29Si NMR and UV鈭扲aman Investigation of Initial Oligomerization Reaction Pathways in Acid-Catalyzed Silica Sol鈭扜el Chemistry

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• 作者：Anouschka Depla ; David Lesthaeghe ; Titus S. van Erp ; Alexander Aerts ; Kristof Houthoofd ; Fengtao Fan ; Can Li ; Veronique Van Speybroeck ; Michel Waroquier ; Christine E. A. Kirschhock ; Johan A. Martens
• 刊名：Journal of Physical Chemistry C
• 出版年：2011
• 出版时间：March 10, 2011
• 年：2011
• 卷：115
• 期：9
• 页码：3562-3571
• 全文大小：991K
• 年卷期：v.115,no.9(March 10, 2011)
• ISSN：1932-7455

文摘

The initial molecular steps of the acid-catalyzed silica sol鈭抔el process de-parting from tetraethylorthosilicate (TEOS) were investigated by in situ ²⁹Si NMR and UV鈭扲aman spectroscopy. The use of a substoichiometric H₂O:TEOS molar ratio (r-value 0.2鈭?.2) slowed the silicate oligomerization reaction and allowed unraveling the initial steps of silica condensation. Molecular modeling confirmed Raman signal and ²⁹Si NMR shift assignment. A comprehensive listing of all Raman and ²⁹Si NMR assignments is provided, including unique Raman assignments of cyclosilicates and the linear tetramer. The combination of experiment and modeling allowed an analysis of the reaction kinetics. The derived kinetic model and the experimental observation both revealed that the H₂O:TEOS molar ratio had a strong influence on the reaction kinetics but not on the reaction pathways. The multianalytical approach led to development of an oligomerization scheme. As dominant oligomerizations, chain growth, cyclodimerization, and branching were identified. Under the investigated conditions, chains did not grow longer than pentamer, and ring sizes were limited to 6-rings. Chains of 4 Si atoms and 4-rings were abundant species. Branched rings and chains were formed by attachment of dimers and trimers. Gelation proceeded from branched 4-rings and branched chains with limited hydroxyl functionalities.