Influence of preparation parameters on pore structure of silica gels prepared from tetraethoxy orthosilicate

The influence of hydrolysis conditions (pH) and of the drying temperature on the structural properties of porous silica gels derived from tetraethoxy orthosilicate (TEOS) has been investigated using nitrogen adsorption and 29Si MAS-NMR. Acid-catalyzed hydrolysis produced silica gels containing mainly micropores. Upon drying at higher temperature (870 K) the weakly cross-linked network collapsed due to dehydration and further cross-linking. This is indicated by diminished pore volume and by a shift in relative number of SiO4 centers with 2- and 3-Si nearest neighbors to more 4-Si nearest neighbor centers. Additionally, the cross-polarization times TSiH increased with drying, indicating a decrease in hydroxyl content. The general isotherm shape was unchanged with drying. The highly-branched cluster aggregates formed during base-catalyzed hydrolysis of TEOS yield a dried gel with a mesoporous silica network. Cross polarization times TSiH for this gel are significantly shorter than for the gels prepared by acid hydrolysis, suggesting closer proximity of the Si centers to hydroxyl groups than in the latter, and/or maintenance of an abundance of hydroxyl groups despite the drying treatment. © 1991, Elsevier Science %26amp; Technology. All rights reserved.

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