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Ultralow Temperature Synthesis of Ordered Hexagonal Smaller Supermicroporous Silica Using Semifluorinated Surfactants as Template Yan Di, Xiangju Meng, Lifeng Wang, Shougui Li, and Feng-Shou Xiao* State Key Laboratory of Inorganic Synthesis and PreparatiVe Chemistry and Department of Chemistry, Jilin UniVersity, Changchun 130012, P. R. China ReceiVed August 4, 2005. In Final Form: NoVember 9, 2005 Ordered hexagonal smaller supermicroporous silica (JLU-14L and JLU-11L) has been successfully synthesized at ultralow temperature (0 ∼ -20 °C) by semifluorinated surfactants, which is extensively investigated by X-ray diffraction, N2 adsorption, and transmission electron microscopy. X-ray diffraction shows that the peak intensity of the samples increases with decreasing synthesis temperatures in the range of 0 ∼ -20 °C, indicating that the ordering of porous silica improves with decreasing synthesis temperature. N2 adsorption/ desorption isotherms show that JLU-14L has both ordered smaller hexagonal supermicropores (1.27 and 1.28 nm) and disordered micropores (0.67 nm). Such hierarchically porous materials with micro/supermicroporosity should be potentially important for fast diffusion of reactants and products in catalytic reactions. The ultralow temperature synthesis is a crucial factor for the formation of ordered smaller supermicropores and the control of microporosity in JLU-14L.
1. Introduction Since the successful preparation of mesoporous silica of the M41S family by Mobil scientists in 1992,1,2 these ordered porous materials (pore size > 2 nm) have been widely investigated because of their potential applications in catalysis, biomolecular immobilization, adsorption, and separation.1-7However, much less effort has been invested in the research of ordered supermicroporous materials with pore size distribution in the range of 1-2 nm. These materials are very important in industrial applications because they bridge the gap of microporous zeolites and mesoporous materials.8-21Supermicroporous silica is expected to have the potential of size and shape selectivity for those organic molecules that are too large to assess the pores of zeolites or are too small to be separated by mesopores. * To whom correspondence should be addressed. Fax: +86-(431)5168590. E-mail:
[email protected]. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vautuli, J. C.; Beck, J. S. Nature 1992, 352, 710. (2) Beck, J. S.; Vautuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T-W.; Olson, D. H.; Sheppard, E. W.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (4) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (5) Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242. (6) Prouzet, E.; Pinnavaia, T. J. Angew. Chem., Int. Ed. 1997, 36, 516. (7) Corma, A. Chem. ReV. 1997, 97, 2373. (8) Bagshaw, S. A.; Hayman, A. R. Chem. Commun. 2000, 533. (9) Bagshaw, S. A.; Hayman, A. R. AdV. Mater. 2001, 13, 1011. (10) Bagshaw, S. A.; Hayman, A. R. Micro. Meso. Mater. 2001, 44-45, 81. (11) Zhou, Y.; Antonietti, M. AdV. Mater. 2003, 15, 1452. (12) Zhou, Y.; Antonietti, M. Chem. Commun. 2003, 2564. (13) Zhou, Y.; Antonietti, M. Chem. Mater. 2004, 165, 44. (14) Ryoo, R.; Park, I.-S.; Jun, S.; Lee, C. W.; Kruk, M.; Jaroniec, M. J. J. Am. Chem. Soc. 2001, 123, 1650. (15) Kruk, M.; Asefa, T.; Jaroniec, M.; Ozin, G. A. J. Am. Chem. Soc. 2002, 124, 6383. (16) Zhao, X. S.; Lu, G. Q.; Hu, X. Chem. Commun. 1999, 1391. (17) Serrano, D. P.; Aguado, J.; Escola, J. M.; Garagorri, E. Chem. Commun. 2000, 2041. (18) Sun, T.; Wong, M. S.; Ying, J. Y. Chem. Commun. 2000, 2057. (19) Bastardo-Gonzalez, E.; Mokaya, R.; Jones, W. Chem. Commun. 2001, 1016. (20) McInall, M. D.; Scott, J.; Mercier, L.; Kooymanb, P. J. Chem. Commun. 2001, 2282. (21) Yano, K.; Fukushima, Y. J. Mater. Chem. 2003, 13, 2577.
