Characterization and temperature evolution of iron-containing species in HZSM-5 zeolite prepared from different iron sources
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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.
KeywordsDFT Density functional modeling Zeolite EXAFS Quantum-chemical calculations UV–Vis
NS, SB, WP, SL, and JW acknowledge the financial support by the Thailand Research Fund (TRF MRG5480049), Center of Excellence for Innovation in Chemistry (PERCH-CIC), Khon Kaen University, and Beamline 2.2, Synchrotron Light Research Institute (SLRI), Thailand. IZK, HAA, and GNV gratefully acknowledge the support by the Horizon 2020 program of the European Union (project Materials Networking - grant agreement 692146, and COST Action MP1306) and computational resources provided by the Bulgarian supercomputer Avitohol. HAA and IZK acknowledge financial support by the Bulgarian Science Fund (project DCOST01/18).
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