Magnetic core–shell Fe3O4@Cu2O and Fe3O4@Cu2O–Cu materials as catalysts for aerobic oxidation of benzylic alcohols assisted by TEMPO and N-methylimidazole

Abstract

In this work, core–shell Fe₃O₄@Cu₂O and Fe₃O₄@Cu₂O–Cu nanomaterials for aerobic oxidation of benzylic alcohols are reported with 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and N-methylimidazole (NMI) as the co-catalysts. To anchor Cu₂O nanoparticles around the magnetic particles under solvothermal conditions, the magnetic material Fe₃O₄ was modified by grafting a layer of L-lysine (L-Lys) to introduce –NH₂ groups at the surface of the magnetic particles. With amine groups as the anchor, Cu(NO₃)₂ was used to co-precipitate the desired Cu₂O by using ethylene glycol as the reducing agent. Prolonging the reaction time would lead to over-reduced forms of the magnetic materials in the presence of copper, Fe₃O₄@Cu₂O–Cu. The nanomaterials and its precursors were fully characterized by a variety of spectroscopic techniques. In combination with both TEMPO and NMI, these materials showed excellent catalytic activities in aerobic oxidation of benzylic alcohols under ambient conditions. For most of the benzylic alcohols, the conversion into aldehydes was nearly quantitative with aldehydes as the sole product. The materials were recyclable and robust. Up to 7 repeat runs, its activity dropped less than 10%. The over-reduced materials, Fe₃O₄@Cu₂O–Cu, exhibited slightly better performance in durability. The magnetic properties allowed easy separation after reaction by simply applying an external magnet.