Fe3O4@SiO2–TEMPO as a Magnetically Recyclable Catalyst for Highly Selective Aerobic Oxidation of 5‐Hydroxymethylfurfural into 2,5‐Diformylfuran under Metal‐ and Halogen‐Free Conditions

A highly selective aerobic oxidation of 5‐hydroxymethylfurfural (5‐HMF) into 2,5‐diformylfuran (DFF) by employing a magnetically separable 2,2,6,6‐tetramethylpiperidine‐N‐oxide (TEMPO) catalyst (Fe3O4@SiO2–TEMPO) under metal‐ and halogen‐free conditions is described. Quantitative yields of DFF with excellent selectivities as high as >99 % were achieved under several versatile reaction conditions in a catalytic system consisting of Fe3O4@SiO2–TEMPO as a catalyst, tert‐butyl nitrite as a co‐catalyst, acetic acid as an additive, and toluene as a solvent. The results clearly demonstrate that the present catalytic system for selective conversion of 5‐HMF into DFF is comparable or even superior to the many systems based on transition‐metal catalysts. In addition, the catalyst can be successfully recovered and reused in five consecutive reaction runs by an external magnetic source with keeping its catalytic performance and selectivity.