Cation-terminated structural clusters as a model for the interpretation of zeolite vibrational spectra

Vibrational spectra have been applied to study the spatial framework of the zeolites containing double 4- and 6-membered rings as characteristic structural units. Using ab initio methods, theoretical vibrational spectra of isolated D4R and D6R units have been calculated. All the units have been terminated by different cations, i.e. K+, Na+ and Zn2+ to balance their negative charge. Based on the results of calculations, normal vibrations of the units have been visualized and the characteristic ring- or pore-opening vibrations, occurring in these units, have been identified. The effect of cation substitution in terminal positions and Al -> Si substitution in tetrahedral positions on the obtained spectra shape has been analyzed. The studied D4R and D6R units constitute the model of secondary building units (SBU) of two important structural zeolite groups. Therefore, the obtained theoretical results have been applied to interpret experimental MIR, FIR and Raman spectra of selected zeolites. In particular, vibrational spectra of zeolite A (LTA), whose structure is composed exclusively of spatially connected D4R units have been analyzed. Spectra of LTA, which was subjected to ion exchange process, have been compared to complete the interpretation procedure. (c) 2006 Elsevier B.V. All rights reserved.