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mesoporous silica and carbon monoliths by using a commercial polyurethane foam as sacrificial template
Materials Letters 61 (2007) 2378 – 2381 www.elsevier.com/locate/matlet
Synthesis of macro/mesoporous silica and carbon monoliths by using a commercial polyurethane foam as sacrificial template Sonia Álvarez, Antonio B. Fuertes ⁎ Department of Chemistry of Materials, Instituto Nacional del Carbón (CSIC), P.O. Box 73, E-33080, Oviedo, Spain Received 17 July 2006; accepted 8 September 2006 Available online 26 September 2006
Abstract A commercial macrocellular polyurethane foam was used as template to fabricate macro/mesoporous silica and carbon monoliths. These materials have a cellular structure which is a faithful replica of that of the polymeric foam. In addition, they have a high surface area and a large porosity made up of accessible mesopores. The synthesis of silica monoliths was carried out by impregnating the polymeric foam with a mixture of a silica precursor and a surfactant. The carbon monoliths were prepared by using the silica monoliths as sacrificial templates. They retain the foamy vesicular structure and exhibit a high surface area of 1800 m2 g− 1 and a large porosity made up of framework-confined mesopores of around 3.4 nm. © 2006 Elsevier B.V. All rights reserved. Keywords: Carbon; Macrocellular; Mesopores; Monolith; Porosity; Polymer
1. Introduction Mesostructured porous materials obtained by nanocasting porous inorganic material have attracted widespread interest during recent years. This interest has focused mainly on the preparation of mesoporous carbons by means of a template approach that employs mesoporous silica hosts [1,2]. These carbons have a high surface area and a large pore volume made up of uniform mesopores. They are expected to be useful in a wide range of applications such as the separation or adsorption of large molecules (i.e. immobilization of enzymes) and as catalytic supports or for energy storage in double layer capacitors [3–5]. Until now, most of these silica and carbon materials have been obtained in the form of a powder formed by small particles of sizes in the 1–10 μm range. However, under certain circumstances this small particle size might limit the applicability of these materials, especially in those applications where a low pressure drop is required (e.g. chromatographic columns). One way to circumvent this problem could be to fabricate these materials in the form of monoliths with a porous structure that combines large macropores with small mesopores ⁎ Corresponding author. E-mail address: [email protected] (A.B. Fuertes). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.09.017
which would provide large surface areas. The macroporous skeleton would guarantee the rapid transport of species inside the monolith, whereas the mesoporous structure would provide a large and accessible surface area useful for catalytic or adsorption processes. The synthesis of porous silica and carbon monoliths has been reported by several authors [6–8]. Nevertheless, in most cases these materials do not have much practical use because their porosity is made up exclusively of mesopores, which limits the accessibility of the reactants to the inner part of the monolith. More recently, works have appeared that report the preparation of silica and carbon monoliths that
Fig. 1. Photographs of (a) polyurethane foam, (b) silica monolith and (c) carbon monolith.
S. Álvarez, A.B. Fuertes / Materials Letters 61 (2007) 2378–2381
contain both macropores and mesopores [9–12]. For example, silica monoliths with these characteristics have been obtained by a sol–gel process as described by Nakanishi et al. [13]. Our group has recently reported the synthesis of a meso/macroporous carbon monolith [14] using a polystyrene-based mesostructured silica monolith as template [9]. This silica monolith was prepared by using as template a polystyrene foam, prepared by the phase inversion method [15]. However, the method of obtaining this polystyrene foam at the laboratory scale is both complex and expensive. In the present work we
2379
explore the use of an inexpensive and widely available commercial polymeric foam as template for the synthesis of silica and carbon monoliths. This paper is, to the best to our knowledge, the first to describe the use of commercial foams as scaffolds for fabricating silica and carbon monoliths. 2. Experimental We selected a polyurethane commercial foam, employed for varnishing, as sacrificial scaffolds to prepare the silica
Fig. 2. SEM images of the (a) polyurethane foam, (b) silica and (c) carbon monoliths, and TEM images of the porous structure of (d) silica and (e) carbon.
