Mechanistic Investigations of Ethylbenzene Disproportionation over Medium-Pore Zeolites with Different Framework Topologies

Ethylbenzene disproportionation has long been recognized as a useful test reaction for distinguishing between medium- and large-pore zeolites because medium-pore materials show neither an induction period nor a diethylbenzene deficit, unlike large-pore ones. Hence, the mechanism of this reaction over all medium-pore zeolites has been regarded as simply monomolecular to date. Here, we demonstrate that the pore architecture of medium-pore zeolites strongly influences the type of main reaction intermediates of ethylbenzene disproportionation and, thus, that of the prevailing reaction mechanism: dual-cycle diphenylethane-mediated, bimolecular diphenylethane-mediated, or monomolecular ethyl-transfer reaction pathway. To our knowledge, our study provides more manifest experimental evidence for the transition state shape selectivity in zeolite catalysis than has ever been reported.