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Chem. Mater. 2005, 17, 2114-2119
Monolithic Periodic Mesoporous Silica with Well-Defined Macropores Tomohiko Amatani,† Kazuki Nakanishi,*,† Kazuyuki Hirao,† and Tetsuya Kodaira‡,§ Department of Material Chemistry, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, Nanoarchitectonics Research Center, National Institute of Science and Technology, 1-1-1, Higashi, Tsukuba-shi, Ibaraki 305-8565, Japan, and PRESTO, Japan Science and Technology Agency (JST), 4-1-8, Honcho, Kawaguchi City, Saitama 332-0012, Japan ReceiVed NoVember 1, 2004. ReVised Manuscript ReceiVed February 7, 2005
Monolithic pure silica gels with hierarchical macro-mesoporous structure have been synthesized via a spontaneous sol-gel process from silicon alkoxide using a structure-directing agent and a micelle-swelling agent. A monolithic body with well-defined co-continuous macropores is a result of concurrent phase separation and sol-gel transition induced by the polymerization reaction, whereas the mesopores are templated by the cooperative self-assembly of inorganic species, a structure-directing agent, and a micelleswelling agent. These bimodal pore systems are formed spontaneously in a closed condition at a constant temperature. The following removal of surfactants by heat-treatment gives silica gels with hierarchical and fully accessible pores in discrete size ranges of micrometers and nanometers. The highly ordered 2D-hexagonal arrays of mesopores have been confirmed by X-ray diffraction measurements and FESEM observations. Furthermore, by further additions of the micelle-swelling agent, the mesostructural transition from well-ordered 2D-hexagonal arrays to mesostructured cellular foams (MCF) has been induced accompanied by minor modifications of the micrometer-range structure.
* To whom correspondence should be addressed. Phone: +8175-383-2411. Fax: +8175 383 2412. E-mail:
[email protected]. † Kyoto University. ‡ National Institute of Science and Technology. § PRESTO.
On the other hand, a novel sol-gel process to fabricate well-defined macroporous monoliths has been established on the basis of the polymerization-induced phase separation in various silica-based sol-gel systems.4 The diameter of macropores can be controlled by adjusting the starting composition or reaction conditions, because the timing of the phase separation relative to the sol-gel transition determines the dimension of the macropores. The cocontinuous macroporous structure in which both separated phases are interconnected has been attracting substantial attention in the field of separation science.5,6 Recently, the preparation of an organic-inorganic hybrid gel with hierarchical macro-mesoporous structure has been reported in the system containing 1,2-bis(trimethoxysilyl)ethane (BTME) as a Si source and P123 as a structuredirecting agent.7 The highly ordered mesopores have been revealed by X-ray diffraction (XRD) measurements. The hybrid monolithic gels have the advantage of possessing the surface on which both organic and inorganic sites are available. On the other hand, pure silica gels/glasses have a lot of advantages, such as high mechanical strength, thermal stability, and chemical durability. By using tetramethoxysilane or tetraethoxysilane as the Si source, amorphous or disordered mesopores with sharp size distributions can be embedded in gel networks that constitute a co-continuous macroporous structure.8,9 In some approaches, it was reported
(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.; McCulle, S. B.; Higgins, J. B.; Schlender, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) Inagaki, S.; Fukushima, Y.; Kuroda, K. J. Chem. Soc., Chem. Commun. 1993, 8, 680.
(4) Nakanishi, K. J. Porous Mater. 1997, 4, 67-112. (5) Tanaka, N.; Kobayashi, H.; Nakanishi, K.; Minakuchi, H.; Ishizuka, N. Anal. Chem. 2001, 73, 420A. (6) Tanaka, N.; Kobayashi, H.; Ishizuka, N.; Minakuchi, H.; Nakanishi, K.; Hosoya, K.; Ikegami, T. J. Chromatogr., A 2002, 965, 35. (7) Nakanishi, K.; Kobayashi, Y.; Amatani, T.; Hirao, K.; Kodaira, T. Chem. Mater. 2004, 16, 3652.
