Aromatization of light alkanes over ZSM-5 catalysts: Influence of the particle properties of the zeolite

Abstract

Activity/selectivity of ZnZSM-5 catalysts (0.04–0.369 mmol Zn/g zeolite) prepared by ion-exchange from two samples of ZSM-5 zeolites having different size of particles (≤1 and ∼4 μm) in aromatization of light alkanes have been investigated. As a probe molecule n-hexane was used. Acid HZSM-5 samples have comparable composition (0.101–0.11 mmol Al/g) and acidity measured by TPDA method (0.92–0.91 mmol acid sites/g). Introduction of Zn into zeolites does not significantly affect the total acidity determined by TPDA but causes the redistribution in the type of acid sites. From the activity/selectivity of ZnZSM-5 catalysts followed that by introduction of Zn into HZSM-5 zeolites the aromatization proceeds very significantly via bifunctional mechanism. Results showed that the activity/selectivity of ZnZSM-5 catalysts strongly depends on the synergism of the particle size, acidity, aluminum distribution in the zeolite framework, and of the amount and the type of Zn species located in the cationic positions in the zeolite. The type of Zn species is strongly affected by the distribution of Al atoms in the zeolite framework. It was published that isolated Zn 2+ or oxygen-bridged Zn–O–Zn 2+ species can be formed in ZSM-5 zeolite depending on the distance between the adjacent aluminum atoms in the framework and that this species have a very different activity in aromatization. It was concluded that the observed differences in the performance of the investigated ZnZSM-5 catalysts at low Zn concentration are caused by the different size of the zeolite particles and at higher amount of Zn by the different distribution of Al atoms in the framework of the zeolite samples.