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Anionic Microemulsion-Mediated Low Temperature Synthesis of Anisotropic Silicalite-1 Nanocrystals Seungju Lee, C. Shane Carr, and Daniel F. Shantz* Department of Chemical Engineering, Texas A&M University, 3122 TAMU, College Station, Texas 77843-3122 Received August 10, 2005. In Final Form: September 25, 2005 The low-temperature (368 K) synthesis of silicalite-1 nanocrystals in anionic microemulsions is reported. In the presence of AOT/isooctane mixtures silicalite-1 nanocrystals can be formed that are coffin-shaped and approximately 100 × 40 × 200 nm in size. This is in contrast to samples made without the microemulsion under the same conditions where irregular spherical particles approximately 100 nm in diameter are formed. The current work shows that, in contrast to previous work in this area, the anionic microemulsions cannot stabilize colloidal silica due to the strong repulsive electrostatic forces between the anionic silicate species and the surfactant headgroup. The crystal morphology of the silicalite-1 obtained is also shown to be sensitive to the surfactant identity as syntheses using SDS/heptane/butanol mixtures lead to different morphologies. It is also possible to uncouple zeolite nucleation from growth in these systems. This was demonstrated by adding a solution containing 25 nm silicalite-1 nanocrystals to the AOT/isooctane mixture, which leads to large micron-sized spheres of silicalite-1 containing large mesopores. This report demonstrates that anionic microemulsions lead to fundamentally different crystal habits than the nonionic or cationic microemulsions investigated previously. The future outlook for the use of microemulsion-mediated zeolite growth is also discussed.
* Corresponding Author. Phone: (979) 845-3492. Fax: (979) 8456446. E-mail:
[email protected].
employing low-solubility silicon sources, fluoride as the mineralizing agent, and high temperatures. There has been much less work demonstrating the ability to synthesize zeolite crystals of controllable morphology. One notable exception is the synthesis of silicalite-1 using different structure-directing agents (SDAs).15-17 These studies have shown that different crystal habits can be obtained by using different SDAs (e.g., TPA dimer/trimer versus TPA). Moreover Lai and co-workers showed that oriented silicalite-1 thin films with superior sieving properties could be obtained by using these different SDAs in secondary growth,18,19 demonstrating one of the potential applications available if one could control zeolite morphology. Such materials could find use as novel adsorbents or catalysts, be integrated into microdevices, be utilized as templates for growing high-density areas of carbon nanotubes or metal nanowires, or be used in thin film formation. On a related note, many recent studies have grown zeolites in media including hydrogels, starches, and carbons.10,20-24 These reports generally had the goal of introducing hierarchical structure
(1) Barrer, R. M. Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982. (2) Breck, D. W. Zeolite Molecular Sieves: Structure, Chemistry and Use; Wiley: New York, 1974. (3) Csicsery, S. M. Zeolites 1984, 4, 202-213. (4) Catalysis and Zeolites: Fundamentals and Applications, 1st ed.; Springer: Berlin, 1999. (5) Wojciechowski, B. W.; Corma, A. Catalytic Cracking: Catalysis, Chemistry, and Kinetics; Dekker: New York, 1986. (6) Zeolite Synthesis; Springer-Verlag: Berlin, 1998; Vol. 1. (7) Cundy, C. S.; Lowe, B. M.; Sinclair, D. M. J. Cryst. Growth 1990, 100, 189-202. (8) Persson, A. E.; Schoeman, B. J.; Sterte, J.; Ottesstedt, J. E. Zeolites 1994, 14, 557-567. (9) Mintova, S.; Olson, N. H.; Valtchev, V.; Bein, T. Science 1999, 283, 958-960. (10) Tosheva, L.; Valtchev, V. P. Chem. Mater. 2005, 17, 2494-2513. (11) Charnell, J. F. J. Cryst. Growth 1971, 8, 291-294. (12) Sun, Y. Y.; Song, T.; Qiu, S.; Pang, W.; Shen, J.; Jiang, D.; Yue, Y. Zeolites 1995, 15, 745-753. (13) Shimizu, S.; Hamada, H. Micropor. Mesopor. Mater. 2001, 48, 39-46. (14) Di Renzo, F. Catal. Today 1998, 41, 37-40.
