Role of Zinc in Zn-Loaded ZSM-5 Zeolites in the Aromatization of n-Hexane

Abstract

Aromatization of n-hexane over zinc-modified ZSM-5 zeolites was investigated. It was shown that incorporation of zinc by ion exchange into cationic positions of NH4-ZSM-5 zeolite causes acid-site strength redistribution and generation of new relatively strong Lewis acid sites in zeolite increasing the selectivity of n-hexane aromatization in comparison with the parent NH4-ZSM-5 zeolite. Simultaneous presence of Lewis and Broensted acid sites in ZSM-5 zeolite does not affect the strength of Broensted acid sites in zeolite. For the activity/selectivity of aromatization of n-hexane on Zn-modified ZSM-5 zeolites, the amount of Zn and its localization in the cationic positions are decisive. The reaction of n-hexane can be also initiated by the Zn species alone in the cationic positions. ZnO species alone as an extraframework phase was found inactive in the catalyst for aromatization of n-hexane. The influence of ZnO addition on the performance of pure ammonium forms of ZSM-5 zeolites in n-hexane conversion is a result of partial migration of zinc into cationic positions of zeolite by solid-state ion exchange.

Interaction of ZnO with ZSM-5 Zeolites

“…These results confirm our presumption 9 that ZnO as an extra-framework phase is an inactive component of the catalyst in the n-hexane aromatization.…”

“…In our previous work 9 it was shown that ZnO as an extra-framework phase is not able to activate n-hexane under given conditions (420°C). The presented activity/selectivity in the test reactions and TPDA data of thermally treated mechanical mixture ZnO + H-ZSM-5 are different from those of pure acid ZSM-5 zeolite.…”

“…On the other hand, in our previous work 9 it was observed that both mechanical mixtures of ZnO with alumina and sodium form of ZSM-5 were totally inactive in n-hexane conversion at 420°C, indicating that ZnO as an extra-framework phase is not able to activate n-hexane under these conditions. We have only observed an increased selectivity to aromatics on a mechanical mixture ZnO + NH 4 -ZSM-5 as compared with NH 4 -ZSM-5.…”

Aromatization of light alkanes over ZSM-5 catalystsInfluence of the particle properties of the zeolite

“…In our previous work [15] the FT-IR measurements showed that in ion-exchanged ZnZSM-5/66 samples Zn species are located in cationic positions and represent relatively strong Lewis acid sites. FT-IR spectra of HZSM-5/70 and ZnZSM-5/70 samples are given in Figs ion-exchange by Zn.…”

Influence of boron promotion on the physico-chemical properties and catalytic behavior of Zn/ZSM-5 in the aromatization of n-hexane

“…The introduction of Zn is known to increase the selectivity to aromatics by providing an alternative path via dehydrogenation of the alkane [10]. Moreover, the presence of Zn species results in significant changes of the acidic properites of ZSM-5, reducing the Brönsted acid site density proportionally to the Zn loading, and generating new relatively strong Lewis acid sites [11,12].…”