Ethylbenzene isomerization over bifunctional platinum alumina–EUO catalysts: Location of the active sites

The transformation of ethylbenzene (EB) was carried out on intimate mixtures of Pt/Al2O3 (PtA) and HEUO (Si/Al = 15) catalysts with different composition under the following conditions: fixed bed reactor, temperature 683 K, pressure of hydrogen and ethylbenzene equal to 8 and 2 bar respectively, weight hourly space velocity between 5 and 60. Products result from the following transformations of ethylbenzene or/and of the reaction products: the bifunctional desired isomerization of EB into xylenes (X), the acidic reactions of disproportionation, dealkylation of EB (followed by ethylene hydrogenation) and EB–X transalkylation, the hydrogenation of EB followed by ethylcyclohexane isomerization and cracking. The selectivity to isomers, which is very low on the fresh catalysts, increases significantly during an initial period of fast deactivation, essentially at the expense of disproportionation and dealkylation by products. This deactivation as well as the increase in the selectivities to isomers were shown to result from the blockage by carbonaceous deposits of the access to the inner sites of the EUO micropores. The selectivity to isomers of EB transformation over these inner sites is very low, which could be due to the trapping of ethylcyclohexene intermediates within the monodimensional channels of EUO zeolite. In contrast, over the stabilized catalysts for which only the protonic sites of large side pockets located on the outer surface can be active, the selectivity to isomers is very high, which could be related to the environment of these sites as well as to their easy access by cycloalkene intermediates.