An in situ Fourier transform infrared study of zeolitic vibrations : dehydration, deammoniation, and reammoniation of ion-exchanged Y zeolites

Dehydration, deammoniation, and reammoniation of ion-exchanged Y zeolites are studied in situ with FTi.r. spectroscopy in the range 40-4000 cm-1. This broad spectroscopic range enables the monitoring of the cationic vibrations, the zeolitic lattice modes, the bending and stretching modes of ammonium ions, and adsorbed water molecules and the stretching modes of the acidic hydroxyls under experimental conditions. Dehydration of zeolites with monovalent cations does not lead to significant migration of these cations and only minor changes are observed for the lattice modes. Dehydration of zeolites with divalent cations results in a migration of these cations into the hexagonal prisms, giving rise to large changes for the zeolitic lattice modes. Deammoniation results in the formation of acidic hydroxyls by protonation of the lattice, which strongly affects the zeolitic lattice modes. The extent of the changes for the lattice modes is related to the amount of protons present. The observed changes for the lattice modes can be explained by an increase in electrostatic interactions between the cations and the zeolitic framework, by deformations of the flexible zeolitic lattice, and by coupling of the lattice modes with the in-plane and out-of-plane bending modes of the acidic bridging hydroxyl groups