NANO LETTERS
Hollow Spheres of Mesoporous Aluminosilicate with a Three-Dimensional Pore Network and Extraordinarily High Hydrothermal Stability
2003 Vol. 3, No. 5 609-612
Yongsheng Li, Jianlin Shi,* Zile Hua, Hangrong Chen, Meiling Ruan, and Dongsheng Yan State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding-xi Road, Shanghai 200050, P. R. China Received March 5, 2003; Revised Manuscript Received March 26, 2003
ABSTRACT A hollow-structured and highly ordered mesoporous aluminosilicate with a 3D pore network and significantly improved hydrothermal stability has been successfully prepared by a simple method. To elucidate its novel structure and improved hydrothermal stability, a model for the formation of such a hollow spherical material has been proposed.
Hollow spherical materials with ordered pore structures have attracted much attention because of their potential applications in drug storage and release, confined-space catalysis, separation, chromatography, and large biomolecular-release systems.1-5 A number of papers have described the formation of hollow spherical materials,6-12 and several of them are on mesoporous silica materials.9-12 A general approach for preparing hollow spheres of mesoporous materials was based on sol-gel/emulsion technologies or the use of organic polymer beads as the templates that control the void formation and its volume. However, the pore structures of these hollow spherical mesoporous materials were hexagonally ordered, and they were hydrothermally unstable, which limited their applications in practice. The combination of a 3D pore network and a hollow spherical morphology, in addition to the remarkable thermal and hydrothermal stability, is significantly important for the mesoporous materials to be used in chemical catalysis and molecular separations.13 In this paper, we report a simple method to synthesize hydrothermally stable and hollow spherical cubic mesoporous aluminosilicate (HSCM). A model for the formation of such a hollow spherical material with improved hydrothermal stability has been proposed. A typical synthesis route of the hydrothermally stable hollow spherical material was as follows. Solution A was * Corresponding author. E-mail:
[email protected]. Tel: 86-2152414802. Fax: 86-21-52413903. 10.1021/nl034134x CCC: $25.00 Published on Web 04/16/2003
© 2003 American Chemical Society
prepared by adding tetrapropylammonium hydroxide (TPAOH) and NaOH to a mixed aqueous solution of Al2(SO4)3‚18H2O and tetraethyl orthosilicate (TEOS) with stirring. To control the hydrolysis process of TEOS, the temperature was kept under 18 °C. The resulting solution was aged for 20 h at the same temperature and then dropped into an aqueous solution of cetyltrimethylammonium bromide (CTAB) under vigorous stirring. Stirring was continued for another 2 h. The mole composition of the gel was (0.5-2)Al2O3:100SiO2:(10-16)Na2O:16TPAOH:(8-12)CTAB:16 600H2O. The resulting gel was sealed in Teflon-lined autoclaves and heated at 130 °C for 12-48 h. The solid product was recovered by filtration and dried in an oven at 100 °C overnight. The as-synthesized material was then calcined in air at 550 °C for 10 h to remove the templates. For comparison, conventional MCM-48 was synthesized according to the published procedure.14 SEM images of HSCM (see Supporting Information) show that the particles are uniform in size with a spherical shape, and the average size of the particles is ∼600 nm. The XRD pattern of the hollow spherical aluminosilicate (2θ(211) ) 2.45, d(211) ) 3.60 nm) is compared with that of conventional MCM-48 material (2θ(211) ) 2.61, d(211) ) 3.39 nm) as shown in Figure 1. For HSCM (Figure 1a), several Bragg diffraction peaks at low angles can be observed, which is typical for MCM-48. No diffraction peaks were observed in the region of higher angles (5-40°, inset), which indicates the absence of large microporous crystals in the sample, suggesting that
Figure 1. XRD patterns of (a) calcined HSCM and (b) calcined conventional MCM-48.
Figure 3. HRTEM images of an ultrathin microtome section of HSCM along the [110] direction.
