Fabrication of extra–framework Al in ZSM–5 to enhance light olefins production in catalytic cracking of n–pentane

Abstract

ZSM–5 zeolite catalysts are widely used for catalytic cracking of hydrocarbons to produce light olefins. In this study, secondary addition of Al was introduced in aluminum–silicon zeolites (ZSM–5) by impregnation to modify their acid properties. Characterizations revealed that extra–framework Al (EFAL) formed after calcinations both on the internal and external surface of the catalysts, which provided certain Lewis acid sites. Interestingly, the amount of strong Brønsted acid increased significantly in catalysts with the introduction of EFAL. In cracking of n-pentane, the modified ZSM-5 zeolites gave obviously enhanced TOF values based on Brønsted acid sites, and a significantly improved n-pentane conversion was realized than the pristine catalyst. Moreover, the isolated strong Brønsted acid sites with certain spatial barriers triggered the cracking reaction happening through monomolecular route, which inhibited the hydride transfer of light olefins. The optimized ZSM–5 with Si/Al ratio of 50 (Z–50(75)) gave 45.2 wt% yield of light olefins at 600 °C with an excellent catalytic stability.