Characterization and temperature evolution of iron-containing species in HZSM-5 zeolite prepared from different iron sources

Abstract

Iron-containing HZSM-5 zeolites are materials with important industrial applications as catalysts. Their characterization is difficult due to the various possible Fe-containing species which can exist in the pores of HZSM-5 zeolites and their dependence on the preparation technique. Three Fe–HZSM-5 samples were prepared by ion-exchange technique using different iron precursors: containing only Fe²⁺ or Fe³⁺ ions, and containing equimolar mixture of Fe²⁺ and Fe³⁺ ions. The samples were characterized by various experimental techniques (XRD, FTIR, UV–Vis spectroscopy and XANES/EXAFS) in order to clarify the type of the Fe-containing species existing in the samples. Periodic density functional calculations were also performed to help in elucidation of the obtained structural information with the EXAFS data and in clarification of the relative stability of the various Fe-containing species in the pores of the HZSM-5 zeolites. In the samples prepared with only Fe²⁺ or Fe³⁺ ions dominate isolated iron species but binuclear FeOFe²⁺ species and small iron oxide clusters are also present. In the third sample, prepared from a precursor containing iron ions in both oxidation states, most of the iron is included in the iron oxide clusters or small nanoparticles. The time-resolved XAS for the sample containing equimolar Fe²⁺ and Fe³⁺ ions revealed existence of two types of dominant iron oxide species—small oligonuclear clusters in the temperature region 100–300 °C and larger more oxidized moieties (nanoparticles) after heating the sample up to 400 and 500 °C.