One-step solvothermal synthesis of Al-promoted Fe3O4 magnetic catalysts for the selective oxidation of benzyl alcohol to benzaldehyde with H2O2 in water

Abstract

Aluminium (Al)-promoted Fe₃O₄ magnetic catalysts were prepared by in situ solvothermal synthesis in ethylene glycol and characterized by XRD, SEM, XPS, BET and VSM techniques. The Al promoter significantly enhanced the catalytic activity of Fe₃O₄ microspheres in the selective oxidation of benzyl alcohol to benzaldehyde with H₂O₂ in water (yield: 5.6% vs. 43.7% before and after the addition of 1.42 wt% of Al, respectively). The effects of Al source and content, catalyst loading, H₂O₂ dosage, reaction temperature and time on the catalytic performance of Al-promoted Fe₃O₄ were also investigated. The Al-promoted Fe₃O₄ catalysts were magnetically recoverable and reusable at least five times without loss of catalytic activity and selectivity after being rejuvenated with NaBH₄ reduction. Theoretical and experimental investigations confirmed that the Al promoter introduced new Al active sites besides the primary iron (Fe) sites on the surface of Fe₃O₄ microspheres, facilitated the formation of Al–peroxo (Al–OOH) intermediates and reduced the apparent activation energy of the decomposition of H₂O₂ from 52.3 kJ mol⁻¹ to 41.1 kJ mol⁻¹. The synergistic effect between Fe and Al sites contributes to a much better catalytic activity of Al-promoted Fe₃O₄ as compared to pure Fe₃O₄, γ-Al₂O₃ or their physical mixtures as catalysts.