Crystallization mechanism of Al-Ti-beta zeolite synthesized from amorphouswetness impregnated xerogels

The crystallization mechanism of Al-Ti-beta zeolite has been investigated w hen this material is synthesized from amorphous SiO2-TiO2 xerogels wetness impregnated with NEt4OH solutions. The final crystalline product presents m ost of the titanium atoms occupying tetrahedral positions in the framework as concluded from DR UV-VIS spectra, whereas bulk TiO2 phases are not detec ted. Likewise, Al-27 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 amo rphous gel consisting of primary particles with sizes around 10 nm. The app earance of the first crystallization nuclei is closely related to the incor poration of Al into these primary units. Both crystallinity and solid yield increase initially very rapidly until total incorporation of the Al specie s 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 specie s from the solution with a slight increase in the solid yield, which result s in a densification of the secondary particles and their transformation in to zeolite crystals.