A comprehensive density functional theory study of ethane dehydrogenation over reduced extra-framework gallium species in ZSM-5 zeolite

Abstract

The stability of various gallium species (Ga +, GaH 2 + , and GaH +2) as models for the active sites in reduced Ga/ZSM-5 and the possible reaction paths of alkane dehydrogenation were studied using a density functional theory cluster modeling approach. In general, alkanes are preferentially activated via an “alkyl” mechanism, in which gallium acts as an acceptor of the alkyl group. A comparison of the computed energetics of the various reaction paths for ethane indicates that the catalytic reaction most likely proceeds over Ga +. The initial step of C H activation is the oxidative addition of an alkane molecule to the Ga + cation, which proceeds via an indirect heterolytic mechanism involving the basic oxygen atoms of the zeolite lattice. Although the catalytic reaction can also occur over GaH 2 + and GaH +2 sites, these paths are not favored. Decomposition of GaH 2 + leading to formation of Ga + during the catalytic cycle is more favorable than regeneration of these sites. The reactivity of GaH +2 ions is strongly dependent on the distance between the stabilizing aluminum-occupied oxygen tetrahedra. In cases of greater Al Al distances, the stability of the GaH +2 species is very low, and it decomposes to Ga + and a Brønsted acid site, whereas when Al atoms are located more closely, the charge-compensating GaH +2 ions are the most stable and exhibit the lowest activity for the initial C H bond cleavage reaction.