Regionally spatial framework Al distribution in MFI channels and its impact on the n-butane cracking reaction pathways

The regulation of the engineering regionally spatial framework aluminum (AlF) distribution in MFI channels has been considered as an effective way for the production of light olefins by targeted fluid catalytic cracking (TCO) technology. In this work, ZSM-5 zeolites with controlled AlF distributions are facilely synthesized by tuning the combinations of pentaerythritol (PET), tetrapropylammonium (TPA) and Na cations. The AlF atoms in the prepared HZ5-[PET + Na] and HZ5-[TPA] samples are enriched in the straight channels and intersection channels, respectively, which are favorable to the n-butane monomolecular reaction pathway, promoting the propylene production. Whereas the AlF atoms in the HZ5-[TPA + Na] sample are simultaneously distributed in both the straight and intersection channels, the n-butane molecules preferentially take place monomolecular reaction in the straight channels to generate carbonium ions (e.g., C2H5+, C3H7+ and C4H9+). Subsequently, all of them enter into the intersection channels to induce the occurrence of bimolecular reaction. Such a process is conducive to the formation of ethylene. These insights can help to clarify the catalytic behavior of regional framework acid sites in ZSM-5 channels and then provide an effective approach to design efficient TCO catalysts.

Automated summary

ZSM-5 zeolites with controlled AlF distributions were synthesized by tuning the combinations of PET, TPA, and Na cations. The enriched AlF atoms in HZ5-[PET + Na] and HZ5-[TPA] were beneficial for the n-butane monomolecular reaction pathway, promoting the production of propylene. The AlF atoms in HZ5-[TPA + Na] were simultaneously distributed in both straight and intersection channels, leading to the generation of carbonium ions in the straight channels and inducing bimolecular reactions in the intersection channels, favoring the formation of ethylene. These findings provide insights into the catalytic behavior of regional framework acid sites in ZSM-5 channels and offer an effective approach for designing efficient TCO catalysts.