Introduction Since the discovery of ordered mesoporous materials in the early 1990s,1-3 related materials have attracted much attention due to their fascinating structures and wide potential applications in the fields of separation, optics, drug delivery, and supports for catalysts. In many cases, these materials have been prepared by supramolecular-templating methods using surfactant as a structure-directing agent, where the mesopores are templated by the cooperative self-assembly of inorganic species and structure-directing agents. Mesoporous materials are now available in the forms of fine powders, fibers, thin films, and controlled-size particles, and even in monoliths. Despite dramatic progress in the synthesis of mesoporous materials, the preparation of materials with simultaneous control of morphology and pore structure in different length scales still remains a challenging task. In particular, the synthesis of well-defined macroporous monoliths with highly ordered mesopores is difficult to achieve. If these monoliths with well-defined pore structures in discrete length scales can be synthesized, a lot of benefits arise from such integrated pore systems.
10.1021/cm048091c CCC: $30.25 © 2005 American Chemical Society Published on Web 03/16/2005
Macro-Mesoporous Pure Silica Monolith
that only a part of the material retained periodic arrangement of mesopores.10 However, there has never been any report demonstrating that the highly ordered mesopores revealed by XRD could be obtained in pure silica monoliths with welldefined macropores. Here, we present the successful preparation and characterization of pure silica monoliths having a hierarchical pore structure, which has both well-defined co-continuous macropores and highly ordered mesopores. By this approach, monolithic periodic mesoporous silica with well-defined macropores can be synthesized spontaneously in closed conditions. Experimental Section Materials Synthesis. The tetramethoxysilane (TMOS) used as the silica source was purchased from Shin-Etsu Chemical Co. Ltd. As a structure-directing agent, poly(ethyleneoxide)-block-poly(propyleneoxide)-block-poly(ethyleneoxide), with an average molecular mass of 5800, was obtained from Aldrich (eqiuivalent to Pluronic P123, BASF). All of the solvents and other reagents including 1,3,5-trimethylbenzene (TMB) were purchased from Wako Chemicals and were used as received. The synthesis of wellordered hexagonal mesoporous silica structures using Pluronic P123 or a P123/expander (TMB) mixture as a template was first reported in 1998.11 Sample gels were prepared as follows. First, the appropriate amount of P123 was homogeneously dissolved in 1.0 M (M; mol dm-3) aqueous solution of nitric acid, and then a given amount of TMB was added under stirring. TMOS was then added at 0 °C under vigorous stirring for hydrolysis. After 10 min, the resultant homogeneous solution was poured into a glass vessel. The glass vessel was sealed and kept at 60 °C for gelation. Subsequently, the wet gel was aged at 60 °C for about 5 times the gelation time and was evaporation-dried at 60 °C. All of the gels were obtained as white or translucent monolithic pieces after drying. The heattreatment was carried out from room temperature to 650 °C in 650 min, and by holding at the same temperature for 5 h in air to remove residual organic substances including P123. Measurements. The morphology of the dried or heat-treated sample gels was observed via SEM (S-2600N, Hitachi Ltd., Japan, with Pt coating) and FE-SEM (JSM-6700F, JEOL Ltd., without coating). The X-ray powder diffraction (XRD) data were collected at room temperature on a MAC Science MXP-3TZ powder diffractometer with vertical θ:θ geometry and Cu KR radiation.12 Its goniometer has two characteristics to dramatically improve the XRD data at low 2θ region: (1) a pair of long Soller slits that improve the diffraction profiles as symmetrically as possible, and (2) variablewidth divergence and scattering slits that suppress the background level. In the present work, the variable-width slits were fixed to irradiate a sample width of 23 mm.12 The size distribution of macropores was measured by a mercury porosimeter (PORESIZER-9320, Micromeritics Co., USA), and the (8) Sato, Y.; Nakanishi, K.; Hirao, K.; Jinnai, H.; Shibayama, M.; Melnichenko, Y. B.; Wignall, G. D. Colloids Surf., A 2001, 187/188, 117-122. (9) Nakanishi, K.; Sato, Y.; Ruyat, Y.; Hirao, K. J. Sol-Gel Sci. Technol. 2003, 26, 567-570. (10) Shi, Z.-G.; Feng, Y.-Q.; Xu, L.; Da, S.-L.; Ren, Y.-Y. Microporous Mesoporous Mater. 2004, 68, 55. (11) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. (12) Ikeda, T.; Kodaira, T.; Oh, T.; Nisawa, A. Microporous Mesoporous Mater. 2003, 57, 249.
