Mechanisms of Gmelinite Zeolite Formation from Faujasite Zeolite via Solvent-Free Interzeolite Transformation

Interzeolite transformation, a class of zeolite synthesis procedures, has attracted attention as an eco-friendly fabrication route for zeolites. In particular, interzeolite synthesis without organic structure-directing agents (OSDAs), which typically determine the zeolite frameworks, is imperative for the development of green synthesis methods. However, hydrothermal synthesis, a general method for the preparation of zeolites, requires a large amount of solvent. In this study, gmelinite (GME) zeolite was synthesized from proton-type faujasite (FAU) zeolite via an OSDA/solvent-free and steam-assisted interzeolite transformation process. The FAU zeolite was directly transformed into GME zeolite without passing through an amorphous phase. In addition, FE-SEM observations revealed the formation of hexagonal prismatic particles of the GME zeolite from primary particles of the FAU zeolite. 23Na, 27Al, and 29Si magic-angle spinning nuclear magnetic resonance analyses clarified the state of the intermediate products in the interzeolite transformation from FAU to GME zeolite under steam-assisted conditions. Therefore, the transformation mechanism from FAU to GME zeolite was elucidated from the atomic scale to the particle scale. Notably, this study provides mechanistic insights into the crystallization of GME zeolite via interzeolite transformation, including the evolution of the framework structure and the formation of stacking faults, rather than merely demonstrating an OSDA-free synthesis route. These results provide not only a new green synthesis procedure for GME zeolite but also an in-depth understanding of the mechanism for the interzeolite transformation process under steam-assisted conditions.