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

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 A g(I)-O interaction potential was parametrized based on ab initio data and i ts 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 t he range 2.32-2.36 Angstrom and coordination to two framework oxygen atoms were found, in good agreement with available EXAFS data. The interaction en ergies of Ag+ with zeolite (127-132 kcal mol(-1) for sites on the channel i ntersection) are about 20 kcal mol(-1) smaller than those found for the Cu/ZSM-5 system. The calculated vertical excitation energies of the T-1 <-- S -0 transition show a strong correlation with the coordination number of the Ag+ ions. Since two-coordinated sites on the channel intersection are stro ngly preferred by the Ag+ ions only a single absorption peak is predicted f or the Ag/ZSM-5 system, in agreement with experimental observation.