Synthesis of core-shell structured MFI-type composite zeolites by isomorphous epitaxial growth

MFI/MFI core-shell composite zeolites with a low-silica ZSM-5 core and a high-silica shell were successfully synthesized by secondary hydrothermal crystallization on the low-silica ZSM-5 cores that was pretreated with a basic TPAOH aqueous solution; the preparation parameters for shell growth including the pH value, water amount, and crystallization time were well considered. The crystal structure, surface morphology, core/shell interface, textural properties and surface acidity of the resultant core-shell zeolites were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectrometer, transmission electron microscopy, N₂ physisorption and NH₃ temperature programmed desorption. The results indicated that in the core-shell composite zeolites, high-silica ZSM-5 shell with a particle size of 200 nm is well developed on the surface of core crystal. The growth of ZSM-5 shell results in an increase of external surface area, decrease of external acid density and increase of weak acid sites without influencing pore structure. The isomorphous epitaxial growth of high-silica ZSM-5 shells can be effectively controlled under the conditions of a pH value of 8.5, H₂O/SiO₂ mol ratio of 30 and crystallization for 24 h during the secondary crystallization.