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 systems containing multiple species into ones with a single classical or non-classical growth path is valuable for understanding their respective mechanisms. However, most zeolite growth involves intertwined classical and non-classical mechanisms, making it crucial to distinguish and modulate their contributions in the original synthetic system. In this study, we presented a method to distinguish and directly quantify the contributions of classical and non-classical crystallization pathways in MFI zeolite synthesis, demonstrating that the dominant pathway could be shifted from non-classical to classical by varying the H₂O/SiO₂ and ethanol/SiO₂ ratios. Our findings showed that reducing the H₂O/SiO₂ ratio favored the non-classical pathway, while increasing the ethanol/SiO₂ ratio promoted the classical mechanism. However, these changes had minimal effect on their crystallization sequences: the non-classical pathway predominated initially, but both pathways intertwined as crystallization progressed. Notably, the shift in crystallization pathway did not significantly affect the acidic properties of the zeolites but had a direct impact on their catalytic performance. The catalytic activity of the resulting ZSM-5 zeolites in furfuryl alcohol etherification correlated with the contribution of the classical pathway, 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.