Comparison of the structural properties of isomorphously substituted Fe in mordenite zeolites prepared by different methods.

Fe was introduced in mordenite zeolite by means of ion exchange either in solid or in liquid state. The iron loading (50--200 wt%), iron precursor (FeSO4.7H2O and FeCl3), and mordenite starting material (NH4M, HM, and NaM) were varied during the exchange processes. The Fe species were characterized by N2 adsorption measurements as well as by XRD and Mössbauer spectroscopies. The Fe-mordenite samples prepared by liquid-state ion exchange attained remarkable Fe dispersion and surface areas higher than those of the parent. It was found that Fe3+ ions, which substituted the framework Al and accordingly occupied tetrahedral sites, were decreased with Fe loadings with concomitant increase in Fe3+-occupied octahedral sites. The latter sites disappeared at 20 K to provoke the superparamagnetic alpha-Fe2O3 in different particles size. The acid leaching (0.1 M HCl, 333 K, 3 h) of the samples showed the disappearance of the most highly distorted extra-framework Fe3+ species, providing an indication of their presence on the external surface. On the other hand, a hematite phase was detected in the solid-state ion exchange of FeCl3 with either HM or NH4M at the loading of 100% Fe. More correlations between Mössbauer data on one hand and XRD and texturing properties on the other hand were evaluated and discussed.