of C6- and C18modified alumina supports. Nitrogen permeance of the as-synthesized membrane was low (~ 10-9 mol/s.m2.Pa, close to the lower limit of the permeation apparatus) indicating that the support pores are filled by grown silica (Table 1). The membranes, after surfactant removal, had a nitrogen permeance of 4.10-4.50u10-8 mol/s.m2.Pa, which is about 20 folds smaller than that of the supports. Permeance of various gases at different temperatures on the C18modified alumina supports, exhibit of Knudsen type permeance behavior, thus confirming the mesoporous structure of the silica membrane. The results indicate that the small pored hydrophobic supports inhibit the transport of silica precursor and the surfactant and their subsequent micelle arrangement with the supports pore is hindered. Hence, silica plugs cannot form in the support pores and amorphous silica deposition is seen on the TBOS side due the transfer of only water through the hydrophilic pores of the support. The larger pore supports were seen to have better plugging, showing that the facilitated transport is one of the key factors for the growth of silica membranes by the CDSA approach. The hydrophobic modification of the support showed a much improved
367 membrane, as there is improved transport of the silica precursor through the pores of the support [11]. From the above results, it followed that use of large pore supports was highly desired. Also, our goal is to obtain straight pores perpendicular to the support surface. It is expected that if the surfactant is introduced inside the pores of the support, they will align parallel to the pore wall and lead to mesopores with an orientation analogous to the shape of the pore. With these two ideas in mind, the CDSA growth was tried in straight pore macroporous polycarbonate track etch membranes. Table 1 Single gas permeance of CDSA silica membranes on ceramic supports at 170 kPa feed pressure and 298 K Membrane Permeance (mol/s.m2.Pa) Pore size Sample (μm) Support As-synthesized Surfactant-free D-A15a
0.35
8.04×10-7
2.01×10-7
7.76×10-7
BSF b
3.08
9.44×10-5
9.36×10-6
2.19×10-5
BSM b
5.83
1.78×10-4
5.19×10-6
-
-7
-4
b
Quartz
C18-DA15 a a
21.29 0.35
1.96×10 7.20×10-7
1.14×10 0.07×10-7
0.45×10-7
Nitrogen gas permeance ,b Helium gas permeance
3.2 CDSA growth in straight pore supports 3.2.1. CDSA growth by Method A: Both hydrophobic and hydrophilic polycarbonate membranes where used for this study. Fig. 2 shows SEM micrographs of hydrophobic (Fig. 2b) and hydrophilic (Fig. 2c) polycarbonate supports after CDSA growth of silica plugs, and a fresh polycarbonate support is shown for comparison. As shown, silica plugs filled all the pores of both supports. With the CDSA method, we were able to grow plugs that ran the entire length of the support pore. The morphology is typical of MCM-41 fibers, as shown in Fig. 2d. However, the XRD studies showed no ordered peaks. It is not clear whether this is due to the small amount of silica (as the supports have less than 15% porosity)which was insufficient to deflect X-rays, or due the disordered nature of the silica plugs. The SEM micrographs also revealed that there is slight gap between the silica plug and the support pore. The gaps were most likely formed due to shrinkage upon drying of the plugs. It is seen that the gap between the plug and the pore is about 0.5μm in the case of the hydrophobic supports and about 1μm in the case of hydrophilic support. This is in keeping with our expectation that the hydrophobic support will give better quality membranes.
368
(a)
(c)
(b)
(d)
silica plug
Fig. 2. SEM of polycarbonate track etch supports showing complete plugging of the support by Method A. (a) Top view of fresh track etch membrane before CDSA growth. (b) Top view of hydrophobic membrane after CDSA growth, showing complete plugging (c) Top view of hydrophilic membrane after CDSA growth, showing complete plugging, (d) Cross sectional view of membrane showing silica plug running through the length of the pore of the membrane
3.2.2 CDSA growth by Method B: A thin layer of ordered mesoporous silica was deposited on the surface of the support in contact with the water phase, as was confirmed by small angle XRD. SEM micrographs show that only about 10% of pores are filled with the silica plugs for the hydrophobic membrane support, but there was no plugging in the case of the hydrophilic membranes. The deposition of particles on the support and not within the support pores is probably due to poor alignment of the polycarbonate membrane support at the surface of the water phase during synthesis. Since the height of the water phase is adjusted to the height at which the polycarbonate support is held, it is possible that the excess water phase was added. This moves the reaction interface above the point where the support is held. Consequently, the reaction will take place, not on the surface of the polycarbonate support, but some distance above the surface of the support. This would lead to particles being deposited on the surface of the polycarbonate support and not within the pores of the support. 3.2.3. CDSA growth by Method C: It is observed that the membranes synthesized from Method C have a thin film on the surface of the polycarbonate support on the TBOS side. The SEM micrographs reveal that 80-90% of the pores are plugged in the case of hydrophobic supports, as seen in Fig. 3a. The presence of mesoporous silica is verified by small angle XRD data shown in Fig. 3a. The hydrophilic support exhibits heavy particulate deposition and no plugs, as seen in Fig. 3b. The plugs in the hydrophobic support seem to be well formed and uniform. The absence of plugging of all the pores of the support can be due to incomplete saturation of pores of the support with TBOS when being placed at the interface. Additionally, these plugs seem to be attached to the thin
369 film on top of the support. The accidental removal of the thin film during preparation of sample for SEM studies might have also resulted in the removal of these plugs.
