A theoretical study of adsorption of carbon monoxide on Ag-ZSM-5 zeolite

Adsorption of carbonmonoxide on the silver ion-exchanged Ag-ZSM-5 zeolite has been studied using both quantum cluster and embedded cluster at the models B3LYP/6-31G(d,p) level of theory. The Ag...Oz distance in Ag-ZSM-5 complexes is predicted to be 2.26 Å for the quantum cluster and 2.32 Å for the embedded cluster, the latter is in agreement with the experimental value of 2.30 + 0.03 Å. The optimized Ag...CO distances in the Ag-ZSM5 are evaluated to be 2.04 and 2.07 Å for quantum and embedded clusters, respectively. The corresponding CO stretching mode in the Ag-ZSM-5/CO is calculated to be 2157 cm for the quantum cluster and shifted to 2171 cm when the zeolite lattice framework is taken into account ,which is close to the observed values by FITR (2190-2192 cm). Similar trends were observed for the Cu-ZSM-5/CO complexes: Cu-Oz = 2.00 (2.00 + 0.02) Å , Cu-CO = 1.85 (1.89) Å , ∆Eads = -33.56 (-28.7) kcal/mol; values in parentheses are those obtained from experimental results. For the Ag-ZSM-5/CO complex, ∆Eads amounts to 60% of the adsorption energy value for the Cu-ZSM-5/CO complex which is due to the lengthened Ag...CO distance as compared to the Cu...CO distance in the Cu-complex. The results obtained for the Ag-ZSM-5 complex indicates that correction of the long-range electrostatic potential of the zeolitic crystal in the quantum cluster is found to yield reliable results when compared to the experiment observation.