Heat-treatment of isomorphously substituted ZSM-5 (MFI) zeolites: An ESR and Mössbauer spectroscopy and kinetic study Original Research Article

Fe-ZSM-5 zeolites comprise of three types (I, II and III) of iron (Fe3+) sites in both as synthesized (AS) and heat-treated states. Previous evidence could be confirmed by the assignment of ESR lines, registered in the X-band, stating: (i) type I framework Fe3+ sites (in Th oxygen coordination), where charge compensation occurs mainly by Na+ ions and the crystal field (cf) approximates cubic symmetry, produce resonances at near to g = 2.00; (ii) type II and probably type III binuclear Fe...Fe dioxo- and oxo-bridges manifest themselves in producing slightly distorted surroundings of axial symmetry, and the relevant powder-averaged subspectrum is superimposed on the previous one in the g ≅ 2.45–1.98 interval. As the temperature of the heat-treatment (HT) is raised, type II and III sites will be annihilated with preference, under concomitant production of amorphous Fe2O3 possessing molecular dispersity. (iii) The weak ESR signal (2–4%) at g ≅ 4.27 is attributed to Th coordinated framework sites in the surface layers (≈ 1 u.c. thick) under the influence of solid surface tension, giving rise to fully rhombic cf symmetry.The size of the ejected Fe2O3 particles, as estimated from Mössbauer spectra (no hyperfine structure at 77 K) is less than 2.9 nm. The extremely large electron affinity of Fe3+ ions manifests itself in decreased ionicity (and unexpectedly dominant cubic symmetry), activity in biomimetic oxidations, autoreduction and spontaneous reoxidation etc.