日本ゼオライト学会　刊行物 Publication of Japan Zeolite Association

The formation of mesopores during the dealumination process of zeolites has not been fully understood due to the complexity of the generation of dealumination species and the detailed changes in the pore structure. In this study, we directly observed the internal （cross-section） structural and compositional changes of zeolite Y (FAU-type framework) with a wide range of silicon-to-aluminum atomic (Si/Al) ratios obtained through a series of dealumination treatments using a high spatial resolution field-emission scanning electron microscope (FE-SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS) instrument. Cross-sectional observations of zeolite Y particles revealed that steaming and calcination lead to the formation of complex local structures with bright contrast originating from a dense Al-rich amorphous matter in scale regions of several tenth nanometer. Pore analysis by Ar physisorption suggested that steaming and calcination forced to release Al atoms from the framework to form mesopores. These extra-framework Al (EFAl) atoms, however, do not fill the micropores of FAU, contradicting the previously proposed mechanism. Furthermore, local condensation of EFAl leads to partial collapse of the framework, transforming it into segregated Al-rich amorphous aluminosilicate. Further removal of segregated amorphous aluminosilicate by acid leaching is the origin of the formation of additional mesopores. Especially, with regard to the internal structure, our concept of a direct visualization approach can be effectively used as a versatile technique to reveal detailed characterization of dealuminated species that correlate with the stepwise mesopore formation during zeolite dealumination.