(96e) Expanding the Scope of Fluoride-Free Pure Silica Zeolite Syntheses

Zeolites are crystalline microporous materials, with wide applications as solid catalysts, adsorbents and ion exchangers. Pure silica zeolites show high hydrothermal stability, hydrophobicity and dielectric properties, making them ideally suited in applications such as gas separation membranes, low-k dielectrics and drug delivery. However, of the 232 zeolite frameworks recognized by the International Zeolite Association, less than 50 have been synthesized in pure silica form. Most pure silica zeolites are synthesized in the presence of hydrogen fluoride, which hinders their commercial applications because of cost and safety-related issues. Only a few zeolite structures can be synthesized in their pure silica form without the use of hydrogen fluoride.

Dry gel conversion (DGC) is one of the techniques that has been used in the past to synthesize pure silica forms of BEA, MFI and TON frameworks. The technique basically involves drying the alkaline synthesis solution to a dry solid, followed by steam-assisted crystallization (SAC). In the case of BEA framework, DGC is the only known method to synthesize the pure silica form without the use of hydrogen fluoride. In this study, we report the use of the DGC technique to synthesize pure silica CHA framework zeolite without the use of hydrogen fluoride for the first time. ²⁹Si MAS NMR studies were used to understand the synthesis mechanism involved and make improvements. In addition, we propose a new metric for analyzing pure silica zeolites and that the uniqueness of the DGC technique allows it to be used as a general technique for the fluoride-free synthesis of a number of new pure silica zeolites.