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 H-2 dissociation and which for the recombination and desorption of H-2. The latter processes, crucial for the dehydrogenation of alkanes over such zeolites, are assumed to involve Zn species. We described heterolytically dissociated H-2 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-H bond of such Zn2+ complexes, 1935 and 1943 cm(-1), agree well with experimental values for dissociatively adsorbed H-2 at Zn-containing H-ZSM-5 zeolite, 1934-1936 cm(-1). Due to the preference for H-2 dissociation, bare Zn2+ species in zeolite are not considered as catalytic sites for H-2 recombination. However, if an additional OH- or H2O ligand is coordinated at a Zn2+ center, H-2 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. (c) 2006 Elsevier B.V. All rights reserved.