Synergy in magnetic NixCo1Oy 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 Ni x Co 1 O y ( x  = 1, 3 and 5) bimetallic oxides can induce reactive oxygen defects and simultaneously stabilize small-sized metallic Au nanoparticles in the Au/Ni x Co 1 O y catalysts. Such catalytic features render effective adsorption and activation of O 2 , OH and C O groups, realizing selective oxidation of HMF to FDCA . On a series of magnetic Au/Ni x Co 1 O y 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 CoO y . 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., O 2 − and O − ) and electron-rich Au 0 sites. The optimum Au/Ni 3 Co 1 O y catalyst exhibits a highest productivity of FDCA (12.5 mmol FDCA mol Au - 1 h −1 ) among all the Au catalysts in the literature and achieves > 99% yield of FDCA at 120 °C and 10 bar of O 2 . 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.