Reconstruction of Al species enables binder-free hierarchical monolithic ZSM-5 zeolite with unique Al distribution

Recrystallization of molded zeolite catalysts enables transformation into binder-free zeolite catalysts, effectively solving the negative effects of binders. However, the evolution and underlying origin of acid properties still remain unclear. In this study, the changes of acid properties, the reconstruction behaviors of Al species, and the chemical interactions between parent ZSM-5 zeolite and SiO₂ binder during the recrystallization process were systematically investigated, aiming to clarify the fundamental causes of changes in acid properties. Upon complete recrystallization, the binder-free monolithic ZSM-5 zeolite exhibited significant changes in acid properties (including acid amounts, acid strength, acid types and spatial distribution of acid centers). Notably, the spatial distribution of acid centers closely resembled that of hydrothermally synthesized ZSM-5 zeolite with the same Si/Al₂ ratio, directly indicating Al species’ reconstruction. Further analysis confirmed that recrystallization process involves changes in Al coordination states and spatial distribution. Moreover, comparison of transformation behaviors of parent ZSM-5 zeolite and SiO₂ binder further revealed that recrystallization of molded ZSM-5 zeolite is not a simple superposition of post-treatment of parent ZSM-5 zeolite and separate crystallization of SiO₂ binder. Instead, strong chemical interactions exist between the two processes, in which Si and Al species released from dissolved ZSM-5 crystals directly participate in the recrystallization of SiO₂ binder. This provides a fundamental explanation for the reconstruction of Al species and changes of acid properties. These insights not only advance the knowledge of Al species during liquid-solid phase transformations but also provide theoretical guidance and practical implications for the targeted synthesis of binder-free zeolites.