High-silica hollow Y zeolite by selective desilication of dealuminated NaY crystals in the presence of protective Al species
Highly crystalline hollow Y zeolite has been obtained by post-synthesis modification of conventional NaY crystals. The top-down transformation involves substantial dealumination of the zeolite framework by silicon tetrachloride followed by acid leaching and finally selective dissolution of the crystal core in the presence of protective aluminium species. The formation of internal cavities depends on the extent of dealumination of the zeolite as well as on the efficiency to remove extraframework Al species prior to realumination. Hollow Y zeolite crystals possess more or less regular internal cavities of ca. 0.8 μm diameter with external size and shape similar to those of the original NaY crystals. They differ from those recently reported in the literature by much higher Si/Al ratios, higher crystallinities and pure microporous shells of ca. 0.12 μm thickness. As a result of these differences, the family of hollow faujasites is significantly enlarged towards more siliceous and less defective materials. Their formation is most likely based on the presence of an Al-rich surface generated by NaAlO2 species during the last stage of the process, which protects the outer parts of the crystals from desilication.