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Templating Behavior of a Long-Chain Ionic Liquid in the Hydrothermal Synthesis of Mesoporous Silica Tongwen Wang,†,‡ Helena Kaper,† Markus Antonietti,† and Bernd Smarsly*,† Max Planck Institute of Colloids and Interfaces, Research Campus Golm, D-14424 Potsdam, Germany, and Department of Chemistry, Yunnan Normal UniVersity, Kunming 650092, P. R. China ReceiVed August 22, 2006. In Final Form: October 11, 2006 The long-chain ionic liquid (IL) 1-hexadecyl-3-methylimidazolium chloride (C16mimCl) was used as a template to prepare porous silica with a two-dimensional hexagonal p6mm mesopore structure (MCM-41-type) as well as with a cubic Ia3d (gyroid, MCM-48-type structure) framework in basic synthesis medium via a hydrothermal synthesis procedure. A systematic study was carried out addressing the influence of the relative concentration of the IL and the pH on the mesostructure. As a main result, the IL template shows a broad range of conditions that allow the synthesis of the gyroid mesostructure with improved reproducibility. Unexpectedly, the formation of the hexagonal phase is less favorable, as the latter is quite sensitive to variations in parameters. This preference for the bicontinuous structure can be attributed to the special headgroup packing of ILs containing imidazolium groups.
Introduction In the past decade, significant effort has been focused on developing ordered periodic mesoporous materials with welldefined controlled pore channels (pore size > 2 nm) by sol-gel templating using amphiphilic surfactants as templates.1-4 The preparation methods can be classified into two strategies: in the so-called “nanocasting” process (true liquid-crystal templating), a concentrated lyotropic mesophase is converted into its mesoporous replica in a 1:1 copy process, enabling the tuning of the mesopore morphology by appropriate choice of the surfactant.5 In contrast, hydrothermal methods are usually based on recipes starting from dilute solutions. Also in this case, a variety of mesophase configurations can be obtained, such as two-dimensional (2D) hexagonal (p6mm) MCM-41,1,2 SBA-3,6,7 cubic (Ia3d) MCM-48,1,2 three-dimensional hexagonal (P63/mmc) SBA-2, and cubic (Pm3m) SBA-11.8 For the synthesis of MCMtype materials, usually a “cooperative” interaction between ionic surfactants and siliceous precursors leads to the desired structure. Especially recipes resulting in sought-after cubic bicontinuous structures, such as the gyroid structure, involve certain difficulties, which is due to a narrow region in the phase diagram. In addition, the cubic bicontinuous structure is, so far, only accessible by a rather limited number of different surfactants and peculiar preparation conditions.9 Hence, in spite of the significant progress in this field, there is still a need for alternative structure-directing agents with improved templating properties. * Corresponding author. E-mail:
[email protected]. Tel.: +49 331 567 9509; Fax: +49 331 567 9502. † Max Planck Institute of Colloids and Interfaces. ‡ Yunnan Normal University. (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) Corma, A. Chem. ReV. 1997, 97, 2373. (4) De Vos, D. E.; Dams, M.; Sels, B. F.; Jacobs, P. A. Chem. ReV. 2002, 102, 3615. (5) Attard, G. S.; Glyde, J. G.; Goltner, C. G. Nature 1995, 378, 366-368. (6) Huo, Q.; Margolese, D. I.; Ciesla, U.; Feng, P.; Gier, T. E.; Sieger, P.; Leon, R.; Petroff, P. M.; Schu¨th, F.; Stucky, G. D. Nature 1994, 368, 317. (7) Huo, Q.; Leon, R.; Petroff, P. M.; Stucky, G. D. Science 1995, 268, 1324. (8) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024.
