Activation of Light Alkanes over Zinc Species Stabilized in ZSM-5 Zeolite: A Comprehensive DFT Study

The mechanism of alkane dehydrogenation over Zn/ZSM-5 zeolite was studied by means of density functional theory using the cluster modeling approach. Three types of active sites were considered: Zn 2+ ions stabilized in conventional ion-exchange sites (ZnZ s ), Zn 2+ ions stabilized in cation sites with distantly placed aluminum ions (ZnZ d ), or as binuclear [ZnOZn] 2+ cations (ZnOZn). A comparison of the computed energetics of various reaction paths for ethane indicates that the catalytic reaction proceeds most easily over the ZnZ d sites. The enhanced Lewis acidity of these sites facilitates the subsequent heterolytic C−H bond cleavage. The most favorable proton-accepting sites are not those of the framework ring to which Zn 2+ is attached, but O sites of a neighboring ring. Although the reactivity of isolated Zn 2+ ions strongly depends on the distribution of framework aluminum in ZSM-5 zeolite, the adsorption of alkane molecules is only slightly influenced by this factor. After the heterolytic C−H bond cleavage step, the zinc−alkyl group and acidic proton recombine via a cyclic transition state resulting in a one-step formation of an alkene and H 2 . This process is both thermodynamically and kinetically preferred for isolated Zn 2+ sites over the consecutive mechanism. The activation barrier for the one-step elimination reaction strongly depends on the relative position of the reacting zinc−alkyl and framework attached H + ions. Despite initial heterolytic C−H bond dissociation being strongly favored on the [ZnOZn] 2+ cations, the activation energy for the subsequent decomposition of the resulting products is high.