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 cleavag...