Elucidation of medium-range structure in a dry gel-forming *BEA-type zeolite

High-energy X-ray diffraction, ²⁹Si nuclear magnetic resonance and Raman spectroscopy were applied to investigate medium-range structures in *BEA-type zeolite containing tetraethylammonium (TEA⁺) cation. In particular, the intermediates to form *BEA were focused the subject of focus. Double three-membered silicate rings (D3Rs) stabilized by TEA⁺ cation existed in the mother TEA⁺-silicate solution to prepare a dry gel that gave *BEA-type zeolite by the steam-assisted crystallization method. The D3R units transformed "4-2"-type secondary building units (SBUs) in the course of preparing the dry gel. Such "4-2"-type SBUs remained in the dry gel and played a significant role in the crystallization of *BEA-type zeolite. On the basis of these observations, we proposed that the presence of "4-2"-type SBUs in the dry gel is essential for the formation of *BEA-type zeolite in the steam-assisted crystallization, suggesting a plausible formation scheme of the periodic structure of *BEA-type zeolite in a long-range order via rearrangement of "4-2"-type SBUs.