Microemulsion-mediated syntheses of silicalite-1

Abstract

The cationic microemulsion-mediated synthesis of silicalite-1 has been studied. The focal point of this work is to determine if these microemulsions present a confined space in which zeolite particles of novel morphology can be formed. The phase behavior of the microemulsions has also been determined to help rationalize the results of the zeolite syntheses. The phase behavior studies of the microemulsions indicate that one-phase, optically transparent microemulsions can be formed at 368 K in the presence of linear and branched cationic surfactants, which can solubilize approximately ten weight percent zeolite synthesis mixture. The results from the phase behavior studies were used to guide the zeolite syntheses. The effects of synthesis parameters such as TPAOH content, microemulsion composition, surfactants and temperature on the morphology of silicalite-1 were analyzed. Using microemulsions with branched cationic surfactant at 95° C, silicalite-1 of novel morphology was synthesized. Disc like silicalite-1, around 400 nm in size, was synthesized from microemulsion-mediated syntheses, while spherical crystals of smaller size were synthesized in the absence of the microemulsion. However, when linear cationic surfactant is used, mesoporous silica is synthesized. At higher zeolite contents in the microemulsion, better crystallinity and shape of silicalite-1 were obtained, while at lower zeolite content (less than ten weight percent) amorphous materials were synthesized. By decreasing the TPAOH content, the shape of the crystals synthesized using microemulsion are more disc-like and slightly larger. Silicalite-1 of 400 nm in size was synthesized with zeolite mixture molar composition of 1 TEOS: 0.18 TPAOH: 20 H₂O, while smaller crystals, 200 nm, were observed at higher TPAOH content (above 0.30). Salt has deleterious effect on silicalite-1 formation at 95° C. Mesoporous silica is synthesized when NaCl is added into the synthesis mixture. Finally, our results indicate that the "nanoreactor concept" may not be valid, observed from the studies of mixtures before and after the solids sediment out of the synthesis mixture. Powder diffraction pattern of material extracted from microemulsion after the mixture phase separates indicates that silicalite-1 is synthesized and its FE-SEM image shows ordered shape. However, very poorly crystalline material is synthesized from one-phase synthesis mixtures.