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Microwave-Assisted Preparation of Hierarchical Mesoporous-Macroporous Boehmite AlOOH and γ-Al2O3 Tie-Zhen Ren, Zhong-Yong Yuan, and Bao-Lian Su* Laboratory of Inorganic Materials Chemistry, The University of Namur (FUNDP), 61 rue de Bruxelles, B-5000 Namur, Belgium Received November 19, 2003. In Final Form: December 2, 2003
Porous aluminum oxides are of great interest because of their widespread applications in catalysis (as catalyst and catalyst support), adsorption, and separation. Many attempts have been made to synthesize ordered mesoporous alumina by the supramolecular templating method.1 Either nonionic2-4 or anionic5-7 or cationic8,9 surfactants, or nonsurfactant molecules,10 were used as structural directing agents for the synthesis of mesoporous aluminas. However, most of the previously reported mesoporous aluminas were comprised mainly amorphous framework walls, which would limit their hydrothermal stability and greatly compromise their usefulness in catalytic applications. Mesoporous aluminum oxide materials with crystalline AlOOH (boehmite) and γ-Al2O3 were then recently demonstrated by Pinnavaia et al. using amine and PEO surfactants as structure directors, respectively.11,12 The occurrence of macroporous structures in the mesoporous metal oxides is important and useful for the treatment of bulky molecules because the textural mesopores and intrinsic interconnected pore systems of macrostructures should efficiently transport guest species to framework binding sites.13 More recently, hierarchical mesoporous-macroporous zirconias,13 aluminas,9 and aluminosilicates14 were synthesized in the presence of surfactant molecules. However, these macro-mesostructured materials were amorphous. In this paper, we report the first synthesis of hierarchical mesoporous-macroporous crystalline boehmite AlOOH and γ-Al2O3. Besides conventional hydrothermal synthesis, microwave heating was also employed in this work since it can accelerate reactions and may produce better quality products.15,16 * To whom correspondence may be addressed. Telephone: +3281-724531. Fax: +32-81-5414. E-mail:
[email protected]. (1) Schu¨ch, F. Chem. Mater. 2001, 13, 3184. (2) Bagshaw, S. A.; Pinnavaia, T. J. Angew. Chem., Int. Ed. Engl. 1996, 35, 1102. (3) Gonzalez-Pena, V.; Diaz, I.; Marquez-Alvarez, C.; Sastre, E.; PerezPariente, J. Microporous Mesoporous Mater. 2001, 44, 203. (4) Yang, P.; Zhao, D.; Margolese, D. I.; Chmelka, B. F.; Stucky, G. D. Chem. Mater. 1999, 11, 2813. (5) Yada, M.; Machida, M.; Kijima, T. Chem. Commun. 1996, 769. (6) Vaudry, F.; Khodabandeh, S.; Davis, M. Chem. Mater. 1996, 8, 1451. (7) Valange, S.; Guth, J. L.; Kolenda, F.; Lacombe, S.; Gabelica, Z. Microporous Mesoporous Mater. 2000, 35, 597. (8) Cabrera, S.; EI Haskouri, J.; Alamo, J.; Beltran, A.; Bltran, D.; Mendioroz, S.; Marcos, M. D.; Amoros, P. Adv. Mater. 1999, 11, 379. (9) Deng, W.; Toepke, M. W.; Shanks, B. H. Adv. Funct. Mater. 2003, 13, 61. (10) Liu, X.; Wei, Y.; Jin, D.; Shih, W. H. Mater. Lett. 2000, 42, 143. (11) Hicks, R. W.; Pinnavaia, T. J. Chem. Mater. 2003, 15, 78. (12) Zhang, Z.; Hicks, R. W.; Pauly, T. R.; Pinnavaia, T. J. J. Am. Chem. Soc. 2002, 124, 1592. (13) Blin, J. L.; Le´onard, A.; Yuan, Z. Y.; Gigot, L.; Vantomme, A.; Cheetham, A. K.; Su, B. L. Angew. Chem., Int. Ed. 2003, 42, 2872. (14) Le´onard, A.; Blin, J. L.; Su, B. L. Chem. Commun. 2003, 2568.
Figure 1. X-ray diffraction patterns of the samples: (a) hydrothermal synthesized; (b) after calcination of (a) at 500 °C; (c) microwave synthesized; (d-f) after calcination of (c) at 300, 400, and 500 °C, respectively. Inset is the low-angle diffraction patterns of (a) and (f).
