pubs.acs.org/Langmuir © 2010 American Chemical Society
Formation Mechanism Studies of Phenylene-Bridged Periodic Mesoporous Organosilicas (PMOs) V. Rebbin,*,† A. Rothkirch,† N. Ohta,‡ and S. S. Funari† †
HASYLAB@DESY, Notkestr. 85, 22607 Hamburg, Germany, and ‡SPring-8, JASRI, 1-1-1 Kouto, Sayo, Hyogo 679-5198, Japan Received December 22, 2009. Revised Manuscript Received February 22, 2010
The formation of phenylene-bridged periodic mesoporous organosilicas (PMOs) in the presence of three diblock copolymers differing in hydrophobic hydrocarbon chain lengths was investigated. Hexaethylene glycol octadecylether (C18(EO)6), hexaethylene glycol hexadecylether (C16(EO)6), and hexaethylene glycol dodecylether (C12(EO)6) were chosen in order to obtain insight on the influence of the hydrocarbon chain length on the mesostructure. 1,4Bis(triethoxysilyl)benzene (BTEB) was used as organosilica precursor under mild acidic conditions. The reactions were followed by in situ small-angle X-ray diffraction (SAXD) on-time in a capillary flow setup. It was found that during the reaction the formation of different structures was observed, which is ascribed to the hydrolysis and condensation of the organosilica precursor. In all cases, different structures evolve with time and phase transitions are observed during the measurements independent of the hydrocarbon chain length.
Introduction Periodic mesoporous materials have a wide field of applications in different separation techniques (e.g., HPLC)1 as well as catalysis.2 In 1992, the so-called highly ordered mesoporous silica materials of the M41S family were introduced by the Mobil Oil researchers.3 These materials are typically synthesized by applying an amphiphilic substance (e.g., cetyltrimethylammonium bromide) as structure directing agent and a silica precursor (e.g., tetraethyl orthosilicate) which hydrolyzes and condensates under the given synthesis conditions. The amphiphilic molecules, for example, surfactants or block copolymers, tend to form a liquid crystalline phase in the solution. The ordering of the micellar aggregates is highly dependent on the surfactant concentration and molecular properties such as, for instance, the size and charge of the headgroup (hydrophilic part) and the nonpolar chain. The formation of each structure can be explained with the help of the concept of the ion pair packing parameter g, which was originally designed for the description of surfactant organization in amphiphilic liquid crystalline arrays in classic micelle chemistry. The ion pair packing parameter g is defined as g = V/(a0l) (V = total volume of the surfactant chains plus any organic cosolvent molecules between the chains, a0 = the effective headgroup area at the micelle surface, and l = kinetic surfactant tail length or the curvature elastic energy). Packing parameters in the range between 1/3 and 1/2 are usually found for micellar cubic phases (close packed spherical micelles), values around 1/2 for 2D hexagonal structures, the range between 1/2 and 2/3 for bicontinuous cubic structures, and finally 2/3 to 1 for lamellar structures.4 The reactions of the silica precursor also influence the formation of the structure due to the varying number of charges on the silica species at the surfactant headgroup/silica species interface. Previous studies of the so-called liquid crystal templating mechanism *To whom correspondence should be addressed. E-mail: Vivian.Rebbin@ desy.de.
(1) Rebbin, V.; Schmidt, R.; Fr€oba, M. Angew. Chem., Int. Ed. 2006, 45, 5210. (2) Weitkamp, J.; Hunger, M.; Rymsa, U. Microporous Mesoporous Mater. 2001, 48, 255. (3) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (4) Huo, Q.; Margolese, D. I.; Stucky, G. D. Chem. Mater. 1996, 8, 1147.
