The siting of Cu(II) in mordenite: a theoretical spectroscopic study

Abstract

The siting of Cu(II) in mordenite has been studied by ab initio calculations on large cluster models, representing the cation exchange sites in mordenite. Partial geometry optimizations, based on density functional theory (DFT), were performed to obtain the structure of the coordination environment of Cu(II) at the different sites. The ligand field spectra and EPR g-tensors of these clusters were then calculated by means of multiconfigurational perturbation theory (CASPT2). The calculated results were compared with experimental information, obtained by diffuse reflectance spectroscopy (DRS) and EPR. The calculations indicate that at low exchange levels Cu( II) is coordinated to oxygen six-rings in the main channel of mordenite, in the presence of two aluminiums. At higher loadings, six- or five-rings containing only one aluminium also become occupied, where Cu(II) is coordinated as a single ion, not as (CuOH)(+). The calculations indicate also that in fully dehydrated mordenite, the twisted eight-ring (site A) is not occupied by Cu(II).