HZSM-5 zeolites undergoing the high-temperature process for boosting the bimolecular reaction in n-heptane catalytic cracking

Abstract

High-temperature treatment is key to the preparation of zeolite catalysts. Herein, the effects of high-temperature treatment on the property and performance of HZSM-5 zeolites were studied in this work. XRD, N2 physisorption, 27Al MAS NMR, and NH3-TPD results indicated that the high-temperature treatment at 650 °C hardly affected the inherent crystal and texture of HZSM-5 zeolites, but facilitated the conversion of framework Al to extra-framework Al, reducing the acid site and enhancing the acid strength. Moreover, the high-temperature treatment improved the performance of HZSM-5 zeolites in n-heptane catalytic cracking, promoting the conversion and light olefins yield while inhibiting coke formation. Based on the kinetic and mechanism analysis, the improvement of HZSM-5 performance caused by high-temperature treatment has been attributed to the formation of extra-framework Al, which enhanced the acid strength, facilitated the bimolecular reaction, and promoted the entropy change to overcome a higher energy barrier in n-heptane catalytic cracking.