(15) Beck, L. W.; Davis, M. E. Micropor. Mesopor. Mater. 1998, 22, 107-114. (16) Dı´az, I.; Kokkoli, E.; Terasaki, O.; Tsapatis, M. Chem. Mater. 2004, 16, 5226-5232. (17) Bonilla, G.; Dı´az, I.; Tsapatis, M.; Jeong, H.-K.; Lee, Y.; Vlachos, D. G. Chem. Mater. 2004, 16, 5697-5705. (18) Lai, Z. P.; 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. (19) Lai, Z.; Tsapatis, M.; Nicolich, J. R. Adv. Func. Mater. 2004, 14, 716-729. (20) Garcia-Martinez, J.; Cazorla-Amoros, D.; Linares-Solano, A.; Lin, Y. S. Microporous Mesoporous Mater. 2001, 42, 255-268. (21) Zhang, B. J.; Davis, S. A.; Mann, S. Chem. Mater. 2002, 14, 1369-1375. (22) Wang, H. T.; Holmberg, B. A.; Yan, Y. S. J. Am. Chem. Soc. 2003, 125, 9928-9929. (23) Kim, S. S.; Shah, J.; Pinnavaia, T. J. Chem. Mater. 2003, 15, 1664-1668. (24) Jacobsen, C. J. H.; Madsen, C.; Houzvicka, J.; Schmidt, I.; Carlsson, A. J. Am. Chem. Soc. 2000, 122, 7116-7117.
Introduction Zeolites are crystalline, microporous (alumino)silicates that have been extensively used in heterogeneous catalysis, separations, and ion-exchange operations.1,2 It has long been understood that particle size and morphology strongly influence zeolite performance in catalysis and separations.3-5 This is particularly acute in catalytic applications where the particle size can have a dramatic effect on the product distribution due to differences in rates of transport/diffusion and reaction. As such there is interest in developing synthetic approaches to control zeolite crystal size and morphology. In general terms, the zeolite science community has developed reproducible approaches to synthesize submicron crystals6-10 as well as very large crystals.11-14 The former relies on syntheses where nucleation rates are very high, often involving monomeric silica and alumina sources, high alkalinity, and low temperatures. The latter relies on syntheses
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into the solid obtained or to control particle size; control of crystal morphology was not the focus of these works. Another approach is to use microemulsions as confined spaces or “nanoreactors” for zeolite growth, as has been shown to be very successful in the synthesis of metal, metal oxide, and metal sulfide nanoparticles.25-27 This area has experienced considerable growth since the initial reports by Dutta and co-workers of zeolite A and zincophosphate FAU analogue growth in microemulsions.28-30 Yates and colleagues have reported microemulsionmediated growth schemes to form AlPO4-5 and silicalite-1 materials at high temperatures,31-34 and Yan’s lab has recently reported the synthesis of zeolite A nanocrystals in cationic microemulsions using microwave heating.35 Our lab has also been actively involved in this area studying silicalite-1 syntheses at low36-38 and high temperatures,39 as well as the growth of zeolite A in nonionic microemulsions.40 Our initial view of this synthetic approach was that there are two balancing or competing factors in this approach. On one hand, the microemulsion constitutes a confined space that will potentially modulate nucleation and growth (nanoreactor). On the other hand, the surfactant will coordinate to the crystallographic faces of the growing crystal and affect the growth rates depending on the crystallographic orientation of the surface and the strength of the organic-inorganic interactions. The latter effect should be tunable in a rational manner by changing the chemical nature of the surfactant. A key issue in determining the formation and growth of microporous materials in microemulsions is the nature and strength of the interactions between the surfactants forming the microemulsion and the zeolite particles or silicate precursors. Our previous results