Broadening the Scope for Fluoride-Free Synthesis of Siliceous Zeolites

Abstract

Siliceous zeolites are ideally suited for emerging applications in gas separations, sensors, and the next generation of low-k dielectric materials, but the use of fluoride in the synthesis significantly hinders their commercialization. Herein, we show that the dry gel conversion (DGC) technique can overcome this problem. Fluoride-free synthesis of two siliceous zeolites-AMH-4 (CHA-type) and AMH-5 (STT-type), has been achieved for the first time using the method. Siliceous *BEA-, MFI-, and *MRE-type zeolites have also been synthesized to obtain insights into the crystallization process. Charge-balancing interactions between the inorganic cation, organic structure-directing agent (OSDA), and Si-O^- defects are found to be an essential aspect. We quantify this factor in terms of the "OSDA charge/silica ratio" of the as-made zeolites and demonstrate that the DGC technique is broadly applicable and opens up new avenues for fluoride-free siliceous zeolite synthesis.

Keywords: CHA zeolite; dry-gel conversion; fluoride-free synthesis; hydrothermal synthesis; zeolites.