Speciation and siting of divalent transition metal ions in silicon-rich zeolites. An FTIR study

Abstract

Speciation and location of Co2+, Mn2+ and Ni2+ in the extraframework positions of the dehydrated zeolite matrix of ferrierite structure were studied in detail using FTIR spectroscopy of antisymmetric T–O–T vibrations of the zeolite framework. Me2+–ferrierites were prepared by the ion exchange of the NH4– and Na–zeolite forms and by impregnation of the NH4 form. Bare Me2+ occupies all three known cationic sites in dehydrated cationic zeolite. The wavenumbers of bands of individual cations in individual sites were identified. At low Me2+ loadings (Me2+/Al < 0.15), Me2+ replaces two protonic sites and exclusively bare Me2+ is present in dehydrated samples. Sets of such samples were employed for the estimation of extinction coefficients of Co2+, Mn2+ and Ni2+ in cationic sites. These coefficients differ for individual cations but are the same for a cation at different sites. Ion exchange to the NH4 form allows preparation of samples with maximum possible loading of bare Me2+ only for Co2+. In the case of Mn2+, exchange to the Na-parent zeolite or impregnation is required for this purpose while samples with maximum loading by bare Ni2+ can be prepared only by impregnation.