Chem. Mater., Vol. 17, No. 8, 2005 2115 Table 1. Starting Composition and Resultant Morphology of the Gel Samples in the TMOS-P123-1.0 M HNO3aq-TMB System (Unit: grams) sample
TMOS
P123
MP4 MP4-T045 MP4-T065 MP4-T085 MP4-T090 MP4-T125 MP4-T310
5.15 5.15 5.15 5.15 5.15 5.15 5.15
4.0 4.0 4.0 4.0 4.0 4.0 4.0
TMB
1.0 M HNO3aq
morphology
0.45 0.65 0.85 0.90 1.25 3.10
12.0 12.0 12.0 12.0 12.0 12.0 12.0
co-continuous isolated pores co-continuous co-continuous co-continuous co-continuous co-continuous
size distribution of mesopores was determined by a nitrogen adsorption-desorption measurement (ASAP-2010, Micromeritics Co., USA) using heat-treated gels. For nitrogen adsorption, the pore size distribution was calculated by the BJH method using the adsorption branch of the isotherm.
Results and Discussion As has been investigated extensively, various macroporous morphologies can be obtained by inducing the phase separation parallel to the sol-gel transition in the alkoxy-derived sol-gel system.4 The transient structure of the spinodal decomposition with comparable volume fractions of the conjugate phases typically becomes the co-continuous structure, where both the gel phase and the solvent phase are cocontinuous. Similarly in the systems containing TMOS and P123 reported earlier,8 the co-continuous macroporous structure was obtained in a limited composition region without TMB by reaction at 60 °C. The starting compositions and the resultant morphology of the gel samples in this study are listed in Table 1. Figure 1 shows SEM photographs of micrometer-range morphologies of the gels prepared with different amounts of TMB. Without TMB, the well-defined co-continuous macroporous gel was obtained (Figure 1a). With an addition of TMB, the morphology turns from isolated to co-continuous macropores with cylindrical or fibrous appearance accompanied by a gradual increase in the domain size (Figure 1b-d). Further additions of TMB result in the isotropic co-continuous structure again accompanied by a slight relative thickening of the gel skeletons (Figure 1e,f). Figure 2 shows the pore size distributions determined by Hg porosimetry for the representative samples. In all samples, being independent of the long-range ordering of the mesopores described below, pores in the micrometer range are sharply distributed. Figure 3 shows the nitrogen adsorption-desorption isotherms and the corresponding pore size distributions of the heat-treated gels listed in Table 1. All of the samples give type IV isotherms with well-defined capillary condensation. The co-continuous macroporous gel prepared without TMB gave a broad peak centered at ∼3.8 nm in pore size distributions. With an addition of TMB, drastic changes both in adsorption-desorption isotherms and in pore size distributions can be observed. The shape of the hysteresis loop of the adsorption-desorption isotherm changed to H1-type (by IUPAC classification) with an increased adsorption volume. This behavior is typical of adsorption for mesoporous materials with 2D-hexagonal structures.11 A well-defined step occurs at approximately p/p0 ) 0.62-0.77, which is the
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Figure 1. SEM photographs of heat-treated gel samples prepared with different amounts of TMB: (a) MP4, (b) MP4-T045, (c) MP4-T085, (d) MP4-T090, (e) MP4-T125, and (f) MP4-T310.
Figure 2. Pore size distribution of heat-treated gel samples determined by mercury porosimetry. O, MP4; 9, MP4-T085.