lend credence to this hypothesis, as they suggest that it is the strength and nature of the surfactant-silicate interactions, not the confined space afforded by the microemulsion, that determine the size and morphology of the zeolite crystals obtained. In the current report, we describe our efforts to synthesize silicalite-1 at low temperatures (368 K) in the presence of anionic microemulsions. The synthesis of silicalite-1 at 368 K from clear solution is studied given that the growth kinetics of the parent synthesis mixture are well understood. Here, in contrast to our previous work, there should be strong repulsive forces between the surfactant and the silicate species/growing zeolite crystal in solution. The results presented here show clear differences from our previous work both in terms of the (25) Osseo-Asare, K.; Arrigagada, F. J. Colloids Surf. 1990, 50, 321339. (26) Pileni, M. P. Langmuir 1997, 13, 3266-3276. (27) Zarur, A. J.; Hwu, H. H.; Ying, J. Y. Langmuir 2000, 16, 30423049. (28) Dutta, P. K.; Robins, D. Langmuir 1991, 7, 1048-1050. (29) Dutta, P. K.; Jakupca, M.; Reddy, K. S. N.; Salvati, L. Nature 1995, 374, 44-46. (30) Singh, R.; Doolittle, J.; George, M. A.; Dutta, P. K. Langmuir 2002, 18, 8193-8197. (31) Yates, M. Z.; Ott, K. C.; Birnbaum, E. R.; McCleskey, T. M. Angew. Chem., Int. Ed. 2002, 41, 476-478. (32) Lin, J.-C.; Dipre, J. T.; Yates, M. Z. Chem. Mater. 2003, 15, 2764-2773. (33) Lin, J.-C.; Dipre, J. T.; Yates, M. Z. Langmuir 2004, 20, 10391042. (34) Lin, J.-C.; Yates, M. Z. Langmuir 2005, 21, 2117-2120. (35) Chen, Z.; Li, S.; Yan, Y. Chem. Mater. 2005, 17, 2262-2266. (36) Lee, S.; Shantz, D. F. Chem. Commun. 2004, 680-681. (37) Lee, S.; Shantz, D. F. Chem. Mater. 2005, 17, 409-417. (38) Axnanda, S.; Shantz, D. F. Microporous Mesoporous Mater. 2005, 84, 236-246. (39) Lee, S.; Shantz, D. F. Microporous Mesoporous Mater. 2005, 86, 268-276. (40) Carr, S. C.; Shantz, D. F. Microporous Mesoporous Mater. 2005, 85, 284-292.
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microemulsion stability and the zeolite crystals obtained. The results are discussed and then placed into the larger context of the capabilities and limitations of microemulsion-mediated syntheses. Experimental Section Materials. The anionic surfactants sodium bis(2-ethylhexyl) sulfosuccinate (AOT, 100% surfactant) and sodium dodecyl sulfate (SDS, 98.5+%) were used as received from EM science and Aldrich, respectively. Tetraethylothosilicate, TEOS (99+% Aldrich), ACS reagent grade heptane, isooctane (99% Mallinckrodt Baker), butanol (VWR), and tetrapropylammonium hydroxide, TPAOH (40% w/w Alfa Aesar), were used as received. Zeolite Synthesis. Zeolite samples grown in oil/water/ surfactant mixtures were prepared as follows: fixed amounts of isooctane or heptane, surfactant, and butanol were mixed together in a Pyrex screw-cap test tube. In the case of AOT/isooctane mixtures, butanol was not added unless noted, and in the SDS/ heptane mixtures, butanol was always added (2:1 surfactant: butanol by weight) unless stated otherwise. The mixtures were then allowed to age at room-temperature overnight to dissolve the surfactant. The corresponding amounts of TPAOH, deionized water, and TEOS were mixed together in a separate Teflon container and stirred overnight at room temperature. Unless noted otherwise the zeolite synthesis mixture has a molar composition of 1 TEOS: 0.36 TPAOH: 20 H2O. The zeolite synthesis mixture was then added into the test tube, and the test tube was closed, vigorously shaken for five minutes, put in an oven at 368 K, and allowed to react under quiescent conditions for a given period of time. The test tubes were typically 60-70% full (by volume) at room temperature. In the results section, mixture