Mechanism Insights into the Aerobic Oxidation of 5-Hydroxymethylfurfural to 2,5-Furandicarboxylic Acid over MnO2 Catalysts

5-Hydroxymethylfurfural (HMF) and its derivatives, especially 2,5-furandicarboxylic acid (FDCA), as important renewable energy sources which can be an excellent substitute for terephthalic acid in the field of polymers such as polyamides and polyesters, have received a lot of attention recently. Notably, a simple nonprecious-metal catalyst system based on MnO 2 to convert HMF into FDCA by aerobic oxidation with O 2 as a sole oxidant (2C 6 H 6 O 3 + 3O 2 → 2C 6 H 4 O 5 + 2H 2 O) has been reported and receiving much attention very recently. However, the mechanism of HMF oxidation catalyzed by MnO 2 is uncertain. Herein, we proposed the reaction mechanism, through two oxidative dehydrogenations via lattice oxygen (O bri ) to generate −CHO and HO bri groups first; then, the H involved in the −CHO group was further oxidized by O bri to form −CO and −HO bri, followed by the −HO bri group inserting into the −CO group to form a carboxyl group (−COOH); the excess −HO bri then releases the H by an O 2 -assisted mechanism, and at last, the consumed lattice oxygen was recovered by O 2 . Furthermore, we explored the structure–activity relationship of MnO 2, finding that the active sites on the β-MnO 2 surface are all frustrated Lewis pairs, which promote the activation of C–H and O–H and facilitate the progress of the catalytic reaction. Besides, the reaction activity of the lattice oxygen on the catalyst surface is critical to the catalytic performance: the better the lattice oxygen activity, the better the catalytic performance; oxygen vacancy (O vac ) formation energy can be a good descriptor for reaction activity. At last, the microkinetic modeling results show that the catalytic activity is optimal when the adsorption energy of FFCA is between −0.7 and −0.4 eV and the O vac formation energy is between 0.1 and 0.5 eV. Hopefully, this may act as a favorable guidance for the filtrate of catalysts in the experiment.