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showed that, during the hydrolysis/condensation, phase transitions can take place within minutes.5-10 In situ X-ray diffraction allows the observation of the mesostructure formation at short time scales. In particular, the formation mechanism of highly ordered pure silica materials from the SBA and the M41S families have been studied.5-10 Following the introduction of periodic mesoporous organosilica materials in 1999,11-13 several methods have been used to vary the structure, the specific surface areas, the pore sizes, and the chemical properties of these materials (for an overview, see ref 14). Different organic units were incorporated into the pore walls by using different organosilica precursors of the formula (RO)3Si-R0 -Si(OR)3 with R usually being methyl or ethyl and R0 being any organic unit. The formation of a highly ordered mesoporous structure depends, besides the organic functionality -R0 -, on the surfactant properties, reactant concentrations, and reaction temperature. Different organosilica precursors have already been used for the synthesis of PMOs,14 but the most well-defined structures showing more than one diffraction peak in the small-angle X-ray scattering (SAXS) region were synthesized with rigid molecules (e.g., molecules with aromatic units), whereas an increase in flexibility often leads to nonordered and mostly nonporous materials. Syntheses of materials with more (5) Flodstr€om, K.; Teixera, C. V.; Amenitsch, H.; Alfredsson, V.; Linden, M. Langmuir 2004, 20, 4885. (6) Kodakov, A. Y.; Zholobenko, V. L.; Imperor-Clerc, M.; Durand, D. J. Phys. Chem. B 2005, 109, 22780. (7) Kipkemboi, P.; Fodgen, A.; Alfredsson, V.; Flodstr€om, K. Langmuir 2001, 17, 5398. (8) Flodstr€om, K.; Wennestr€om, H.; Teixera, C. V.; Amenitsch, H.; Linden, M.; Alfredsson, V. Langmuir 2004, 20, 10311. (9) Linden, M.; Schunk, S. A.; Sch€uth, F. Angew. Chem., Int. Ed. 1998, 37, 821. (10) O’Brien, S.; Francis, R. J.; Fogg, A.; O’Hare, D.; Okazaki, N.; Kuroda, K. Chem. Mater. 1999, 11, 1822. (11) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 1999, 121, 9611. (12) Asefa, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 402, 867. (13) Melde, B. J.; Holland, B. T.; Blanford, C. F.; Stein, A. Chem. Mater. 1999, 11, 3302. (14) Hoffmann, F.; Cornelius, M.; Morell, J.; Fr€oba, M. Angew. Chem., Int. Ed. 2006, 45, 3216.
Published on Web 04/16/2010
DOI: 10.1021/la904837v
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Table 1. Molar Concentrations and Molar Ratios of BTEB/Surfactant of the Investigated Samples sample S1 S2 S3 S4 S5 S6 S7 S8 S9
C18(EO)6 [mol]
C16(EO)6 [mol]
C12(EO)6 [mol]
BTEB/ surfactant
0.0120 0.0238 0.0357
42:1 21:1 14:1 42:1 21:1 14:1 42:1 21:1 14:1
0.0120 0.0238 0.0357 0.0120 0.0238 0.0357
than one organic functionality have also been described in the literature.15,16 Pore sizes obtained for mesoporous silica and organosilica materials varied in the range of 2-30 nm, using different ionic or nonionic surfactants or block copolymers.17-19 However, highly ordered materials with pore sizes larger than 10 nm or smaller than 2 nm could not be obtained up to now. Many references concerning the formation mechanism of mesoporous materials have been published up to now.5-10 In most cases, the surfactant concentration in the solution is below the second critical micelle concentration (cmc2) where the liquid crystal is formed. Typically, the formation of the liquid crystalline phase takes place when the silica precursor is added to the solution. In situ X-ray diffraction offers the opportunity to investigate the formation of different structures during the reaction, which are initiated by micelles of different sizes and shapes. In this study, we present results dealing with the influence of different hydrocarbon chain lengths on the formation process of phenylene-bridged PMO materials by keeping the headgroup at a constant size, following the changes in the lattice parameters. For these experiments, C18(EO)6, C16(EO)6, and C12(EO)6 were used as structuredirecting agents and 1,4-bis(triethoxysilyl)benzene (BTEB) as organosilica precursor.