Recently, ionic liquids (ILs) have attracted considerable interest not only in chemistry, but also in materials science.10,11 ILs have been utilized as clean solvents and catalysts for green chemistry10 and as electrolytes for batteries, photochemistry, and electrosynthesis,12,13 yet their potential as templates and reaction media for nanostructured materials is less commonly known. ILs derived from 1-alkyl-3-methylimidazolium are of particular interest because, by changing the alkyl chain length or the anion, a wide variation of properties such as hydrophobicity, viscosity, density, and solvation strength can be obtained.14 The long-chain ILs display the behavior of both lyotropic and thermotropic liquid crystals,15-20 and, recently, the advantages of amphiphilic IL derivatives in the introduction of ordered self-organized structures have received more attention.21-26 For example, the long-chain 1-hexadecyl-3-methylimidazolium chloride (C16mimCl) displayed significantly stronger tendency toward self-aggregation and supramolecular templating in the preparation of supermicroporous lamellar silica by nanocasting, owing to the distinct polarizability of the head groups and the special high-concentration phases of those ILs.21-24 Recently, Adams demonstrated that long-chain ILs (among them also C16mimCl) can be used to generate mesoporous silica with a 2D hexagonal structure of cylindrical mesopores (MCM-41-type material) using a hydrothermal synthesis approach,26 but no other mesopore morphologies have been reported. The objective of the present study is twofold: First, it is intended to demonstrate that long-chain ILs can also serve as templates (9) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147. (10) Welton, T. Chem. ReV. 1999, 99, 2071. (11) Seddon, K. R. Nat. Mater. 2003, 2, 363. (12) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Exnar, I.; Gratzel, M. J. Am. Chem. Soc. 2003, 125, 1166. (13) Dzyuba, S. V.; Bartsch, R. A. Angew. Chem. Int. Ed. 2003, 42, 148. (14) Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H. D.; Broker, G. A.; Rogers, R. D. Green Chem. 2001, 3, 156. (15) Bleasdale, T. A.; Tiddy, G. J. T.; Wyn-Jones, E. J. Phys. Chem. 1991, 95, 5385. (16) Neve, F.; Francescangeli, O.; Crispini, A. Inorg. Chim. Acta 2002, 338, 51. (17) Lee, C. K.; Huang, H. W.; Lin, I. J. B. Chem. Commun. 2000, 19, 1911. (18) Yoshio, M.; Mukai, T.; Kanie, K.; Yoshizawa, M.; Ohno, H.; Kato, T. AdV. Mater. 2002, 14, 351 (19) Holbrey, J. D.; Seddon, K. R. J. Chem. Soc., Dalton Trans. 1999, 13, 2133. (20) Neve, F.; Francescangeli, O.; Crispini, A.; Charmant, J. Chem. Mater. 2001, 13, 2032.
10.1021/la062470y CCC: $37.00 © 2007 American Chemical Society Published on Web 12/08/2006
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for the cooperative templating approach to generate ordered MCM-48-like silica with an Ia3d gyroid structure, as exemplified for C16mimCl. Second, our study addresses the question of whether these ILs, owing to their headgroup, indeed show a different or even improved templating behavior in hydrothermal syntheses, compared to the commonly used cetyltrimethylammonium bromide (CTAB) and cetyltrimethylammonium chloride (CTAC) possessing identical alkyl chains. Hence, for both cubic and 2D hexagonal mesopore structures, the relative concentrations were varied to assess the differences in the templating performance. Such a systematic comparison can contribute to a better understanding of the special role of the head group of templates in sol-gel templating in general. The materials were characterized by transmission electron microscopy (TEM), nitrogen sorption measurements, small-angle X-ray scattering (SAXS), and infrared spectroscopy (IR). Experimental Section Preparation of C16mimCl. The IL C16mimCl was synthesized according to a reported route.24,27 All chemicals were purchased from Aldrich and used as received. As a typical synthesis, an excess of 1-hexadecylchloride (33.41 g, 0.128 mol) was mixed with 1-methylimidazole (10.26 g, 0.125 mol). The mixture was put into a 250 mL flask, refluxed at 90 °C for 48 h, and then cooled to room temperature. The product was further purified by recrystallization in tetrahydrofuran (THF). After being washed several times with THF, the crystalline C16mimCl white powder was collected by vacuum filtration and dried in air at room temperature. Preparation of Cubic and Hexagonal Mesostructured Silica with C16mimCl as the Template. Cubic (MCM-48) and hexagonal (MCM-41) mesoporous samples were prepared in basic medium by a hydrothermal synthesis procedure