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S. Álvarez, A.B. Fuertes / Materials Letters 61 (2007) 2378–2381
added under stirring to a solution of HCl in ethanol (0.14 M). The mixture was subjected to vigorous stirring for 1 h at 70 °C in a closed vessel. Then, the surfactant (Pluronic 127) was added to the mixture and stirred until dissolution. The starting mole ratio is: TEOS/Pluronic 127/HCl/H2O/EtOH = 1:0.01:0.03:12:50. A piece of the polyurethane foam was introduced in a closed vessel that contained the surfactant mixture. The impregnated polymeric material was dried for 24 h at room temperature. The impregnation–drying cycle was repeated up to three times in order to improve the silica yield. The resulting silica–polymer composite was calcined in air at 600 °C (2 °C/min) for 4 h. The carbon monoliths were prepared by using the silica monoliths as templates. The carbon was introduced into the porosity of the silica by the vapour deposition polymerization method [14,17,18]. The macro/mesoporous silica monolith was immersed in a solution of paratoluene sulfonic acid (0.1 M in ethanol) and dried at 80 °C. Next, the impregnated silica monolith was placed in a closed Teflon vessel, in which also there was a small container with furfuryl alcohol. The vessel was closed and maintained at 40 °C for 14 h. The monolith was then heated in air up to 80 °C (15 h) in order to polymerize the furfuryl alcohol into polyfurfuryl alcohol. Next, it was carbonised at 800 °C (2 °C/min) in N2 for 1 h. Finally, the silica framework was removed with HF (48%) giving rise to the carbon monolith. Small-angle X-ray diffraction (XRD) patterns were measured by means of a Siemens D5000 instrument employing CuKα radiation (λ = 0.15406 nm). Transmission electron micrographs (TEM) and scanning electron micrographs (SEM) were taken on a JEM-2000 EX II and Zeiss DSM 942 microscopes, respectively. Adsorption measurements were performed using a Micromeritics ASAP 2010 gas analyser. The BET surface areas were deduced from the isotherm analysis in the relative pressure range of 0.04–0.20. The total pore volumes were calculated from the amount of nitrogen adsorbed at a relative pressure of 0.99. The pore size distributions (PSD) were deduced from the adsorption branch of nitrogen isotherms by means of the Kruk–Jaroniec–Sayari (KJS) algorithm [19]. 3. Result and discussion Photographs of the polyurethane foam and the silica and carbon monoliths are shown in Fig. 1. From these images it is clear that the macroscopic morphology of both the silica and carbon monoliths is the same as for the initial polyurethane foam. They also reveal that during the replication process, from the polymeric foam to the silica and Fig. 3. (a) Low-angle XRD patterns for the silica and carbon materials and (b) nitrogen sorption isotherms and pore size distributions (inset) of the silica and carbon monoliths.
monoliths. These polymeric foams have a low density and very open macrocellular structure formed by voids of a size within the 50–250 μm range (see Fig. 2a). The procedure used to prepare the silica monoliths is based on the method reported by Zhao et al. [16] to obtain mesostructured SBA-16 silica materials. In a typical synthesis, the silica source (Tetraethyl orthosilicate, TEOS, Aldrich) was
Table 1 Physical properties of the framework-confined porosity of the silica and carbon monoliths Sample
SBET (m2 g− 1)
Vp (cm3 g− 1) a
Vmesop (cm3 g− 1) b
δKJS (nm) c
FWHM (nm) d
Silica Carbon
560 1860
0.7 1.9
0.6 1.7
7.2 3.4
1.9 2.8
a b c d
Total pore volume of the framework-confined pores. Volume of the mesopores. Maximum of PSD. Full width at the half maximum of PSD.
