Study of the coordination of Cu2+ in zeolite Y: Interaction with water and ammonia

Abstract

The ligand field spectrum of Cu(II ) exchanged zeolite Y, obtained after saturation with H2O and NH3 and during the gradual desorption of these ligands, was measured by diffuse reflectance spectroscopy (DRS). DFT and ab-initio calculations on several model clusters were performed to interpret the spectra. The structure of the model clusters was optimized by means of density functional theory (DFT), using the B3LYP functional. The electronic spectra of the models were calculated using multiconfigurational perturbation theory based on a CASSCF wavefunction (CASPT2) and compared with the DRS spectra. Firstly, several [Cu(NH3)y(H2O)x]2+ complexes were studied. It was shown that in fully hydrated Cu(II )Y, a [Cu(H2O)6]2+ complex can be present in the cages of the zeolite. In Cu(II )Y, saturated with NH3, the [Cu(NH3)4]2+ complex is present, but the Cu2+ center in this complex must still be coordinated to one or two lattice oxygens in the zeolite. Secondly, calculations were performed on large cluster models, representing the adsorption complexes of one H2O or one NH3 ligand on Cu2+ in the six-ring sites in zeolite Y. The ligand field spectrum of partially dehydrated and deammoniated Cu(II )Y shows d–d transitions at a lower energy than the spectrum of fully dehydrated Cu(II )Y, which is confirmed by the CASPT2 results of the six-ring clusters.