using tetraethylorthosilicate (TEOS) as the silica source and C16mimCl as the structure-directing agent. In the synthesis of the gyroid phase, C16mimCl and NaOH were dissolved in deionized water under mild magnetic stirring. After homogenization of the mixture, TEOS was added dropwise at 45 °C. The pH of the reaction mixture was 13. The molar compositions of the starting mixtures were 1 TEOS/n C16mimCl/ 0.512 NaOH/56 H2O (n ) 0.109, 0.218, 0.272, 0.325, 0.543, or 1.086). The resulting mixtures were stirred at 45 °C for 60 min and then transferred into a PTFE-lined steel autoclave and heated at 100 °C for 3 days. After hydrothermal treatment, the mixtures were filtered, washed with deionized water, dried under atmosphere at room temperature, and finally calcined at 550 °C for 5 h with a temperature ramp of 2 °C min-1 under static air conditions to remove the template. The final product was ground into powder for further characterization. The 2D hexagonal mesoporous samples were synthesized by the same hydrothermal procedure, except that the molar compositions of the starting mixtures were 1 TEOS/n C16mimCl/0.45 NaOH/111.5 H2O (n ) 0.107, 0.213, 0.319, 0.531, 0.743, or 1.065), and, after adding TEOS, strong hydrochloric acid was dripped into this mixture to adjust the pH value to 9. Preparation of Hexagonal Mesostructured Silica with CTAB as the Template. Mesostructured silica using CTAB as the template was synthesized using a mixture of 1 TEOS/0.10 CTAB/0.3 NaOH/ 60 H2O. First, 0.61 g of CTAB (1.7 mmol) and 0.2 g of NaOH (5 mmol) were dissolved in 18 mL of water. To this mixture, 3.5 g of TEOS was added dropwise under mild stirring. The starting solution (21) Antonietti, M.; Kuang, D. B.; Smarsly, B.; Yong, Z. Angew. Chem., Int. Ed. 2004, 43, 4988. (22) (a) Zhou, Y.; Antonietti, M. Chem. Commun. 2003, 2564. (b) Kuang, D.; Brezesinski, T.; Smarsly, B. J. Am. Chem. Soc. 2004, 126, 10534. (c) Smarsly, B.; Kuang, D.; Antonietti, M. Colloid Polym. Sci. 2004, 282, 892. (d) Zhou, Y.; Schattka, J. H.; Antonietti, M. Nano. Lett. 2004, 4, 477. (23) Zhou, Y.; Antonietti, M. AdV. Mater. 2003, 15, 1452. (24) Zhou, Y.; Antonietti, M. Chem. Mater. 2004, 16, 544. (25) Cooper, E. R.; Andrews, C. D.; Wheatley, P. S.; Webb, P. B.; Wormald, P.; Morris, R. E. Nature 2004, 430, 1012. (26) Adams, C. J.; Bradley, A. E.; Seddon, K. R. Aust. J. Chem. 2001, 54, 679. (27) Seddon, K. R.; Stark, A.; Torres, M. J. Pure Appl. Chem. 2000, 72, 2275.
Figure 1. SAXS pattern of C16mimCl-templated cubic mesoporous silicas. Molar composition of the reaction mixture: 1.0 TEOS/n C16mimCl/0.512 NaOH/55.77 H2O, with n ) (I) 0.22, (II) 0.27 (a: calcined sample, b: uncalcined), and (III) 0.54. was stirred at 45 °C for 1 h and then transferred to a poly(tetrafluoroethylene) (PTFE)-lined steel autoclave and heated at 100 °C for 3 days. After hydrothermal treatment, the mixture was filtered and washed with deionized water. Characterization. SAXS measurements were performed using a D8 diffractometer from Bruker instruments (wavelength 0.154 nm). TEM images with high contrast were obtained using a Zeiss EM 912Ω at an acceleration voltage of 120 kV. Samples were ground in a ball-mill and taken up in ethanol. One droplet of the suspension was applied to a 400-mesh carbon-coated copper grid and left to dry in air. Nitrogen sorption experiments were conducted using a Micromeritics Tristar 3000 automated gas adsorption analyzer. Before sorption measurements, the samples were degassed in a Micromeritics VacPrep061 degasser overnight at 150 °C under 100 µTorr pressure. A Bio-Rad 6000 Fourier transform infrared (FT-IR) spectrometer equipped with a single reflection diamond attenuated total reflection was employed for recording IR spectra. Simulation of the spectra of C16mimCl and CTAB were carried out using B3LYP/6-311 in Gaussian 03 after geometry optimization.
Results and Discussion C16mimCl-Templated Cubic Gyroid Mesoporous Structure. Similar to previous studies using CTA-based surfactants, the concentration of C16mimCl in reactant compositions is the primary dictating factor for the formation of the cubic mesostructure. Figure 1 shows the SAXS powder pattern of calcined samples prepared using C16mimCl as the structure-directing agent
C16mimCl Templating BehaVior in Silica Synthesis
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Figure 3. N2 sorption isotherm of calcined C16mimCl-templated cubic mesoporous silica with a C16mimCl/Si molar ratio of 0.27. The inset shows a plot of the pore size distribution (calculated from the desorption branch by the BJH method).
