Fabrication of spinel CoMn2O4 hollow spheres for highly selective aerobic oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran

Spinel CoMn2O4 hollow spheres were prepared by a solvothermal method, where uniform cobalt-manganese glycerate spheres as the precursor were self-assembled and converted into hollow spheres by calcination. The as-prepared catalysts were characterized by field emission scanning electron microscopy (FESEM), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption and desorption isotherms, thermogravimetric analysis, and H2 temperature programmed reduction (H2-TPR). The results show that body-centered tetragonal and face-centered cubic spinel structures of CoMn2O4 hollow spheres can be tuned by different Co/Mn molar ratios. Moreover, the spinel CoMn2O4 hollow spheres exhibit high catalytic activity for the selective oxidation of 5-hydroxymethylfurfural (HMF) into 2,5-diformyfuran (DFF). Importantly, the spinel CoMn2O4 hollow spheres with Co/Mn molar ratio of 2/3 shows the highest catalytic performance with a 41.6% HMF conversion and a 100% selectivity to DFF. On one hand, the spinel CoMn2O4 hollow spheres with large surface area have sufficient exposed active sites for the oxidation of HMF and oxidized product DFF can fast leave the catalyst surface leading to high selectivity, avoiding further oxidation. On the other hand, the synergistic effect between Co and Mn species in the binary oxides by the formation of heteronuclear cluster complexes CoMn2O4, which affects and promotes the electron-transfer process, results in significant improvement in catalyst performance. Additionally, the effects of different Co/Mn molar ratios, catalyst amounts, reaction time and temperatures of the catalyst for HMF oxidation were also investigated. Furthermore, the spinel CoMn2O4 hollow spheres can be used for six consecutive runs without significant loss of its catalytic activity.