Recently, there have been a few successful approaches for the preparation of ordered supermicroporous silica materials.8-22 As examples, ordered supermicroporous silica with pore size of 1.96 nm is synthesized from a template of double-chain surfactant.14 Highly ordered silica materials with pore sizes of 1.4-2.0 nm are templated by ω-hydroxy-bolaform surfactants.8-10 A series of lamellar supermicrostructured silica with pore sizes of 1.2-1.5 nm are prepared by nanocasting with ionic liquids (ILs).11-13 Ordered supermicroporous organic-inorganic materials with pore sizes of 1.4-2.5 nm are obtained by modification of unprecedented loading of pendant vinyl groups.15 Notably, it is very difficult to obtain ordered porous silica with a pore size of less than 1.3 nm, due to the limitation of hydrocarbon chains (larger than C8) for the surfactants. If hydrocarbon chains are shorter than C8 in the surfactants, the hydrophobicity of hydrocarbon chains is not enough for formation of the surfactant micelle. If a C8-hydrocarbon surfactant is used as a model micelle, the calculated diameter would be 1.78 nm.23 It has been reported that synthesis temperature strongly influences the pore sizes of mesoporous materials, and lower temperatures in the synthesis generally result in the formation of mesoporous silica with smaller pore sizes.24-27 For example, Prouzet et al. effectively control pore sizes (2.0-4.5 nm) of mesoporous silica (MSU-1) by synthesis temperatures (25-65 °C).24 Kim et al. and Che et al. successfully synthesize ordered mesoporous materials (SBA-1) with various pore sizes (1.4-2.4 nm) by changing temperatures (-5-40 °C).25,26 Beaudet et al. prepare hybrid organic-inorganic nanoporous silica microspheres with various pore sizes at the temperatures of 0-71 °C.27 Notably, it is difficult to synthesize mesoporous silica materials at temperatures lower than -5 °C, due to the limitation of ordered micelle in the starting reaction gels.28 (22) Meng, X.; Di, Y.; Zhao, L.; Li, S.; Jiang, D.; Xiao, F.-S. Chem. Mater. 2004, 16, 5518. (23) Frisch, M. J.; Trucks, G. W.; Schlegel H. B.; et al. GAUSSIAN 03, Revision B. 03, Gaussian, Inc., Pittsburgh, PA, 2003. (24) Prouzet, E.; Pinnavaia, T. J. Angew. Chem., Int. Ed. Engl. 1997, 36, 516. (25) Kim, M. J.; Ryoo, R. Chem. Mater. 1999, 11, 487. (26) Che, S.; Sakamoto, Y.; Terasaki, O.; Tatsumi, T. Chem. Mater. 2001, 13, 2237. (27) Beaudet, L.; Hossain, K.-Z.; Mercier, L. Chem. Mater. 2003, 15, 327.
