Siting and Distribution of the Co Ions in Beta Zeolite: A UV–Vis–NIR and FTIR Study

The Co(II) ion sites in dehydrated beta zeolites, consisting of polymorphs A, B, and C, were identified using an approach based on a similarity between visible spectra of the Co(II) ions in mordenite, ferrierite ZSM-5, and beta zeolites, and similarity in local framework structures accommodating the Co(II) ions in these zeolites. The results were supported by FTIR spectra in the region of skeletal vibrations showing the characteristic shifts of framework T–O–T vibrations induced by the framework perturbations due to the Co(II) ions at cationic sites. Three different cationic sites accommodating divalent metal ions in beta zeolite were found. Their positions in the channel system and local framework arrangements in different polymorphs of beta zeolites were suggested. The α-site is formed in polymorph C of the beta structure by an elongated six-member ring composed of twofold connected five-member rings. The Co(II) ions at the α-site exhibit an absorption band at 14,600 cm−1 in the visible spectrum and a band at 901 cm−1 in the IR spectra. The β-site present in polymorphs A and B is formed by a planar deformed six-member ring of the hexagonal cage. The Co(II) ions at the β-site are characterized by four bands, at 15,500, 16,300, 17,570, and 21,700 cm−1 in the visible spectrum and by a band at 918 cm−1 in IR spectra. The γ-site occurring in polymorphs A and B of the beta structure is located inside the hexagonal cage. The Co ions at the γ-site are reflected in a doublet, at 18,500 and 20,100 cm−1 in the visible spectra.