Preparation of stable mesoporous inorganic oxides via nano-replication technique

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1 Catalysis Today (2004) Preparation of stable mesoporous inorganic oxides via nano-replication technique Min Kang a, Dongchan Kim a, Seung Hwan Yi b, Jae Uk Han b, Jae Eui Yie b, Ji Man Kim a, a Functional Materials Laboratory, Department of Molecular Science and Technology, Ajou University, Suwon , Republic of Korea b Catalyst and Surface Laboratory, Department of Applied Chemistry, Ajou University, Suwon , Republic of Korea Abstract Various kinds of mesoporous metal oxides with high thermal stabilities as well as crystalline frameworks have been successfully prepared using a mesoporous carbon as a template via nano-replication technique. This noble synthesis strategy for the preparation of mesoporous materials was carried out by the impregnation with the desired metal precursor. The mesoporous metal oxides calcined at 823 K exhibited m 2 /g of BET surface areas and 3 12 nm of pore sizes. The frameworks of the mesoporous metal oxide materials were crystallized by heat treatment at 1073 K in N 2 flow before the template removal. The results indicate that the mesoporous metal oxides can be obtained by the faithful negative replication of the structure of the mesoporous carbon Elsevier B.V. All rights reserved. Keywords: Mesoporous metal oxide; Nano-replication; Mesoporous carbon; Template 1. Introduction It is well known that mesoporous materials are synthesized by the synergistic self-assembly between surfactant micelles and inorganic species to form mesoscopically ordered composites [1 3]. Since the discovery of mesoporous silica materials [1], increasing attention has been focused on surfactant-templated synthesis of non-siliceous mesoporous materials based on metal oxides [4]. Especially, many effects have been devoted to the synthesis of mesoporous transition metal oxides which are expected to be very useful for several applications. The mesoporous transition metal oxide with ordered mesoporous structure, high surface area and crystalline framework structures are expected to have a lot of advantages for catalysis and nano-science. There have been several reports concerning the synthesis of mesoporous metal oxides, such as titanium, vanadium, zirconium, tungsten, niobium and tantalum oxides via surfactant-templated [5], ligand-assisted [6] and polymer templated [7] pathways. However, the synthesis of mesoporous metal oxides has been less successful compared with that of silica materials. One difficulty may be a facile crystallization of most Corresponding author. Tel.: ; fax: address: jimankim@ajou.ac.kr (J.M. Kim). metal oxides, accompanied by structural collapse, during the mesostructure formations and the removal of the organic templates [4]. Moreover, most of these methods require either the handling of large amounts of liquids or suspension, complex reaction sequences, and expensive precursors. Recently, our group [8] and others [9] have reported that ordered mesoporous carbon, obtained from the mesoporous silica by nanocasting route [10], can be reversibly used as a template for the order mesoporous silica materials. Because the mesoporous carbons have well-ordered mesoporosity and rigidity, it can be an excellent template for the porous inorganic materials via nano-replication technique. Mesoporous metal oxides have also been prepared by this nano-replication technique [8] using mesoporous carbon, CMK-3 as the template. These efforts have opened a new possibility to synthesize ordered mesoporous materials using other rigid templates such as carbon instead of the conventional surfactant templates. More recently, Dong et al. [11], have also reported the synthesis of mesoporous spheres of metal oxides and phosphates using mesoporous carbon. They have used the spherical mesoporous silica as the template for the mesoporous carbon, which has been obtained from alkylamine surfactant. Therefore, the carbon material thus obtained may exhibit disordered structures [11] because the spherical mesoporous silica is known to be disordered structure [12] /$ see front matter 2004 Elsevier B.V. All rights reserved. doi: /j.cattod