10.1021/la0521342 CCC: $33.50 © 2006 American Chemical Society Published on Web 03/02/2006
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On the other hand, it is well-known that the hydrophobicity of fluorocarbon is much stronger than that of hydrocarbon,29,30and such strong hydrophobicity of short fluorocarbon chain has an advantage for the formation of surfactant micelle with relatively small diameter.29 Moreover, the starting reaction gel containing fluorocarbon surfactants have an advantage for the formation of ordered micelle at lower temperatures than that containing hydrocarbon surfactants.28 More recently, there are a number of successful examples for synthesis of ordered mesoporous silica materials templated from ionic fluorocarbon surfactants or nonionic semifluorinated surfactants.22,30-35 Particularly, high-temperature synthesis of stable ordered mesoporous silica-based materials is performed by using fluorocarbon and hydrocarbon mixtures as templates.30 We demonstrate here that when a nonionic semifluorinated surfactant with shorter fluorocarbon chains (CF3(CF2)4(EO)10, FSO-100) is used as a template, ordered hexagonal smaller supermicroporous silica materials (JLU-14L) have been successfully synthesized at relatively low temperatures (0 ∼ -20 °C). The ultralow temperature synthesis is a crucial factor for the formation of ordered smaller supermicropores and the control of microporosity in JLU-14L. 2. Experimental Section 2.1 Materials. Tetraethyl orthosilicate (TEOS) and hydrochloric acid were purchased from Beijing Chemical Co. (China). Semifluorinated surfactants of FSO-100 (CF3(CF2)4(EO)10) and FSN100 (CF3(CF2)5(EO)14) were purchased from DuPont Co. 2.2 Synthesis. In a typical synthesis for JLU-14L, 1.6 g of FSO100 (CF3(CF2)4(EO)10) was dissolved in 50 mL of H2O with 6 mL of HCl (10 M/L), followed by addition of 2.4 mL of TEOS. The mixture (SiO2/HCl/FSO/H2O with molar ratio at 1/5.8/0.2/267) were cooled to specific low temperature (0, -10, -15, and -20 °C) and remained for 3 days with vigorous stirring. The products were collected by filtration, washed with distilled water, dried in air, and calcined at 600 °C for 4 h to remove the semifluorinated surfactant. The samples were denoted as JLU-14L(0), JLU-14L(-10), JLU-14L(-15), and JLU-14L(-20), respectively. For comparison, JLU-14 samples have also been prepared at room temperature and 40 °C using FSO-100 semifluorinated surfactant, which were denoted as JLU-14RT and JLU-1440. In a typical synthesis for JLU-11L, 2.2 g of FSN-100 (CF3(CF2)5(EO)14) was dissolved in 50 mL of H2O with 6 mL of HCl (10 M/L), followed by addition of 4 mL of TEOS. The mixture (SiO2/HCl/ FSN/H2O with molar ratio at 1/3.4/0.13/160) was cooled to low temperature of 0 °C and remained for 3 days with vigorous stirring. The products were collected by filtration, washed with distilled water, dried in air, and calcined at 600 °C for 4 h to remove the semifluorinated surfactant. 2.3 Characterizations. X-ray diffraction patterns were obtained with a Siemens D5005 diffractometer using Cu KR radiation. Transmission electron microscopy experiments were performed on a JEM-3010F electron microscope (JEOL, Japan) with an acceleration voltage of 300 kV. Scanning electron microscopy experiments were performed on a S-5200 electron microscope (Hitachi, Japan). The nitrogen isotherms at the temperature of liquid nitrogen were (28) Zhao, G.; Zhu, B. Principles of Surfactant Action; China Light Industry Press: Beijing, 2003. (29) Muto, Y.; Esumi, K.; Meguro, K.; Zana, R. J. Colloid Interface Sci. 1987, 120, 162. (30) Han, Y.; Li, D.; Zhao, L.; Song, J.; Yang, X.; Li, N.; Di, Y.; Li, C.; Wu, S.; Xu, X.; Meng, X.; Lin, K.; Xiao, F.-S. Angew. Chem., Int. Ed. 2003, 42, 3633. (31) Blin, J. L.; Lesieur, P.; Ste´be´, M. J. Langmuir 2004, 20, 491. (32) Blin, J. L.; Ste´be´, M. J. Phys. Chem. B 2004, 108, 11399. (33) Blin, J. L.; Ge´rardin, G.; Rodehu¨ser, L.; Selve, C.; Ste´be´, M. J. Chem. Mater. 2004, 16, 5071. (34) Tan, B.; Dozier, A.; Lehmler, H.-J.; Knutson, B. L.; Rankin, S. E.; Langmuir 2004, 20, 6981. (35) Tan, B.; Lehmler, H.-J.; Vyas, S. M.; Knutson, B. L.; Rankin, S. E. Chem. Mater. 2005, 17, 916.
