Influence of the amount and the type of Zn species in ZSM-5 on the aromatisation of n-hexane

摘要

The aromatisation of n-hexane over a series of zinc modified (0.035-0.50 mmolZn/g) acid ZSM-5 zeolite (1.09 mmolAl/g) prepared by ⅰ) ion exchange or by ⅱ) mechanical admixing of ZnO was investigated. The reaction of aromatisation was carried out at 420°C at atmospheric pressure. At this temperature thermal cracking of n-hexane doesn't contribute to the conversion. Over ion exchanged samples with increasing amount of zinc in the zeolite the conversion of n-hexane comparing with acid ZSM-5 at first continuously decreases goes through the minimum and than begins to rise, while the selectivity to aromatics continuously raises. As was shown the differences in the acidity of samples are very low so from the results we conclude that the role of the Zn in cationic positions in n-hexane conversion depends on the zinc concentration. Zinc species at low concentration probably due to their localization are not active in n-hexane activation but only in dehydrogenation of the oligomerized products. Mechanical mixtures of ZnO + acid ZSM-5 in contrary to the ion-exchanged samples had constant activity /selectivity independently on the amount of ZnO. Their activity and selectivity are roughly on the level of ion-exchanged sample containing 0.085 mmol Zn/g. From the results follows that at 420°C ZnO species in mechanical mixtures are not active in n- hexane conversion and the catalytic performance of the mixtures is probably the consequence of a solid state ion exchange of Zn into zeolite from ZnO species. From the constant activity/selectivity of the mechanical mixtures containing different amount of ZnO follows that the extent of assumed solid state ion exchange is limited and is controlled probably by the surface characteristics of the zeolite and not by the amount of ZnO.