2 696 M. Kang et al. / Catalysis Today (2004) Table 1 BET surface areas and total pore volumes of the mesoporous metal oxides obtained via nano-replication technique from the mesoporous carbon Materials Framework structure a S b BET (m 2 /g) V p c (cm 3 /g) Pore size d (nm) Al 2 O 3 -Al 2 O TiO 2 Rutile ZrO 2 Tetragonal V 2 O 5 Orthorhombic MoO 3 Orthorhombic WO 3 Orthorhombic Fe 2 O 3 Cubic MnO 2 Tetragonal a The framework structures were determined by high angle XRD patterns. b BET surface areas obtained from N 2 adsorption isotherms. c Total pore volumes. d Pore sizes were calculated from N 2 desorption branches using BJH model. In the present work, we have utilized a highly ordered mesoporous silica SBA-15 as the template for the highly ordered mesoporous carbon CMK-3, and obtained various kinds of mesoporous metal oxides with high thermal stability as well as crystalline frameworks through the nano-replication technique using the CMK-3. We will discuss the preparation of mesoporous metal oxide materials using the ordered mesoporous carbon as a template. As shown in Table 1, our results cover the various kinds of metal oxides such as Al 2 O 3,TiO 2, ZrO 2,V 2 O 5, MoO 3, WO 3,Fe 2 O 3 and MnO Experimental 2.1. Materials and synthesis A mesoporous SBA-15 silica was prepared according to the typical synthetic process reported by literature [8,13]. Triblock copolymer P123 and 2 M HCl were dissolved in water to form a clear solution. Then TEOS was added to that solution under stirring condition then further stirred for 24 h at 313 K and then for 24 h at 373 K under static condition. Mesoporous carbon CMK-3 were obtained following the procedures described elsewhere [10], using a calcined SBA-15 as the template and sucrose as the carbon framework source. SBA-15 silica was well mixed with an aqueous solution consisting of sucrose, H 2 SO 4 and H 2 O. The resultant viscous mixture was dried at 373 K in a drying oven and the oven temperature was subsequently increased to 433 K. The silica carbon mixture was heated to 1173 K in N 2 flow and then washed with HF solution. The CMK-3 material thus obtained was impregnated with aqueous solution of metal precursors by wetness method using a rotary evaporator, and subsequently the sample was hydrothermally treated under acidic or basic conditions to obtain the fully condensed inorganic framework. Na 2 SiO 3, TiCl 4, ZrOCl 2 and NaAlO 2 were used as the precursors of silica, titania, zirconia and alumina frameworks, respectively. Finally, the samples were heated at 1073 K in nitrogen flow and then were calcined at 823 K in air under static condition to remove the carbon frameworks Characterization The structure and order of mesoporous transition metal oxide were determined by X-ray powder diffraction (XRD) analyses. The XRD patterns were obtained by using Cu K radiation using a Rigaku D/MAX-III instrument at room temperature. N 2 adsorption desorption isotherms at liquid N 2 temperature were measured using an Autosorb-1 apparatus (Quantachrome), and pore size distributions were calculated by the BJH method. Before the measurements, samples were degassed for 12 h at 550 K. Scanning electron microscopic (SEM) images were collected with a Sterescan 440 microscope (Leica Cambridge Ltd.) operating at 15 kv. Transmission electron microscopic (TEM) images were obtained using CM 20 (Philips) apparatus operating at 200 kev. 3. Results and discussion Fig. 1 shows XRD patterns of the SBA-15 and CMK-3, and the mesoporous silica, alumina, zirconia and titania materials, which are obtained by nano-replication technique using the CMK-3 as the template. In case of SBA-15 and CMK-3 materials, XRD patterns with a very intense diffraction peak and two or more weak peaks, which are characteristic of 2D hexagonal (P6mm) structures [1 3]. The mesoporous silica material, obtained from the mesoporous carbon, also exhibits, highly ordered 2D hexagonal structure Fig. 1. X-ray diffraction patterns for SBA-15 and CMK-3, and mesoporous SiO 2, ZrO 2,Al 2 O 3 and TiO 2 materials, which are obtained from CMK-3 via nano-replication technique.