Figure 2. HRTEM images of calcined HSCM along the [311] direction.
the HSCM sample is a pure mesostructured phase. Nitrogen adsorption-desorption measurements show a typical IV isotherm (IUPAC nomenclature). A sample with a specific surface area of 1005 m2/g and a pore diameter of 2.9 nm (3.21 nm according to the calibrated BJH method15-17) was obtained using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively. Figure 2 is the HRTEM image of HSCM along the [311] direction. Image contrast suggests that the spherical particles might be hollow in the center, though the mesopore structure has a highly cubic order. To make the structure of the particles more clear, the HRTEM images of the calcined sample in an ultrathin microtome section were examined. As shown in Figure 3, it can be clearly found that the particles have a hollow spherical morphology and that the shell of the hollow particles consists of uniformly and cubically ordered mesopores. Because the microtome section for HRTEM observation is only ∼70 nm thick, some ordered pores may have been destroyed somewhat, and the hollow sphere may be deformed into an elliptical one during the cutting process. The diameters of the hollow sections and the thicknesses of the shells are in the range of 150-200 nm and 200-250 nm, respectively. The comparable dimensions of the hollow core and the length of the mesopore channels, combined with the 3D pore network, are favorable for facilitating the access of guest molecules.13 Lin9 has found that the shells of the hollow spheres of MCM-41 are composed of a hexagonal arrangement of nanochannels in latitude directions, which limits molecular diffusion, so they 610
Figure 4. XRD patterns of (a) HSCM after treatment in boiling water for 120 h and (b) HSCM after treatment in pure-water steam for 60 h.
Figure 5. FT-IR spectra of (a) MCM-48 and (b) HSCM.
prepared hollow spheres of MCM-41 with two tiny holes at the poles so that large entities could penetrate the spheres. The hydrothermal stability of HSCM was evaluated by the treatment in 120 °C pure steam and boiling water. The XRD patterns of HSCM after treatment in pure-water stream for 60 h and boiling water for 120 h are shown in Figure 4. The pore structure of HSCM is still in cubic order. HRTEM Nano Lett., Vol. 3, No. 5, 2003
Figure 6. Schematic drawing of the suggested formation process of a hollow structure.
image of HSCM after treatment with boiling water for 120 h (see Supporting Information) shows that the particles are integral and that the mesostructure of the particle is cubically ordered on a long-range scale after the treatment. The N2 adsorption-desorption isotherm shows clearly a typical IV isotherm, and the pore diameter distribution is very narrow. The specific surface area is 759 m2/g after the boiling-water treatment, which demonstrates a significant improvement as compared to the report that the mesostructured silica hollow sphere would lose its periodicity only after 10 h of heating in boiling water.10 Figure 5 gives the FT-IR spectra of HSCM and conventional MCM-48. The FT-IR spectrum of conventional MCM-48 shows a broad band at 460 cm-1 in the region of 400-600 cm-1, which is similar to those of amorphous materials. However, HSCM exhibits a distinctive absorption band in the region of 550-600 cm-1, which is similar to that of characteristic units consisting of eight fivemembrered rings of T-O-T (T ) Si of Al) in the microporous ZSM-5 zeolite18-20 but not present in MCM48. These results verify that zeolite primary building units are present within HSCM walls. Kawi et al.21 have reported that the small amount of Al substituted in the framework of Si-Al-MCM-41 could give a remarkable improvement of the hydrothermal stability of Si-Al-MCM-41 in boiling water for days. However, it is found that conventional MCM48 with the same composition as HSCM would become disordered after treatment in boiling water for 24 h. These results demonstrate that the zeolite primary building units existing within the HSCM walls are responsible for the improved hydrothermal stability.22-25 A model for the formation of the hollow structure is suggested as sketched in Figure 6. Adding TEOS to an aqueous solution containing Al2(SO4)3‚18H2O with vigorous stirring creates an oil(TEOS)-in-water(aqueous solution) emulsion (Figure 6a). When the solution containing TPAOH and NaOH was dropped gradually into the emulsion, TPAOH as a surfactant was located preferentially at the liquid-liquid interface. Thus, it is possible that the structure directing by the template and the hydrolysis of TEOS take place simultaneously. Because of the low TPAOH content and the continuous addition of alkaline solution, some hydrolyzed TEOS condensed along the interface, which could prevent TEOS in the droplets from hydrolyzing further. In this way, some unhydrolyzed TEOS was trapped in the droplets Nano Lett., Vol. 3, No. 5, 2003
