Synthesis and Characterization of Mixed Fe–Cr Oxide Pillared α-Zirconium Phosphate Materials

Abstract

Mixed Fe/Cr hydroxyacetate oligomers ranging in composition from 90/10 to 10/90 have been prepared by mixing Fe3+and Cr3+nitrate solutions and then addingn-propylammonium acetate up to an acetate/Cr3+ratio of 2.8 and pH 4. The oligomers were intercalated into colloidal α-zirconium phosphate or precipitated with excessn-propylamine. The precipitates are all amorphous and show differential thermal behavior in comparison with the intercalates. Upon calcination in air, the precipitates become crystalline, showing the characteristic XRD peaks of mixed Fe/Cr oxides. These mixed oxides present unusually high BET surface areas (up to 276 m2· g−1), especially those with low Fe/Cr ratios. The intercalates are poorly crystalline or amorphous at room temperature and upon calcination at 400°C in air or under N2, which indicates that there is no oxide segregation from the interlayers. From these results it is suggested that mixed Fe/Cr oxides are cross-linked to the phosphate layer, forming a pillared structure. Significantly, the mixed oxides segregated from the phosphate surface at 1000°C present XRD patterns identical to the mixed oxides obtained from the precipitates with the same Fe/Cr ratios. The pillared materials show high BET surface areas, from 200 to 306 m2/g, and narrow pore size distributions with pore radius ranging from 8.5 to 13.8 Å. XPS analysis reveals a higher affinity of the phosphate surface for Cr3+at high Fe/Cr ratios from 70/30 to 90/10. In these samples the partial oxidation of Cr(III) to Cr(VI) was observed in the XPS spectra, in spite of the materials being calcined under N2.