Theoretical investigation of ethane dehydrogenation on cationic Zn species in ZSM-5 zeolites—The second Al center in vicinity of the cation is essential for the accomplishment of the complete catalytic cycle

Ethane dehydrogenation on Zn,H-ZSM-5 was modeled by density functional calculations on isolated zeolite fragment. Three types of cationic Zn species, that are assumed to exist in the samples prepared by ion exchange, were considered as active sites for the process: Zn2+ in the vicinity of paired Al centers as well as ZnOH+ and ZnH+ in the vicinity of one isolated Al site. For all modeled mechanisms and active sites we accounted for the influence of the entropy. The results suggest that the process on Zn2+ cation at paired Al site has the lowest free energy of activation. The calculations also highlight the essential role of the second Al center in vicinity of the zinc cation for the reaction since it generates basic framework oxygen center participating in the reaction steps with lowest activation barrier. In absence of a second Al center close to the zinc species (as in the models with isolated Al sites) the reaction is blocked due to formation of rather stable intermediates ZnH+ or Zn(C2H5)+ that cannot be decomposed due to the too high activation barriers on the next stages of the dehydrogenation process.