C. Kirschhock, R. Ravishankar, Frederik Verspeurt
May 27, 1999
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Journal
Journal of Physical Chemistry B
Abstract
29Si liquid NMR and in situ infrared spectroscopy was used to investigate the polycondensation process of tetraethyl orthosilicate (TEOS) in a concentrated aqueous solution of tetrapropylammonium hydroxide (TPAOH) at low temperatures. The composition was characterized by a molar hydrolysis ratio (H2O/TEOS) of 6 and a molar TPAOH/TEOS ratio of 0.37. The 29Si NMR spectra and the infrared spectra of the samples recorded at different reaction times and temperatures were assigned to a limited number of specific silicate polyanions containing three and five rings. The structure directing action of tetrapropylammonium cations was evidenced by the formation of silicate polyanions with a curved hydrophobic SiO2 surface, such as the bicyclic pentamer, pentacyclic octamer, and the tetracyclic undecamer. At room temperature, the polycondensation process leads to the selective formation of a species containing 33 Si atoms. It occluded a tetrapropylammonium molecule and had the same framework connectivity as in bulk MF...