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 (Al F ) 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 Al F distributions are facilely synthesized by tuning the combinations of pentaerythritol (PET), tetrapropylammonium (TPA) and Na cations. The Al F 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 Al F 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., C 2 H 5 + , C 3 H 7 + and C 4 H 9 + ). 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.
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