In a typical synthesis, 6.67 g of surfactant Brij 56 (Aldrich) was dissolved in a H2SO4 aqueous solution of 70 mL (pH ) 2) with stirring for 3 h at 70 °C, followed by the addition of aluminum sec-butoxide (Fluka, 4.2 g) with a surfactant/aluminum molar ratio of 0.5. After further stirring for 1 h, the mixture was transferred into a Teflonlined autoclave and was hydrothermally treated at 80 °C for 24 h or microwave treated at 80 °C for 30 min. Microwave heating was carried out in a Milestone MLS1200 microwave oven with a temperature-controllable program, operating at a constant output power of 500 W. After cooling, the resulting product was filtered, washed by Soxhlet extraction with ethanol solution for 36 h to remove the surfactant species, and dried at 40 °C. Calcination was employed at 300, 400, and 500 °C for 1 h to investigate the structural stability of the product. Figure 1 shows the X-ray diffraction (XRD) patterns of the as-synthesized and calcined products. The assynthesized samples obtained from either hydrothermal treatment or microwave heating exhibit diffraction lines assignable to the boehmite phase AlOOH (JCPDS card no. 21-1307). The weak and broad diffraction peaks indicate that the samples were composed of small crystals with a crystalline size in nanometer scale. The crystalline structure of boehmite can be kept after calcination at 300 °C. After calination at a temperature higher than 400 °C, the boehmite structure transformed to γ-alumina (JCPDS card no. 10-0425), which is in agreement with a previous report that the γ-Al2O3 phase is formed upon the dehydration of aluminum oxyhydroxide boehmite at temperatures ranging from 400 to 700 °C.17 The low-angle (15) Zhang, Q. H.; Luo, J.; Vileno, E.; Suib, S. L. Chem. Mater. 1997, 9, 2090. (16) Ma, Y.; Vileno, E.; Suib, S. L.; Dutta, P. K. Chem. Mater. 1997, 9, 3023.
10.1021/la0361767 CCC: $27.50 © 2004 American Chemical Society Published on Web 01/10/2004
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Figure 2. Representative scanning electron microscopic images of the as-synthesized products from (a) hydrothermal treatment and (b) microwave heating.
diffraction patterns of both boehmite of as-synthesized form and γ-Al2O3 obtained after calcination gave one single peak, which is indicative of a disordered mesostructure without long-range order in the pore arrangement.
Notes
Figure 2 displays the typical scanning electron microscopic images of the obtained products. The particles of both hydrothermal and microwave synthesized products are tens to several tens of micrometers in size with a regular macroporous structure. The macropores are in channel-like shape with a uniform distribution having diameters of 0.8-2 µm, which are larger than those previously observed in macroporous zirconia13 and alumina.9 Local regular array of the macropores can clearly be seen in both as-synthesized boehmite and γ-Al2O3 obtained after a calcination of 500 °C. A careful analysis of these macro-mesostructured particles shows that the macropore distribution in the microwave-synthesized products is more uniform than that in the hydrothermalsynthesized products. Such well-ordered macroporous structures are further confirmed by the transmission electron microscopy (TEM) of the microtomed specimens (Figure 3). The wall thickness between the macropores with circular openings is around 0.4-1.4 µm. Highmagnification TEM images (panels d and e of Figure 3) of the cross-sectional boehmite and γ-Al2O3 specimens also reveal that their macroporous frameworks are composed of fibrous nanoparticles of boehmite and γ-Al2O3 with a scaffold-like array of hierarchical ordering, respectively. The mesostructures of boehmite and γ-Al2O3 are formed through the scaffold-like aggregation and intergrowth of the boehmite and γ-Al2O3 nanofibers, respectively. The nanofibers are of more or less regular size and wellstructured to give accessible and interconnected mesochannels, which is consistent with the small-angle diffraction peaks observed in the XRD patterns in Figure 1. The retainability of the scaffold-like mesostructure and well-defined macroporous structure in the γ-Al2O3 materials indicate that the topochemical transformation of macro-mesostructured boehmite to γ-Al2O3 did not result
Figure 3. Cross-sectional TEM images of (a) as-synthesized boehmite from hydrothermal treatment, (b) as-synthesized boehmite from microwave heating, and (c) 500 °C calcined product γ-Al2O3 of (b). (d) and (e) are the high-magnification images of a macroporous region of (a) and (c), respectively.
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Figure 4. N2 adsorption-desorption isotherms (left) and the corresponding pore-size distributions (right) of the hydrothermalsynthesized (lower) and microwave-synthesized (upper) samples.
in the macro-mesostructural change. The crystalline walls of macro-mesostructured boehmite and γ-alumina were also verified by the observable polycrystalline diffraction rings in their corresponding selected area electron diffraction patterns. The N2 adsorption-desorption isotherms of the samples from hydrothermal treatment and microwave heating are shown in Figure 4. The isotherms of the samples of both as-synthesized and calcined forms are of classical type IV with a hysteresis loop, indicative of mesoporosity according to the IUPAC. The hysteresis loops seem to be intermediate between type H2 and H1, rather than being type H1, indicating that the materials have good pore connectivity with channel-like or ink-bottle pores. The pore size distributions, determined from the adsorption branch of the isotherms by the BJH method, of these materials are a little broad, in comparison with mesostructures formed through a liquid-crystal template pathway.17 The pore sizes increased by the calcination temperature during the transformation process from boehmite to γ-Al2O3 structure. The textural properties of these macro-mesostructured boehmite and γ-Al2O3 materials are provided in Table 1. The retention of high surface areas and large pore volumes is seen during the conversion of macromesostructured boehmites to γ-Al2O3 by calcination at 500 °C. It is particularly noteworthy that the macromesostructured γ-Al2O3 reported in Table 1 have BET surface areas and pore volumes (around 430 m2/g and 1.00 cm3/g, respectively) substantially larger than those of conventional γ-Al2O3 (normally < 250 m2/g and < 0.50 cm3/g),18 and those of mesostructured γ-Al2O3 previously reported (300-350 m2/g and 0.45-0.75 cm3/g, respectively).19 Moreover, the macro-mesostructured boehmite and γ-Al2O3 materials obtained from microwave heating exhibit higher surface areas, larger pore volumes, and more uniform pore-size distributions than the products (17) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (18) Misra, C. Industrial Alumina Chemicals; ACS Monograph 184; American Chemical Society: Washington, DC, 1986. (19) Zhang, Z.; Pinnavaia, T. J. J. Am. Chem. Soc. 2002, 124, 12294.
