Ethylbenzene isomerization on bifunctional platinum alumina-mordenite catalysts 1. Influence of the mordenite Si/Al ratio

The transformation of ethylbenzene was carried out on intimate mixtures of 0.5 wt% Pt/Al2O3 (75 wt%) with a platinum dispersion of 100% and of HMOR ze olites (25 wt%) with Si/Al ratios between 6.6 and 180. Products result from six main transformations of ethylbenzene or/and of the reaction products: the desired isomerization of ethylbenzene into xylenes (reaction 1), dispro portionation of ethylbenzene (2), dealkylation followed by hydrogenation of ethylene (3), secondary ethylbenzene-xylene transalkylation (4), hydrogena tion of ethylbenzene followed by ethylcyclohexane isomerization (5), and se condary cracking of C-8 naphthenes (6). From the comparison of the product distributions over Pt/Al2O3, over HMOR and over the bifunctional catalysts, reactions 2-4 were confirmed to occur through acid catalysis and hydrogena tion through metal catalysis, and reactions 1 and 6 through bifunctional ca talysis. Whatever the catalyst, the selectivity to isomers increases with e thylbenzene conversion, which is due to thermodynamic limitations in reacti ons 2 and 5. A significant increase in selectivity to isomers is also obser ved when the Si/Al ratio of the mordenite component increases from 20 to 18 0. With these bifunctional catalysts, the initial rate of disproportionatio n is proportional to the square of the concentration of protonic sites, whi ch suggests that this bimolecular reaction requires two protonic sites for its catalysis; furthermore, the rate of dealkylation. (per protonic site) d epends only on the strength of the protonic sites and the change of the iso merization rate with the balance between the metallic and acidic functions is the one expected from a classical bifunctional mechanism. For the bifunc tional catalysts with HMOR6.6 and -10 as acidic components, the rates of di sproportionation, dealkylation, and isomerization are lower than expected a nd the selectivity to isomers is higher. This can be explained by strong di ffusion limitations in these zeolites which, in contrast to the other sampl es, do not present mesopores in addition to micropores. (C) 2001 Academic P ress.