Figure 2. TEM images of calcined C16mimCl-templated cubic mesoporous silica recorded along the (a) 110 and (b) 111 directions. The sample was prepared from a reaction mixture with a molar composition of 1.0 TEOS/0.27 C16mimCl/0.512 NaOH/56 H2O.
from a reaction mixture with a molar composition of 1 TEOS/x C16mimCl/0.512 NaOH/56 H2O. The patterns consist of several distinguishable Bragg peaks that can be indexed to different hkl reflections with reciprocal spacing, 1/dhkl, ratios of 31/2, 2, 71/2, 81/2, 101/2, 111/2, 121/2, and so forth. These well-defined SAXS peaks verify the presence of a cubic gyroid phase (Ia3d) with a unit-cell dimension of ∼9 nm, in good agreement with the patterns from the siliceous MCM-48 prepared with quaternary ammonium surfactants.9 SAXS data reveal that, with an increasing molar ratio of C16mimCl/Si, a gradual transition from a gyroid phase with certain polydispersity (Figure 1-I) over a highly ordered gyroid structure (Figure 1-II) to a less ordered structure (Figure 1-III) occurs, spanning a relatively large range of relative IL/ SiO2 concentrations in the starting solution. The effect upon calcination is demonstrated in Figure 1-II: reflections from the corresponding calcined sample (Figure 1-IIa) are more pronounced than those of the as-synthesized one because of the increase in the scattering contract (Figure 1-IIb), also indicating that the degree of order is not disturbed by template removal. TEM images of C16mimCl-templated SiO2 with gyroid structure in Figure 2a,b show projections along the [110] and [111] zone axes of the cubic phase, respectively, corresponding to TEM studies on MCM-48 templated by quaternary ammonium ions.28,29 Also the representative nitrogen sorption isotherm shown in Figure 3 (type IV according to IUPAC30,31) suggests a uniform (28) Alfredsson, V.; Anderson, M. W. Chem. Mater. 1996, 8, 1141. (29) Romero, A. A.; Alba, M. D.; Zhou, W.; Klinowski, J. J. Phys. Chem. B 1997, 101, 5294. (30) Thommes, M.; Ko¨hn, R.; Fro¨ba, M. J. Phys. Chem. B 2000, 104, 7933. (31) Sing, K. S. W.; Everett, D. H.; Houl, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603.
pore size, which is confirmed by the pore size distribution of the adsorption branch (inset). Next to that, in some cases, a hysteresis at a relatively high relative pressure above p/p0 ) 0.5 could be observed. Since the desorption loop closes at ∼p/p0 ) 0.42, this phenomenon is probably attributable to grain boundaries, creating small cavities between adjacent ordered regions. In this case, such larger “pores” are connected to the surrounding region via smaller pores, and Schumacher et al. observed this type of hysteresis in some MCM-48 materials,32 which was attributed to the filling of a secondary pore structure. While the SAXS, TEM, and sorption results themselves are in agreement with previously reported continuous cubic structures, it is more interesting in the present context to study the impact of the initial IL/SiO2 ratio on the final mesostructure and porosity to assess the peculiarities of the IL template. Thus, it is crucial to investigate whether the final material is homogeneous, what amount of template is integrated and how (self-assembled or molecularly), and, finally, whether the pores are accessible. Therefore, one important parameter to look at is the volume fraction of pores in the final material obtained from nitrogen sorption. When compared to the theoretical possible volume fraction, information about the homogeneity of the material and the integration of the template in the material is gained. Since the IL aggregates into micelles, it would be more accurate to look at the weight fraction of C16mimCl instead of the volume fraction, but, from nitrogen sorption measurements, only the volume fraction of pores is obtained as a direct parameter. Thus, in Table 1, the volume fraction φ obtained from nitrogen sorption, φvolume, the theoretical volume fraction of C16mimCl, and φweight, the theoretical weight fraction of C16mimCl in the silica, are compared. It is seen that the final volume fraction of mesopores in the solid material is already quite high (φ ) 0.56) at moderate IL/SiO2 solution ratios, quickly reaching a maximum value of ∼0.7-0.75 for all other concentrations. This finding implies that increasing the relative IL template concentration into the starting solution beyond a minimal critical value has no effect on the volume fraction of the final product, although the quality of ordering can still vary within one sample. This also means that precipitation of the hybrid materials occurs only until the IL is depleted. Hence, already at relative moderate molar ratios of IL/Si ) 0.33 (in the starting composition), optimum templating conditions are fulfilled to obtain the demanded final gyroid structure. Next to that, the mesopore size and the d211-spacing remain almost unchanged with increasing concentration of C16(32) Schumacher, K.; Ravikovitch P.; Du Chesne, A.; Neimark, A. V.; Unger, K. K. Langmuir 2000, 16, 4648.
