Extra-Framework Al-Modified Defective Hierarchical ZSM-5 Zeolite for Catalytic Cracking of n-Pentane to Olefins

Abstract

In the catalytic cracking of naphtha to light olefins over conventional microporous ZSM-5 zeolite, issues such as high mass-transfer resistance leading to low conversion and high acid-site density causing olefin consumption are commonly encountered. In this work, a hierarchical nanosponge ZSM-5 zeolite (NP) with a Si/Al ratio of 20 was synthesized. Acid washing was applied to reduce the acid-site density and introduce defects (NP-Aci). Furthermore, extra-framework Al was introduced via impregnation to modify the defective hierarchical-porous ZSM-5 (Al/NP-Aci). Characterization results confirm that the impregnated Al is anchored at defect sites, existing as extra-framework Al that acts as a Lewis acid. In n-pentane cracking, the Lewis-acid-modified Brønsted acid sites promote the monomolecular cracking of n-pentane, while the lowered acid density suppresses the bimolecular hydrogen transfer of the olefins. At 600 °C, the Al/NP-Aci catalyst achieves 98.03% n-pentane conversion, with ethylene and propylene selectivity of 30.12% and 19.63%, respectively. Moreover, the Al/NP-Aci catalyst exhibits excellent cracking stability, showing a deactivation rate of only 3.81% after 16 h of reaction at 600 °C under a weight hourly space velocity (WHSV) of 6.26 h^–1.