Peculiarities of palladium-containing MFI-type zeolites as catalysts of isomerization of linear alkanes

Abstract

Platinum group metals are widely used as a hydrogenating-dehydrogenating component of a number of petroleum refining and petrochemical catalysts, in particular for isomerization of linear alkanes. The main direction in improving the preparation of these catalysts is to reduce their cost by optimizing the metal component amount. However, insufficient attention was paid to the method of introduction of an active metal into the carrier; at the same time, this issue is especially important in case of zeolite catalysts, for which ion exchange can be used in addition to traditional impregnation. Therefore, the purpose of this work was to compare the catalytic efficiencies of Pd-containing MFI zeolites in which metal was introduced by two methods: impregnation from a solution of palladium chloride and ion exchange from tetraamminepalladium(II) chloride in the amount of 0.5 wt.% in terms of pure metal. Study of texture characteristics by nitrogen low-temperature adsorption/desorption technique showed that the specific surface area of samples and the total sorption volume remained practically unchanged, regardless of the procedure of metal component introduction. A significant decrease in average pore size was observed only in case of ion-exchange metal introduction; this indicated the localization of palladium mainly in zeolite pores, which was confirmed by transmission electron microscopy. The stage of transformation from ammonium form to hydrogen one strongly affects the activity of samples, this stage should precede the final stage of metal recovery. The highest yield of hexane isomers of about 46.5 wt.% with the selectivity of 88.7% was observed over a catalyst with Pd introduced by ion-exchange method with the smallest palladium particles (3–7 nm).