Table 1. Textural Properties of the As-Synthesized and Calcined Products sample processing
condition
hydrothermal as-synthesized treatment calcined 300 °C calcined 400 °C calcined 500 °C microwave as-synthesized heating calcined 300 °C calcined 400 °C calcined 500 °C
pore pore av pore SBET volumea sizeb diameterc (m2/g) (cm3/g) (nm) (nm) 381 400 428 424 469 454 684 435
0.682 0.739 0.786 1.013 0.916 1.022 1.499 0.980
3.0 3.0 3.0 8.8 4.7 5.9 6.4 7.2
5.5 5.9 5.7 7.3 5.9 6.7 6.8 7.8
a Nitrogen pore volumes at P/P ) 0.993. b BJH pore diameter 0 as determined from the adsorption branch. c BJH adsorption average pore diameter (4V/A).
obtained from hydrothermal treatment. This indicates that microwave heating can produce better quality products in a very short time of only 30 min. The surface areas of the synthesized boehmites to γ-Al2O3 could be affected by the surfactant/aluminum ratio in the synthesis. In the present work, the surfactant content is high with a surfactant/aluminum ratio of 0.5. Increasing the aluminum content to a surfactant/ aluminum ratio of 0.25 resulted in improvement of the surface area of the microwave-synthesized boehmite to 518 m2/g, but little boehmite particles presented a welldefined macroporous structure. This suggests that high surfactant content may be necessary in the system for the formation of macro-mesostructured boehmites to γ-Al2O3. During the present synthesis, aluminum alkoxide is hydrolyzed via the reaction of Al(OR)3 + 2H2O f AlOOH + 3ROH to form AlOOH nanoclusters with boehmite phase, which could react with surfactant molecules to form nanoparticles of mesostructured surfactant/boehmite composites with a large number of surface hydroxyl groups due to incomplete condensation. Meanwhile, the surfactant molecules in solution can be adsorbed onto the surface of these primary mesostructured nanoparticles to form a bilayer structure at the interface. The further aggregation of these primary hybrid particles and surfactant molecules could result in the vesiculation of bilayer structures and
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the formation of supermicelles by the coalescence of multiple micelles and the interaggregate interactions. This self-assembly process under aging would lead to the production of an ordered array of macrochannels.13,20 The removal of the surfactant species then affords a hollow macropore structure with mesoporous walls. Subsequent calcination of the macro-mesostructured boehmite can result in topochemical transformation to γ-Al2O3 with the retained macro-mesostructure, according to the dehydration reaction of 2AlOOH f Al2O3 + H2O. In conclusion, the hierarchically mesoporous-macroporous aluminum oxides with crystalline boehmite AlOOH phase have been synthesized by either hydrothermal treatment or microwave heating. Macro-mesostructured γ-aluminas were formed by the calcination of these boehmites at 500 °C, retaining the structural integrality of mesoporous-macroporous framework. The well-defined macroporous frameworks of these materials were com(20) Yuan, Z. Y.; Vantomme, A.; Le´onard, A.; Su, B. L. Chem. Commun. 2003, 1558. (21) Hem, S. L.; White, J. L. Pharm. Biotechnol. 1995, 6, 249.
Notes
posed of fibrous nanoparticles of boehmite and γ-Al2O3 with a scaffold-like array of hierarchical ordering to form mesochannels. Better quality products were produced by microwave heating in a very short time of only 30 min. High surface areas and pore volumes (467 m2/g and 0.92 cm3/g for boehmite and 435 m2/g and 0.98 cm3/g for γ-Al2O3, respectively) were obtained for these macro-mesostructured aluminum oxide materials, which should be promising for multiapplications in catalysis and separation technology. The macro-mesoporous boehmites may also find potential application as vaccine adjuvants,21 since the vaccine potency is dependent on the textural properties of adjuvant boehmite. Acknowledgment. This work was financially supported by the European Program of InterReg III (Program France-Wallonie-Flandre, FW-2.1.5) and the Belgian Federal Government PAI-IUAP 01/5 project. LA0361767
