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.
Abs~aetAromatization of n-hexane and disproportionation of toluene over H-ZSM-5 and Zn-ZSM-5 prepared by ion-exchange with aqueous solution has been studied.Introduction of zinc into ZSM-5 does not si~ificantly affect the total acidity determined by TPDA, but causes redistribution in the strength and type of acid sites. From the results it was concluded that the first step in the aromatization of n-hexane on Zn-ZSM-5 is probably the formation of carbonium ions by hydride abstraction with acid sites in zeolite and that Zn species have a decisive role in oligomerization of cracking products and in dehydrogenation of cyclic intermediates.
The possibility of introduction of zinc from ZnO into cationic positions of H-and NH 4 -ZSM-5 zeolites by solid-state ion exchange was investigated. It was concluded that during thermal pretreatment of a mechanical mixture of ZnO with acid forms of ZSM-5 zeolite, introduction of Zn species into cationic positions as a consequence of the proposed solid-state ion exchange proceeds, but its degree is limited. It indicates that the extent of interaction of Zn species is probably controlled by the concentration of cationic sites on the external surface of the zeolite.It has been reported that mechanical mixtures of ZnO with acid forms of ZSM-5 zeolites are highly active and selective in aromatization of lower alkanes 1-3 . The role and the state of metallic species in the mechanical mixtures under the reaction conditions have also been the subject of a wide discussion 2-8 .Kanai and Kawata 2 have reported that a mechanical mixture of ZnO and NH 4 -ZSM-5 is a bifunctional catalyst in which ZnO in the n-hexane aromatization catalyzes dehydrogenation of n-hexane into hexene and of the oligomerized products into aromatics.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. We have explained this fact by partial migration of Zn from ZnO into cationic positions due to solid-state ion exchange during thermal pretreatment of the mechanical mixture before measurement.
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