S. Álvarez, A.B. Fuertes / Materials Letters 61 (2007) 2378–2381
carbon monoliths, contraction takes place. However, in spite of this contraction, both the silica and carbon materials maintain their structural integrity and no cracks are detected in the monolithic structure. Fig. 2b and c shows the SEM microphotographs obtained for the polyurethane foam and the templated silica and carbon materials. These images reveal that the foamy structure characteristic of a polymer consisting of large interconnected vesicles (Fig. 2a) is preserved in the templated monoliths of silica (Fig. 2b) and carbon (Fig. 2c). Thus, these materials have an internal structure made up of large interconnected voids (approx. 200 μm) separated by thin walls, which form the skeleton of the monolith. More important, the walls of the silica and carbon skeleta contain a framework-confined porosity that is made up of quite uniform and well-ordered mesopores. From the TEM images it can be seen that the porosity exhibits an ordered pattern, which is characteristic of the cubic porous structure of SBA-16 silica [16] (Fig. 2d) and its carbon replica (Fig. 2e). These results are confirmed by the X-ray diffraction analysis performed at the low-angle range (2θ b 5°). Fig. 3a contains the XRD patterns obtained for the silica monolith and its carbon replica. They exhibit an intense diffraction peak (2θ ∼ 1°) and one weaker peak (2θ ∼ 1.5°), which can be assigned to the (100) and (200) reflections characteristic of the wellordered cubic porous structure of SBA-16 [16]. A d-spacing of 9.7 nm is deduced from the (100) reflection for the silica monolith. These results show that the walls of the carbon and silica monoliths have a well-ordered porosity. The physical properties of the framework-confined porosity in the silica and carbon monoliths were studied by means of nitrogen adsorption. These monoliths have high BET surface areas and large framework-confined pore volumes (see Table 1). Around 90% of the pore volume is made up of mesopores in the 2–10 nm range. Fig. 3b shows the nitrogen sorption isotherms and the PSDs (inset) obtained for the silica and carbon materials. The PSD (Fig. 3b, inset) reveals that the porosity of the silica is made up of mesopores of sizes centred at around 7.2 nm. These mesopores are very uniform, as evidenced by the narrow PSD being the full width at the half maximum (FWHM) of 1.9 nm (see Table 1). As in the case of silica, the carbon monolith also shows the existence of a mesopore system as confirmed by the PSD, which exhibits a maximum at 3.4 nm. The mesopores in the carbon material are very uniform (FWHM = 2.8 nm).
4. Conclusions In summary, a facile and new route to prepare macro/ mesoporous silica and carbon monoliths is presented. These materials are obtained by using a low-cost commercial polyurethane foam as a sacrificial scaffold. The macrocellular structure of the polyurethane foam is preserved in the silica and carbon
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monoliths, which exhibit a fully interconnected macroporous network. Moreover, these materials have a well-ordered porosity made up of uniform mesopores with sizes centred at around 7.2 nm (silica) and 3.4 nm (carbon). They also have high BET surface areas (1860 m2 g− 1 for carbon and 560 m2 g− 1 for silica) and large pore volumes (1.9 cm3 g− 1 for carbon and 0.7 cm3 g− 1 for silica). This synthetic strategy opens up a novel and costeffective route to fabricate monoliths that combine a very open and interconnected macroporous structure with a large internal surface area and porosity. Acknowledgements The financial support for this research work provided by the Spanish MCyT (MAT2005-00262) is gratefully acknowledged. S. Alvarez thanks the Spanish MCyT for her FPI (BES-20030134) grant. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
J. Lee, S. Han, T. Hyeon, J. Mater. Chem. 14 (2004) 478. S. Alvarez, A.B. Fuertes, Carbon 42 (2004) 437. H.F. Yang, Y. Yan, D.Y. Zhao, J. Ind. Eng. Chem. 10 (2004) 1146. E. Frackowiak, F. Beguin F, Carbon 39 (2001) 937–950. S. Álvarez, M.C. Blanco-López, A.J. Miranda-Ordieres, A.B. Fuertes, T.A. Centeno, Carbon 43 (4) (2005) 866. B-H. Han, W. Zhou, A. Safari, J. Am. Chem. Soc. 125 (2003) 3444. H. Yang, Q. Shi, X. Liu, S. Xie, D. Jiang, D. Zhao, Chem. Commun. (2002) 2842. Z-G. Shi, Y-Q. Feng, L. Xu, S.-.L. Da, Y-Y. Ren, Microporous Mesoporous Mater. 68 (2004) 55. H. Maekawa, J. Esquena, S. Bishop, C. Solans, B.F. Chmelka, Adv. Mater. 15 (2003) 591. B. Zhang, S.A. Davis, S. Mann, Chem. Mater. 14 (2002) 1369. A. Taguchi, J-H. Smatt, M. Linden, Adv. Mater. 15 (2003) 1209. Z. Shi, Y. Feng, L. Xu, S. Da, M. Zhang, Carbon 41 (2003) 2677. Y. Sato, K. Nakanishi, K. Hirao, H. Jinnai, M. Shibayama, Colloids Surf., A Physicochem. Eng. Asp. 187 (2001) 117. S. Alvarez, J. Esquena, C. Solans, A.B. Fuertes, Adv. Eng. Mater. 6 (11) (2004) 897. J. Esquena, G.S.R.R. Sankar, C. Solans, Langmuir 19 (2003) 2983. D. Zhao, Q. Huo, B.F. Feng, G.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. A.B. Fuertes, Carbon 40 (2002) 1600. A.B. Fuertes, D.M. Nevskaia, J. Mater. Chem. 13 (2003) 1843. M. Kruk, M. Jaroniec, A. Sayari, Langmuir 13 (1997) 6267.
