Hydrothermal and Postsynthesis Surface Modification of Cubic, MCM-48, and Ultralarge Pore SBA-15 Mesoporous Silica with Titanium

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• 作者：Mark S. Morey ; Stephen O'Brien ; Stephan Schwarz ; and Galen D. Stucky
• 刊名：Chemistry of Materials
• 出版年：2000
• 出版时间：April 2000
• 年：2000
• 卷：12
• 期：4
• 页码：898 - 911
• 全文大小：177K
• 年卷期：v.12,no.4(April 2000)
• ISSN：1520-5002

文摘

We describe the introduction of titanium centers to cubic MCM-48 and SBA-15 mesoporoussilica by hydrothermal and postsynthetic grafting techniques. MCM-48 was hydrothermallyprepared with a gemini surfactant that favors the cubic phase and leads to a high degree oflong-range pore ordering. This phase was chosen due to its high surface area (1100-1300m²/g) and its three-dimensional, bicontinuous pore array. SBA-15, synthesized with a blockcopolymer template under acidic conditions, has a surface area from 600 to 900 m²/g and anaverage pore diameter of 69 Å, compared to 24-27 Å for MCM-48. Alkoxide precursors oftitanium were used to prepare samples of Ti-MCM-48 and Ti-SBA-15. We have detailed thebulk and molecular structure of both the silica framework and the local bonding environmentof the titanium ions within each matrix. X-ray powder diffraction and nitrogen adsorptionshows the pore structure is maintained despite some shrinkage of the pore diameter at highTi loadings by grafting methods. UV-visible and Raman spectroscopy indicate that graftingproduces the least amount of Ti-O-Ti bonds and instead favors isolated tetrahedral andoctahedral titanium centers. High-resolution photoacoustic FTIR spectra demonstrated thepresence of intermediate range order within the silicate walls of MCM-48, established theconsumption of surface silanols to form Si-O-Ti bonds by grafting, and resolved thecharacteristic IR absorbance at 960 cm^-1, occurring in titanium silicates, into two components.All three spectroscopic techniques, including in situ Raman, reveal the reactive intermediatesformed when the materials are contacted with hydrogen peroxide.