Aluminosilicate species in the hydrogel phase formed during the aging process for the crystallization of FAU zeolite

Faujasite zeolite is synthesized hydrothermally after an aging process under various conditions. Hydrogel is formed during the aging, and the amount increases with increasing the duration. Aluminosilicate species included in the hydrogel phase during the aging is investigated for the changes in Si/Al ratios and Q units during the aging and the crystallizing processes by means of Si-29 magic-angle spinning NMR spectroscopy. Aging for more than 1 day is necessary to obtain faujasite with high crystallinity. The distribution in Q(4) units in the hydrogel is gradually changed, and the aluminosilicate species having mainly a Q(4)(4Al) unit is obtained after 7 days of aging, while 2 days of aging results in remanent silicate species derived from the starting silica source. The former gives faujasite crystallites with higher crystallinity, sharper particle size distribution, and a lower Si/Al ratio. On the basis of these findings, the crystallization mechanism is discussed.