Synthesis of macro/mesoporous silica and carbon monoliths by using a commercial polyurethane foam as sacrificial template Sonia Álvarez, Antonio B. Fuertes ⁎ Department of Chemistry of Materials, Instituto Nacional del Carbón (CSIC), P.O. Box 73, E-33080, Oviedo, Spain Received 17 July 2006; accepted 8 September 2006 Available online 26 September 2006
Abstract A commercial macrocellular polyurethane foam was used as template to fabricate macro/mesoporous silica and carbon monoliths. These materials have a cellular structure which is a faithful replica of that of the polymeric foam. In addition, they have a high surface area and a large porosity made up of accessible mesopores. The synthesis of silica monoliths was carried out by impregnating the polymeric foam with a mixture of a silica precursor and a surfactant. The carbon monoliths were prepared by using the silica monoliths as sacrificial templates. They retain the foamy vesicular structure and exhibit a high surface area of 1800 m2 g− 1 and a large porosity made up of framework-confined mesopores of around 3.4 nm. © 2006 Elsevier B.V. All rights reserved. Keywords: Carbon; Macrocellular; Mesopores; Monolith; Porosity; Polymer
1. Introduction Mesostructured porous materials obtained by nanocasting porous inorganic material have attracted widespread interest during recent years. This interest has focused mainly on the preparation of mesoporous carbons by means of a template approach that employs mesoporous silica hosts [1,2]. These carbons have a high surface area and a large pore volume made up of uniform mesopores. They are expected to be useful in a wide range of applications such as the separation or adsorption of large molecules (i.e. immobilization of enzymes) and as catalytic supports or for energy storage in double layer capacitors [3–5]. Until now, most of these silica and carbon materials have been obtained in the form of a powder formed by small particles of sizes in the 1–10 μm range. However, under certain circumstances this small particle size might limit the applicability of these materials, especially in those applications where a low pressure drop is required (e.g. chromatographic columns). One way to circumvent this problem could be to fabricate these materials in the form of monoliths with a porous structure that combines large macropores with small mesopores ⁎ Corresponding author. E-mail address: [email protected] (A.B. Fuertes). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.09.017
which would provide large surface areas. The macroporous skeleton would guarantee the rapid transport of species inside the monolith, whereas the mesoporous structure would provide a large and accessible surface area useful for catalytic or adsorption processes. The synthesis of porous silica and carbon monoliths has been reported by several authors [6–8]. Nevertheless, in most cases these materials do not have much practical use because their porosity is made up exclusively of mesopores, which limits the accessibility of the reactants to the inner part of the monolith. More recently, works have appeared that report the preparation of silica and carbon monoliths that
Fig. 1. Photographs of (a) polyurethane foam, (b) silica monolith and (c) carbon monolith.
S. Álvarez, A.B. Fuertes / Materials Letters 61 (2007) 2378–2381
contain both macropores and mesopores [9–12]. For example, silica monoliths with these characteristics have been obtained by a sol–gel process as described by Nakanishi et al. [13]. Our group has recently reported the synthesis of a meso/macroporous carbon monolith [14] using a polystyrene-based mesostructured silica monolith as template [9]. This silica monolith was prepared by using as template a polystyrene foam, prepared by the phase inversion method [15]. However, the method of obtaining this polystyrene foam at the laboratory scale is both complex and expensive. In the present work we
2379
explore the use of an inexpensive and widely available commercial polymeric foam as template for the synthesis of silica and carbon monoliths. This paper is, to the best to our knowledge, the first to describe the use of commercial foams as scaffolds for fabricating silica and carbon monoliths. 2. Experimental We selected a polyurethane commercial foam, employed for varnishing, as sacrificial scaffolds to prepare the silica
Fig. 2. SEM images of the (a) polyurethane foam, (b) silica and (c) carbon monoliths, and TEM images of the porous structure of (d) silica and (e) carbon.
