Molecular Level Processes and Nanostructure Evolution During the Formation of the Cubic Mesoporous Material KIT-6

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• 作者：Sharon Ruthstein ; Judith Schmidt ; Ellina Kesselman ; Ronit Popovitz-Biro ; Liora Omer ; Veronica Frydman ; Yeshayahu Talmon ; Daniella Goldfarb
• 刊名：Chemistry of Materials
• 出版年：2008
• 出版时间：April 22, 2008
• 年：2008
• 卷：20
• 期：8
• 页码：2779 - 2792
• 全文大小：2090K
• 年卷期：v.20,no.8(April 22, 2008)
• ISSN：1520-5002

文摘

This study focuses on the formation mechanism of the bicontinuous cubic Ia3̅d mesoporous material KIT-6, both on the molecular and on the mesoscopic levels. KIT-6 is synthesized with Pluronic P123 (PEO₂₀PPO₇₀PEO₂₀), low acid concentration, and n-butanol at 40 °C. Through in situ EPR measurements on a series of spin-labeled Pluronic molecules introduced at minute quantities into the reaction mixture, changes in the hydrophobicity and the mobility of the polymer chains during the reaction were observed. In addition, to learn more on the functionality of the butanol in this synthesis, freeze-quench electron spin–echo envelope modulation (ESEEM) measurements on reaction mixtures in D₂O and in butanol-d₁₀ were preformed. The above experiments gave information on variations in the butanol location and content in the micellar structures during the formation of KIT-6. The evolution of the solution nanostructures was determined by cryo-TEM. Five main stages were resolved: the first two occurred during the first 140 min of the reaction, where condensation of the silica oligomers takes place at the micellar/water interface; this induces depletion of water and butanol molecules from the core−corona interface and reduces the mobility of the ends of the Pluronic chains located at the corona−water interface. This in turn leads to a transition from spheroidal micelles to threadlike micelles and to their aggregation toward the end of the second stage. During the third stage, precipitation (140–160 min), reorganization in the micellar structure, and a change in the relative sizes of core and corona take place. The fourth stage, that ends around 6 h, involves the formation of a hexagonal phase, through accelerated condensation of silica oligomers in the corona, accompanied by extensive depletion of water and butanol molecules. The presence of butanol in the micelle corona is essential in the last stage, 6–24 h, where the cubic phase is formed. We show that the addition of butanol to the reaction mixture of SBA-15 after the formation of the hexagonal phase leads to the formation of the cubic phase.