IRON-SUBSTITUTED BETA-MOLECULAR SIEVE - SYNTHESIS AND CHARACTERIZATION

A synthesis of crystalline ferrisilicate having zeolite BEA topology (Fe-Beta) and containing significant quantities of iron in the framework (22 Fe atoms per unit cell based on 192 T-atoms per unit cell) has been reported. The synthesis of Fe-Beta was carried out using tetraethylammonium hydroxide (TEAOH), 25% methanolic instead of an aqueous solution, as a source of organic template. X-Ray powder diffraction (XRD) and scanning electron micrography (SEM) were used to check the structural identity and phase purity of the Fe-Beta samples. A thermogravimetric analysis (TGA) study showed that the amount of TEAOH ions interacting with Fe-Beta framework increases with increase in the iron content of the sample. The Fe ions in Fe-Beta were in the trivalent oxidation state which was confirmed by an X-ray photoelectron spectroscopy (XPS) study. A comparison of Si/Fe ratios determined by chemical analysis and XPS method allowed us to conclude that Fe3+ species are distributed homogeneously from the bulk to the surface of Fe-Beta crystallites. However, the O1s XP spectra indicated the presence of small amounts of extraframework iron oxidic impurities in as-synthesized as well as calcined samples. The infrared (IR) spectra showed three types of hydroxyl groups in Fe-Beta located at 3740, 3670 and a very broad band at approximately 3540 cm-1. The band at 3680 cm-1 was found to be affected by pyridine adsorption and therefore was assigned to structural bridging hydroxyl groups (Si-OH-Fe) formed by substitution of iron in the zeolite framework. The shift in the IR OH stretching frequency towards higher wave number and the desorption of NH3 at lower temperature for Fe-Beta samples as compared to Al-Beta zeolite allowed us to conclude that the strength of Bronsted acid sites in Fe-Beta materials is lower than those Bronsted acids sites in Al-Beta zeolites.