Electronic view on ethene adsorption in Cu(I) exchanged zeolites

Ethene adsorption on isolated Cu(I) sites in two types of zeolites (faujasite and MFI) is investigated by means of the embedded cluster method. Structures, energetic stabilities and C=C stretching vibrations in adsorption complexes are discussed. Furthermore, for interpretative purposes, the interaction energies are decomposed, using novel approaches based on so called natural orbitals for chemical valence. Ethene is always symmetrically bound to Cu(I) ion by both C atoms. In some cases two local minima of similar stability on the potential energy surface, differing by Cu(I) site relaxation can be found that may be simultaneously populated in equilibrium. Binding energies usually decrease with the degree of reconstruction of Cu(I) site after adsorption, however, in particular cases, a more distorted structure can be slightly more stable if favorable pi* back donation overwhelms the distortion effects. Calculated values of binding energies for Cu(I)-Y zeolite (about 80 kJ mol(-1)) agree well with microcalorimetric data. We predict that ethene binding in MFI is over two times stronger (to the best of our knowledge no experimental data are available). The C=C stretching frequency is not site specific, but depends only on the type of copper connectivity to oxygen nodes. The appearance of two C=C bands in IR spectra of Cu(I)-faujasite can be explained as the effect of coexistence of two types of adsorption complexes, with Cu(I) coordinated to one or two framework tetrahedrons, respectively. In Cu(I)-MFI, only one type of adsorption complex with Cu(I) ion coordinated to a single tetrahedron exists, as only a single C=C band is present in IR spectra.