Figure 1. (A) XRD patterns of calcined JLU-14L samples (JLU14L(0), JLU-14L(-10), JLU-14L(-15), and JLU-14L(-20)) synthesized at (a) 0 °C, (b) -10 °C, (c) -15 °C, and (d) -20 °C, respectively; (B) XRD patterns of calcined JLU-11L samples (JLU11L(0), JLU-11L(-5), and JLU-11L(-15)) synthesized at (a) 0 °C, (b) -5 °C, and (c) -15 °C, respectively. measured using a Micromeritics ASAP 2020M system. The samples were degassed for 10 h at 200 °C before the measurements.
3. Results and Discussions 3.1. X-ray Diffraction (XRD). Figure 1A shows the XRD patterns of JLU-14L samples synthesized at various temperatures (0 ∼ -20 °C) using semifluorinated surfactants of FSO-100 as a template. Notably, all samples exhibit only one peak, given at almost the same angle. For example, the peak of JLU-14L reflects a d spacing of near 3.3 nm (Table 1). In contrast, we have used hydrocarbon surfactants such as CTAB and P123 to template mesoporous silica at the temperature of -10 °C, and the reaction gels are completely frozen. Therefore, the synthesis of mesoporous silica at ultralow temperatures is not successful. All of these results indicate that fluorocarbon surfactants play a key role for the ultralow temperature synthesis of mesoporous or supermicroporous silica-based materials. Furthermore, ultralow temperature synthesis is extended to synthesize smaller supermicroporous silica (JLU-11L) by using a template of semifluorinated surfactant of FSN-100, and obtained XRD patterns also show one peak (Figure 1B). These results suggest that ultralow temperature synthesis of smaller super-
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Table 1. Textural Parameters of Calcined JLU-14L Samples Synthesized at Low Temperatures (0 ∼ -20 °C)
samples
d100 (nm)
a0 (nm)
BJH pore size (nm)
HK pore size (nm)
BET surface area (m2/g)
mesopore volume (cm3/g)
micropore volume (cm3/g)
total pore volume (cm3/g)
JLU-14L(0) JLU-14L(-10) JLU-14L(-15) JLU-14L(-20)
3.34 3.36 3.31 3.35
3.86 3.88 3.82 3.87
1.28 1.28 1.27 1.27
0.67 0.67 0.68 0.68
624 729 706 717
0.28 0.33 0.31 0.31
0.06 0.15 0.05 0.20
0.34 0.48 0.36 0.51
microporous silica is not limited to the use of FSO-100, and different semifluorinated surfactants such as FSN-100 are also suitable. Interestingly, the peak intensity of the samples increases with decreasing synthesis temperature (Figure 1), indicating that the ordering of porous silica improves with decreasing synthesis temperature. For example, the sample synthesized at 0 °C shows relatively low intensity (Figure 1A-a and Figure 1B-a). However, when the synthesis temperature is reduced to -20 °C, the sample intensity is very high (Figure 1A-d and Figure 1B-c). This phenomenon is quite different from the syntheses of ordered mesoporous silica at the temperatures of rt-100 °C reported previously, in which the ordering of mesoporous silica increases with synthesis temperature.14 Compared with room temperature, it is obvious that hydrolysis of TEOS and crystallization of silica
at lower synthesis temperatures (0 ∼ -20 °C) are slow.36 Possibly, a slower rate for hydrolysis and crystallization would proceed under more thermodynamically controlled, near-equilibrium conditions, leading to more ordering of supermicroporous silica samples. 3.2. Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). Figure 2 shows the TEM images of a calcined JLU-14L sample synthesized at -15 °C. Notably, the sample exhibits ordered hexagonal-like arrays of supermicropores with uniform pore size.14 From bright-dark contrast in the TEM image of the sample (Figure 2a,b), the distance between supermicropores is estimated to be 3.3 nm, in good agreement with the value determined from XRD (Figure 1). Figure 3 shows the SEM images of calcined JLU-14L and JLU-14 synthesized at -15, 25, and 40 °C, respectively. Notably,
Figure 2. TEM images of calcined JLU-14L(-15) sample taken in [100] (a) and [110] (b) directions.
