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+) ex changed in Na-ferrierite and ammonia adsorption/desorption was studied by FTIR spectroscopy in the range 4000-400 cm(-1). The complete catio n exchange eliminated zeolite bridging acidic hydroxyls and provided f or monitoring of the stretching and bending modes of ammonia interacti ng with cations and the accompanying parallel changes of the zeolite l attice vibrations induced by bare cations and cation-ligand complexes at various ammonia coverage. Dehydration of ferrierites with ion-excha nged divalent cations gives formation of new LR bands in the transmiss ion window between antisymmetric (around 1070 cm(-1)) and symmetric (a round 780 cm(-1)) T-O-T stretching lattice modes. These new bands disa ppear upon full saturation by ammonia and reappear under complete ammo nia desorption. They are assigned to antisymmetric stretching T-O-T mo de of zeolitic lattice shifted to a lower frequency hue to the reversi ble local deformation of the flexible zeolitic lattice by the interact ion 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 dec reased by formation of such cation-extraframework ligand complex. By t aking the position of the T-O-T antisymmetric band of the parent hydra ted Na-ferrierite at 1070 cm(-1) as reference, this relaxation effect, caused by bonding of a guest molecule, was found to amount to about 1 5-20% of the spectral shift produced by bonding of a bare divalent cat ion to the framework.