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)2His1.2·5H2O has been selected for its higher His content and remarkable interlayer spacing of 18.4 A. 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)2His1.2·5H2O 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.