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Altering the Crystal Morphology of Silicalite-1 through Microemulsion-Based Synthesis J-C. Lin and M. Z. Yates* Department of Chemical Engineering and Laboratory for Laser Energetics, University of Rochester, Rochester, New York 14627 Received October 28, 2004. In Final Form: January 25, 2005 The crystal morphology of silicalite-1 was adjusted through a microemulsion-based hydrothermal synthesis. The surfactant cetyltrimethylammonium bromide (CTAB) with cosurfactant butanol was used to form water-in-oil microemulsions containing the silicalite-1 synthesis gel. The crystal morphology of silicalite-1 was adjusted from coffin-shaped to novel rod-shaped and to irregular-shaped nanoparticles by varying the microemulsion composition. Silicalite-1 synthesized in the microemulsion has a smaller size and a more narrow size distribution than that produced by conventional synthesis without the microemulsion. The novel morphology of silicalite-1 may facilitate assembly into films and find applications in separation and catalysis.
Introduction Silicalite-1 is a porous silica crystal that has two interconnected channel systems with pore diameters of around 0.55 nm.1,2 It is the all-silica analogue to the aluminosilicate zeolite ZSM-5. The sub-nanometer-scale pores make silicalite-1 attractive for molecular sieving membranes,3 catalysis,4 and adsorption.5 Silicalite-1 holds promise as a low dielectric constant material for microelectronics.6,7 When the pores of the crystals are filled with optically functional molecules, they display nonlinear optical properties due to restricted rotation of the guest molecules in the pores.8,9 Many advanced applications of silicalite-1 and other zeolites and molecular sieves require control of pore orientation and the controlled assembly of small crystals to create functional devices. In these advanced applications, the control of crystal size and shape is critical.10 The synthesis of silicalite-1 typically occurs by hydrothermal crystallization in the presence of an organic structure directing agent. A variety of silica precursors and structure directing agents may be employed for silicalite-1 synthesis.11,12 Many investigations have examined the effect of synthesis conditions on crystallization rate, crystal size, or the recrystallization of different silicates to silicalite-1.13-15 Relatively few silicalite-1 synthesis studies have attempted to control crystal morphology and size.3,16 In some cases, the choice of organic (1) Kokotailo, G. T.; Lawton, S. L.; Olson, D. H. Nature 1978, 272, 437-438. (2) Olson, D. H.; Kokotailo, G. T.; Lawton, S. L.; Meler, W. M. J. Phys. Chem. 1981, 85, 2238-2243. (3) Lai, Z.; Bonilla, G.; Diaz, I.; Nery, J. G.; Sujaoti, K.; Amat, M. A.; Kokkoli, E.; Terasaki, O.; Thompson, R. W.; Tsapatsis, M.; Vlachos, D. G. Science 2003, 300, 456-460. (4) Flego, C.; Dalloro, L. Microporous Mesoporous Mater. 2003, 60, 263-271. (5) Sivasankar, N.; Vasudevan, S. Catal. Lett. 2004, 97, 53-58. (6) Wang, Z. B.; Mitra, A.; Wang, H. T.; Huang, L. M.; Yan, Y. H. Adv. Mater. 2001, 13, 1463. (7) Wang, Z. B.; Wang, H. T.; Mitra, A.; Huang, L. M.; Yan, Y. S. Adv. Mater. 2001, 13, 746. (8) Gao, F.; Zhu, G.; Chen, Y.; Li, Y.; Qiu, S. J. Phys. Chem. B 2004, 108, 3426-3430. (9) Kim, H. S.; Lee, S. M.; Ha, K.; Jung, C.; Lee, Y.-J.; Chun, Y. S.; Kim, D.; Rhee, B. K.; Yoon, K. B. J. Am. Chem. Soc. 2004, 126, 673682. (10) Davis, M. E. Nature 2002, 417, 813-821. (11) Kida, T.; Kojima, K.; Ohnishi, H.; Guan, G.; Yoshida, A. Ceram. Int. 2004, 30, 727-732. (12) de Moor, P.-P. E. A.; Beelen, T. P. M.; van Santen, R. A.; Beck, L. W. B.; Davis, M. E. J. Phys. Chem. B 2000, 104, 7600-7611.
