Direct Visualization of the Dealumination Process on Zeolite Y: How Was the Mesoporous Architecture Formed?

The general aspects in the formation mechanism of mesoporous architecture during the dealumination of zeolites are not fully elucidated owing to their complexity, wherein the creation of dealuminated species and pore structural change can occur in diverse ways. In particular, there is still a lack of direct evidence of intermediate states of the mesopore formation, i.e., the detailed location, precise structure, and behavior of the dealuminated species. Herein, integrated techniques of recently developed high-resolution, low-accelerating-voltage field-emission scanning electron microscope (FE-SEM) and energy-dispersive X-ray spectroscopy (EDS) enable direct observation and comparative investigation of the structural and compositional evolution inside zeolite Y with a wide range of Si/Al ratios realized by sequential dealumination. A systematic FE-SEM observation in the cross-section of zeolite Y (fabricated by Ar-BIB milling) revealed that steaming and calcination created a complex local structure in submicrometer-scale regions with bright contrast originating from high-density Al-rich amorphous components. Results of Ar physisorption analyses suggested that steaming and calcination force to eject Al atoms from the zeolite Y framework and create mesopores in it, but this extra-framework Al (EFAl) does not fill micro- and mesopores, which strictly contradicts the previous mechanism. Local condensation of the EFAl leads to a partial collapse of the framework, which transforms into segregated Al-rich amorphous aluminosilicate regions. Further removal of segregated amorphous aluminosilicate via acid leaching significantly led to the additional formation of mesopores. Especially with regard to the internal structure, our concept of the direct visualization approach can be effectively used as a versatile technique to unveil the detailed features of dealuminated species correlated with step-by-step mesopore formation in the dealumination of zeolites.