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Table 1. Structural Parameters of the Calcined Cubic MCM-48 Mesoporous Materials with Different IL/Si Molar Ratiosa
sample
IL/Si molar ratio
d211 (nm)
SBET (m2 g-1)
Vt (cm3 g-1)
DBJH (nm)
a0 (nm)
φ
φvolume
φweight
a b c d e f
0.11 0.22 0.27 0.33 0.54 1.09
3.5 3.4 3.7 3.7 3.6 3.8
880 1300 1250 1290 1050 1260
0.58 0.94 1.04 1.33 1.07 1.29
2.5 2.5 2.6 3.0 3.2 3.3
8.5 8.2 9.0 9.0 8.7 9.2
0.56 0.67 0.70 0.75 0.70 0.74
0.61 0.76 0.80 0.83 0.89 0.94
0.42 0.59 0.64 0.68 0.78 0.88
a Abbreviations: d211, interplanar spacing; SBET, BET specific surface area; Vt, total pore volume; DBJH, BJH desorption average pore diameter; a0, the lattice parameter calculated from SAXS data using the formula a0 ) d211x6; φ, volume fraction of pores obtained from the total pore volume; φvolume, theoretical volume fraction of C16mimCl calculated from the amount of C16mimCl under the assumption that all IL aggregates into micelles inside the reaction mixture φ ) V(IL)/(V(IL)+V(SiO2)). The density of the IL was approximated as 1.0 mL/g, and that of SiO2 was approximated as 2.2 mL/g. φweight is the theoretical weight fraction of IL in the silica.
mimCl, and the pore size increases only slightly within a certain range of experimental accuracy. This independence of the porosity upon variation of the initial relative IL concentration also suggests a low amount of disordered, nontemplated silica. Hence, the highly structured MCM-48 samples probably contain only a negligible amount of nontemplated or unimolecular templated silica, that is, they are quite homogeneous. Only at high ratios of IL/SiO2, the material becomes more inhomogeneous. The porosity data thereby show that, within a certain, broad concentration range, a situation of “ideal” templating is achieved, that is, the final material consists only of mesostructured ILSiO2 hybrids and practically no nonorderd silica precipitates. The fact that a well-defined gyroid structure with a high relative template fraction and excellent inner order is obtained at already low IL/SiO2 ratios points to a significant bonding strength between the IL and the growing silicate species. C16mimCl-Templated 2D Hexagonal Mesoporous Structure. Hexagonal mesostructures were obtained when C16mimCl was used at a lower concentration and pH compared to that of the cubic mesophase. Figure 4-I shows the SAXS patterns of as-synthesized and calcined samples prepared from a reaction mixture with a molar composition of 1 TEOS/0.32 C16mimCl/ 0.45 NaOH/112 H2O. The pattern features characteristic Bragg reflections of the 2D p6mm hexagonal mesostructure, and the d100-spacing is in the same size range as that described for the siliceous MCM-41 prepared using quaternary ammonium ion surfactants.1,2 TEM images along the [100] (Figure 5a) and [001] (Figure 5b) directions also reveal a highly ordered hexagonal arrangement of one-dimensional channels. Nitrogen physisorption further confirms the existence of uniform mesopores (Figure 6, Table 2). However, the synthesis is characterized by low reproducibility and a related high sensitivity to small changes in the reaction conditions. As seen in Figure 4-II, the product synthesized via the same route with an only slightly lower concentration of C16mimCl is much less ordered. At higher concentrations of C16mimCl, the structure gets significantly disordered (Figure 4-III), probably because of a substantial mismatch of the IL and silica concentrations. As a further marked difference toward the recipes leading to the gyroid mesostructure, no regular trends regarding the surface area, d-spacing, and pore size can be observed for the hexagonal IL-silica (Table 2), which could be explained, for instance, by the presence of larger fractions of disordered, nontemplated silica in the precipitate. Although the volume fraction of pores eventually reaches high values up to 0.7, a pronounced deviation is observed in relation to the φvolume and φweight calculated from the initial IL/SiO2 ratio. In summary, in the present set of experiments, cubic (Ia3d, gyroid structure) and cylindrical mesopore silica frameworks
Figure 4. SAXS pattern of calcined C16mimCl-templated hexagonal mesoporous silicas. Molar composition of the reaction mixture: 1.0 TEOS/n C16mimCl/0.45 NaOH/111.5 H2O, with n ) (I) 0.32 (a: as-synthesized, b: calcined), (II) 0.3, and (III) 0.74.
could be obtained using a long-chain IL as the structure-directing agent. The option of preparing gyroid mesopore structures is notable itself because only few surfactants have so far been reported to generate this delicate pore morphology. Hence, differences in the hydrothermal synthesis compared to standard surfactants such as CTAB/CTAC are to be discussed. In general, the formation mechanism of mesoporous silica by the cooperative mechanism was previously subject to detailed mechanistic studies and explained by an electrostatic charge density matching pathway between the structure-directing cationic surfactant (S+, 1-methylimidazolium or CTA+ cation, etc.) and the frameworkforming silicate oligomers (I-).9,33 As one of the common features
C16mimCl Templating BehaVior in Silica Synthesis
Figure 5. TEM images of calcined C16mimCl-templated hexagonal mesoporous silica recorded along the (a) 100 and (b) 001 directions. The sample was prepared from a reaction mixture with a molar composition of 1.0 TEOS/0.21 C16mimCl/0.45 NaOH/112 H2O.
