Synergy in magnetic Ni Co1O oxides enables base-free selective oxidation of 5-hydroxymethylfurfural on loaded Au nanoparticles

The base-free aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) in water is recognized as an important and sustainable upgrading process for cellulosic carbohydrates. However, selectivity control still remains a challenge. Here, we disclose that the unique synergy in magnetic NixCo1Oy (x = 1, 3 and 5) bimetallic oxides can induce reactive oxygen defects and simultaneously stabilize small-sized metallic Au nanoparticles in the Au/NixCo1Oy catalysts. Such catalytic features render effective adsorption and activation of O2, OH and CO groups, realizing selective oxidation of HMF to FDCA. On a series of magnetic Au/NixCo1Oy catalysts with almost identical Au loadings (ca. 0.5 wt%) and particle sizes (ca. 2.7 nm), the variable Ni/Co molar ratios give rise to the tunable electron density of Au sites and synergistic effect between NiO and CoOy. The initial conversion rates of HMF and its derived intermediates (i.e., DFF, HMFCA and FFCA) show a volcano-like dependence on the number of oxygen defects (i.e., O2− and O−) and electron-rich Au0 sites. The optimum Au/Ni3Co1Oy catalyst exhibits a highest productivity of FDCA (12.5 mmolFDCA molAu-1 h−1) among all the Au catalysts in the literature and achieves > 99% yield of FDCA at 120 °C and 10 bar of O2. In addition, this catalyst can be easily recovered by a magnet and show superior stability and reusability during six consecutive cycling tests. This work may shed a light on Au catalysis for the base-free oxidation of biomass compounds by smartly using the synergy in bimetallic oxide carriers.