compositions are described using weight fractions with the following notation: S, surfactant (+ butanol); H or I, heptane or isooctane; M, silicalite-1 mixture. As an example 0.2-0.6-0.2 S-H-M is 20% weight in surfactant and butanol, 60% weight in heptane, and 20% weight in silicalite-1 mixture. Unless stated otherwise, the synthesis duration was 96 h. Samples were collected by filtration, washed with copious quantities of ethanol, acetone, and water and air-dried. Characterization. Powder X-ray diffraction (PXRD) measurements were performed using a Bruker D-8 X-ray diffractometer with CuKR radiation. Samples were analyzed over a range of 5-40° 2θ using a step scan mode with a step size of 0.04° and a step rate of 3 s/step. Peak intensities and 2θ values were determined using the Bruker program EVA. Field-emission scanning electron microscopy (FE-SEM) measurements were performed using a Zeiss Leo-1530 microscope operating at 1-5 kV. Transmission electron microscopy (TEM) was performed on a JEOL 2010 microscope with a lanthanum hexaboride filament and an excitation voltage of 200 kV. The samples were mortar and pestled, dispersed in ethanol (100%, Aldrich), and then placed on a 400-mesh copper grid. Nitrogen adsorption experiments were performed on a Micromeritics ASAP 2010 micropore analyzer with a turbopump capable of obtaining relative pressures of less than 10-6. The samples used in adsorption experiments were calcined to remove the TPAOH by heating from room temperature to 723 K at a rate of 1 K/min and were then heated at 723 K for 8 h. Samples were degassed at 373 K for 4 h and then at 573 K overnight before performing the measurements. The isotherms were measured over the relative pressure range of 10-6 to 0.98. Thermogravimetric analyses (TGA) were performed using a NETZSCH TG 209 instrument. The heating profile was as follows: ramp from room temperature to 373 K at a rate of 5 K/min, hold for 1 h at 373 K, and then ramp at 5 K/min to 823 K. Dynamic light scattering (DLS) measurements were performed using a BIC ZetaPALS at room temperature using a quartz cuvette (Starna Cell Co., 1-Q-10-GL 14-C). The wavelength of the incident laser beam was 658 nm. The detecting angle was 90 degrees. For each sample, three measurements were performed and the running time of each measurement was 5 min to ensure high signal-to-noise ratio and good counting statistics.
Results and Discussion Effect of Anionic Microemulsion. Figure 1 shows the PXRD patterns and FE-SEM images of samples made
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Figure 2. FE-SEM images of silicalite-1 samples made with (left) 0.2 AOT: 0.8 zeolite and (right) 0.2 AOT + butanol: 0.8 zeolite.
Figure 1. FE-SEM and PXRD of silicalite-1 made in the (left) absence and (right) presence of an AOT/isooctane microemulsion (0.213-0.637-0.15 S-I-M).
in the presence (0.213-0.637-0.15 S-I-M) and absence of the microemulsion. The crystals formed in the AOT/ isooctane mixture possess a different size and morphology as compared to crystals obtained without the microemulsion. The crystals made in the mixture of AOT/isooctane microemulsions (Figure 1) are coffin-shaped and approximately 200 nm in length and 40 nm in thickness. Note also that the surfaces are smooth and have welldefined crystal edges. By contrast, silicalite-1 crystals formed without the microemulsion are spherical and approximately 100 nm in diameter. Thus, the AOT/ isooctane mixture perturbs the synthesis leading to the observed morphology. These crystals appear qualitatively similar to the coffin-shaped crystals often observed in silicalite-1 syntheses; however, they are much smaller than is typically observed, very few twinned crystals are observed, and they are very thin (
