Microstructure and porosity of silica xerogel monoliths prepared by the fast sol-gel method

An adaptation of the fast sol-gel method to the synthesis of xerogel monoliths using tetramethoxysilane (TMOS) as the alkoxide precursor is described in this paper. The procedure involves running the reaction at 70-80 degrees C in an open vessel, which accelerates hydrolysis and condensation and reduces the amount of liquid by expelling excess methanol through outdistillation. This procedure yields crack-free monoliths. The porosity and microstructure of these xerogel monoliths were studied by using N-2 adsorption and desorption and scanning electron microscopy (SEM). The SEM data show that the solid skeletal phase has a globular morphology with particles, 20-40 nm in diameter, arranged into agglomerates a few hundred nm in diameter. The microstructure of the acid-catalyzed xerogel is a consolidation of these agglomerates. The isotherm data show these xerogels to be microporous. In contrast, the base-catalyzed xerogel has a hierarchical morphology with the clusters of agglomerates organized into larger clusters approaching 1 mu m in diameter. An analysis of the isotherm data shows these xerogels to be less microporous with a narrow distribution of mesopores having an average diameter of 50 Angstrom.