(Figure 6b). After the addition of this emulsion to the surfactant solution, surfactant is enriched at the oil-water interface and contributes to the stabilization of the droplets (Figure 6c and d). When the temperature is raised during hydrothermal treatment, diffusion out and hydrolysis of TEOS in the center of the droplets will continue and form the hollow core. More primary building units of zeolite form and further condense. At the same time, these zeolite primary building units assemble with surfactant molecules around the interface, resulting in the formation of an ordered cubic mesostructured shell around the hollow core (Figure 6e). During this process, zeolite primary building units can be successfully incorporated into the inorganic walls of the mesoporous materials, and hence the hydrothermal stability of the material was greatly improved. This formation process is different from that of ultrastable mesostructured silica vesicles, which is created through minimizing the surface energy by hydrogen bonding between electrically neutral gemini surfactants and silica precursors.13 It is also found that the hydrolysis temperature of TEOS is a key to the successful synthesis of hollow structures with highly ordered cubic mesopore shells. If the temperature is higher than 18 °C, then the ultimate product is not hollow spheres, and the hydrothermal stability is much worse than that of HSCM. These effects may be due to the complete hydrolysis of TEOS before mixing with surfactant at higher temperature. Therefore, it is believed that the void volume of the spheres was made from the diffusion out and hydrolysis of TEOS during hydrothermal treatment; TEOS was previously trapped in the center of the droplets and stabilized by TPAOH and surfactant molecules at low temperature. In conclusion, hollow-structured and highly ordered cubic mesoporous material with significantly improved hydrothermal stability has been synthesized by a simple method. This material is expected to have a high potential for use in practical applications (e.g., the hollow spheres with a cubic meoporous wall structure could be used as controlled drugdelivery systems). Its improved hydrothermal stability makes it possible to be used as a catalyst and a support under hydrothermal conditions as well. Acknowledgment. We gratefully acknowledge the support of the K.C. Wong Education Foundation, Hong Kong, 611
the China Postdoctoral Science Foundation, and the National Natural Science Foundation of China (50232050). Supporting Information Available: FE-SEM images of calcined HSCM samples at low and high magnification and HRTEM image of HSCM after being treated in boiling water for 120 h. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Nature (London) 1994, 368, 321. (2) Schacht, S.; Huo, Q.; Voigt-Martin, I. G.; Stucky, G. D.; Schuth, F. Science (Washington, D.C.) 1996, 273, 768. (3) Lin, H.-P.; Mou, C.-Y. Science (Washington, D.C.) 1996, 273, 765. (4) Yang, P. D.; Zhao, D. Y.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1998, 10, 2033. (5) Huang, H.; Remsen, E. E.; Kowalewski, T.; Wooley, K. L. J. Am. Chem. Soc. 1999, 121, 3805. (6) Mecking, S.; Thomann, R. AdV. Mater. 2000, 12, 953. (7) Yin, Y.; Liu, Y.; Gates, B.; Xia, Y. Chem. Mater. 2001, 13, 1146. (8) Zhong, Z.; Yin, Y.; Gates, B.; Xia, Y. AdV. Mater. 2000, 12, 206. (9) (a) Lin, H.-P.; Cheng, Y.-R.; Mou, C.-Y. Chem. Mater. 1998, 10, 3772. (b) Lin, H.-P.; Mou, C.-Y.; Liu, S.-B.; Tang, C.-Y. Chem. Commun. 2001, 1970. (10) Zhu, G.; Qiu, S.; Terasaki, O.; Wei, Y. J. Am. Chem. Soc. 2001, 123, 7723.
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Nano Lett., Vol. 3, No. 5, 2003
