Selective and metal-free oxidation of biomass-derived 5-hydroxymethylfurfural to 2,5-diformylfuran over nitrogen-doped carbon materials

Abstract

The development of new strategies for metal-free promoted reactions has recently received great interest from chemists because these processes benefit from the use of inexpensive catalysts and from avoidance of contamination of the desired products by the metal catalysts. In this study, a method for HNO₃-promoted oxidation of biomass-derived 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) was developed in the presence of nitrogen-doped carbon materials using molecular oxygen as the terminal oxidant under mild reaction conditions. The catalytic activity of the nitrogen-doped carbon materials was discovered to greatly increase with increasing pyrolysis temperature; it also depended on both the type of nitrogen atoms in the catalyst and the surface area of the catalyst. The NC-950 catalyst demonstrated unprecedented catalytic activity towards the oxidation of HMF to DFF. Several reaction parameters, such as the reaction solvent, the reaction temperature, the amount of HNO₃, the catalyst loading and the oxygen pressure, have been investigated. The highest DFF yield of 95.1% was attained with full HMF conversion after 14 h at 100 °C under 10 bar oxygen pressure in the presence of a catalytic amount of HNO₃. Control experiments indicated that the oxidation of HMF was initially promoted by HNO₃, and O₂ acted as a secondary oxidant for HNO₃ recovery. In particular, the NC-950 catalyst had remarkable reusability without significant losses in its activity and selectivity after six consecutive usages.