Approaches for the Synthesis of Ultra-Large and Ultra-Small Zeolite Crystals

Ultra-large crystals: crystals of which the unit cell of framework atoms can be regularly repeated in three directions are the object of this section. The perfectness of the crystal and the framework are only subject to the ability to elucidate the fractional coordinates of the atoms comprising the unit cell. The size of those crystals clearly depends on the type of zeolite. Of zeolite beta, even the smallest single crystal is not known, while analcime forms single crystals of a few millimeters. However, crystals may also be considered ultra-large in cases where one or two dimensions are well developed, such as elongated prismatic MFI of 60 µm × 60 µm × 350 µm [1] and ferrierite tablets of 1000 µm × 1000 µm × 100 µm [2]. The availability of ultra-large crystals of zeolites and other microporous materials enables fundamental studies to be carried out, providing insights and achieving results which would hardly be possible using small crystals. In particular, measurement of the diffusional properties of species which diffuse quickly through the porous network and accurate determination of zeolite structures, down to the atomic positions, with single-crystal X-ray techniques are possible. Measurement of micropore diffusivities is essential for the optimization and design of catalytic reactors and adsorptive separation processes. The construction of ordered composites of zeolite crystals containing guest species allows non-linear optical devices, quantum-confined semiconductors, and other such materials to be realized.