(647e) Mechanistic Links between Mesoporous and Microporous Silicate Synthesis

The link between M41S mesoporous silicates and all-silica zeolites during the initial stages of synthesis is analyzed with small-angle scattering and conductivity experiments. The phase behavior of silica in aqueous solutions is studied for a family of quaternary ammonium hydroxides, from tetramethylammonium to dodecyltrimethylammonium. All solutions exhibit a critical aggregation concentration at a 1:1 SiO₂:OH^– molar ration with solutions containing short chain compounds forming optically transparent nanoparticle suspensions, and solutions of long chain compounds phase separating into a solid and liquid phase. The solid phases have the hexagonal structure of MCM-41. The role of attractive hydrophobic interactions is also studied using water/ethanol solutions. In ethanolic solutions, the entire family of quaternary ammounim compounds are found to form stable nanopraticle suspensions. The main conclusion is that silica nanoparticles observed in the early stages of zeolite synthesis should be considered the same class of objects as mesoporous silicates. In zeolites, the structural evolution continues to form nuclei and crystals, while with mesoporous silicates, the structure is ‘frozen' into a thermodynamic local energy minimum. This is evident from the analysis of the heat evolution of the synthesis of mesoporous silicates versus heat evolution of silicalite-1 synthesis.