structure directing agent determines the crystal morphology.12,17,18 Other compounds may also be added to the synthesis gel to alter crystallization. For example, benzene1,2-diol added to the synthesis gel of silicalite-1 results in larger crystals due to the formation of a silicon-benzene1,2-diol complex that leads to fewer nuclei in the system.16 One promising approach that can be used to control both the morphology and size of microporous materials is microemulsion-based synthesis.19-23 Confinement of the synthesis gel to the small microemulsion droplets influences crystal nucleation, and preferential adsorption of surfactant onto certain faces of the growing crystal can influence crystal shape.21,22 Silicalite-1 synthesized in a bicontinuous anionic microemulsion was shown to produce twinned crystals.24 More recently, crystallization of silicalite-1 within nonionic microemulsions resulted in silicalite-1 in spherical or platelike morphology, depending on the synthesis conditions.20 In our previous work, we have demonstrated that cationic microemulsions can be used to alter the crystal size and shape of the molecular sieve aluminum phosphate 5 (AlPO4-5).21-23 In the present study, we show that the synthesis of silicalite-1 in a cationic microemulsion results in a significant alteration in crystal size and morphology. The results demonstrate the potential for microemulsions to control the crystal morphology of a variety of zeolites and molecular sieves. Results and Discussion The silicalite-1 synthesis was investigated using a standard gel consisting of ammonium fluoride, tetrapropylammonium bromide (TPABr), water, and colloidal silica (13) Crea, F.; Nastro, A.; Nagy, J. B.; Aiello, R. Zeolite 1988, 8, 262267. (14) Feng, F. X.; Balkus, K. J. Microporous Mesoporous Mater. 2004, 69, 85-96. (15) Valtchev, V. P.; Faust, A. C.; Lezervant, J. Microporous Mesoporous Mater. 2004, 68, 91-95. (16) Shao, C. L.; Li, X. T.; Qiu, S. L.; Xiao, F. S.; Terasaki, O. Microporous Mesoporous Mater. 2000, 39, 117-123. (17) Kuhl, G. H.; Hill, C. U.S. Patent 4,585,638, 1986. (18) Beck, L. W.; Davis, M. E. Microporous Mesoporous Mater. 1998, 22, 107-114. (19) Dutta, P. K.; Jakupca, M.; Reddy, K. S. N.; Salvatl, L. Nature 1995, 374, 44-46. (20) Lee, S.; Shantz, D. F. Chem. Commun. 2004, 680-681. (21) Lin, J-C.; Dipre, J. T.; Yates, M. Z. Chem. Mater. 2003, 15, 27642773. (22) Lin, J-C.; Dipre, J. T.; Yates, M. Z. Langmuir 2004, 20, 10391042. (23) Yates, M. Z.; Ott, K. C.; Birnbaum, E. R.; McCleskey, T. M. Angew. Chem., Int. Ed. 2002, 41, 476-478.
10.1021/la0473456 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/16/2005
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Figure 1. Single phase diagrams for microemulsion formation at room temperature with the aqueous phase consisting of silicalite-1 synthesis gel and the surfactant phase consisting of surfactant CTAB and cosurfactant 1-butanol in a mass ratio of 2:1. The regions enclosed by lines on the phase diagram are compositions that form single phase microemulsions. The solid line is for an aqueous gel with the standard amount of tetrapropylammonium bromide (TPABr), and the dashed line is for double the amount of TPABr. Compositions A-C are different microemulsion compositions chosen for hydrothermal synthesis and are discussed in detail in the text.
