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 ZnH 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.

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