Evidence for controlled insertion of Fe ions in the framework of clinoptilolite natural zeolites

This work is focused on the spectroscopic characterization of a set of materials obtained by hydrothermal treatments from a natural clinoptilolite zeolite, NZ. OPAZ (orthophosphoric acid treatment on NZ), Fe²⁺-OPAZ (ferrous sulphate acid treatment on OPAZ) and Fe³⁺-OPAZ (ferric sulphate acid treatment on OPAZ) samples were characterized by HRTEM, UV-Vis, temperature dependent EPR and Infra Red spectroscopies in order to describe the structure, nuclearity and distribution of iron ions and iron oxides clusters and particles. Complementary pieces of information were obtained from the different techniques. In particular, HRTEM analysis suggested an effect of the hydrothermal treatments not only on the Fe speciation but also on the zeolite particles morphology and surface. The same technique could not clearly detect the presence of iron oxide phases, suggesting their very small size and high dispersion. A more detailed description of the isolated and clustered species could be in turn obtained by UV-Vis spectroscopy. This was confirmed by temperature dependent EPR spectroscopy, suggesting the presence of nanosized ferromagnetic magnetite-like particles. Finally, IR spectroscopy gave clear cut evidence about the insertion of Fe³⁺ ions in framework positions, with consequent generation of distinct Bronsted acidity, after hydrothermal treatments with iron salts. This transformation could be explained through the reaction with defective hydroxyl nests formed by mildly acid hydrothermal treatment with isolated Fe³⁺ ions.