(9c) Broadening Framework Types of Zeolites in the Seed-Assisted, OSDA-Free Synthesis

Organic structure-directing agents (OSDAs) have been often employed for the synthesis of zeolites, and many kinds of useful zeolites with novel framework types have been synthesized. In the state-of-the-art synthesis, however, the structures of those OSDAs become more complex, and their cost become more expensive. The use of OSDAs also resulted in complex synthesis process, huge energy consumption, and environmental burdens due to the waste water treatments and to the removal of organics in the pore of obtained zeolites by calcination in the commercial production. Therefore, a versatile OSDA-free synthesis route of zeolites has been strongly desired.

Recently, the seed-assisted, OSDA-free syntheses of useful zeolites such as beta^1,2 (*BEA), RUB-13³ (RTH), and ZSM-12^4,5 (MTW) have been reported by several research groups, although it had been believed that these zeolites could be essentially synthesized with OSDAs. In these synthesis methods, the zeolite seeds synthesized using OSDAs are added after calcinations to the reactant gel without any organics; however, the underlying crystallization mechanism has not been fully understood yet. In our previous paper on the crystallization behavior of beta⁶, it was found that the beta seeds in the sodium-aluminosilicate gel system were partially dissolved during hydrothermal treatment, and new beta crystallized on the surface of the residual beta seeds by liquid-mediated supply of the precursors of zeolite.

On the basis of the comprehensive study on the effects of the seeds and the structural comparison between seed zeolite (e.g., beta) and zeolite obtained from OSDA-free gel without any seeds (e.g., mordenite), the following working hypothesis was proposed. For broadening zeotypes in the seed-assisted, OSDA-free synthesis, seeds should have at least one common composite building unit with the zeolite to be synthesized in the synthesis gel without seeds. In addition, the other requirements for the successful synthesis of zeolites by this method were summarized as follows; 1) zeolite seeds should not dissolve completely during the hydrothermal treatment, and the SiO₂/Al₂O₃ratio of the seeds should be optimized, 2) the spontaneous nucleation should not occur prior to the completion of the crystal growth of the target zeolite, 3) the chemical composition of the gel to which the seeds are added should be optimized for the crystal growth of the target zeolite. Although zeotypes already obtained by this method have been limited to aluminosilicate zeolites, the new hypothesis should be useful for broadening the scope of zeotypes including other metalosilicates.

In this presentation, other aluminosilicate zeolite with PAU-type structure was also successfully synthesized by seed-assisted, OSDA-free synthesis. Detailed crystallization behaviors and characterizations of obtained zeolites will be presented and discussed in the meeting.

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Chem. C 115, 744, 2011