in the molar ratio of 1:0.2:40:1. The gel composition was based on a synthesis method from the literature.25 The synthesis gel was used as the aqueous phase for forming microemulsions. Toluene was the oil phase. The surfactant phase was comprised of cetyltrimethylammonium bromide (CTAB) and n-butanol as cosurfactant in a mass ratio of 2:1. The microemulsion was treated as a pseudoternary system with oil, “surfactant”, and “aqueous” components, and phase diagrams were determined as described previously.21-23 The phase diagrams were used as a guide to select different microemulsion compositions for hydrothermal synthesis. According to our previous studies, a higher concentration of structure directing agent (SDA) is required for microemulsion synthesis than for bulk hydrothermal synthesis. Therefore, the phase diagram was also measured with double the molar ratio of the SDA tetrapropylammonium bromide (0.4 mol of TPABr/mol of ammonium fluoride). For selected compositions, hydrothermal synthesis was conducted in a Teflon-lined vessel at 180 °C for 6 days with agitation (see the Supporting Information for the detailed Experimental Section). Figure 1 shows two phase diagrams for the microemulsion formed with the surfactant phase of CTAB and butanol in a 2:1 weight ratio. Regions on the phase diagram enclosed by lines are compositions that result in the formation of an optically transparent single phase microemulsion. The solid line is for the microemulsion with an aqueous phase containing the standard molar ratio of SDA, and the dashed line is for double the molar ratio of SDA. Three microemulsion compositions chosen for hydrothermal synthesis are marked as A, B, and C and will be discussed in detail below. The single phase regions on the phase diagrams are similar for standard and double the molar ratio of SDA. Doubling the molar ratio of TPABr increases the ionic strength of the aqueous phase. A higher ionic strength can affect microemulsion phase behavior by screening electrostatic repulsion between ionic surfactants. However, in the range of adjustment made by altering the TPABr molar ratio, the effect of ionic strength on the microemulsion phase behavior is small. Hydrothermal synthesis was conducted using the CTAB microemulsion at compositions A, B, and C marked in Figure 1. Silicalite-1 is produced at all three compositions (24) Manna, A.; Kulkarni, B. D.; Ahedi, R. K.; Bhaumik, A.; Kotasthane, A. N. J. Colloid Interface Sci. 1999, 213, 405-411. (25) Robson, H. Microporous Mesoporous Mater. 1998, 22, 628.
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both with the standard molar ratio of SDA and with double the molar ratio of SDA. The morphology of the crystals was significantly altered and found to depend on the microemulsion composition, as shown in Figure 2 for selected compositions. The full result of all compositions with standard and double the molar ratio of SDA is in the Supporting Information. Parts A and B of Figure 2 show crystals that were synthesized from compositions A (32 wt % toluene, 49 wt % CTAB/butanol, and 19 wt % synthesis gel) and B (52 wt % toluene, 34 wt % CTAB/ butanol, and 14 wt % synthesis gel), respectively, on the phase diagram with the standard amount of SDA. For composition A, the crystals are in a typical coffin morphology of silicalite-1 with a size of about 20 µm in length and 2 µm in width. Composition B results in a rod-shaped crystal morphology with sizes of about 20 µm in length and 2-3 µm in diameter. The morphology reverts back to the typical coffin shape when moving to composition C with the standard molar ratio of SDA (Supporting Information). With double the molar ratio of SDA, composition C (74 wt % toluene, 15 wt % CTAB/butanol, and 11 wt % synthesis gel) results in irregular-shaped silicalite-1 nanoparticles about 50 nm in width (Figure 2C). For comparison, Figure 2D shows silicalite-1 synthesized in a control experiment without the microemulsion using the standard amount of SDA. The crystals synthesized without the microemulsion are coffin shaped and have a large size