Distinguishing and unraveling classical and non-classical pathways in MFI zeolite crystallization: Insights into their contributions and impact on the final product

Abstract

Simplifying complex synthesis system containing multiple species into ones with single classical or nonclassical growth path is valuable for understanding their respective mechanisms. However, most zeolites growth involves intertwined classical and non-classical mechanisms, making it crucial to distinguish and modulate their contributions in original synthetic system. In this study, we provide a method to distinguish and directly quantity the contributions of classical and non-classical crystallization pathways in MFI zeolite synthesis, demonstrating that the dominant pathway can be shifted from non-classical to classical by varying the H2O/SiO2 and ethanol/SiO2 ratios. Our findings show that reducing H2O/SiO2 favors the non-classical pathway, while increasing ethanol/SiO2 promotes the classical mechanism. However, these changes have minimal effect on their crystallization sequences: the non-classical pathway predominates initially, but both pathways intertwine as crystallization progresses. Notably, the shift in crystallization pathway does not significantly affect the acidic properties of the zeolites but has a direct impact on their catalytic performance. The catalytic activity of the resulting ZSM-5 zeolites in furfuryl alcohol etherification correlates with the classical pathway contribution, with higher contributions leading to enhanced catalytic activity. This study provides new insights into the zeolite crystallization process, offering a valuable approach for optimizing synthesis conditions and improving catalyst performance.