(a)
(b)
Fig. 3. SEM and XRD of polycarbonate track etch supports showing membranes after CDSA growth by Method C. (a) Hydrophobic support showing plugging, (b) hydrophilic support showing particulate deposition
It is seen that the CDSA growth method yields good quality silica plugs in the case of hydrophobic supports by methods A and C. The lack of plugs by method B could be due to miss alignment of the polycarbonate support at the interface. Method A also yielded good quality silica plugs in the hydrophilic polycarbonate membrane support. 4. CONCLUSION The counter diffusion self assembly (CDSA) approach is effective in synthesizing mesoporous silica membranes. CDSA growth did not give a high quality membrane on tortuous hydrophilic alumina supports of 0.35 Pm pore diameter, due to inhibited transfer of the hydrophobic precursors through the pores. The modification of the support with hydrophobic organic groups improved the quality of the membrane. Gas permeance of the good quality silica membranes is of Knudsen type (0.45u10-7 mol N2/s.m2.Pa) confirming the mesoporous structure of the silica membrane. Preliminary results on straight pore polycarbonate track etch membranes have shown that good quality membranes can be synthesized with the CDSA growth approach. The key factor for membrane formation is the facilitated transport of precursors. Macroporous straight pore supports facilitate diffusion of the hydrophobic silica precursor and the amphiphilic surfactant leading to formation of ordered mesoporus structure with in support pores. The surface chemistry of the polycarbonate supports does not play a significant role in the formation of the plugs as long as the support pore is large enough to allow precursor transport. The structure of the silica plugs formed is influenced by the morphology of the pore. ACKNOWLEDGEMENT The authors are grateful to the Petroleum Research Fund, administrated by the American Chemical Society, for the support on this project (grant 42086-AC5).
370 REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
C. T. Kresge, M. E Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. S. H. Zhong, C. F. Li, Q. Li and X. F. Xiao, Sep. Purif. Technol., 32 (2003) 17. L. Huang, S. Kawi, K. Hidajat and S. C, Ng, Microporous Mesoporous Mater.,88 (2006) 254. H. Miyata,T. Noma, M. Watanabe and K Kuroda, Chem. Mater.,14, (2002),766 Y. Yamauchi, M. Sawada, T. Noma, H. Ito, S. Furumi, Y. Sakka and K. Kuroda, J. Mater. Chem., 15 (2005) 1137. H. W. Hillhouse, T. Okubo, J. W. van Egmond and M. Tsapatsis, Chem. Mater., 9 (1997) 1505. J. Otomo, S. Wang, H. Takahashi and H. Nagamoto, J. Membr. Sci.,279 (2006) 256. A. Yamaguchi, F. Uejo, T. Yoda, T. Uchida, T. Tanamura, T. Yamashita and N. Teramae, Nature Mater., 3 (2004) 337. H. M. Alsyouri and Y. S. Lin, Chem. Mater., 15 (2003) 2033. H. M. Alsyouri and J. Y. S. Lin, J. Phys. Chem. B, 109 (2005) 13623. H. M. Alsyouri , D. Li, Y. S. Lin, Z. Ye and S. P. Zhu, J. Membr. Sci., 282 ,( 2006) 266 H. M. Alsyouri, Ph. D. Dissertation, University of Cincinnati, Cincinnati, OH, 2004, 19. Y. S. Lin and A. J. Burggraaf, J. Membr. Sci., 79 (1993) 65. David Grosso, Florence Cagnol, Galo J. de A. A. Soler-Illia, Eduardo L. Crepaldi, Heinz Amenitsch, Aline Brunet-Bruneau, Alexi Bourgeois and Clement Sanchez, Adv. Funct. Mater., 14 (2004) 309.