distribution. The size range was 20-100 µm in length and 2-10 µm in thickness, with the majority of the product being large crystals. For the control experiment with twice the molar ratio of SDA (not shown), the crystal morphology is the same as with the standard molar ratio of SDA but the crystal size is larger (up to 200 µm in length and 20 µm in thickness). The scanning electron microscopy (SEM) images reveal that the microemulsion-based synthesis results in a smaller crystal size and a more narrow crystal size distribution than the control experiment. The particles shown in Figure 2A and B appear similar at lower magnification. However, close-up images of the crystals reveal distinctly different crystal morphologies. Figure 3A is a close-up image of a crystal from the same sample in Figure 2A. The close-up image reveals the faceted end of the coffin-shaped crystals. The rod-shaped crystals shown in Figure 2B appear rounded at a higher magnification, as illustrated in Figure 3B. A comparison of the images in Figures 2 and 3 demonstrates that the microemulsion clearly alters the crystal size and morphology. All products were examined by powder X-ray diffraction of the as-synthesized crystals. The diffraction patterns shown in Figure 4A-D correspond to the samples shown in Figure 2A-D, respectively. The diffraction patterns are all consistent with the MFI crystal structure of silicalite-1 and appear the same as the pattern from a sample synthesized by the same recipe.26 The broad peak around 20-25° in Figure 4B and C indicates some amorphous material is present. For Figure 4B and C, there is a notable enhancement in the reflected intensity of the peak at the angle 2Θ of 18.34°. The peak is present in all samples, including the control experiment, but the relative intensity varies. The enhancement of the peak intensity indicates an increase in the relative number of X-ray reflections from a specific set of planes as compared to the other crystal planes. Such enhancement can arise from the alteration in crystal morphology resulting in a relative increase in the number of planes parallel to the sample (26) http://www.iza-synthesis.org/Recipes/XRD/Silicalite-1.jpg.
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Figure 2. Silicalite-1 produced from different microemulsion compositions. Parts A-C correspond to compositions A-C in Figure 1. Parts A and B are with the standard amount of structure directing agent, and part C is with double the amount. Part D is the control experiment without microemulsion with the standard amount of structure directing agent. The scale bars are 20 µm for parts A, B, and D and 200 nm for part C.
Figure 3. Close-up SEM images of the crystals of Figure 2A and B. Part A shows the faceted end of the coffin-shaped crystals in Figure 2A, and part B shows the rounded end of the rod-shaped crystals in Figure 2B. The scale bars are 1 µm for part A and 2 µm for part B.
holder from the deposited powder. The peak at 18.34° has been identified as originating from the (140) crystal plane for calcined silicalite-1 without fluoride and is also present in the reference spectra from a sample synthesized by the fluoride route.26,27 It is likely that the peak is from the (140) plane of the silicalite-1. However, we cannot definitely assign the peak to a crystal plane without a more thorough X-ray analysis. An alteration in crystal morphology is a result of the difference in growth rates on different crystal planes. Comparing Figures 2 and 4
shows that crystals with the typical coffin-shaped morphology do not exhibit enhanced reflection from the plane, while crystals with altered morphology (rod-shaped and irregular nanoparticles) do show the enhanced reflection. It is likely that the coffin-shaped crystals have a preferred orientation when deposited. However, there will be no preferred orientation for the irregular nanoparticles. The preferred orientation also appears lost for the rod-shaped (27) Treacy, M. M. J.; Higgins, J. B. Collection of Simulated XRD Powder Patterns for Zeolites, 4th ed.; Elsevier: New York, 2001.
