Preparation and characterisation of histidine– and iron–histidine–α-zirconium phosphate intercalation compounds. Catalytic behaviour of the iron derivatives in oxidation reactions with H2O2

Abstract

The amino acid histidine (His) easily diffuses into layered α-zirconium phosphate if the layers are first pre-swelled via preparation of a metastable ethanol intercalate α-Zr(HPO₄)₂·2EtOH. Different materials are obtained by varying the contact time and the His solution concentration. A new compound of composition α-Zr(HPO₄)₂His_1.2·5H₂O has been selected for its higher His content and remarkable interlayer spacing of 18.4 Å. The FT-IR spectra of this compound show that His interacts with the P–OH groups of the host through the NH₂ primary group of the amino acid. Zr(HPO₄)₂His_1.2·5H₂O takes up Fe(II) and Fe(III) ions from the respective iron sulfate solutions by ion-exchange. This process, accompanied by a partial release of His to the contact solutions, gives rise to intercalates in which the metal ion∶His molar ratios are close to 1∶1. The reflectance spectra account for the presence, in the interlayer region, of a Feⁿ⁺∶His = 1∶1 coordinated species. The iron–histidine intercalation compounds have been tested as catalysts in the oxidation of an organic substrate, Indigo Carmine, by H₂O₂ and the results compared with those obtained with the H₂O₂ solution alone, with Fenton's and Ruff's reagents (a H₂O₂ aqueous solution containing respectively iron(II) or iron(III) ions as catalysts) or with H₂O₂ solutions containing the Feⁿ⁺–His (1∶1) complex species. The solid catalysts, recovered after reaction, have been re-used for further runs.