Sintesis De Alumina Mesoporosa

COMMUNICATION

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Sol-gel synthesis of ordered mesoporous alumina
Krisztian Niesz, Peidong Yang and Gabor A. Somorjai*
Received (in Cambridge, UK) 23rd December 2004, Accepted 3rd February 2005 First published as an Advance Article on the web 16th February 2005 DOI: 10.1039/b419249d

Well-ordered mesoporous alumina materials with high surface area and anarrow pore size distribution were synthesized using a sol-gel based self assembly technique. Since the first successful synthesis of well-ordered, periodically organized mesoporous silica materials, such as the members of the M41S family1 and SBA-15,2 efforts have been directed toward extending the group of mesoporous materials to non-silica systems.3 In this contribution, we report the synthesis ofordered mesoporous Al2O3 which is an interesting and attractive material because of its applications as absorber or catalyst support in heterogeneous catalytic reactions.4 Although several papers were published in this direction using anionic, cationic or non-ionic templates as structure directing agents, none of the reported procedures resulted in ordered mesoporous alumina structures.5–10 Thepresent paper describes a highly reproducible process for the synthesis of mesoporous alumina with an SBA-15-like arrangement of ordered channels, with high surface area and a narrow pore size distribution. During the synthesis, in solution A 1 g of Pluronic P123 (EO20PO70EO20, EO 5 ethylene oxide, PO 5 propylene oxide) was dissolved in 12 ml of absolute ethyl alcohol and stirred for 15 min at 40 uC.Meantime solution B was prepared from various amounts of 37 wt% hydrochloric acid and 6 ml of absolute ethyl alcohol. Water was not added to the synthesis mixture in addition to the HCl solution. Aluminium tri-tert-butoxide (2.46 g) was portioned slowly to solution B under vigorous stirring. After 15 min stirring, the two solutions were mixed together and further stirred at 40 uC. The molar ratioof Al3+ : Pluronic P123 : EtOH in the final solution was fixed at 1 : 0.017 : 30, and the hydrolysis rate was varied by changing the [H2O] : [Al3+] ratio between the values of 2 and 12 and the [HCl] : [Al3+] ratio from 0.6 to 3.6. The homogeneous sol was poured into a teflon container for aging for three days at 40 uC under N2 flow. Eventually the white sticky material that was produced wascalcined at 400 uC in a flow of oxygen (flow rate 5 100 cm3 min21) for 4 h (1.5 uC min21 ramping rate) in a tube furnace to remove the polymers from the pores. The formation of the hybrid mesophase between the inorganic precursor (aluminium tri-tert-butoxide) and the structure directing agent (Pluronc P123) occurred during the slow evaporation of the solvent from the diluted ethanol solution. Theprecisely controlled conditions we used allowed us to slow down the hydrolysis/ condensation reactions and prevent the uncontrolled phase separation between the inorganic and organic components so that a higher degree of cross linking between the precursor molecules

occurs during the formation of the mesoscopic assemblies. The water concentration of the final solution turned out to have a stronginfluence on the nature of the final product. Only using Al3+ : Pluronic P123 : EtOH : H2O : HCl 5 1 : 0.017 : 30 : 6 : 1.8 resulted in the formation of mesoporous alumina with a hexagonally ordered channel system as Fig. 1 shows. As the TEM images indicate{, the mesoporous structure is highly ordered ˚ with a pore size of 68 A which is in a good agreement with the data obtained from thephysisorption and the small-angle X-ray measurements. The SAXS pattern of the well-ordered mesoporous alumina (Fig. 2A) shows three peaks, the (100), (110) and (200) reflections, which represent a 2D hexagonal mesostructure with ˚ space group P6mm. The d100 distance of 83 A was calculated using the Bragg equation. The physisorption measurements reveal a high BET surface area (y410 m2 g21) and a narrow pore...