Isomorphous substitution of Fe³⁺ in the framework of aluminosilicate mordenite by hydrothermal synthesis

Fe³⁺ framework-substituted mordenites have been hydrothermally synthesized under template-free conditions using the gel compositions with SiO₂/(Al₂O₃ + Fe₂O₃) molar ratio of 20, 15 and 10 where Fe₂O₃/Al₂O₃ molar ratio was varied from 0 to 1000. Crystalline materials with Fe incorporated into the framework have been obtained from starting gels containing up to 0.5, 0.63 and 0.75 mol fraction of Al₂O₃ replaced by Fe₂O₃ at SiO₂/(Al₂O₃ + Fe₂O₃) ratio of 20, 15 and 10, respectively. Using gels with lower SiO₂/(Al₂O₃ + Fe₂O₃) ratio has extended the maximum substitution level of Fe for Al. XRD, UV visible. IR spectroscopy and chemical analysis have provided evidence for the presence of framework Fe. When Al atoms were gradually replaced by Fe, unit cell parameters increased and an asymmetric stretching vibration of Si-O-T was shifted to lower wavenumbers. A new band around 3630 cm^-1 due to Si(OH)Fe groups observed in the IR spectra of Fe-containing mordenite also supports the framework incorporation of Fe.