ISOMERIZATION OF N-HEXANE ON PLATINUM DEALUMINATED MORDENITE CATALYSTS .3. INFLUENCE OF HYDROCARBON IMPURITIES

The effect which alkanes, naphthenes and aromatics have on the rate of hydroisomerization of n-hexane when amounts (1 or 5 wt.-%) was determined at 250-degrees-C, pH2 = 0.8 bar, pn-hexane = 0.2 bar on 0.3 wt.-% platinum mordenite catalysts with Si/Al ratios equal to 8 (PtHMOR8) and to 68 (PtHMOR68). On both catalysts aromatics and naphthenes (toluene, 1-methylnaphthalene, methylcyclohexane) reduce the n-hexane hydroisomerization rate significantly. This inhibiting effect is due to (i) a competition in adsorption on the acid sites between aromatics (added or formed by naphthene dehydrogenation) and the olefinic intermediates of n-hexane hydroisomerization and (ii) to diffusional limitations caused by adsorbed aromatic species. With PtHMOR8 but not with PtHMOR68 aromatics, naphthenes and heavier than n-hexane alkanes (n-heptane, 2-methylhexane) increase the deactivation. This deactivating effect is due to a rapid coke formation from aromatics and from alkenes resulting from cracking of alkanes or naphthenes.