ZSM-5 with optimized acid center position for enhancing monomolecular cracking of pentene

The competition of monomolecular and bimolecular reactions significantly influences the conversion behavior of C−C and C−H bonds in olefin molecules during the catalytic cracking process of olefins. Monomolecular cracking of pentene (C₅⁼) represents an ideal reaction pathway for maximizing the production of ethylene and propylene, and the development of highly efficient ZSM-5 zeolites has consistently been central to achieving this goal. This study aims to elucidate the previously unclear structure-activity relationship between the spatial distribution of acid sites in ZSM-5 zeolites and the competition between monomolecular and bimolecular reaction pathways of C₅⁼. A strategy combining bottom-up synthesis with top-down modification was developed to prepare ZSM-5 zeolites possessing comparable acid amount and strength, yet distinct spatial distribution of acid sites in the ten-membered-ring (10-MR) channels and intersection cavities. Catalytic cracking tests of C₅⁼ demonstrated that ZSM-5 zeolites with a higher concentration of acid sites in the 10-MR channels favored the monomolecular pathway, whereas acid sites in the intersection cavities promoted the bimolecular pathway. Further, an efficient ZSM-5 zeolite (NBA(51)-1.6P) was designed for the monomolecular cracking of C₅⁼, with 76.3% total ethylene and propylene selectivity and a 0.94 propylene-to-ethylene molar ratio, indicating a predominant monomolecular reaction characteristic. This is the first report to clarify the regulatory mechanism of the spatial distribution of acid sites in ZSM-5 zeolites on the competition between monomolecular and bimolecular reaction pathways of C₅⁼, thereby providing a scientific foundation for developing efficient zeolite catalysts for enhanced production of ethylene and propylene.