Atom-Selective Analyses Reveal the Structure-Directing Effect of Cs Cation on the Synthesis of Zeolites

To understand the crystallization mechanism of zeolites, it is important to clarify the detailed role of the structure-directing agent, which is essential for the crystallization of zeolite, interacting with an amorphous aluminosilicate matrix. In this study, to reveal the structure-directing effect, the evolution of the aluminosilicate precursor which causes the nucleation of zeolite is analyzed by the comprehensive approach including atom-selective methods. The results of total and atom-selective pair distribution function analyses and X-ray absorption spectroscopy indicate that a crystalline-like coordination environment gradually forms around Cs cations. This corresponds to the fact that Cs is located at the center of the d8r units in the RHO structure whose unit is unique in this zeolite, and a similar tendency is also confirmed in the ANA system. The results collectively support the conventional hypothesis that the formation of the crystalline-like structure before the apparent nucleation of the zeolite.