Ethylbenzene Isomerization on Bifunctional Platinum Alumina−Mordenite Catalysts. 2. Influence of the Pt Content and of the Relative Amounts of Platinum Alumina and Mordenite Components

Abstract

Ethylbenzene transformation was carried out over intimate mixtures of Pt/Al2O3 (PtA) and HMOR zeolites under the following conditions: fixed-bed reactor, temperature 683 K, pressures of hydrogen and of ethylbenzene equal to 8 and 2 bar, respectively, weight hourly space velocity between 5 and 200. A significant number of these bifunctional PtA/HMOR catalysts were used, differing by the platinum content of PtA (0.5, 1.1 and 2.3 wt %), the framework Si/Al ratio of the mordenite (from 6.6 to 180), and especially by the relative proportions of the PtA and HMOR components. With all of the bifunctional catalysts, ethylbenzene undergoes isomerization into xylenes, disproportionation, dealkylation, secondary ethylbenzene−xylene transalkylation, hydrogenation followed by ethylcyclohexane isomerization, and secondary cracking of C8 naphthenes. The rate of isomerization which occurs through bifunctional catalysis was shown to depend mainly on the balance between hydrogenating and acid functions, taken here as the ratio between the concentration of accessible platinum atoms and of protonic acid sites (nPt/nH+). The rate of disproportionation per acid site does not depend on the relative proportion of zeolite and of Pt/Al2O3 in the bifunctional catalysts but increases with the density of protonic sites in the mordenite component. This suggests that this bimolecular reaction requires more than one acid site for its catalysis. Ethylbenzene dealkylation does not depend on the density of protonic sites but seems very sensitive to their strength. For these three reactions, the apparent activity of the protonic sites is significantly greater when the HMOR components of the bifunctional catalysts are dealuminated, hence present mesopores; most of the protonic sites of these dealuminated mordenites would be active, whereas for mordenites without mesopores, it would be the case only for the protonic sites of the shell of the crystallites. High selectivities to xylenes (≈75% at 35% conversion) were shown to be obtained on bifunctional catalysts with values of nPt/nH+ sufficiently high to have the acid isomerization of olefinic intermediates as the limiting step of ethylbenzene isomerization.