[Zurück]

E. Uzunova, H. Mikosch:
"Adsorption and Activation of Ethene in Transition Metal Exchanged Zeolite Clinoptilolite: a Density Functional Study";
ACS Catalysis, 3 (2013), 12; S. 2759 - 2767.

Ethene adsorption in transition-metal exchanged clinoptilolite with cations of the d-elements
(Fe-Cu) and also Pd, was examined by density functional theory with the B3LYP functional,
using the embedded cluster method in ONIOM. The preferred extraframework cation sites for
the divalent cations are those in the large channel A. The monovalent cations Ni+ and Cu+ show little preference towards the available cation sites: they approach a center of negative framework charge, forming shorter M-O bonds as compared to the divalent cations. Periodic model calculations were applied to validate the extraframework site preference of Cu+ cations and they confirm the ONIOM results, though periodic calculations systematically predict smaller energy gaps between the different cation site occupancies. A dominant component in the formation of the metal cation-adsorbate π-complexes is the electron charge transfer from the filled d-orbitals of the transition metal cations to the unoccupied π* orbitals of ethene. Significant contribution of the framework oxygen atoms as electron-donors to the cations was revealed. This results in lengthening of the C=C bond and a red shift of the corresponding stretching vibration, which is most pronounced in the adsorption complexes with monovalent cations, (Ni+, Cu+, Pd+). The hydrogen atoms in the ethene molecule become non-equivalent upon adsorption. The C-H bond lengthening is more significant for the adsorption complexes in Channel B and on Ni+ cations in Channel A. The deformation density maps, derived from the B3LYP calculated densities provide insight into the role of the framework in the charge-transfer from the metal cations to the ethene molecule.

Electronic Structure; Density Functional Calculations; Natural Zeolite; ONIOM, Clinoptilolite, Ethene adsorption

http://dx.doi.org/10.1021/cs4005997