Experimental Section Small-angle X-ray diffraction (SAXD) experiments were carried out at beamlines A2 (DORIS III@DESY, Hamburg),20 BL40B2 (BIOSAXS II) (SPring-8, Japan),21 and DUBBLE BM26B (ESRF, France).22 In all cases, data acquisition - i.e. one diffraction pattern (frame) - was taken every 180 s. All data have been calibrated to physical quantities of interest to allow comparison. Hexaethylene glycol octadecylether (C18(EO)6), hexaethylene glycol hexadecylether (C16(EO)6), and hexaethylene glycol dodecylether (C12(EO)6) were purchased from Nikko Chemicals Co., LTD, Tokyo, Japan. 1,4-Bis(triethoxysilyl)benzene (BTEB) was purchased from Sigma-Aldrich Co. Measurements applying the nonionic surfactant Brij 76 were described by Wang et al.23 and led to a 2D ordered hexagonal (15) Morell, J.; G€ungerich, M.; Wolter, G.; Jiao, J.; Hunger, M.; Klar, P. J.; Fr€oba, M. J. Mater. Chem. 2006, 16, 2809. (16) Burleigh, M. C.; Jayasundera, S.; Spector, M. S.; Thomas, C. W.; Markowitz, M. A.; Gaber, B. P. Chem. Mater. 2004, 16, 3. (17) Muth, O.; Schellbach, C.; Fr€oba, M. Chem. Commun. 2001, 2031. (18) Burleigh, M. C.; Markowitz, M. A.; Wong, E. M.; Lin, J.-S.; Gaber, B. P. Chem. Mater. 2001, 13, 4411. (19) Goto, Y.; Inagaki, S. Chem. Commun. 2002, 2410. (20) Meyer, A.; Dommach, M.; D€ohrmann, R. HASYLAB Annual Report I (2001), 57 (21) Outline Beamline 40B2 at Spring 8, JASARI, Japan, http://www.spring8. or.jp/wkg/BL40B2/instrument/lang-en/INS-0000001280/instrument_summary_ view, accessed November 2009. (22) Bras, W.; Dolbnya, I. P.; Detollenaere, D.; van Tol, R.; Malfois, M.; Greaves, G. N.; Ryan, A. J.; Heeley, E. J. Appl. Crystallogr. 2003, 36, 791. (23) Wang, W.; Zhou, W.; Sayari, A. Chem. Mater. 2003, 15, 4886.
9018 DOI: 10.1021/la904837v
Figure 1. (a) Temporal evolution of the SAXD patterns of sample S2 at 60 °C (BTEB/C18(EO)6 = 21:1). (b) Selected diffraction patterns of the in situ SAXD measurement of the sample S2, illustrating the time evolution of the synthesis. A phase transition, in this case from intermediate to 2D hexagonal, can be clearly seen from a comparison of the patterns at 103 and 127 min. structure. Our own studies with Brij 58 led to mostly cubic ordering. Motivated by these results, we adapted the molar ratios for the three nonionic surfactants C18(EO)6, C16(EO)6, and C12(EO)6. Three different surfactant concentrations were investigated. The molar concentrations and molar ratios of BTEB/surfactant of the samples S1 to S9 are listed in Table 1. The molar concentrations of BTEB (0.5 mol), HCl (0.36 mol), and H2O (64.71 mol) were kept constant. Samples (S1-S9) were prepared by dissolving the surfactant in slight acidic aqueous solution. This mixture was filled into a four neck flask with a reflux condenser placed on a magnet stirrer and heater. The reaction vessel was connected to a flow capillary placed in the X-ray beam by flexible tubing and the solution was pumped through at 5 mL/min. Measurements were carried out at 60 °C. Diffraction patterns of the solution containing only the surfactant in hydrochloric acid (0.1 mol/L) were taken first. After the addition of the organosilica precursor BTEB to the solution, the reaction started at the desired concentrations of the reagents. Data acquisition was started immediately after the addition of BTEB and ended when the capillary was blocked by the resulting solid formed in the solution.
Results Syntheses with C18(EO)6 as Structure-Directing Agent. Three different concentrations were investigated at a constant reaction temperature of 60 °C (BTEB/C18(EO)6 = 42:1 (S1); 21:1 (S2), 14:1 (S3)). Sample S1 (with the lowest surfactant concentration) did not show any diffraction peak within 5 h reaction time, indicating that the surfactant concentration was too low to form a liquid crystalline phase. The sample S2 reaches a 2D hexagonal mesophase after going through an intermediate structured transition state (Figure 1a). Langmuir 2010, 26(11), 9017–9022
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Figure 2. Temporal evolution of the repeat distances of samples S2 and S3. No liquid crystalline phase was found for the synthesis of S1. Symbols: (9/0) intermediate phase; (b/O) (100), 2D hexagonal; (2) (110), 2D hexagonal; (1) (200), 2D hexagonal. Table 2. Repeat Distances (d), Appearing and Vanishing Times (tstart, tend), and Shifts of the Repeat Distances (Δ) of the Reflections of Samples S2 and S3