Figure 6. N2 sorption isotherm of calcined C16mimCl-templated hexagonal mesoporous silica with a C16mimCl/Si molar ratio of 0.21:1. The inset shows a plot of the pore size distribution (calculated by the BJH method using the desorption branch).
of the diverse MCM-48 syntheses, a high concentration of surfactants (especially CTAB) and a high CTAB/Si or CTAC/Si ratio as well as the presence of swelling agents such as ethanol or hydrophobic organic additives (e.g., trimethyl benzenes) were reported to be essential conditions.9,34,35 In Table 3, different recipes for the synthesis of MCM-48 are compared. A representative synthesis of MCM-48 was presented by Monnier et al.34 In this system, the CTAC/Si molar ratio and the concentration of CTAC were 0.65:1 and 13 wt %, respectively. So far, only a few surfactants other than CTAB allowed the synthesis of MCM-48, for instance, cetyldimethylbenzylammonium (CDBA).9
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In our case, for C16mimCl, the cubic mesopore structure is obtained at rather low IL/SiO2 ratios in the starting solution, compared to CTAB/CTAC, and also the formation of the gyroid structure is favored compared to the 2D hexagonal structure, even in the absence of any additives. These differences can be attributed to two particular features of imidazolium-containing surfactants, namely, a stronger binding strength to charged silica species and also a higher binding density; the relatively low IL/SiO2 ratio, at which mesostructured SiO2 is formed, has to be attributed to the strong interaction between the imidazolium group and the charged silica oligomers. Interestingly, nature takes advantage of the special behavior of the imidazolium ring: marine organisms synthesize structured silica using imidazoliumcontaining proteins as catalysts, and the role of the imidazolium has been investigated in several studies.36 In order to understand the mechanism of the formation of the mesostructured silica, the phase diagram of the template and the solvent as well as interactions between the template and the precursors have to be taken into account. The phase diagrams of CTAC and C16mimCl in water both do not reveal a cubic phase, but only 2D cylindrical and lamellar structures at concentrations higher than 60 wt % and a micellar solution below.37,38 The absence of certain mesophases in aqueous phases, which, however, are observed in silica mesostructures, is frequently observed for various surfactants. The critical micelle concentrations of CTAC and C16mimCl differ only slightly (CTAC: 1.3E-3 mol/L, C16mimCl: 1.0E-3 mol/l).38 Therefore, the interaction between the solvent and the structure-directing agent are nonpredominant. More relevant are the interactions between the structure-directing agent and the silica species, as common for the cooperative mechanism in hydrothermal synthesis. In comparison to CTAC, C16mimCl possesses a different binding ability to Si oligomers. Because of interactions between the extended hydrogen-bridge system of imidazoliumbased ILs and the silica oligomers, high cooperative binding at a lower relative template concentration can be obtained. Thus, the point where the silica precipitates occurs at a lower absolute concentration of the structure-directing agent. The effect of binding can be observed when comparing the IR spectra of the pure IL to the one of the IL/SiO2 hybrid (Figure 7). In comparison to the IR spectrum of the pure IL, the IR spectrum of the cubic IL/SiO2 hybrid vibration bands that are related to the imidazolium headgroup disappear or are shifted. In Table 4, the vibrations of the recorded pure C16mimCl and the C16mimCl-SiO2 nanocomposite and the simulated spectrum of C16mimCl are compared. The assignment of the bands was carried out according to the simulation using B3LYP/6-311 in Gaussian 03 after geometry optimization and is similar to that of refs 39 and 40. In the spectrum of the IL/SiO2 hybrid, the bands at 3460 cm-1 and 3408 cm-1 that are present in the spectrum of the pure IL (33) (a) Oliver, S.; Kuperman, A.; Coombs, N.; Lough, A.; Ozin, G. Nature 1995, 378, 47. (b) Tanev, P. T.; Pinnavaia, T. J. Science 1996, 271, 1267. (c) Kim, S. S.; Zhang, W. Z.; Pinnavaia, T. J. Science 1998, 282, 1302. (d) Mercier, L.; Pinnavaia, T. J. Chem. Mater. 