Upgrade of 5-Hydroxymethylfurfural to Dicarboxylic Acids onto Multifunctional-Based Fe3O4@SiO2 Magnetic Catalysts

The oxidation of 5-hydroxymethylfurfural (HMF) to valuable dicarboxylic acids has been investigated on magnetic separable nanocatalysts in the search for selectivity enhancement. Cationic Ru(4 wt %)-Fe3O4@SiO2 catalyst led to 2,5-furandicarboxylic acid (FDCA), with a selectivity of 80.6% for HMF conversion of 92%. Replacing the deposited noble metal with a transitional oneand NaOH with n-butylamine ensured further advantages, such as the increase of HMF stability and the possibility to tune the selectivity for other valuable products. Thus, under similar reaction conditions, the Fe3O4@SiO2-CoOx (10 wt % CoOx) catalyst provided a selectivity to succinic acid (SA) of 92.7% for a HMF conversion of 78.6%, and the Fe3O4@SiO2-MnOx (10 wt % MnOx) catalyst led to 72% selectivity to maleic acid (MAc) for HMF conversion of 5%. However, the performances in this reaction have been improved (85% selectivity to MAc for HMF conversion of 35.6%) working with a metal-free catalyst (Fe3O4@SiO2-NH2), in which the -NH2 functionalization substituted the role of the homogeneous strong base. All the investigated catalysts showed very good stability.