Heterolytic dissociation and recombination of H₂ over Zn,H-ZSM-5 zeolites-A density functional model study

This computational study aims at clarifying which type of cationic Zn species in Zn,H-ZSM-5 zeolite are suitable catalytic sites for H₂ dissociation and which for the recombination and desorption of H₂. The latter processes, crucial for the dehydrogenation of alkanes over such zeolites, are assumed to involve Zn species. We described heterolytically dissociated H₂ on four types of zeolite-supported Zn species by applying a density functional method to suitable cluster models. We determined the dissociation of hydrogen on Zn²⁺ species to be exothermic (by 14 and 30 kJ/mol, depending on the model). Corroborating this result, the calculated vibrational frequencies of the Zn{single bond}H bond of such Zn²⁺ complexes, 1935 and 1943 cm^-1, agree well with experimental values for dissociatively adsorbed H₂ at Zn-containing H-ZSM-5 zeolite, 1934-1936 cm^-1. Due to the preference for H₂ dissociation, bare Zn²⁺ species in zeolite are not considered as catalytic sites for H₂ recombination. However, if an additional OH^- or H₂O ligand is coordinated at a Zn²⁺ center, H₂ recombination becomes exothermic. Thus, Zn(H₂O)²⁺ and ZnOH⁺ species in Zn-exchanged zeolites are suggested to be involved in the dehydrogenation of alkanes over Zn,H-ZSM-5.