Co3O4 NPs decorated Mn-Co-O solid solution as highly selective catalyst for aerobic base-free oxidation of 5-HMF to 2,5-FDCA in water

Oxidative transformations of HMF are key routes toward a wide variety of chemicals and biofuels through an environmental friendly process. Co3O4 NPs decorated Mn-Co-O solid solution, donated as Co3O4/MnXCo, was prepared by co-precipitation method and used as non-precious metal catalyst for base-free oxidation of HMF to FDCA in the presence of O2 and water. Comprehensive characterizations such as XRD, H2-TPR, O2-TPD, XPS, NH3/CO2-TPD and TEM were used to study materials properties especially the oxygen mobility and acidity-basicity. The effect of Mn/Co molar ratio in catalyst and the reaction conditions were investigated and optimized. In addition, the kinetics of the reaction was monitored to reveal the reaction route and rate-determining step. The variation of Mn/Co molar ratio was found to affect not only HMF conversion but also distribution of products. When the Mn/Co ratio was of 0.2, the Co3O4/Mn0.2Co catalyst showed the optimal catalytic performance (i.e., >99% yield of FDCA obtained under 1 bar O2 at 140 °C after 24 h) due to the highest content of both Lewis and Brønsted acid sites on the surface, leading to the strongest ability of HMF adsorption and COOH group formation, as well as the enhanced oxygen mobility. This catalyst was shown stable after a minor deactivation (≤8%) during six recycling uses and its activity can be entirely regenerated by calcination in air.