Theoretical study on reactions catalyzed by gallium-substituted zeolites

The dehydrogenation and cracking reactions of light alkanes in gallium-containing zeolites were studied using density functional theory. Gallium isomorphically substituted, generating Brønsted acid sites, was used in the computations. The following reactions were examined: dehydrogenation of methane, ethane, propane, isobutane and cracking of ethane, propane and isobutene, all catalyzed by the framework gallium species. The cracking reaction seems to be favored relative to the dehydrogenation when framework gallium species are used. This behavior is also observed in aluminum-containing zeolites (H-ZSM5). The geometries and energetics of the transition states found for the gallium zeolites were compared with theoretical data for the same transition states in aluminum zeolites. There seems to be no significant difference between framework gallium and framework aluminum species. Therefore the framework gallium should not be the species responsible for the catalytic enhancement observed in gallium-containing zeolites.From the Proceedings of the 28th Congreso de Químicos Teóricos de Expresión Latina (QUITEL 2002)