Formation Mechanism of Al Pairs in ZSM-5 Synthesized in a Quasi-Solid Phase System: Al Source, Alkalinity, and Crystallization Kinetics

As the predominant contributor of Brønsted acid sites, the spatial distribution of framework aluminum in the ZSM-5 zeolite critically governs its catalytic performance in hydrocarbon conversion reactions, such as cracking, isomerization, and aromatization. This study systematically analyzed the effect of aluminum coordination state on the Al pair formation in ZSM-5 synthesis by the QSP method. A comparative analysis was conducted on six aluminum sources in conjunction with the JMAK kinetic model and multiscale characterization. The results show that the coordination state and spatial distribution of aluminum species directly determine the content and distribution of aluminum pairs in the ZSM-5 zeolite. By increasing the alkalinity of the system, the conversion of Al[(OH2)6]3+ to Al(OH)4– was promoted and the content of Al pairs increased from 34 to 74%. It was further found that the aluminum source has a significant effect on the crystallization path by adjusting the nucleation energy barrier (En = 32.23–64.31 kJ/mol) and the growth size. This work illustrates the unique advantages of the QSP method in accurately regulating Al pairs and provides theoretical support for the green synthesis of ZSM-5 catalysts.