III International Conference

PP-I-75(Zn Z d ). The catalytic cycle starts with theheterolytic C-H bond cleavage of the C 2 H 6molecule strongly adsorbed on the Zn 2+cation with distantly located chargecompensating[AlO 2 ] - framework units.Initial proton abstraction is preferred to theO-sites neighboring to the ring where thecation is located rather than to the oxygenatoms of the same zeolitic ring. Theresulting intermediate II decomposes viaone-step formation of H 2 and C 2 H 4(II→V), whereas the consecutive ethyleneand hydrogen elimination (outer cycle inScheme 1) is unfavored. The activationScheme 1energy for the one-step reaction is stronglydependent on the relative position of [Zn-C 2 H 5 ] + and H + species. Therefore, the presence ofacidic protons in the catalyst can promote the regeneration of the active sites. Subsequentdesorption of ethylene from the thus formed molecular complex V regenerates the initialactive site. To our knowledge such mechanism (inner cycle in Scheme 1) has never beenreported before for this type of catalytic reactions.Bivalent zinc ions stabilized at the conventional ion-exchange sites (Zn Z s ) are less likelyactive sites. The low-energy path for the catalytic reaction over these sites consists of the sameelementary steps as for the Zn Z d , which, however, face significantly higher activation energies.The [Zn-OH] + ions show remarkably lower activity in the initial C-H activation ascompared to the parental isolated Zn 2+ cations. Moreover, it is found that the catalytic reactionover these sites leads to their decomposition via water desorption and generation of the morereactive Zn 2+ ions. On the other hand, although binuclear [ZnOZn] 2+ sites show the highestinitial activity, the heterolytic C 2 H 6 dissociation results in formation of very stable species.Subsequent transformations of the thus formed intermediates are found to be stronglyunfavored. We conclude that these oxygen-containing zinc species cannot be responsible forthe dehydrogenation activity of Zn/ZSM-5 zeolites, while the most probable active sites arethe isolated Zn 2+ ions stabilized at the distant anionic sites of the zeolite.References1. A. Hagen, F. Roessner, Catal. Rev. 42 (2000) 403.2. V.B. Kazansky, I.R. Subbotina, N. Rane, R.A. van Santen, E.J.M. Hensen, Phys. Chem. Chem.Phys. 7 (2005) 3088; and references therein.142