Crystallization mechanism of Al–Ti-beta zeolite synthesized from amorphous wetness impregnated xerogels

Abstract

The crystallization mechanism of Al–Ti-beta zeolite has been investigated when this material is synthesized from amorphous SiO₂–TiO₂ xerogels wetness impregnated with NEt₄OH solutions. The final crystalline product presents most of the titanium atoms occupying tetrahedral positions in the framework as concluded from DR UV-VIS spectra, whereas bulk TiO₂ phases are not detected. Likewise,²⁷Al MAS-NMR indicates the presence of Al with tetrahedral coordination in the Al–Ti-beta framework. The use of Al isopropoxide as the aluminium source leads to shorter synthesis times and higher incorporation degrees of aluminium compared to Al nitrate. The crystallization mechanism involves dissolution of the raw xerogel and subsequent formation of an amorphous gel consisting of primary particles with sizes around 10 nm. The appearance of the first crystallization nuclei is closely related to the incorporation of Al into these primary units. Both crystallinity and solid yield increase initially very rapidly until total incorporation of the Al species into the solid phase occurs. Thereafter, the primary particles undergo an aggregation process into larger secondary units. The last crystallization stage is characterized by slow incorporation of silicon and titanium species from the solution with a slight increase in the solid yield, which results in a densification of the secondary particles and their transformation into zeolite crystals.