3 M. Kang et al. / Catalysis Today (2004) as reported elsewhere [8,9]. The result indicates that the mesoporous silica can be reversibly obtained by faithful negative nano-replication from the mesoporous carbon, and it is possible to use the mesoporous carbon as a good templating material for the production of mesoporous inorganic oxides. The XRD patterns of zirconia, alumina and titania materials in Fig. 1 show a single diffraction peak at low angle, which are broader than those of the SBA-15 and the silica replica material. Moreover, there is no long range order peaks in these cases. This means that the non-silica mesoporous materials in the present work have disordered channel arrangements, compared with the highly ordered mesostructure of silica material. These may be due to the mesostructural reconstruction during the calcination at 823 K to remove the carbon template. Even though the mesostructures of the non-silica materials obtained from the mesoporous carbon are poorly ordered, it is very interesting that the materials shows the XRD peaks after the calcination at high temperature. Various kinds of mesoporous non-silica materials can also be successfully obtained by using this synthetic strategy, as listed in Table 1. It is reasonable that the rigid nature and well-defined mesoporosity of the mesoporous carbon framework help to form and stabilize the mesostructured inorganic frameworks, and prevent from loss of mesostructures during the calcination. Elemental analysis of the materials indicates that there is no carbon residue after calcination. Further evidences for mesostructures of the non-silica materials are provided by the TEM images in Fig. 2. Mesoporous alumina shows relatively ordered structure compared with other materials. Fig. 2(a) shows that the channels of the mesoporous alumina material run parallel with the c-axis, which means that the material may have ordered 2D hexagonal structure. However, it is very difficult to find such an ordered region from the mesoporous zirconia and titania materials. As shown in Fig. 2(b), no structural orders have been found from the TEM image of the mesoporous zirconia material. The mesostructures seems to be a three-dimensionally disordered network of wormhole-like channels, which may be constructed with aggregation of lots of nanoparticles. This kind of mesostructures is believed to be useful for the catalytic applications from the diffusional point of view. Very similar microscopic images are obtained from the mesoporous titania and other materials. Fig. 3 shows nitrogen adsorption desorption isotherms and corresponding pore size distribution curves for the mesoporous non-silica materials. All the isotherms in Fig. 2 are type IV with hysteresis loops, which are characteristics of the mesoporous materials. The adsorption jumps in the isotherms appear between partial pressures p/p 0 of Fig. 2. Transmission electron microscopic images of (a) mesoporous Al 2 O 3 and (b) mesoporous ZrO 2 materials. Fig. 3. N 2 adsorption and desorption isotherms for mesoporous ZrO 2, TiO 2 and Al 2 O 3 materials, and corresponding pore size distribution curves (insets) obtained from desorption branches of the isotherms using BJH method.

4 698 M. Kang et al. / Catalysis Today (2004) due to the capillary condensation in the mesopore. BET surface areas, total pore volume and BJH pore diameters for the materials are listed in Table 1. The physical properties in Table 1 indicate that the surface areas and pore volumes are much larger than those of the conventional non-silica materials. As shown in Fig. 3, pore size distribution curves of the mesoporous non-silica materials are broader than that of the mesoporous silica [2] due to their poorly ordered structures. In case of the mesoporous alumina, the pore size is very similar with that of mesoporous silica obtained by nano-replication. This is probably due to somewhat ordered structure as shown in TEM image of Fig. 2(a). The mesoporous zirconia and titania materials exhibit large pore diameters (8.8 and 11.4 nm). The relatively small pore sizes around 3 nm for other materials in Table 1 is believed to be large lattice contractions depending on the thermal stabilities of the framework constituents. It is noteworthy to look at the morphologies of the materials during the nano-replication. Fig. 4 shows the SEM images of the SBA-15, CMK-3 and the mesoporous alumina material. SEM images reveal that the morphology of the alumina material is very similar to those of the SBA-15 and CMK-3. This means that the morphology is well preserved in the replica materials in the present work, even though the ordered channel structure of mesoporous carbon turns to disordered channel arrangement in the mesoporous non-silica materials. This is also one of the evidences that the nano-replication technique can be useful to produce the mesoporous materials. More interesting and important results are high angle XRD patterns of the materials in Fig. 5. Mesoporous non-silica materials can also be synthesized by the direct pathways using surfactants and polymers as the structure-directing agents [4 7]. However, the mesostructures based on the non-silica materials is often collapsed by the thermal damage of structural integrity during the calcination. Although there are several reports concerning the thermally stable non-silica mesoporous materials, the frameworks are generally amorphous or semicrystalline [4 7], and it is difficult to control the crystallinity of the frameworks. The high angle XRD patterns of the mesoporous zirconia, alumina and titania materials in Fig. 5 shows that the materials have well-crystalline tetragonal-zro 2, -Al 2 O 3 and rutile-tio 2 frameworks, respectively. The crystalline structures of the non-silica mesoporous materials obtained by the nano-replication pathway are listed in Table 1. To prepare the materials in the present work, the void mesoporous space of the CMK-3 is filled with inorganic precursors and the carbon-inorganic composites thus obtained are heated at high temperature (e.g K) in the inert atmosphere. We believe that the thermally stable nature of the carbon template in this heating conditions help to make the inorganic frameworks crystalline before removal of templates. Moreover, this technique promises the successful preparation of Fig. 4. Scanning electron microscopic images of the SBA-15, CMK-3 and mesoporous Al 2 O 3 materials. Fig. 5. High angle X-ray diffraction patterns for the mesoporous ZrO 2, Al 2 O 3 and TiO 2 materials.

