Isomerization of n-hexane on platinum dealuminated mordenite catalysts I. Influence of the silicon-to-aluminium ratio of the zeolite

Abstract

n-Hexane transformation was carried out at 250°C under hydrogen at atmospheric pressure and at high pressure (30 bar) on a series of 0.3 wt.-% platinum mordenite samples differing in their Si/Al ratio (7–68). Under atmospheric pressure, the activity decreases when the acid site density decreases i.e., when the Si/Al ratio increases. Under high pressure the activity passes through a maximum for the value of Si/Al under which none of the AlO − 4, hence none of the acid sites, is in the nearest neighbourhood. Above this value the activity is proportional to the number of acid sites. Whatever the operating pressure, the apparent reaction scheme is parallel for catalysts with a high acid site density and successive for catalysts with a low density. The lower the site density the greater the catalyst stability. All these observations can be explained by agreeing with the fact that n-hexane transformation occurs through a bifunctional process in which the limiting step is the transformation of olefinic intermediates on the acid sites. Differences observed between reactions under atmospheric and high pressures should be related to the limiting effect of hydrogen on the residence time of olefinic intermediates in the micropores of the catalysts.