2000, 12, 188. (34) Monnier, A.; Schuth, F.; Huo, Q.; Kumar, D.; Margolese, D.; Maxwell, R. S.; Stucky, G. D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Science 1993, 261, 1299. (35) Vartuli, J. C.; Schmitt, K. D.; Kresge, C. T.; Roth, W. J.; Leonowicz, M. E.; McCullen, S. B.; Hellring, S. D.; Beck, J. S.; Schlenker, J. L.; Olson, D. H.; Sheppard, E. W. Chem. Mater. 1994, 6, 2317. (36) (a) Zhou, Y.; Shimizu, K.; Cha, J. N.; Stucky, G. D.; Morse, D. E. Angew. Chem., Int. Ed. 1999, 38, 779. (b) Delak, K. M.; Sahai, N. Chem. Mater. 2005, 17, 3221. (c)Tahir, M. N.; The´ato P.; Mueller, W. E. G.; Schroeder, H. C.; Janshoff, A.; Zhang, J.; Huth, J.; Tremel, W. Chem. Commun. 2004, 24, 2848. (d) Patwardhan, S. V.; Clarson, J. J. Inorg. Organomet. Polym. 2003, 13, 49. (37) Blackmore, E. S.; Tiddy, G. J. T. Liq. Cryst. 1990, 8, 131. (38) Kaper, H.; Smarsly, B. Z. Phys. Chem. 2006, 220, 1455. (39) Fitchett, B. D.; Conboy, J. C. J. Phys. Chem. 2004, 108, 20255. (40) Perchard, C.; Novak, A. Spectrochim. Acta 1967, 23A, 1953.
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Table 2. Structural Parameters of the Calcined Hexagonal MCM-41 Mesoporous Materials with Different IL/Si Molar Ratiosa, Illustrating the Low Reproducibility for This Type of Mesostructure sample
ILs/Si ratio
d100 (nm)
SBET (m2 g-1)
Vt (cm3 g-1)
DBJH (nm)
a0 (nm)
φ
φvolume
φweight
a b c d e f
0.10 0.21 0.32 0.53 0.74 1.07
4.0 3.9 4.0 3.9 4.3 3.7
890 1100 1200 950 900 850
1.02 0.99 1.04 0.64 1.08 0.78
4.2 3.3 3.4 3.1 3.6 3.2
4.6 4.5 4.6 4.5 5.0 4.2
0.69 0.69 0.70 0.59 0.71 0.63
0.61 0.76 0.82 0.89 0.92 0.94
0.41 0.58 0.68 0.78 0.83 0.88
a Abbreviations: d 100, interplanar spacing; SBET, BET specific surface area; Vt, total pore volume; DBJH, BJH desorption average pore diameter; a0, the lattice parameter calculated from SAXS data using the formula a0 )2d100/x3; φ, volume fraction of pores obtained from the total pore volume; φvolume, volume fraction of C16mimCl calculated from the amount of C16mimCl in the reaction mixture φ ) V(IL)/(V(IL)+V(SiO2)). The density of the IL was approximated as 1.0 mL/g, and that of SiO2 was approximated as 2.2 mL/g. φweight is the theoretical weight fraction of IL in the silica.
Table 3. Comparison of Recipes for the Synthesis of MCM-48-like Mesoporous Silicaa author
TEOS
template
NaOH
this recipe Monnier31 Vartuli35 Kresge1 Huo7 Huo9
1 1
C16mimCl 0.11-1.09 CTAC 0.65 CTAB/Si ) 1.2-2 CTAB/Si > 1 gemini 0.05 CDBA 0.1
0.5 0.25
1 1
0.62 n.a.
H2O 56 62 115 n.a.
a Given are the molar ratios of the educts according to the literature, as far as available.
Table 4. Wavenumbers (cm-1) of the Vibrational Modes of the Pure IL, the IL/SiO2 Hybrid Prior to Calcination and the Calculated Spectra of the Pure IL Using B3LYP/6-311 in Gaussian 03 after Geometry Optimization vibration assignment
pure IL
Γ Γ γ γ γ δ ∆ ∆ νs CH2 chain νas CH2 chain νas CH3 chain νs N-CH3 νas CH (4,5) νs CH (4,5) ν1 H2O ν3 H2O
622 665 713 800 865 1178 1472 1570 2849 2915 3053 3084 3142 3155 3408 3460
silica-IL nanocomposite 620 650 720 788 n.m. 1162 1467 1573 2850 2919 n.m. n.m. 3155 broad n.m. n.m.
simulated 659 678 830 830 1170 1587 1612 3025 3083 3108 3118 3337 3337 n.s. n.s.
Notations: Γ, out of plane deformation of skeleton atoms; γ, out of plane deformation; δ, in plane deformation of X-H bond; ∆, in plane deformation of skeleton atoms; ν, stretching vibration of X-H bond; s, symmetric; as, antisymmetric; n.m., not measurable; n.s., not stimulated.
