Organic structure-directing agents (OSDAs) have been widely used for the synthesis of zeolites. In most cases, OSDAs are occluded in zeolites as an isolated cation or molecule geometrically fitted within the zeolite cavities. This is not the case for zeolite beta synthesized by using tetraethylammonium (TEA⁺) cation as an OSDA, in which a cluster/aggregate of ca. six TEA⁺ cations is occluded intact in the cavity (i.e., the channel intersection) of zeolite beta. The structure direction of TEA⁺ in such a nontypical, clustered mode has remained elusive. Here, zeolite beta was hydrothermally synthesized using TEA⁺ in the absence of other alkali metal cations in order to focus on the structure-directing behaviors of TEA⁺ alone. The solid products formed throughout the hydrothermal synthesis were analyzed by an array of characterization techniques including argon adsorption–desorption, high-energy X-ray total scattering, Raman and solid-state NMR spectroscopy, and high-resolution transmission electron microscopy. It was revealed that the formation of amorphous TEA⁺–aluminosilicate composites and their structural, chemical, and textural evolution toward the amorphous zeolite beta-like structure during the induction period is vital for the formation of zeolite beta. A comprehensive scheme of the formation of zeolite beta is proposed paying attention to the clustered behavior of TEA⁺ as follows: (i) the formation of the TEA⁺–aluminosilicate composites after heating, (ii) the reorganization of aluminosilicates together with the conformational rearrangement of TEA⁺, yielding the formation of the amorphous TEA⁺–aluminosilicate composites with the zeolite beta-like structure, (iii) the formation of zeolite beta nuclei by solid-state reorganization of such zeolite beta-like, TEA⁺–aluminosilicate composites, and (iv) the subsequent crystal growth. It is anticipated that these findings can provide a basis for broadening the utilization of OSDAs in the clustered mode of structure direction in more effective ways.

Glasses with high elastic moduli have been in demand for many years because the thickness of such glasses can be reduced while maintaining its strength. Moreover, thinner and lighter glasses are desired for the fabrication of windows in buildings and cars, cover glasses for smart-phones and substrates in Thin-Film Transistor (TFT) displays. In this work, we report a 54Al₂O₃-46Ta₂O₅glass fabricated by aerodynamic levitation which possesses one of the highest elastic moduli and hardness for oxide glasses also displaying excellent optical properties. The glass was colorless and transparent in the visible region and its refractive indexn_dwas as high as 1.94. The measured Young’s modulus and Vickers hardness were 158.3 GPa and 9.1 GPa, respectively, which are comparable to the previously reported highest values for oxide glasses. Analysis made using²⁷Al Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) spectroscopy revealed the presence of a significantly large fraction of high-coordinated Al in addition to four-coordinated Al in the glass. The high elastic modulus and hardness are attributed to both the large cationic field strength of Ta⁵⁺ions and the large dissociation energies per unit volume of Al₂O₃and Ta₂O₅.

Significance: Zeolites have greatly contributed to modern industries. Consumption of zeolites is expected to increase with the emergence of newly commercialized applications. Typical synthesis of zeolites relies on batchwise hydrothermal synthesis, which usually takes tens of hours or even several days to complete. People have thus long believed that the crystallization of zeolites is very slow in nature. We herein demonstrate the continuous flow synthesis of ZSM-5, an industrially important zeolite, on the order of seconds. Crystallization from amorphous state to full crystallinity could be completed in tens of or even several seconds. The synthesis on the order of seconds provides a great potential to facilitate the mass production as well as to deepen the fundamental understanding of zeolite crystallization.