The effects of rhenium and sulfur on the activity maintenance and selectivity of platinum/alumina hydrocarbon conversion catalysts

Abstract

During n-hexane conversion at atmospheric pressure and 500 °C, coimpregnated PtRe Al 2O 3 , unlike a physical mixture of Pt Al 2O 3 and Re Al 2O 3 , displays a remarkably high selectivity for cracking which is characteristic of alloy formation. Presulfidation alters significantly the activities and selectivities of Pt Al 2O 3 and PtRe Al 2O 3 . Hydrogenolysis is reduced for both catalysts. The rate of activity decline is also reduced for PtRe Al 2O 3 . The effects of poisoning by sulfur and deactivation by carbonaceous deposits are in competition such that after some time on stream sulfided PtRe Al 2O 3 displays a significantly higher activity than unsulfided PtRe Al 2O 3 . Conversely, sulfided Pt Al 2O 3 remains less active than unsulfided Pt Al 2O 3 for the full duration (25 h) of the reaction experiments. Another remarkable effect of presulfiding PtRe Al 2O 3 is that dehydrocyclization is enhanced for n-heptane conversion but not for n-hexane conversion. We conclude that the combined action of Re and S is responsible for the differences in performance between sulfided Pt Al 2O 3 and PtRe Al 2O 3 . Sulfur is preferentially adsorbed on the Re atoms of PtRe alloy particles. Adsorbed sulfur on both Pt Al 2O 3 and PtRe Al 2O 3 causes changes in the selectivity patterns which are characteristic of a reduction in the size of Pt ensembles. The superior activity maintenance of PtRe(S) Al 2O 3 is consistent with a tentative model which assumes that adsorbed sulfur, S ads, fixed on Re impedes the reorganization of hydrocarbonaceous fragments into pseudographitic entities which cause irreversible deactivation of the Pt function by carbonaceous deposits.