d001 (start) [nm] t (d001 (start)) [min] d001 (min) [nm] t (d001 (min)) [min] d001 (end) [nm] t (d001 (end)) [min] Δ d001 [nm] d100 (start) [nm] t (d100 (start)) [min] d100 (end) [nm] t (d100 (end)) [min] d110 (start) [nm] t (d110 (start)) [min] d110 (end) [nm] t (d110 (end)) [min] d200 (start) [nm] t (d200 (start))[min] d200 (end) [nm] t (d200 (end)) [min]
S3
S2
6.78 55 6.06 67 6.84 117 0.78 8.41 103 7.83 148 -
6.04 55 5.93 61 6.30 130 0.37 8.11 103 7.14 178 4.45 112 4.16 178 3.91 112 3.58 178
As one can see from Figure 1a, a single diffraction peak from the initial intermediate phase (d = 6.04 nm) appears at 55 min reaction time. During the reaction, the repeat distance associated to this phase goes through a minimum (d = 5.92 nm) at 61 min, later increasing to 6.29 nm until it vanishes after 130 min. Nevertheless, the occurrence of a single diffraction peak makes indexing to a specific structure impossible. Considering the organosilica species which are highly hydrolyzed at this early stage of the reaction, we expect a high charge density on the surface, which is the condition to favor a lamellar structure. Further, we see the onset of the 2D hexagonal phase, whose evolution of the (100) reflection starts after 103 min with both the (110) and the (200) reflections after 112 min. The (100), (110), and (200) reflections of the 2D hexagonal phase show an increase of their intensities and a slight shift toward higher s-values until 150 min reaction time. For a clearer illustration of the time dependent evolution of the structures formed, selected patterns are given in Figure 1b. The sample synthesized with the highest surfactant concentration (S3) shows diffraction patterns with at least two reflections associated to different phases. The evolution of the repeat Langmuir 2010, 26(11), 9017–9022
Figure 3. (a) Temporal evolution of the SAXD patterns of sample S5 at 60 °C (BTEB/C16(EO)6 = 21:1). (b) Patterns of selected frames of the in situ SAXD measurement of sample S5, illustrating the time evolution of the synthesis. A phase transition from intermediate to a 2D hexagonal structure can be clearly seen.
distances during the reaction for samples S2 and S3 is depicted in Figure 2. As one can see from Figure 2, only the (100) diffraction peak of the 2D hexagonal mesostructure on sample S3 could be observed, which is evidence for lesser ordering of the composite material caused by the higher surfactant concentration. Comparing the lattice parameters of the structures of S2 and S3 allow us to assume a 2D hexagonal structure for S3. The values of the repeat distances (d), onset and vanishing times (tstart, tend), and shifts of the repeat distances (Δ) of the reflections of samples S2 and S3 are listed in Table 2. The comparison of the data sets for S2 and S3 shows remarkable trends. First, with increasing surfactant concentration, the lattice parameters increase. Second, the shift (Δ) of the first reflection increases with increasing amount of surfactant in the reaction solution. Syntheses with C16(EO)6 as Structure-Directing Agent. Similar experiments as described for the surfactant C18(EO)6 were carried out with C16(EO)6. The same molar ratios (BTEB/C16(EO)6 = 42:1 (S4), 21:1 (S5), 14:1 (S6)) and reaction temperature of 60 °C were applied. The evolution of the SAXS patterns of sample S5 with the molar ratio BTEB/C16(EO)6 = 21:1 is shown in Figure 3a. Please note that the constant streak around 0.12 nm-1 is an artifact due to the primary beam/diffuse beam stop scattering and that the region of interest is at higher s-values (sufficiently off the artifact); see also Figure 4a. The evolution of the structures and the phase transition are illustrated by extraction of selected patterns shown in Figure 3b. After starting the reaction, it takes 60 min until we observe the diffraction peak of the intermediate structure. After 64 min, the 2D hexagonal mesostructure starts to evolve. The evolution of the SAXS patterns of sample S5 is similar to that of sample S2 synthesized with C18(EO)6 as structure-directing agent. This is not DOI: 10.1021/la904837v
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Rebbin et al. Table 3. Surfactant Concentrations of Samples S1-S9 and Critical Micelle Concentrations of the Hexaethylene Glycol Monoalkyl Ethers C12(EO)6, C16(EO)6, and C18(EO)621
surfactant
cmc [mol/L] (25 °C)
c(Cn(EO)6 for BTEB/Cn(EO)6 21:1 [10-3 mol/L]
c(Cn(EO)6 for BTEB/Cn(EO)6 14:1 [10-3 mol/L]
C12(EO)6 C16(EO)6 C18(EO)6
8.7 10-5 1 10-6