2380
S. Álvarez, A.B. Fuertes / Materials Letters 61 (2007) 2378–2381
added under stirring to a solution of HCl in ethanol (0.14 M). The mixture was subjected to vigorous stirring for 1 h at 70 °C in a closed vessel. Then, the surfactant (Pluronic 127) was added to the mixture and stirred until dissolution. The starting mole ratio is: TEOS/Pluronic 127/HCl/H2O/EtOH = 1:0.01:0.03:12:50. A piece of the polyurethane foam was introduced in a closed vessel that contained the surfactant mixture. The impregnated polymeric material was dried for 24 h at room temperature. The impregnation–drying cycle was repeated up to three times in order to improve the silica yield. The resulting silica–polymer composite was calcined in air at 600 °C (2 °C/min) for 4 h. The carbon monoliths were prepared by using the silica monoliths as templates. The carbon was introduced into the porosity of the silica by the vapour deposition polymerization method [14,17,18]. The macro/mesoporous silica monolith was immersed in a solution of paratoluene sulfonic acid (0.1 M in ethanol) and dried at 80 °C. Next, the impregnated silica monolith was placed in a closed Teflon vessel, in which also there was a small container with furfuryl alcohol. The vessel was closed and maintained at 40 °C for 14 h. The monolith was then heated in air up to 80 °C (15 h) in order to polymerize the furfuryl alcohol into polyfurfuryl alcohol. Next, it was carbonised at 800 °C (2 °C/min) in N2 for 1 h. Finally, the silica framework was removed with HF (48%) giving rise to the carbon monolith. Small-angle X-ray diffraction (XRD) patterns were measured by means of a Siemens D5000 instrument employing CuKα radiation (λ = 0.15406 nm). Transmission electron micrographs (TEM) and scanning electron micrographs (SEM) were taken on a JEM-2000 EX II and Zeiss DSM 942 microscopes, respectively. Adsorption measurements were performed using a Micromeritics ASAP 2010 gas analyser. The BET surface areas were deduced from the isotherm analysis in the relative pressure range of 0.04–0.20. The total pore volumes were calculated from the amount of nitrogen adsorbed at a relative pressure of 0.99. The pore size distributions (PSD) were deduced from the adsorption branch of nitrogen isotherms by means of the Kruk–Jaroniec–Sayari (KJS) algorithm [19]. 3. Result and discussion Photographs of the polyurethane foam and the silica and carbon monoliths are shown in Fig. 1. From these images it is clear that the macroscopic morphology of both the silica and carbon monoliths is the same as for the initial polyurethane foam. They also reveal that during the replication process, from the polymeric foam to the silica and Fig. 3. (a) Low-angle XRD patterns for the silica and carbon materials and (b) nitrogen sorption isotherms and pore size distributions (inset) of the silica and carbon monoliths.
monoliths. These polymeric foams have a low density and very open macrocellular structure formed by voids of a size within the 50–250 μm range (see Fig. 2a). The procedure used to prepare the silica monoliths is based on the method reported by Zhao et al. [16] to obtain mesostructured SBA-16 silica materials. In a typical synthesis, the silica source (Tetraethyl orthosilicate, TEOS, Aldrich) was
Table 1 Physical properties of the framework-confined porosity of the silica and carbon monoliths Sample
SBET (m2 g− 1)
Vp (cm3 g− 1) a
Vmesop (cm3 g− 1) b
δKJS (nm) c
FWHM (nm) d
Silica Carbon
560 1860
0.7 1.9
0.6 1.7
7.2 3.4
1.9 2.8
a b c d
Total pore volume of the framework-confined pores. Volume of the mesopores. Maximum of PSD. Full width at the half maximum of PSD.
S. Álvarez, A.B. Fuertes / Materials Letters 61 (2007) 2378–2381
carbon monoliths, contraction takes place. However, in spite of this contraction, both the silica and carbon materials maintain their structural integrity and no cracks are detected in the monolithic structure. Fig. 2b and c shows the SEM microphotographs obtained for the polyurethane foam and the templated silica and carbon materials. These images reveal that the foamy structure characteristic of a polymer consisting of large interconnected vesicles (Fig. 2a) is preserved in the templated monoliths of silica (Fig. 2b) and carbon (Fig. 2c). Thus, these materials have an internal structure made up of large interconnected voids (approx. 200 μm) separated by thin walls, which form the skeleton of the monolith. More important, the walls of the silica and carbon skeleta contain a framework-confined porosity that is made up of quite uniform and well-ordered mesopores. From the TEM images it can be seen that the porosity exhibits an ordered pattern, which is characteristic of the cubic porous structure of SBA-16 silica [16] (Fig. 2d) and its carbon replica (Fig. 2e). These results are confirmed by the X-ray diffraction analysis performed at the low-angle range (2θ b 5°). Fig. 3a contains the XRD patterns obtained for the silica monolith and its carbon replica. They exhibit an intense diffraction peak (2θ ∼ 1°) and one weaker peak (2θ ∼ 1.5°), which can be assigned to the (100) and (200) reflections characteristic of the wellordered cubic porous structure of SBA-16 [16]. A d-spacing of 9.7 nm is deduced from the (100) reflection for the silica monolith. These results show that the walls of the carbon and silica monoliths have a well-ordered porosity. The physical properties of the framework-confined porosity in the silica and carbon monoliths were studied by means of nitrogen adsorption. These monoliths have high BET surface areas and large framework-confined pore volumes (see Table 1). Around 90% of the pore volume is made up of mesopores in the 2–10 nm range. Fig. 3b shows the nitrogen sorption isotherms and the PSDs (inset) obtained for the silica and carbon materials. The PSD (Fig. 3b, inset) reveals that the porosity of the silica is made up of mesopores of sizes centred at around 7.2 nm. These mesopores are very uniform, as evidenced by the narrow PSD being the full width at the half maximum (FWHM) of 1.9 nm (see Table 1). As in the case of silica, the carbon monolith also shows the existence of a mesopore system as confirmed by the PSD, which exhibits a maximum at 3.4 nm. The mesopores in the carbon material are very uniform (FWHM = 2.8 nm).
