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

With n-hexane conversion at 5000C and 1 bar as the test reaction, PtRe/Al/sub 2/O/sub 3/ prepared by coimpregnation differs markedly from a physical mixture of Pt/Al/sub 2/O/sub 3/ and Re/Al/sub 2/O/sub 3/. The coimpregnated catalyst displays a high selectivity for hydrogenolysis which is characteristic of Pt-Re alloy formation. Presulfidation alters significantly the activities and selectivities of Pt/Al/sub 2/O/sub 3/ and PtRe/Al/sub 2/O/sub 3/, hydrogenolysis being reduced for both catalysts. Most remarkable is, however, the effect of presulfidation on the steady-state activity: it is found to be lowered for Pt/Al/sub 2/O/sub 3/; but enhanced for PtRe/Al/sub 2/O/sub 3/. Evidently, controlled poisoning of the latter catalyst results in a suppression of its deactivation by carbonaceous deposits. The effect of sulfur on the selectivity depends on the hydrocarbon: dehydrocyclization is decreased with n-hexane; but increased with n-heptane. This suggests different ensemble requirements for ring closure involving primary or secondary carbon atoms respectively