filling of the mesopores due to capillary condensation. In pore size distribution, the pore diameter increased (∼6.2 nm) corresponding to the capillary condensation at a higher relative pressure and the peaks became sharper and stronger with an increase of the amount of TMB. From these data, the addition of a small amount of TMB is assumed to enhance the self-organization of P123 to transform the pore system from random to cylindrical in shape. Confirmation of the presence of periodic mesostructure over the whole gel samples was obtained by XRD measurement. Figure 4 gives XRD patterns of the representative heat-
treated gels. The MP4 sample without TMB shows only a weak broad diffraction peak at a 2θ of ∼1.2°. With an addition of TMB, however, a drastic change can be recognized in XRD profiles as well as in the nitrogen adsorptiondesorption isotherms. The MP4-T085 sample show three well-resolved diffraction peaks in the 2θ range of 1-2°. If the sharp first peak was indexed as the (10) diffraction of 2D-hexagonal symmetry, the remaining two weak peaks can be indexed as (11) and (20) diffractions. These results suggest the presence of a periodic arrangement of channels in a 2Dhexagonal symmetry over the whole gel sample. With an addition of TMB, the first diffraction peak becomes stronger, indicating that the short-range order of mesopores is enhanced. As the amount of TMB is increased, the first diffraction peak is shifted to lower angle and the value of the d-spacing of the (10) peaks becomes larger, indicating that the unit-cell parameter (a value) of the sample becomes larger. The structural properties of the heat-treated gels are summarized in Table 2. Pore sizes were determined from nitrogen adsorption-desorption measurements. The wall thicknesses were calculated as: a - pore size (a ) 2 × d(10)/x3). The thickness of the pore wall hardly changed irrespective of the expansion of the pore diameter. The intensities of the (11) and (20) diffractions are increased as the amount of TMB is increased in the synthesis, confirming that the fraction of 2D-hexagonal pore structure in the gel becomes larger. The addition of TMB significantly stabilized
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Figure 3. Nitrogen adsorption-desorption isotherms (data are shifted by 200 cm3/g STP relative to each other for clarity) and corresponding pore size distribution curves calculated by the BJH method using the adsorption branches of the heat-treated gel samples. O, MP4; b, MP4-T045; 9, MP4-T085; 3, MP4-T090; [, MP4-T125; ], MP4-T310.
Table 2. Structural Properties of the Heat-Treated Gel Samples in the TMOS-P123-1.0 M HNO3aq-TMB System
sample
unit cell (nm)
BET surface area (m2/g)
pore size (nm)
pore volume (cm3/g)
wall thickness (nm)
MP4-T045 MP4-T065 MP4-T085 MP4-T090
9.20 10.2 11.0 11.5
573 649 656 605
4.90 5.60 6.20 7.15
0.52 0.64 0.61 0.51
4.30 4.59 4.77 4.30
the cylindrical micelles of P123, leading to the emergence of long-range order (2D-hexagonal array) in the mesophase. Figure 5 shows FE-SEM photographs of fractured surfaces of the macroporous gels. Well-aligned 2D-hexagonal pores can be recognized throughout the cross-section of the gel skeletons, which is consistent with XRD profiles. These FESEM photographs provided additional structural information that 2D-hexagonal mesostructures have straight channels parallel to the gel skeletons. The 2D-hexagonal pores run parallel to the skeleton length possibly due to the tensile stress exerted during the coarsening process of the cocontinuous domains. These gels have the co-continuous macroporous structure with cylindrical or fibrous skeletons as shown in Figure 1. The macroscopic morphology of mesoporous powders (as usually produced) is influenced by the shape of the surfactant micelles and other several complex factors.13 In a similar manner, the local morphology of gel Figure 4. X-ray diffraction profiles of heat-treated gel samples prepared with different amounts of TMB: O, MP4; b, MP4-T045; 0, MP4-T065; 9, MP4-T085.
(13) Zhao, D.; Sun, J.; Li, Q.; Stucky, G. D. Chem. Mater. 2000, 12, 275279.
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Figure 5. FE-SEM photographs of heat-treated gel sample with highly ordered 2D-hexagonal mesopores at different magnifications.
Figure 6. FE-SEM photographs of heat-treated gel sample with mesostructured cellular foams (MCF) at different magnifications.
