Iron-substituted Beta molecular sieve: Synthesis and characterization

Abstract

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 Fe 3+ species are distributed homogeneously from the bulk to the surface of Fe-Beta crystallites. However, the O 1s 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 ∼ 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 NH 3 at lower temperature for Fe-Beta samples as compared to Al-Beta zeolite allowed us to conclude that the strength of Brönsted acid sites in Fe-Beta materials is lower than those Brönsted acids sites in Al-Beta zeolites.