Analysis of Fe species in zeolites by UV-VIS-NIR, IR spectra and voltammetry. Effect of preparation, Fe loading and zeolite type

Three procedures were employed for the preparation of Fe-zeolites with ZSM-5 (MFI), ferrierite (FER) and beta (BEA) structures: ion exchange from FeCl3 solution in acetyl acetonate and solid-state ion exchange from FeCl2 using an oxygen or nitrogen stream. A combination of UV-VIS-NIR spectra, IR spectra of skeletal vibrations and of adsorbed NO, as well as voltammetry provided information on the type of Fe species introduced. Single Fe(III) ion complexes (Fe(H2O)6−xOHx) in hydrated zeolites were reflected in the charge-transfer bands at 33 100, 37 300 and 45 600 cm−1. The single Fe(II) ions at cationic sites in evacuated zeolites yielded (through perturbation of framework T-O bonds) characteristic bands (910-950 cm−1) in the region of the skeletal window. These Fe(II) ions with adsorbed NO were also reflected in vibrations at 1880 cm−1. Dinuclear Fe-oxo complexes yielded the Vis band at 28 200 cm−1. Voltammetry indicated the presence of Fe oxides (hematite) through the reduction peak at −0.7 V. Such oxide-like species were also reflected in the absorption edge at 19 800 cm−1, and a doublet at 11 000 and 11 800 cm−1 in the Vis spectra. Fe(II)-NO vibrations at 1840, 1810 and 1760 cm−1 belonged to the undefined exposed Fe cations, probably originating from supported oxides. Using an ion exchange procedure, employing FeCl3 in acetyl acetonate, exclusively Fe ions at cationic sites could be introduced at low concentrations (Fe/Al < 0.1). At higher Fe loadings, dinuclear Fe-oxo complexes were formed preferably in Fe-ZSM-5, but were absent in Fe-beta. Exclusively single Fe species could not be prepared at Fe concentrations above Fe/Al > 0.2; all three types of Fe species, single Fe ions, dinuclear Fe-oxo complexes and Fe oxides were formed.