skeletons is governed by the shape of the surfactant micelles and the local surface curvature in the present experimental system. At TMB concentrations higher than 0.95 g, the XRD peak intensity and resolution were diminished gradually, indicating that the mesopore structure was transformed from an ordered to a disordered one as TMB increased (not shown). In parallel, the nitrogen adsorption isotherm deforms to exhibit a drastic change from H1 to a broader hysteresis loop of H2-type with an increased adsorption volume at higher relative pressure (Figure 3). These shapes of the isotherm and hysteresis loop suggest that these samples possess “ink bottle” type pores in which large cells are connected by narrower windows. In the corresponding differential pore size distribution, the maximum at around 10-20 nm emerges and grows with an increase of TMB concentration. The inspection by FE-SEM of the MP4-T310 sample (Figure 6) revealed that the mesopores are very similar to those known as “mesostructured cellular foam” (MCF) with well-defined ultralarge mesopores reported recently.14,15 The sizes of their cells are in excellent agreement with those estimated by the
adsorption branch of the isotherm. Unlike the sizes of the cells, in these samples, the sizes of the windows connecting the cells cannot be precisely determined because the cause of loss of nitrogen at a relative pressure of 0.42-0.44 at 77 K is not capillary evaporation but is instability of the meniscus of the condensed nitrogen in the pores.16-18 The windows must be smaller than 5 nm, which is the diameter of a cylindrical pore that undergoes capillary evaporation at this pressure. The micrometer-range morphologies of the gels with MCF have the isotropic co-continuous structure accompanied by a slight relative thickening of the gel skeletons, as shown in Figure 1. This result strongly supports the preceding consideration that the local morphology of gel skeletons is governed by the shape of surfactant micelles and the local surface curvature. As shown in the FE-SEM photographs, the MCF-type mesopores are easily accessible from the macropore regions. If the size of the windows connecting the cells can be controlled, these materials will be applicable to separation media and catalyst supports for polymers or biorelated higher molecular-mass substances.
(14) Schmidt-Winkel, P.; Lukens, W. W., Jr.; Yang, P.; Margolese, D. I.; Lettow, J. S.; Ying, J. Y.; Stucky, G. D. Chem. Mater. 2000, 12, 686696. (15) Schmidt-Winkel, P.; Glinka, C. J.; Stucky, G. D. Langmuir 2000, 16, 356-361.
(16) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982. (17) Inoue, S.; Hanzawa, Y.; Kaneko, K. Langmuir 1998, 14, 3079. (18) Galarneau, A.; Desplantier, D.; Dutartre, R.; Di Renzo, F. Microporous Mesoporous Mater. 1999, 27, 297.
Macro-Mesoporous Pure Silica Monolith
As described above, in the nanometer range, the formation of the highly ordered 2D-hexagonal arrays of mesopores and the transition between 2D-hexagonal and MCF-type mesopores occurred at a TMB-to-P123 mass ratio of ∼0.2 and ∼0.2-0.3, respectively. In the case of the system with the different amount of P123, the formation of the 2D-hexagonal arrays was observed at a TMB-to-P123 mass ratio of ∼0.2. In the range of a TMB-to-P123 ratio of 0.2-0.3, the hysteresis loop of the adsorption-desorption isotherm changed from H1 to H2-type, indicating the transformation of mesopore morphology from 2D-hexagonal to MCF. Confirmation of this transition was obtained by FE-SEM photographs. These results indicate that, while both P123 and TMB influence the phase separation tendency, the formations and transitions of mesostructure are determined mainly by the TMB-to-P123 mass ratio under the fixed amount of TMOS and water. Conclusion In the TMOS-based system, the integration of highly ordered mesopores into homogeneously accessible macroporous arrays can be achieved via the completely spontane-
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ous chemical process in the presence of P123 and TMB. By the addition of TMB, highly ordered 2D-hexagonal arrays of mesopores are generated in the mesophase. By further additions of TMB, the mesostructural transformation from 2D-hexagonal to MCF was induced, accompanied by minor modifications of the well-defined co-continuous macroporous structure. The completely spontaneous formation of hierarchical macro-mesopores has the versatility to prepare porous structure in miniaturized and/or confined spaces. These techniques will be applicable to miniaturized porous devices for separation media and catalyst supports, etc. Acknowledgment. Technical assistance by JEOL Ltd. for FE-SEM operation is gratefully acknowledged. Financial support by a Grant-in-Aid for Scientific Research (No. 15206072) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan, is gratefully acknowledged. Also, this work is partially supported by the 21st century Center Of Excellence (COE) program, COE for a United Approach to New Material Science, which is financially supported by the Ministry of Education, Culture, Sports, Science, and Technology, Japan. CM048091C
