Influence of Sulfur on Activity, Selectivity and Deactivation During Reactions of Methylcyclohexane Over Pt and PtRe Reforming Catalysts

The presence of small amounts of sulfur lowers hydrocracking activity for both Pt and PtRe catalysts. Treatment of coked PtRe in pure H 2 restores substantial hydrogenolysis activity when sulfur is absent, but in the presence of sulfur such activity appears to be irreversibly poisoned. Gravimetric measurementts show that coke deposition on PtRe catalysts is greatly retarded by S and such diminution of coke deposition is closely associated with the retarded rates of deactivation of these catalysts. The observations are generally in accord with a model that includes the following major features: (1) hydrogenolysis at kinked terrace edges of metal crystallites, (2) dehydroaromatization by the concerted action of exposed metal atoms in the terraces together with a disordered carbonaceous overlayer, (3) rearrangment of this disordered layer to a non-catalytic graphitic deposit, (4) deactivation of hydrogenolysis (kinked edge) sites by either sulfur or carbon, especially by sulfur when Re is present and (5) retardation of the growth and 3-dimensional rearrangement of the carbonaceous overlayer by sulfur bound to Re atoms in the surfaces of the crystallite terraces. This model and the results are also compatible with the viewpoint that ensembles of surface Pt are catalytically important and that surface Re in PtRe alloys can reduce the concentration of such ensembles.