Toward Microscopic Design of Zeolite Crystals: Advantages of the Fluoride-Mediated Synthesis

MFI zeolite crystals were organized into three-dimensional functional microscopic assemblies such as star-like or hedgehog morphologies. These zeolite structures were prepared under acidic conditions by fluoride-mediated synthesis. The degree of crystallinity and the specific surface area have been tailored as a function of the fluoride concentration in the synthesis gel. Furthermore, the morphology of the zeolite crystals assembly has also been investigated. Indeed, the crystal growth has been modified by changing the synthesis parameters: silica source, time, pH, and fluoride anion concentration. Depending on the aging time, either a star-like morphology of the MFI crystals was observed for the first time, or a hedgehog-microscopic assembly of the zeolite crystals was observed. It is therefore possible to synthesize zeolites with properties tailored at a molecular level and having an anisotropic assembly of the crystals. These organized microstructures create peculiar diffusion conditions which allow improved catalyst activity and stability, when compared to isolated crystals.