ALKYLENE-BRIDGED SILSESQUIOXANE SOL-GEL SYNTHESIS AND XEROGEL CHARACTERIZATION - MOLECULAR REQUIREMENTS FOR POROSITY

Alkylene-bridged silsesquioxane xerogels with alkylene spacers spanning the range from ethylene to tetradecamethylene have been synthesized. Hydrolysis and condensation conditions using 1 N HCI in tetrahydrofuran or 1 N NaOH in ethanol were employed. The resulting xerogels were characterized by Si-29 solid-state NMR and nitrogen adsorption porosimetry. It was found that base catalysis resulted in a higher degree of condensation than acid catalysis. The acid catalyzed ethylene-bridged xerogel possessed high surface areas by the BET method (400-600 M2 g-1) and was microporous, but materials prepared from the longer alkylene spacers were found to be nonporous with no significant surface area. In contrast, base-catalyzed xerogels were found to be both microporous and mesoporous, with the peak of the pore size distribution in the mesopore range. High surface areas were also found for the base catalyzed xerogels. The surface area decreased as the alkylene spacer length increased to the point where no significant surface area existed for the tetradecamethylene-bridged xerogel. Pore size analysis of the base-catalyzed xerogels showed an increase in the mean pore size distribution, concomitant with decreasing total pore volume, as the alkylene spacer length increases. Comparison with previous work employing arylene-bridged xerogels leads to the conclusion that employing rigid organic spacers with either acid or base catalysis appears to be a requirement for organic-bridged polysilsesquioxane xerogels possessing high surface areas and porosity.