Template-Free Self-Assembly of Mesoporous Organosilicas

Using a known organosilane building block (1,1,3,3,5,5-hexaethoxy-1,3,5-trisilacyclohexane), materials with unexpectedly high surface area (>1200 m(2)/g) and porosity with narrow pore-size distributions were synthesized in the total absence of pore-templating agents. The properties of these mesoporous organosilicas (MOS), closely resembling those of surfactant-templated mesoporous silicas, are found to vary over a wide range depending on synthesis gel pH and gelation temperature and time. Considerable microporosity is observed at short 90 degrees C gelation times at pH 12.5, which diminishes to a very minor component of the porosity at times > 23 h. Under the gelation conditions used here (90 degrees C, pH 12.5), pore volume and pore diameter rise sharply with time, ultimately leveling off at 1.2 cm(3)/g and 70 angstrom at times > 70 h. This building block is shown to be stable in acidic gels but undergoes gradual ring opening at gelation temperatures and times in excess of "standard" conditions employed for alkaline gels.The synthesis and properties of these materials, called EMA-2, are compared with materials made from the same precursor in a templated synthesis and from other organosilane precursors in nontemplated syntheses. Comparative TEM and SAXS analysis of products from identical preparations using tetraethylorthosilicate and the 1,1,3,3,5,5-hexaethoxy-1,3,5-trisilacyclohexane precursors indicates that the latter forms a highly interconnected network gel lacking discrete particle aggregates.