pubs.acs.org/Langmuir © 2009 American Chemical Society
Control of Silica-Alkyltrimethylammonium Bromide Mesophases with 1,3,5-Trialkylbenzenes under Acidic Conditions Ayumu Fukuoka,† Izumi Kikkawa,† Yukichi Sasaki,‡ Atsushi Shimojima,† and Tatsuya Okubo*,† †
Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and ‡Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya 456-8587 Received April 9, 2009. Revised Manuscript Received June 9, 2009
This article reports the structural variation of SBA-type mesostructured silica formed from a mixture of tetraethyl orthosilicate (TEOS), alkyltrimethylammonium bromides, and 1,3,5-trialkylbenzenes under acidic conditions. Swollen 2D hexagonal mesophases were formed from the silica source and hexadecyltrimethylammonium bromide (C16TAB) with varying amounts of trimethylbenzene (TMB) and triethylbenzene (TEB), whereas drastic structural changes were observed with triisopropylbenzene (TIPB). Characterization by X-ray diffraction and transmission electron microscopy observation revealed that the mesophase was changed from hexagonal p6mm to cubic Pm3n to cubic Fm3m with increasing amounts of TIPB. Thus, the addition of TIPB leads to the preferential formation of spherical micelles rather than the swelling of rodlike micelles. When tetradecyltrimethylammonium bomide (C14TAB) was used, similar structural changes were triggered by smaller amounts of TIPB; however, almost no structural change was observed when octadecyltrimethylammonium bromide (C18TAB) was used. These findings provide a better understanding of the roles of 1,3,5-trialkylbenzenes in the structural control of silica-alkyltrimethylammonium mesophases.
Introduction The structural control of mesoporous silica synthesized through the self-assembly of surfactants and siloxane species is a key issue in realizing their wide range of potential applications.1 One effective approach for achieving this aim is the addition of organic molecules during the preparation of silica-surfactant mesostructured composites. Previous work on the nanostructure engineering of mesoporous silica (e.g., M41S- and SBA-type) has shown that the size and shape of the mesopores can be tuned to some extent by the addition of organic additives, such as alkanes,2-5 amines,6 alcohols,7-9 and 1,3,5-trialkylbenzenes.10-12 Of particular interest is the variation of the mesophase with such *Corresponding author. E-mail:
[email protected]. Tel: þ81-3-5841-7348. Fax: þ81-3-5800-3806.
(1) For reviews, see (a) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Angew. Chem., Int. Ed. 1999, 38, 56. (b) Soler-Illia, G. J. A. A.; Sanchez, C.; Lebeau, B.; Patarin J. Chem. Rev. 2002, 102, 4093. (c) Wan, Y.; Zhao, D. Chem. Rev. 2007, 107, 2821. (2) (a) Blin, J. L.; Otjacques, C.; Herrier, G.; Su, B. L. Langmuir 2000, 18, 4229. (b) Blin, J. L.; Su, B. L. Langmuir 2002, 18, 5303. (3) (a) El-Safty, S. A.; Hanaoka, T. Adv. Mater. 2003, 15, 1893. (b) El-Safty, S. A.; Hanaoka, T. Chem. Mater. 2004, 16, 384. (4) (a) El-Safty, S. A.; Hanaoka, T.; Mizukami, F. Chem. Mater. 2005, 17, 3137. (b) El-Safty, S. A.; Mizukami, F.; Hanaoka, T. J. Mater. Chem. 2005, 15, 2590. (5) (a) Sun, J. M.; Ma, D.; Zhang, H.; Wang, C.; Bao, X. H.; Su, D. S.; KleinHoffmann, A.; Weinberg, G.; Mann, S. J. Mater. Chem. 2006, 16, 1507. (b) Sun, J. M.; Ma, D.; Zhang, H.; Wang, C.; Jiang, F.; Cui, Y.; Guo, R.; Bao, X. H. Langmuir 2008, 24, 2372. (6) Sayari, A.; Kruk, M.; Jaroniec, M.; Moudrakovski, I. L. Adv. Mater. 1998, 10, 1376. (7) Feng, P.; Bu, X.; Pine, D. J. Langmuir 2000, 16, 5304. (8) (a) Kleitz, F.; Solovyov, L. A.; Anilkumar, G. M.; Choi, S. H.; Ryoo, R. Chem. Commun. 2004, 1536. (b) Kim, T.-W.; Kleitz, F.; Paul, B.; Ryoo, R. J. Am. Chem. Soc. 2005, 127, 7601. (c) Kleitz, F.; Kim, T.-W.; Ryoo, R. Langmuir 2006, 22, 440. (9) Kao, H.-M.; Cheng, C.-C.; Ting, C.-C.; Hwang, L.-Y. J. Mater. Chem. 2005, 15, 2989. (10) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olsan, D. H.; Higgins, E. W.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (11) (a) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (b) Cao, L.; Man, T.; Kruk, M. Chem. Mater. 2009, 21, 1311. (12) (a) Jana, S. K.; Nishida, R.; Shindo, K.; Kugita, T.; Namba, S. Microporous Mesoporous Mater. 2004, 68, 133. (b) Luechinger, M.; Pirngruber, G. D.; Lindlar, B.; Laggner, P.; Prins, R. Microporous Mesoporous Mater. 2005, 79, 41.