4. Conclusions In summary, a facile and new route to prepare macro/ mesoporous silica and carbon monoliths is presented. These materials are obtained by using a low-cost commercial polyurethane foam as a sacrificial scaffold. The macrocellular structure of the polyurethane foam is preserved in the silica and carbon
2381
monoliths, which exhibit a fully interconnected macroporous network. Moreover, these materials have a well-ordered porosity made up of uniform mesopores with sizes centred at around 7.2 nm (silica) and 3.4 nm (carbon). They also have high BET surface areas (1860 m2 g− 1 for carbon and 560 m2 g− 1 for silica) and large pore volumes (1.9 cm3 g− 1 for carbon and 0.7 cm3 g− 1 for silica). This synthetic strategy opens up a novel and costeffective route to fabricate monoliths that combine a very open and interconnected macroporous structure with a large internal surface area and porosity. Acknowledgements The financial support for this research work provided by the Spanish MCyT (MAT2005-00262) is gratefully acknowledged. S. Alvarez thanks the Spanish MCyT for her FPI (BES-20030134) grant. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
J. Lee, S. Han, T. Hyeon, J. Mater. Chem. 14 (2004) 478. S. Alvarez, A.B. Fuertes, Carbon 42 (2004) 437. H.F. Yang, Y. Yan, D.Y. Zhao, J. Ind. Eng. Chem. 10 (2004) 1146. E. Frackowiak, F. Beguin F, Carbon 39 (2001) 937–950. S. Álvarez, M.C. Blanco-López, A.J. Miranda-Ordieres, A.B. Fuertes, T.A. Centeno, Carbon 43 (4) (2005) 866. B-H. Han, W. Zhou, A. Safari, J. Am. Chem. Soc. 125 (2003) 3444. H. Yang, Q. Shi, X. Liu, S. Xie, D. Jiang, D. Zhao, Chem. Commun. (2002) 2842. Z-G. Shi, Y-Q. Feng, L. Xu, S.-.L. Da, Y-Y. Ren, Microporous Mesoporous Mater. 68 (2004) 55. H. Maekawa, J. Esquena, S. Bishop, C. Solans, B.F. Chmelka, Adv. Mater. 15 (2003) 591. B. Zhang, S.A. Davis, S. Mann, Chem. Mater. 14 (2002) 1369. A. Taguchi, J-H. Smatt, M. Linden, Adv. Mater. 15 (2003) 1209. Z. Shi, Y. Feng, L. Xu, S. Da, M. Zhang, Carbon 41 (2003) 2677. Y. Sato, K. Nakanishi, K. Hirao, H. Jinnai, M. Shibayama, Colloids Surf., A Physicochem. Eng. Asp. 187 (2001) 117. S. Alvarez, J. Esquena, C. Solans, A.B. Fuertes, Adv. Eng. Mater. 6 (11) (2004) 897. J. Esquena, G.S.R.R. Sankar, C. Solans, Langmuir 19 (2003) 2983. D. Zhao, Q. Huo, B.F. Feng, G.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. A.B. Fuertes, Carbon 40 (2002) 1600. A.B. Fuertes, D.M. Nevskaia, J. Mater. Chem. 13 (2003) 1843. M. Kruk, M. Jaroniec, A. Sayari, Langmuir 13 (1997) 6267.