Iron-alumina materials prepared by the non-hydrolytic sol–gel route: Synthesis, characterization and application in hydrocarbons oxidation using hydrogen peroxide as oxidant

Abstract

Novel alumina materials containing FeIII ions were prepared by the non-hydrolytic sol–gel route and applied as heterogeneous catalysts in the oxidation of hydrocarbons (cyclooctene and cyclohexane) by the green oxidant hydrogen peroxide. The synthetic route followed an alkyl halide elimination pathway, via etherolysis/condensation between AlCl3 and diisopropyl ether (iPr2O), in the presence of FeIII ions (FeIII:AlIII:iPr2O molar ratio ∼1:15:30). The obtained xerogel (designated FeAlO-50) was thermally treated at 400, 750, or 1100 °C (samples FeAlO-400, FeAlO-750, and FeAlO-1100, respectively), and the resulting materials were characterized by means of different techniques, such as thermal analyses (TG/DTA), ultraviolet–visible (UV–vis) and infrared (FTIR) absorption spectroscopy, X-ray powder diffraction (XRD) and electron paramagnetic resonance (EPR). The concentration of Brönsted acid sites (BAS) at the surface of the samples was also evaluated by adsorption–desorption of cyclohexylamine. The thermal treatments led to truly heterogeneous catalysts, and the FeAlO-1100 material furnished the highest substrate conversion values. The larger catalytic activity of FeAlO-1100 is probably due to the absence of BAS on its surface. Indeed, the other FeAlO samples presented high BAS concentration, and it is known that H2O2 can be non-productively decomposed in the presence of strong acid sites. The catalytic efficiency of FeAlO-1100 can be considered promising (96% cyclooctene and 22% cyclohexane conversions), while the selectivity cyclohexanone/cyclohexanol achieved in the oxidation of cyclohexane was 1.2 (molar ratio). The catalytic systems obtained here are advantageous because they involve the use of H2O2, a green oxidant, and the temperature employed for the oxidation reactions (50–55 °C) is milder than those reported in the literature for other heterogeneous catalyst/H2O2 systems.