10992 DOI: 10.1021/la901263p
organic additives; however, only a few systematic studies have been reported so far. Kleitz et al. have demonstrated the phase control of highly ordered large-pore cubic (Ia3d, Im3m, and Fm3m) or hexagonal mesoporous silica materials using triblock copolymers (P123 and F127) and n-butanol as an additive.8 We have recently reported the synthesis of 3D cubic mesoporous silica materials by employing a hydrophobic additive, 1,3,5-triisopropylbenzene (TIPB), in the evaporation-induced self-assembly (EISA) process.13 This finding provides convenient access to the 3D cubic phase using hexadecyltrimethylammonium bromide (C16TAB) surfactant, being different from many previous studies using trialkylbenzenes as a pore expander of 2D hexagonal phases.10-12 However, the products had relatively less ordered structure because of uniaxial structural contraction that is generally observed for the materials synthesized through the EISA process,14,15 which hampered a deeper understanding of the phase behavior of silica-C16TABtrialkylbenzene ternary systems. An alternative process is the cooperative organization of silica-surfactant mesophases in relatively dilute solutions, but it is known to be strongly governed by the packing parameter of the surfactant.16-18 Alkyltrimethylammonium-type surfactants such as C16TAB predominantly give 2D hexagonal phases, although cubic Ia3d (MCM-48)10 and Pm3n phases (SBA-1)17,19 are also formed under certain (13) (a) Naik, S. P.; Yokoi, T; Fan, W.; Sasaki, Y; Wei, T. C.; Hillhouse, H. W.; Okubo, T. J. Phys. Chem. B 2006, 110, 9751. (b) Naik, S. P.; Fan, W.; Yokoi, T.; Okubo, T. Langmuir 2006, 22, 6391. (14) (a) Brinker, C. J.; Lu, Y.; Sellinger, A.; Fan, H. Adv. Mater. 1999, 11, 579. (b) Grosso, D.; Cagnol, F.; Soler-Illia, G. J. A. A.; Crepaldi, E. L.; Amenitsch, H.; Brunet-Bruneau, A.; Bourgeois, A.; Sanchez, C. Adv. Funct. Mater. 2004, 14, 309. (15) Klotz, M.; Albouy, P.-A.; Ayral, A.; Menager, C.; Grosso, D.; Lee, A. V. d.; Cabuil, V.; Babonneau, F.; Guizard, C. Chem. Mater. 2000, 12, 1721. (16) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 1 1976, 72, 1525. (17) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147. (18) Ogura, M.; Miyoshi, H.; Naik, S. P.; Okubo, T. J. Am. Chem. Soc. 2004, 126, 10937. (19) (a) Kim, M. J.; Ryoo, R. Chem. Mater. 1999, 11, 487. (b) Che, S.; Sakamoto, Y.; Terasaki, O.; Tatsumi, T. Chem. Mater. 2001, 13, 2237. (c) Chao, M. C.; Wang, D. S.; Lin, H. P.; Mou, C. Y. J. Mater. Chem. 2003, 13, 2853.
Published on Web 07/09/2009
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conditions. From both fundamental and practical standpoints, it is important to tune the mesostructure of the silica-surfactant composites in a predictable way. To the best of our knowledge, systematic control of the silica-alkyltrimethylammonium (halides) mesophase with organic additives has not yet been achieved. Such a process will eliminate the need to employ various surfactants with different packing parameters and regulate the reaction conditions. In this article, we report the structural variations of SBA-3-type17 mesoporous silica formed from a mixture of tetraethyl orthosilicate (TEOS), alkyltrimethylammonium bromides (C14TAB, C16TAB, and C18TAB), and varying amounts of 1,3,5-trialkylbenzenes under acidic conditions. Although trimethyl- and triethyl-benzene act as pore expanders to produce 2D hexagonal phases with larger periodicities, well-defined 3D cubic phases were formed in the presence of TIPB. Structural analyses by X-ray diffraction and transmission electron microscopy revealed that the mesostructure was changed from hexagonal p6mm to cubic Pm3n to cubic Fm3m with increasing amounts of TIPB. These results suggest that TIPB induces the preferential formation of spherical micelles rather than rodlike micelles.