Figure 3. SEM images of JLU-14L(-15) (a, b), JLU-14RT (c), and JLU-1440 (d) samples.
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Figure 4. N2 isotherms for calcined samples of (a) JLU-14L(0), (b) JLU-14L(-10), (c) JLU-14L(-15), and (d) JLU-14L(-20). The isotherms for JLU-14L(-10), JLU-14L(-15), and JLU-14L(-20) are offset from 30, 100, and 150 cm3/g to begin with for clarity, respectively.
the morphology of JLU-14L synthesized at -15 °C is sphere-like with the uniform particle sizes of 2-3 µm (Figure 3a,b). In contrast, the morphology of JLU-14 samples synthesized at relatively higher temperatures (25 and 40 °C) is irregular (Figure 3c,d). Possibly, the formation of sphere-like JLU-14L particles is also attributed to the ultralow-temperature synthesis in which condensation of silica gels would proceed under more thermodynamically controlled, near-equilibrium conditions, leading to more isotropic growth, similar to the previous reports.26 3.3 N2 Adsorption Isotherms. Figure 4 shows N2 isotherms of JLU-14L samples synthesized at various temperatures (0 ∼ -20 °C), and textural parameters are presented in Table 1. Interestingly, all of these isotherms display a transitional type between standard type I and IV curves, exhibiting well-defined capillary condensation steps at a very low relative pressure (P/ P0) of 0.02-0.08. These results indicate the formation of small pores with diameter less than 2.0 nm. Similar phenomena have been observed in the isotherms of other supermicroporous materials reported previously.8-21 Correspondingly, pore sizes for the samples synthesized at various temperatures (0 ∼ -20 °C) show narrow distribution at near 1.27 and 1.28 nm using the BJH model (Figure 5A). Notably, although various researchers have debated the reliability of this model,37,38 it appears to be generally valid in the size range under consideration here. It is very clear that the semifluorinated surfactant of CF3(CF2)4(EO)10 includes hydrophobic fluorocarbon chain (CF3(CF2)4-) and hydrophilic group ((EO)10-). If only fluorocarbon chains act as the rigidly hydrophobic parts, the size of ordered supermicropores in JLU-14L templated by hydrophobic fluorocarbon chains would be estimated as twice (1.04 nm) the length (0.52 nm) for -CF3(CF2)4.23 This value is a slight difference from the experimental supermicropore sizes in JLU-14L (1.27 and 1.28 nm), which might be assigned to following reasons: (1) (EO)10 part still has a little contribution for hydrophobicity of the micelle; (2) because of lipophobic properties of fluorocarbon chains there could be a bit of space between fluorocarbon chains in the core of micelle, leading to larger pore size than twice of -CF3(CF2)4 chains (calculated diameter of the micelle). Furthermore, it is worthy to note that the samples synthesized at various temperatures (0 ∼ -20 °C) show narrow distribution centering at near 0.67 and 0.68 nm using the HK model (Figure 4b). Correspondingly, these samples also show a large amount of micropore volume, given at 0.05-0.20 cm3/g. All of these
Figure 5. (A) BJH pore size distribution of calcined samples (a) JLU-14L(0), (b) JLU-14L(-10), (c) JLU-14L(-15), and (d) JLU14L(-20); (B) HK pore size distribution of calcined samples (a) JLU-14L(0), (b) JLU-14L(-10), (c) JLU-14L(-15), and (d) JLU14L(-20).
