Catalytic Control of SiO 2 Sol-Gel Kinetics - a Mechanistic Study of Bases
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CATALYTIC CONTROL OF SiO 2 SOL-GEL KINETICS A MECHANISTIC STUDY OF BASES JORGE SANCHEZ, MARY REESE, AND ALON McCORMICK
Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455 ABSTRACT The aim of this research was to determine the relative effects of three different base catalysts (NaOH, NH4OH, and RbOH) on the rate of gelation of SiO2 from reactive silicon ethoxide solutions in alcohol (the sol/gel method). The time to gelation was determined using a reaction protocol which assured that condensation was the rate controlling step. NMR spectroscopy was used to verify that the nature of the reacting polymers remained similar as the base catalyst was varied. It was determined that under these conditions the condensation reactions avoid diffusional limitations. Furthermore, the base catalyst serves to modulate both the activation energy and also the Arrhenius preexponential factor. This leads to the proposal of a new condensation mechanism. INTRODUCTION The sol/gel process constitutes the synthesis of an inorganic polymer gel network from a reactive solution of monomeric or sol moieties at or near room temperature. In this work we are interested in the reactions of silicon ethoxide in ethanol solution catalyzed by univalent hydroxides. It is important to better understand the chemistry and physics of this process in order to design materials systems with specified molecular structure and hence desired mechanical, optical, electronic, and chemical properties. Silica glasses and ceramics are currently used as catalysts, ion selective membranes, coatings, optical fibers, and lenses. The sol/gel preparation method is attractive in comparison to traditional methods because of its fast gelation times and its resultant low temperature sintering. Glasses with low thermal expansivity and highly porous powders are thus advantageously produced using the sol/gel method. [1 - 6] The aim of this research was to determine the effects of various base catalysts on the reaction kinetics. Gel times were monitored for a variety of precursor solutions. Apparent activation energies are calculated using a simplified kinetic model incorporating well-justified mechanistic assumptions. The trends of gel times and activation energies were compared to rationalize the action of the catalyst. THEORY With the sol/gel technique, SiO2 gel forms through hydrolysis and condensation of a silicon alkoxide: [1, 7 - 13] Hydrolysis: Si(OR)4 + n H20 = (RO)4.nSi(OH)n + n ROH Mat. Res. Soc. Symp. Proc. Vol. 180. 01990 Materials Research Society
Downloaded from https://www.cambridge.org/core. Access paid by the UCSB Libraries, on 05 Jul 2018 at 16:50:12, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-180-263
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Condensation: Si(OR')4 + HOSi(OR')3 --> (R'O)3SiOSi(OR')3 + R'OH (where R = proton or alkyl group; R' = proton, alkyl, or silicon; n=1,2,3, or 4). Over the course of these reactions the solution increases in viscosity, finally
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