NaA, NaY, Na-Mordenite and ZSM-5 type zeolites were synthesized using fine silica powder of Zeosil 77 and sodium silicate solution as the silica sources, and the effects of starting raw materials, aging, and reaction time on the crystallization rates and crystallinities for those zeolites were investigated.
For NaA, NaY and ZSM-5, aging about 1 day at room temperature was essential, but it showed a negative effect for Na-Mordenite synthesis. Whilst these precursors, such as secondary building unit and nuclei of zeolites, were believed to form in the aging step at romm temperature for A, Y and ZSM-5 type zeolite, the nuclei of Na-Mordenite were formed, with silica as core, during hydrothermal reaction at higher temperature.
The effects of silica sources on the crystallization showed different results depending on the zeolite types.
While higher crystallinity and faster reaction rates were obtained using Zeosil alone as the silica source for Na-Mordenite, for ZSM-5 superior crystallization was resulted when sodium silicate solution was used in conjunction with Zeosil. It is believed that TPA⁺ reacts more easily with silicate or aluminosilicate ions, and effectively promotes the structure directing role in the crystallization of ZSM-5 than with fine silica particles.
When the Zeosil was added to sodium silicate solution to achieve the necessary silica content of the substrates, the rates of crystallization and crystallinity of NaA, NaY and Na-Mordenite were enhanced with increasing wt% of Zeosil, but it showed adverse effect on the crystallization of ZSM-5.