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: Zn2+ ions stabilized in conventional ion-exchange sites (ZnZs), Zn2+ ions stabilized in cation sites with distantly placed aluminum ions (ZnZd), 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 ZnZd 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 Zn2+ is attached, but O sites of a neighboring ring. Although the reactivity of isolated Zn2+ 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 H2. This process is both thermodynamically and kinetically preferred for isolated Zn2+ 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.