Engineering crystal plane of NiCo₂O₄ to regulate oxygen vacancies and acid sites for alkali-free oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid

The catalytic oxidation of HMF involves a cascading reaction with multiple intermediate products, making it crucial to enhance the oriented adsorption capacity of specific functional groups for accelerating the entire process. To achieve the efficient selective oxidation of HMF to FDCA, a series of NiCo₂O₄ catalysts with different morphologies, such as flaky, echinoids, pompon and corolla, were prepared and characterized by XRD, SEM, TEM, BET, XPS, and FTIR. Among the four catalysts, flaky NiCo₂O₄ exhibited the most excellent catalytic activity and stability, with a FDCA yield of 60.1% within 12 h at 80 ℃ without alkali participation. The excellent performance of flaky NiCo₂O₄ catalyst is attributed to the oxygen vacancies and acid sites generated by the exposed (400) facets. The oxygen vacancies and acid sites on the catalyst surface can precisely adsorb -CHO and -CH²-OH of HMF, respectively, and this synergistic effect promotes the efficient production of FDCA. This work is of great significance for fundamentally study the effect of micro-topography or crystal-plane reaction properties on surfaces.