Synthesis of NaY zeolites with exceptionally high SiO2/Al2O3 ratios via a dissolution-recrystallization process

Commercial HY zeolites with SiO2/Al2O3 ratios between 5.1 and 80 have been treated in alkaline solutions in the presence of crown ether molecules. If the zeolites are almost totally amorphized at room temperature due to severe desilication of the framework, they completely recrystallize within less than one day when temperature is increased to 110 °C. Extra-framework species likely present in HY precursors are reincorporated in the framework, providing NaY zeolites with exclusively 4-coordinated Al species. The composition of the newly formed NaY zeolite depends not only on the SiO2/Al2O3 ratio of the HY precursor but also on the alkalinity of the desilicating solution. For each HY, the SiO2/Al2O3 ratio of the recrystallized NaY increases linearly when the Na2O/SiO2 ratio of the alkaline solution decreases, down to a limit beneath which complete recrystallization is no longer possible. FAU zeolites with framework SiO2/Al2O3 ratios up to 25 have been obtained from CBV 760, which constitutes the highest value ever reported in the literature for as-made NaY zeolites. A mechanism involving adsorption of crown ether molecules in the supercages of the partially desilicated framework followed by recrystallization has been proposed. This mechanism supports all experimental results, in particular the presence of more than 1 crown ether molecule per supercage in recrystallized solids.