Skeletal T-O-T vibrations as a tool for characterization of divalent cation complexation in ferrierite

The effect of several divalent cations (Mn2+, Mg2+, Ni2+, and Co2+) exchanged in Na-ferrierite and ammonia adsorption/desorption was studied by FTIR spectroscopy in the range 4000-400 cm(-1). The complete cation exchange eliminated zeolite bridging acidic hydroxyls and provided for monitoring of the stretching and bending modes of ammonia interacting with cations and the accompanying parallel changes of the zeolite lattice vibrations induced by bare cations and cation-ligand complexes at various ammonia coverage. Dehydration of ferrierites with ion-exchanged divalent cations gives formation of new LR bands in the transmission window between antisymmetric (around 1070 cm(-1)) and symmetric (around 780 cm(-1)) T-O-T stretching lattice modes. These new bands disappear upon full saturation by ammonia and reappear under complete ammonia desorption. They are assigned to antisymmetric stretching T-O-T mode of zeolitic lattice shifted to a lower frequency hue to the reversible local deformation of the flexible zeolitic lattice by the interaction with a bare divalent cation (a lower frequency band, B-0) or with a low, probably 1:1 cation-extraframework ligand complex with ammonia molecule (a higher frequency band, B-I). The extent of perturbation of the ferrierite lattice due to a divalent cation is locally partly decreased by formation of such cation-extraframework ligand complex. By taking the position of the T-O-T antisymmetric band of the parent hydrated Na-ferrierite at 1070 cm(-1) as reference, this relaxation effect, caused by bonding of a guest molecule, was found to amount to about 15-20% of the spectral shift produced by bonding of a bare divalent cation to the framework.