Glycol-modified silanes in the synthesis of mesoscopically organized silica monoliths with hierarchical porosity

Silica monoliths exhibiting a unique hierarchical network structure with a bimodal pore size distribution and high surface areas were prepared from three different glycol-modified silanes by sol-gel processing. Tetrakis(2-hydroxyethyl)-, tetrakis(2-hydroxypropyl)-, and tetrakis(2,3-dihydroxypropyl)orthosilicate were obtained by transesterification reaction from tetraethylorthosilicate and the corresponding alcohols. The present work shows that, for ethylene glycol- and propane- 1, 2 -diol- modified silanes, simply the release of the corresponding diols during sol-gel processing in the presence of block copolymeric surfactants such as Pluronic P123 results in phase separation on different levels. In addition to an extraordinary cellular network structure with interconnected macropores of several hundreds of nanometers in diameter, the material exhibits a well-ordered mesostructure with periodically arranged mesopores of about 6-7 nm in diameter. Interestingly, the application of glycerol-modified silanes at the given synthesis conditions results in the formation of a disordered silica mesostructure. The architectural properties and the morphology of the gel network cannot only be controlled by the choice of the glycol but also by the amount of acid catalyst in the starting composition.