Quantifying the modulation of modified ZSM-5 acidity/alkalinity on olefin catalytic pyrolysis to maximize light olefin selectivity

Abstract

Undesirable side reactions often occur during the catalytic pyrolysis of olefins in FCC gasoline to produce light olefins. We proposed a method to maximize light olefin selectivity by suppressing these side reactions. In this study, P-ZSM-5 and Ce–P-ZSM-5 were prepared by the pore volume impregnation method to investigate the regulatory relationship between their acidity/alkalinity and catalytic pyrolysis. The study found that acid sites could promote the olefin conversion and adjust the cracking pathway. Furthermore, the densities of strong B acid sites and base sites could regulate olefin adsorption, affecting hydrogen transfer activity. Acid strength was crucial in controlling the aromatization reaction. It showed that thermal cracking played a minor role in catalytic pyrolysis, with methane generation mainly resulting from acid sites promoting the cracking of terminal C–C bonds. Loading phosphorus strengthens the acidity of ZSM-5, while loading cerium weakens its acidity and enhances its alkalinity. By optimizing the acidity and alkalinity of ZSM-5, side reactions were suppressed, maximizing the selectivity of light olefins. The results showed that the light olefin selectivity of Ce–P-ZSM-5 increased from 33.2% to over 80%, while the side reaction selectivity decreased from over 60% to below 10%. This work will lay a good foundation for developing catalysts for catalytic pyrolysis of olefins to produce light olefins.