Highly ordered, thermally/hydrothermally stable cubic Ia3d aluminosilica monoliths with low silica in frameworks

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• 作者：Sherif El-Safty ; Ahmed Shahat ; Kazuyuk Ogawa ; Takaaki Hanaoka
• 关键词：Monoliths ; Cubic Ia3d ; Aluminosilica ; Thermal/hydrothermal steam stabilities
• 刊名：Microporous and Mesoporous Materials
• 出版年：2011
• 出版时间：February 2011
• 年：2011
• 卷：138
• 期：1-3
• 页码：51-62
• 全文大小：2395 K

文摘

High order aluminosilica monoliths (Al/HOM-5) with cubic Ia3d structures were fabricated with low silica content (Si/Al = 1) by means of a simple, reproducible, and one-pot synthesis strategy. A realistic control over the cubic Ia3d geometry of aluminosilica monoliths was achieved by using microemulsion phases of copolymer P123 (EO₂₀PO₇₀EO₂₀) as soft templates. The textural and geometrical pore structures and acidic properties of Al/HOM-5 were characterized by means of various tools of XRD, N₂ isotherms, HRTEM, FTD, FESEM, ²⁷Al and ²⁹Si MAS NMR, EDX, and NH₃-TPD. The incorporated amounts of Al species in Al/HOM-5 monoliths play a key determinant in the formation of the coordination state of the aluminum species in four (Al^IV, AlO₄)-, five (Al^V, AlO₄)-, and six (Al^VI, AlO₆)-coordinate environments. Results show evidence of the formation of aluminosilicas with disordered distribution of Si and Al sites in the frameworks. In addition, the increase of the aluminum contents enhanced the distortion in the pore uniformity of Al/HOM-5 monoliths. The large amount of acid sites was revealed with the high aluminum contents into the pore framework walls. With high-temperature treatments (1073 K), the γ-Al₂O₃ phase with face-centered-cubic Fd3m symmetry can be formed in the monolithic aluminosilica matrices. This finding might indicate that the dealumination effect at high thermal treatment would be expected to generate many defect sites existing in the Al/HOM-5 frameworks. Accordingly, a large mass transport of aluminum from tetrahedral aluminum (framework) to octahedral aluminum (extra framework) was occurred, leading to form a separate aluminum species into the crystalline alumina matrices. Moreover, cubic Ia3d aluminosilica structures exhibit outstanding steam stability even with high Al content (Si/Al = 1). However, the mesostructured integrity of Al/HOM-5 can be retained under 100 % steam treatments with N₂ flow for 8 h at 1073 K, permitting their desirability in various applications such as catalysis, adsorption, and sensing technologies.