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sorption onto the growing crystals, even though the microemulsion itself becomes destabilized under the hydrothermal synthesis conditions. Our previous studies demonstrated that preferential adsorption of components of the microemulsions onto certain faces of the growing crystal may play a role in altering crystal morphology.21 To probe the influence of components of the microemulsion on crystal morphology, several control experiments were conducted by adding only some of the components required to form the microemulsion. It was observed that individual components of the microemulsion could affect crystal morphology without forming a microemulsion. However, there was poorer crystallization and the crystals were more broadly distributed in size and shape when compared to those synthesized in the microemulsion. For example, the addition of toluene alone produced coffin-shaped crystals up to 700 µm in length with a large fraction of irregularshaped crystals. The addition of CTAB alone produced irregular nanocrystals. The addition of oil and CTAB together resulted in irregular micron-size crystals. From these control experiments, it is clear that the microemulsion offers an environment to produce more homogeneous crystals in addition to altering crystal morphology. Strong adsorption onto a crystal retards growth on that face, so the growth rate in the a, b, or c direction can be altered by adding components that have different interaction energies with different crystal faces (axis as illustrated in Figure 2D). Theoretical studies of the interaction energy of surfactants with different crystal faces may offer a route to select microemulsions to tailor crystal morphology.
Figure 4. (A-D) X-ray diffraction patterns for the products shown in Figure 2A-D, respectively.
particles. It is clear that the microemulsion synthesis acts to alter the crystal morphology of silicalite-1 because the synthesis conditions used in the control experiment and the microemulsion experiments were identical except for confining the gel in the microemulsion. Since the crystal size and shape is altered, it confirms that the microemulsion alters crystal nucleation and growth. Unfortunately, we cannot unambiguously match the crystal planes with the axes of the particles to conclusively state which crystal planes are enhanced. Confinement in the microemulsion leads to smaller crystals than a standard hydrothermal synthesis, presumably because confinement in the microemulsion allows a greater number of nuclei to form. Without microemulsions, the synthesis gel appears milky white and the starting materials are homogenized only by stirring. When confined in the microemulsion, the gel appears transparent due to confinement in nanometer-scale droplets.28 Since the microemulsion self-assembles, it provides a more homogeneous environment without relying on mechanical stirring. The silicalite-1 crystals are smaller and more uniform than those produced without microemulsions, as shown in Figure 2. The size of crystals synthesized in microemulsions can be reduced to one-fifth or even smaller when compared with crystals from the control experiment. It should also be noted that microemulsions only exist at low temperatures. Upon heating above 50 °C, the microemulsion was observed to turn hazy, indicating aggregation and destabilization of the small microemulsion droplets. Therefore, the microemulsion likely plays a major role in the very early stages of nucleation, allowing more nuclei to form. Once nuclei are formed, components of the microemulsion can influence crystal morphology by ad-
Conclusions In summary, the crystal size and morphology of silicalite-1 can be altered through hydrothermal synthesis in microemulsions. Using CTAB as the surfactant for forming the microemulsion results in silicalite-1 crystals in the shape of rods and small irregular-shaped nanoparticles. Generally, the crystal size is smaller and more uniform than those synthesized without the microemulsion. The crystal morphology can be controlled by adjusting the microemulsion composition. The microemulsion synthesis route is promising for controlling the morphology of a variety of zeolites and molecular sieves. Moreover, the novel silicalite-1 crystal shapes may be utilized to control orientation when assembling crystals into thin films. We have recently demonstrated the assembly of oriented thin films of rod-shaped molecular sieve crystals.29 The assembly of elongated silicalite-1 particles using the same approach would allow the creation of membranes with controlled pore direction for application in separation or catalysis. Acknowledgment. We acknowledge support from the University of Rochester, The Laboratory for Laser Energetics, and from the donors of the Petroleum Research Fund, administered by the ACS (grant 37970-G5). The electron microscopy facility at the University of Rochester is supported by NSF CTS-6571042. Supporting Information Available: Experimental Section and table summarizing silicalite-1 crystallization from CTAB microemulsions. This material is available free of charge via the Internet at http://pubs.acs.org. LA0473456 (28) Moulik, S. P.; Paul, B. K. Adv. Colloid Interface Sci. 1998, 78, 99-195. (29) Lin, J-C.; Yates, M. Z.; Petkoska, A.; Jacobs, S. Adv. Mater. 2004, 16, 1944-1948.
