Density Functional Theory Study of CO Adsorption on Cu(I)-ZSM-5

The coordination and local geometry of Cu+ cations in Cu(I)-ZSM-5 and the adsorption of CO to such cations were explored using density functional theory. A thorough examination was made of the effects of placing Cu+ in each of the cation-exchange sites available in the MFI lattice, the location of the Al atom in the site, and the number of Al atoms in the site. Cu+ cations in I2 exchange sites, located at the edge of the main and sinusoidal channels, are coordinated to two framework O atoms before and after CO adsorption. The calculated adsorption energy for CO adsorbed on such cations lies between 30 and 33 kcal/mol, and the C−O vibrational frequency lies between 2150 and 2158 cm-1. Cu+ associated with five- or six-membered rings, i.e., M5, M6, Z5, and Z6 exchange sites, located in the main and sinusoidal channels of the zeolite, are 3-fold coordinated to framework O atoms, but the coordination number can decrease to 2 upon CO adsorption. The calculated CO adsorption energy of Cu+ in such sites is in the range of 25−28 kcal/mol. Cu+ cations located in the basket structures formed by two fused five-membered rings, in M7 sites, bind CO with a calculated adsorption energy of 21 kcal/mol. The results of this study indicate that the presence of Cu+ cations in different exchange sites cannot be identified on the basis of infrared spectra of adsorbed CO and would be difficult to identify by temperature-programmed desorption of adsorbed CO. Evidence for two types of Cu+ coordination can be obtained, though, from Cu K-edge extended X-ray absorption fine structure data. It is also shown that changes in Cu+ coordination occurring upon CO adsorption can be observed by Cu K-edge X-ray absorption near-edge structure data.