High temperature synthesis of silicalite-1 in cationic microemulsions

The current investigation reports on the synthesis and characterization of silicalite-1 made in cationic microemulsions using conventional synthesis conditions (polymeric silica sources, alkaline media, high temperatures (433 K)). Silicalite-1 materials formed in microemulsion-mediated syntheses possess different morphological properties as compared to samples grown using the same synthesis mixture in the absence of the microemulsion. Syntheses are also performed to ascertain how the microemulsion composition, silica precursor, alkali content, presence of salt, and the surfactant identity influence material properties, most notably crystal morphology. The morphology of the crystals obtained is fairly insensitive to surfactant structure and salt, but highly sensitive to the silica source. Polymeric silica sources lead to silicalite-1, whereas monomeric silica sources (e.g. TEOS) lead to amorphous solids. The results also indicate that a key parameter is the ratio of surfactant to structure-directing agent, suggesting that electrostatic forces are important in determining the synthesis outcome. Control experiments show all components of the microemulsion are necessary to obtain the observed crystal morphology. Based on the results presented it is concluded that the surfactant-silicate electrostatic forces are primarily responsible for the modulation of crystal morphology observed. The results indicate that surfactant adsorption on the growing crystal surface, not the confined space afforded by the microemulsion, is essential. The results suggest that this may be a useful approach to controlling zeolite crystal morphology and growth of crystals obtained from conventional high-silica zeolite synthesis procedures.