Determination of Zn Speciation, Siting, and Distribution in Ferrierite Using Luminescence and FTIR Spectroscopy

Abstract

A methodology for the analysis of the siting of Zn2+ ions in extra-framework cationic sites of silicon-rich zeolite was developed and demonstrated on Zn-ferrierite samples (Si/Al 8.5, Zn/Al 0.04-0.33). This methodology is based on the FTIR spectroscopy of antisymmetric tetrahedral-octahedral-tetrahedral (T-O-T) vibrations of the zeolite framework perturbed by Zn2+ ions, combined with a complementary approach based on Zn2+ luminescence monitored by laser-induced time-resolved luminescence spectroscopy. Three vibration bands in the FTIR spectra of antisymmetric T-O-T vibrations of dehydrated zeolite reflect Zn2+ ligation in three extra-framework cationic sites of ferrierite zeolite. The band at 935 cm(-1) corresponds to Zn2+ ions in the alpha-site of ferrierite, the band at 917 cm(-1) correspondsto ions in beta-site, and the band at 902 cm(-1) correspondsto Zn2+ ions in the.-site. The extinction coefficient for quantitative analysis of Zn2+ ions in cationic sites was estimated and exhibited the same value for Zn2+ cations in all cationic sites, epsilon = 49.1 +/- 3.8 cm.mu mol(-1). In all Zn samples, Zn2+ siting in the beta-site prevails, while Zn2+ ions in the gamma-site are of low population or negligible. Time-resolved luminescence showed that bare Zn2+ ions in the extra-framework cationic sites can be distinguished from Zn2+ ions in ZnO by a decay time which is several magnitudes longer and a high sensitivity for quenching. The luminescence spectrum of Zn-ferrierites is composed of three bands at 545, 480, and 425 nm attributable to Zn2+ ions in the alpha-, beta-, and gamma-site with luminescence coefficients (for semiquantitative analysis) zeta(alpha) = 10.1, zeta(beta) = 9.4, and zeta(gamma) = 8.8 mmol/g of Zn2+ ions in the alpha-, beta-, and gamma-sites, respectively. The analysis of Zn ions in ferrierites showed that ZnH-ferrierites exclusively contain Zn2+ ions in cationic sites. In the case of the ZnNa-FER sample with maximum Zn loading (Zn/Al 0.33), a small amount of Zn-oxo species can be formed.