Experimental Section Sample Preparation. Mesoporous silica materials were synthesized under acidic condition using TEOS (Tokyo Kasei Kogyo) as a silica source, C16TAB (Wako Pure Chemical Industry) as a structure-directing agent, and 1,3,5-trimethylbenzene (TMB, Tokyo Kasei Kogyo), 1,3,5-triethylbenzene (TEB, Aldrich), and 1,3,5-triisopropylbenzene (TIPB, Lancaster) as hydrophobic additives. In a typical procedure, TEOS was added to an acidic C16TAB solution containing trialkylbenzenes, and the mixture (pH 2.2) resulted only in the formation of less-ordered Fm3hm mesophases (data not shown). The SEM images of the calcined samples show different particle morphologies depending on the amount of TIPB (Figure 3). The hexagonal p6mm sample prepared without TIPB consists of columnar particles with hexagonal faces, as shown in Figure S2. Such a particle shape is typical of hexagonal mesoporous silica. However, the morphology of the particles was changed to more spherical by adding TIPB (Figure 3a,b), which is consistent with the change in the mesophase from hexagonal p6mm to cubic Pm3hn. In fact, the spherical particles (∼1.5 μm) obtained at x = 0.55 are morphologically similar to the typical Pm3hn silica-templated materials (SBA-1).19 Further increases in (22) (a) Matos, J. R.; Kruk, M.; Mercuri, L. P.; Jaroniec, M.; Zhao, L.; Kamiyama, T.; Terasaki, O.; Pinnavaia, T. J.; Liu, Y. J. Am. Chem. Soc. 2003, 125, 821. (b) Zhao, L.; Zhu, G.; Zhang, D.; Di, Y.; Chen, Y.; Terasaki, O.; Qiu, S. J. Phys. Chem. B 2005, 109, 764.
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Figure 4. Nitrogen adsorption-desorption isotherms of the calcined materials synthesized with TIPB: x=(a) 0, (b) 0.14, (c) 0.28, (d) 0.55, (e) 1.1, and (f) 2.2. The open circles and filled circles denote adsorption and desorption branches, respectively.
the amount of TIPB led to irregular morphologies of the products (Figure 3c,d). The nitrogen adsorption-desorption isotherms of the powder samples are shown in Figure 4. All of the samples exhibit type IV curves characteristic of mesoporous silica. The H2 hysteresis observed for the samples prepared in the presence of TIPB at x g 0.55 is suggestive of the formation of cage-type mesopores with narrow windows23 connecting the adjacent pores.21 The structural and porous properties obtained from the XRD and N2 adsorption-desorption measurements are summarized in Table 1. The BET surface areas for the samples are in the range of 615-1190 m2 g-1, and the total pore volumes are in the range of 0.63-0.97 cm3 g-1. The pore size increased from 3.2 to 4.8 nm with an increase in the TIPB ratio from x=0 to 2.2. It should be noted here that the pore diameter did not increase linearly with (23) Kruk, M.; Jaroniec, M. Chem. Mater. 2001, 13, 3169.
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Table 1. Structural and Porous Parameters of the Calcined Samples Synthesized with Varying Molar Ratios of TIPB (x) x
mesostructure
BET/m2 g-1
unit-cell parameter/nm
0 0.14 0.28 0.55 1.1 2.2
hexagonal (p6mm) hexagonal (p6mm) cubic (Pm3n) cubic (Pm3n) cubic (Fm3m) cubic (Fm3m)
954 862 821 1190 760 615
3.9 4.1 10.3 11.5 6.6 8.2
pore volume/cm3 g-1
NLDFT pore diameter/nm
0.63 0.66 0.73 0.97 0.85 0.68
3.2 3.5 4.3 4.7 4.8 4.8
Figure 5. XRD patterns of the calcined materials synthesized with different molar ratios of TMB: x=(a) 0, (b) 0.28, (c) 0.55, (d) 1.1, and (e) 2.2.
