Fe/ZSM-5 Prepared by Sublimation of FeCl3: The Structure of the Fe Species as Determined by IR, 27Al MAS NMR, and EXAFS Spectroscopy

The state of the iron in two different Fe/ZSM-5 samples prepared by sublimation of FeCl3 was investigated by EXAFS, IR, 27Al MAS NMR, XRD, and nitrogen adsorption measurements. In one Fe/ZSM-5 (Fe/Al=1) sample, EXAFS revealed for the first time the presence of diferric (hydr)oxo-bridged binuclear clusters, whose structures differ from those postulated in the literature, resembling that of the methane monooxygenase enzyme. IR showed that binuclear Fe complexes are located at the ion-exchange positions of the zeolite, compensating one or two lattice charges. The remainder of the charge-compensating sites are Brønsted hydroxyls. On both zeolites, the NMR detection of the framework Al atoms (54 ppm) is strongly perturbed by the paramagnetic effects induced by the Fe ions. The intensity of this peak parallels that of the Brønsted hydroxyls in the IR spectra, thus reflecting the presence of Fe species at ion-exchange positions. In a second Fe/ZSM-5 (Fe/Al=0.8) sample, the iron was present predominantly in the form of large hematite particles (EXAFS, XRD), although a minor fraction of binuclear species might be present as well. The formation of different species seems to be related to different hydrolysis processes occurring on the two zeolites upon washing of the preparation after the sublimation of FeCl3. It is also suggested that the final state of the Fe depends on the presence of extraframework Al species as well as the crystallite size of the zeolite used.