260036 Seed-Assisted Synthesis of *BEA-Type Zincoaluminosilicate Zeolite without Organic Structure-Directing Agent
Zeolites are crystalline microporous materials constructed of tetrahedral TO4 (where, T = Si, Al, P, Zn, etc.) units, and are widely utilized in practical applications such as adsorbents, catalysts, and ion-exchangers. In the syntheses of zeolites, organic compounds called organic structure-directing agents (OSDAs) have been often employed to control pore size or channel system 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 results 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 beta1,2 (Framework Type Code, FTC: *BEA), RUB-133 (RTH), and ZSM-124,5 (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 calcination to the synthesis gel without any organics, however, the underlying crystallization mechanism has not been fully understood yet. In our previous paper on the crystallization behavior of aluminosilicate zeolite 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. Moreover, 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/Al2O3 ratio 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.
In the present study, the OSDA-free synthesis of zinco-aluminosilicate zeolite beta by adding the seeds of zincosilicate zeolite with *BEA topology, named as CIT-6, to aluminosilicate gel is successfully achieved. The chemical compositions of the synthesis gels for the zinco-aluminosilicate beta with high purity and crystallinity are varied in wide range as in the case of the synthesis of aluminosilicate beta2. The result indicates that the important factor is not composition of added beta seeds, but a common composite building unit between seeds and the zeolite obtained from the gel without seeds after long hydrothermal treatment2,5. Moreover, surprisingly, a hierarchical, bimodal pore system consisting of micro- and meso-pores of the obtained zinco-aluminosilicate zeolite beta without any post-synthesis treatment was confirmed by the nitrogen adsorption-desorption measurements. Although the volume of the micropore is almost the same in all obtained zinco-aluminosilicate beta, the volume of mesopore varies depending on the chemical composition of the synthesis gel to which seeds are added. Detailed results of synthesis and characterization of obtained products such as chemical compositions, morphologies, and adsoption-desorption isothems of nitrogen will be reported and discussed in the presentation
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