Sodium exchange over H-EU-1 zeolite. Part II: Catalytic properties

Three different model reactions (methylcyclohexane, m-xylene and 1,3,5-trimethylbenzene transformations) were studied over a series of partially Na-exchanged H-EU-1 zeolites. In the case of methylcyclohexane transformation, the catalytic activity was shown to be directly correlated to the concentrations and strength distribution of the acid sites. Consequently, a maximum in activity and turn-over frequency (TOF) was obtained for an exchange ratio comprised between 26% and 34%, for which the proportion of very strong acid sites was very high. An acidity–activity correlation, based on the Polanyi principle, was successfully applied to this reaction. This was not the case for m-xylene and 1,3,5-trimethylbenzene transformations. During those reactions, the activity and TOF decreased with the protonic site concentration, hence with the increase of the exchange ratio, the activity loss being more pronounced for low exchange ratios. Poisoning experiments using γ-collidine indicated that two kinds of external acid sites could be distinguished, depending on their locations, that is, at the pore mouths and in 12-ring pockets. The fast exchange by sodium cations of the latter sites, which would be particularly active in m-xylene and 1,3,5-trimethylbenzene transformations, could explain the evolution of these activities.

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► Methylcyclohexane cracking activity directly correlated to acid strength.
► Maximum in methylcyclohexane activity for exchange ratios between 26% and 34%.
► Significant decrease of m-xylene and 1,3,5-TMB catalytic activities with exchange.
► Fast exchange of the acid sites of external pocket active in aromatic isomerization.