Characterization of Ag+ Sites in ZSM-5: A combined quantum mechanics/interatomic potential function study

Abstract

The interaction of Ag⁺ ions with the ZSM-5 lattice was studied by means of a combined quantum mechanics/interatomic potential function method. A new Ag(I)–O interaction potential was parametrized based on ab initio data and its quality was tested. The Ag⁺ ions preferentially occupy the sites on the intersection of the main and zig-zag channels of ZSM-5. Ag–O distances in the range 2.32–2.36 Å and coordination to two framework oxygen atoms were found, in good agreement with available EXAFS data. The interaction energies of Ag⁺ with zeolite (127–132 kcal mol⁻¹ for sites on the channel intersection) are about 20 kcal mol⁻¹ smaller than those found for the Cu⁺/ZSM-5 system. The calculated vertical excitation energies of the T₁←S₀ transition show a strong correlation with the coordination number of the Ag⁺ ions. Since two-coordinated sites on the channel intersection are strongly preferred by the Ag⁺ ions only a single absorption peak is predicted for the Ag/ZSM-5 system, in agreement with experimental observation.