Figure 6. XRD patterns of the calcined materials synthesized with different molar ratios of TEB: x=(a) 0, (b) 0.28, (c) 0.55, (d) 1.1, and (e) 2.2.
increasing TIPB ratio; a large increase in the diameter from 3.2 to 4.3 nm was observed with x=0.28, which can be attributed to the drastic change in the mesostructure from 2D hexagonal to 3D cubic as evidenced by XRD and TEM. These results confirmed that the addition of TIPB can effectively induce the phase change from 2D hexagonal to 3D cubic, although it acts as a pore expander of the 2D hexagonal phase when the amount is relatively small. The presence of TIPB in the as-synthesized silica-surfactant composites before calcination was evidenced by FT-IR. The bands associated with aromatic dC-H bending (860 cm-1) and CdC stretching (1600 cm-1) vibrations were observed along with those of C16TAB and silica (Supporting Information, Figure S4). With the increase in the amount of TIPB from x= 1.1 to 2.2, these two bands together with that due to the CH3 stretching vibration (2960 cm-1) arising from both terminal methyl groups of C16TAB and isopropyl groups of TIPB) increased. We note that these bands were still observed even after the samples were washed with hexane. It is therefore reasonable to consider that TIPB molecules are located in the hydrophobic region of C16TAB micelles. The amounts of TIPB contained in the as-synthesized samples (before calcination) prepared with TIPB were evaluated by CHN elemental analysis. The molar ratios of TIPB/hexadecyltrimethylammonium cation were found to be 0.014, 0.019, and 0.083 when x=0.14, 0.55, and
2.2 (TIPB/C16TAB = 0.13, 0.5, and 2), respectively. Thus, the molar ratio of TIPB incorporated into the samples has increased with increasing x, being in agreement with the IR data, although the values are much smaller than those in the starting mixtures. It should be noted here that such a structural change from 2D hexagonal to 3D cubic is not observed when TMB was used as an additive. The XRD patterns of the calcined products prepared with varying amounts of TMB are shown in Figure 5. With the increase in the amount of TMB, the d10 spacing was increased and the peaks gradually became broad. The pore size was increased from 3.2 to 4.6 nm with the increase in the amount of TMB from x = 0 to x = 2.2 (data not shown). Such swelling behavior was originally reported for M41S-type mesoporous silica. When TEB was used as an additive, the 2D hexagonal phase was formed at up to x=0.55 (Figure 6). Although further increases in the amount of TEB resulted in a significant change in the XRD profiles (Figure 6d,e), the observed peaks are broad and ill-defined, suggesting the formation of less-ordered structures. The phase behavior was strongly affected by the difference in the R groups in 1,3,5-trialkylbenzenes. This can be attributed to the different locations of the organic additives in C16TAB micelles, as shown in Figure 7. In general, the morphology of surfactant assemblies is explained in terms of the geometrical packing parameter,g=V/(a0l), where V is the effective volume of
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DOI: 10.1021/la901263p
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Figure 7. Possible locations of (a) TMB, (b) TEB, and (c) TIPB in the silica-surfactant (C16TAB) composite micelles.
the hydrophobic chains, a0 is the effective area of the hydrophilic headgroup, and l is the hydrophobic chain length.16,17 When hydrophobic organic molecules are located between the alkyl chains of C16TAB (Figure 7a), then the V value should be increased and the formation of mesophases with lower surface curvatures is expected. Recent research based on 1H NMR and EPR analyses has revealed that TMB molecules can be located between the alkyl chains of C16TAB micelles.24,25 It is therefore reasonable that swollen 2D hexagonal phases, having lower surface curvatures than the nonswollen 2D hexagonal phase, were formed by the addition of TMB. In contrast, the formation of cubic mesophases in the presence of TIPB can be explained by the localization of TIPB in the middle of the micelles (Figure 7c), presumably as a result of the larger and more bulky nature of TIPB that should inhibit their intercalation between the alkyl chains. In such a case, the packing parameter of C16TAB does not change significantly, favoring the formation of swollen spherical micelles, the surface curvature of which is higher than that for the swollen rodlike micelles but is lower than for nonswollen spherical micelle. Similar results have recently been obtained in the microemulsion system containing a nonionic alkyl polyoxyethylene-type surfactant (Brij56) and n-alkanes. Long-chain alkanes (C9-C19) are solubilized in the deep core of Brij56 micelles, thus increasing the surface curvature of the micelles from p6mm f Pm3n f Fm3m.3 However, a different behavior has been observed for block copolymers forming larger hydrophobic domains. Zhou et al. have recently reported the synthesis of multilamellar vesiclelike silica by using triblock copolymer P123 and the TIPB additive.26 It is proposed that TIPB adjusts the packing parameter of P123, inducing a phase transition from cylindrical micelles (1/3
