Sol-gel synthesis, characterization and catalytic properties of Fe-Ti mixed oxides

Fe2O3-TiO2 solid acid catalysts have been prepared by a sol–gel route, characterized and tested in the isomerization of a-pinene oxide. The textural, morphological and microstructural characterization of the catalysts was carried out by BET surface area measurements, X-ray diffraction (XRD) and scanning electron microscopy with elemental X-ray analysis (SEM-EDX). Characterization data have shown that, on the pure TiO2 catalyst, titania is mainly in the anatase phase. On the contrary the rutile phase and/or Fe2TiO5, amorphous iron–titanium mixed oxides and hematite were found on the Fe-promoted TiO2 catalysts, depending on the Fe content. Temperature programmed desorption (TPD), microcalorimetry and FT-IR spectroscopy of adsorbed probe basic molecules (ammonia or pyridine) have shown that addition of iron to titania favours formation of new acid sites having a stronger Lewis character. Fe2O3-TiO2 catalysts show a higher activity compared to pure titania in the isomerization of a-pinene oxide to campholenic aldehyde. The products distribution was also influenced by the composition of the catalysts. A correlation between the selectivity to campholenic aldehyde and the amount of Lewis acid sites on the catalysts was found. Based on the characterization and catalytic results, Fe–Ti–O mixed structures were proposed to be the active sites for the selective isomerization of a-pinene oxide to campholenic aldehyde.