(61c) Broadening the Scope of Fluoride-Free Siliceous Zeolite Synthesis

Zeolites are crystalline materials, with pore structures ranging from 0.4 to 2 nm. While aluminosilicate zeolites have received considerable attention in recent years due to their wide-ranging applications as solid acid catalysts, adsorbents and ion exchangers, siliceous (or pure silica) zeolites are ideally suited for applications in gas separations, low-k dielectrics and drug delivery due to high hydrothermal stability, hydrophobicity and dielectric properties. However, synthesis of siliceous zeolites continues to be a challenge – of the 235 zeolite frameworks recognized by the International Zeolite Association, only 47 have been synthesized so far in pure silica form.¹ Most of these zeolites are synthesized in the presence of hydrogen fluoride, which hinders their commercial applications because of cost and process safety-related issues. Only about 10% of the known zeolite framework structures can be synthesized in their pure silica form without the use of hydrogen fluoride.

Dry gel conversion (DGC) has been used in the past for the fluoride-free synthesis pure silica forms of BEA, MFI and TON frameworks. In the case of BEA framework, DGC is the only known method to synthesize the pure silica form without the use of hydrogen fluoride. Herein we report the use of the DGC technique for the fluoride-free synthesis of siliceous CHA and STT zeolites for the first time. ²⁹Si MAS NMR studies were used to understand the synthesis mechanism involved and the insights gained were used to improve the synthesis technique. In addition, we propose a new metric for analyzing siliceous zeolites and also 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 siliceous zeolites.

[1] Vattipalli, V.; Paracha, A. M.; Hu, W.; Chen, H.; Fan, W. Angew. Chem. Int. Ed. 2018, 57, 3607 –3611.