Application of IR spectra in the studies of zeolites from D4R and D6R structural groups

This work presents results of FT-IR spectroscopic studies (MIR and FIR range) of zeolites (belonging to D4R and D6R structural groups) after heavy metal cations (Ag+, Cu2+, Cd2+, Pb2+, Zn2+, Ni2+ and Cr3+) immobilization. Sorption has been conducted on monoionic forms of synthetic zeolite A (LTA) and Y (FAU) as well as on natural chabazite (CHA).It was proven that those cations’ sorption causes changes in the spectra of the zeolites. Changes in intensities and positions of the bands corresponding to the characteristic ring and MeO vibrations have been observed. These rings occur in pseudomolecular complexes (built of alumino- and silicooxygen tetrahedra) which constitute the secondary building units (SBU) and form spatial framework of the zeolite. Based on the results of DFT method (B98 HF 6–31G(d)) calculations, normal vibrations of the SBU terminated by different cations (Na+, K+) have been visualized. Obtained results have been applied for interpretation of experimental spectra of selected zeolites. The most significant changes have been determined in the region of pseudolattice vibrations (800–550 cm−1). On the other hand, an introduction of heavy metal cations into the structure of zeolites has caused the considerable modification of bands due to MeO vibrations in FIR spectra (below 400 cm−1).In both cases systematic changes connected with the type of cation (its chemical character) have been revealed. Based on changes in the spectra due to immobilization of heavy metal ions, a scale of sorption can be established.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► We examined the changes in zeolite framework based on the computed spectra of SBU. ► It is possible to assign the bands to individual normal modes. ► We applied computed spectra to interpretation process of experimental once. ► We determined an influence of non-tetrahedral cations on IR spectra of zeolites. ► Changes of the bands due to ring vibrations after ion exchange are observed.