5 M. Kang et al. / Catalysis Today (2004) non-silica mesoporous materials with different crystalline structures, if the heating temperature is controlled in the inert atmosphere. 4. Conclusions Ordered mesoporous carbon can be successfully used as a rigid template for the formation of mesoporous non-silica materials. Various kinds of mesoporous inorganic materials such as alumina, zirconia, titania, etc. can also be obtained from the carbon template by using the nano-replication synthetic strategy. The method is especially suitable for preparation of the materials with highly crystalline frameworks. This noble nano-replication technique for the preparation of mesoporous materials is believed to be a remarkable achievement in the field of porous materials. However, the XRD patterns and other characterization results indicate that the mesoscopic orders of the materials are not in good qualities, compared with that of the silica material. We are developing the optimum conditions in order to get highly ordered mesoporous non-silica materials. Acknowledgements This work was supported by the Korea Research Foundation Grant No. KRF D00079 through grants for upcoming researchers. References [1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. [2] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548. [3] A. Corma, Chem. Rev. 97 (1997) 2373, and references therein. [4] F. Schüth, Chem. Mater. 13 (2001) 3184, and references therein. [5] U. Ciesla, S. Schacht, G.D. Stucky, K.K. Unger, F. Schüth, Angew. Chem. Int. Edit. 35 (1996) 541. [6] D. Antonelli, J.Y. Ying, Angew. Chem. Int. Edit. 35 (1996) 426. [7] P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka, G.D. Stucky, Nature 396 (1998) 152. [8] (a) M. Kang, S.H. Yi, H.I. Lee, J.E. Yie, J.M. Kim, Chem. Commun. (2002) 1944; (b) J.M. Kim, M. Kang, S.H. Yi, J.E. Yie, R. Ryoo, Stud. Surf. Sci. Catal. 146 (2003) 53. [9] (a) A.-H. Lu, W. Schmidt, A. Taguchi, B. Spliethoff, B. Tesche, F. Schüth, Angew. Chem. Int. Edit. 41 (2002) 3489; (b) J. Parmentier, C. Vix-Guterl, S. Saadallah, M. Reda, M. Illescu, J. Werckmann, J. Patarin, Chem. Lett. 32 (2003) 262. [10] (a) S. Jun, S.H. Joo, R. Ryoo, M. Kruk, M. Jaroniec, Z. Liu, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc. 122 (2000) 10712; (b) S.H. Joo, S.J. Choi, I. Oh, J. Kwak, Z. Liu, O. Terasaki, R. Ryoo, Nature 412 (2001) 169. [11] A. Dong, N. Ren, Y. Tang, Y. Wang, Y. Zhang, W. Hua, Z. Gao, J. Am. Chem. Soc. 125 (2003) [12] M. Grün, G. Büchel, D. Kumar, K. Schmuacher, B. Bidlingmaier, K.K. Unger, Stud. Surf. Sci. Catal. 128 (1998) 155. [13] J.M. Kim, G.D. Stucky, Chem. Commun. (2000) 1159.