Figure 7. IR spectra of (a) pure C16mimCl and (b) a SiO2-C16mimCl nanocomposite. The bands marked with a star vanish in the SiO2-C16mimCl nanocomposite
are absent. These two bands can be assigned to the antisymmetric ν3 and symmetric ν1 stretching modes of water, where the water interacts with the anion via H-bonding in a symmetric complex Cl-‚‚‚H-O-H‚‚‚Cl-.41 In the silica, this interaction cannot be present, since water interacts with the silica and the chloride is not necessarily included in the structure. Next to that, the two characteristic bands of the imidazolium ring around 3155 cm-1 and 3142 cm-1, which correspond to the symmetric and asymmetric stretch of the HCCH bond in positions four and five of the imidazolium ring,39 merge into one broadband around 3155 cm-1. These effects already demonstrate a strong interaction between the imidazolium headgroup and the silica matrix. But the modes of the imidazolium ring that appear in the fingerprint region are also shifted and broadened. However, this area should be evaluated with caution since the broad bands of the silica itself are present in this region and overrule the bands of the IL. On the other hand, the strong bands around 2849 cm-1 and 2913 cm-1 belonging to the alkyl chain are still present and not shifted in the spectrum of the hybrid. The asymmetric stretching band (41) Cammarata, L.; Kazarin, S. G.; Salter, P. A.; Welton, T. Phys. Chem. Chem. Phys. 2001, 3, 5192. (42) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Yu-Tai; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152.
at 3084 cm-1 can be assigned to the methyl group bound to the imidazolium ring and vanishes completely when the IL is bound in the silica. The same effect is observed for the IR spectra of CTAB compared to the spectrum of the CTAB/SiO2 hybrid; also, in this case, the corresponding band of the methyl group at 3035 cm-1 disappears (see Supporting Information). The absence of this band is likely due to geometry changes in the alkyl chain inside the pores compared to the bulk material.42 IR studies therefore allow concluding that the imidazolium ring is strongly bound into the silica matrix. The higher packing density of the present template (compared to CTAB/CTAC) obviously complies with the gyroid structure. It is well-known that the disklike imidazolium head groups have a strong tendency to arrange themselves mutually in a parallel fashion, which is further enhanced by hydrogen bonding and the polarizability of the headgroups. Hence, applying the basic principles of self-assembly, this higher binding density results in a mesostructure with a preferably low average curvature, ideally a lamellar or bicontinuous cubic phase such as the gyroid phase. These findings also go well with the recently reported high degree of the mesoscopic order of lamellar SiO2/C16mimCl mesostructures made by nanocasting.22
Conclusion The present study has demonstrated that the long-chain IL C16mimCl is a template for the convenient synthesis of high-
C16mimCl Templating BehaVior in Silica Synthesis
quality cubic gyroid mesoporous silica (MCM-48-type) under basic conditions. By appropriate matching of C16mimCl, the silica precursor, and the reaction composition, the formation of a MCM48-type phase is highly reproducible over a quite wide range of IL/SiO2 ratios, while the formation of a hexagonal phase is observed, but only at the borderlines of the parameter space. Different from previous syntheses (Table 3) of MCM-48 using CTAB, in our C16mimCl-templated systems, the cubic mesophase was synthesized using a relatively low C16mimCl/Si molar ratio (0.27:1) and a low concentration of C16mimCl, owing to the high bonding density and strength of the IL to silicate oligomers. The synthesis with C16mimCl leads to a material with a higher volume fraction of pores and good homogeneity. Similar to the previous successful preparation based on CTAB, our MCM-48-type silica is well ordered according to SAXS, physisorption, and TEM, featuring a typical mesopore size of ∼3 nm, together with large surface areas (1200 m2 g-1). The peculiarities in the usage of C16mimCl as a template can be
Langmuir, Vol. 23, No. 3, 2007 1495
attributed to the anisotropic headgroup, which is in line with our recent findings on the use of ILs for nanocasting. Further work will be undertaken to employ the unique templating behavior of the IL for the generation of other morphologic and structural mesoporous materials. Acknowledgment. The Max-Planck Society is thanked for financial support. T.W. acknowledges the Natural Science Foundation of Yunnan Province/China (2003E0005Z) for financial support. We are indebted to Dr. F. Goettmann for help with the IR simulations. Supporting Information Available: IR spectra of pure CTAB and a CTAB-SiO2 nanocomposite, and the wavenumbers of the vibrational modes of pure CTAB, CTAB-SiO2, and the calculated spectra of the pure IL. This material is available free of charge via the Internet at http://pubs.acs.org. LA062470Y
