Isomorphously substituted Fe-ZSM-5 zeolites as catalysts: Causes of catalyst ageing as revealed by X-band EPR, Mössbauer and 29Si MAS NMR spectra Original Research Article

An unconstrained curve fitting/parameter estimation program adapted to PC was applied for the deconvolution of X-band EPR spectra of Fe(III) in ZSM-5 (MFI) zeolites. The sub-spectra of framework (FW) and extra-framework (EFW) Fe(III) ions sited in environments of different (ligand) symmetry could be identified. The EPR transition probability, r, of Fe(III) in the little oxide clusters (diads, triads) was 13.75 times larger than that incorporated into the lattice. Increase of the cluster size and ordering of the random structure led to reduction of r. When the co-ordination of Fe(III) ions in the oxide clusters approached the cubic symmetry of Fe(III) in the lattice, r converged to 1.0. The state: r=1.0 is equivalent to the development of a crystalline phase. The experimental determination of r revealed important details of catalyst action. The initial activity in the direct oxidation of benzene to phenol (oxidant: N2O) is due to oxide clusters of random structure (known as “ferrihydrite”). Partial and eventually (almost) complete loss of activity occurs when the clusters get ordered to hematite (or in reducing atmosphere to magnetite, as well). Contrary to fully inactive hematite, magnetite retains about 50% of the original activity because it exposes [Fe(III)]Th–O–[FeIII)]Oh linkages supposed to be responsible for oxidation activity.