results suggest that the samples synthesized at the ultralow temperatures (0 ∼ -20 °C) contain uniform micropores. Generally, t-plot curves are always used to confirm the presence of micropores in the mesoporous materials.39,40 The t-plots of conventional mesoporous materials should pass zero of axis, meaning no micropores in them. Miyazawa et al. reported that there are some micropores less than 4 Å in the walls of SBA-15 formed by penetration of hydrophilicpoly (ethylene oxide) chains of triblock coplymer in the silica wall.40 Ryoo et al. also found the network of micropores in the walls of SBA-15.39 Notably, all t-plot curves for JLU-14L samples synthesized at various temperatures (0 ∼ -20 °C) do not pass zero of axis, giving micropore volumes at 0.05-0.20 cm3/g (Figure 6), confirming the presence of a large amount of microporosity. Particularly, JLU-14L(-20) exhibits micropore volume at 0.20 cm3/g (Table 1), giving a very high ratio of micropore volume to total pore volume (over 30%). Such hierarchically porous materials with (36) Antonietti, M.; Berton, B.; Go¨ltner, C.; Hentze, H.-P. AdV. Mater. 1998, 10, 154. (37) Galarneau, A.; Desplantier, D.; Dutartre, R.; Renzo, F. Di. Micro. Meso. Mater. 1999, 27, 297. (38) Lukns, W. W.; Schmidt-Winkel, P.; Zhao, D.; Feng, J.; Stucky, G. D. Langmuir 1999, 15, 5403. (39) Ryoo, R.; Ko, C. H.; Kruk, M.; Antochshuk, V.; Jaroniec, M. J. Phys. Chem. B 2000, 104, 11465. (40) Miyazawa, K.; Inagaki, S. Chem. Commun. 2000, 2121.
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Figure 6. t-Plot of calcined samples of (a) JLU-14L(0), (b) JLU-14L(-10), (c) JLU-14L(-15), and (d) JLU-14L(-20).
micro/supermicroporosity should be potentially important for fast diffusion of reactants and products in catalytic reactions.39 It is reasonably believable that, at the low temperatures of 0 ∼ -20 °C, microporosity in JLU-14L samples is resulted from the hydrophilic nature of EO-blocks of semifluorinated surfactants, which leads to the formation of the silica framework around the EO-blocks, thereby incorporating them in the pore walls.39 Obviously, lower temperatures (0 ∼ -20 °C) in the synthesis have an advantage for increasing hydrophilicity of EO-blocks, which results in the incorporation of more EO-blocks into silica walls in JLU-14L samples. Therefore, JLU-14L samples synthesized at lower temperatures exhibit much more microporosity than conventional mesoporous silica materials. Additionally, it is also found that the microporous sizes (HK model) of JLU-14L samples are much larger than those of SBA15. As shown in Figure 5B, the microporous sizes of JLU-14L are 0.67 and 0.68 nm. In contrast, microporous sizes of SBA-15 samples are less than 0.4 nm.40 The big difference in microporous sizes between JLU-14L and SBA-15 samples is also attributed to the ultralow temperature synthesis. At lower temperatures most of the EO-blocks are incorporated into silica walls in JLU14L samples, leading to larger micropore sizes than those of
SBA-15 synthesized at conventional temperatures (>rt). Apparently, the presence of larger micropore sizes (>0.6 nm) in JLU-14L samples is favorable for fast diffusion of larger molecules such as aromatics (near 0.6 nm) in adsorption, separation, and catalysis.
4. Conclusion In conclusion, ordered hexagonal-like supermicroporous silica (JLU-14L) with microporous walls have been successfully synthesized at relatively low temperatures (0 ∼ -20 °C) using semifluorinated surfactants as templates. The lower temperatures in the synthesis of JLU-14L are a crucial factor for the formation of ordered smaller supermicropores and the control of microporosity. Acknowledgment. We thank Ms. Hong Wang and Prof. Wencai Lu (Institute of Therotical Chemistry, Jilin University, China) for helpful suggestions and discussions. This work is supported by NSFC, BASF, CNPC, State Basic Research Project (2004CB217804 and 2003CB615802), and National High Technology Research and Development Program of China. LA0521342
