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

Abstract

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 MnO2 to convert HMF into FDCA by aerobic oxidation with O2 as a sole oxidant (2C6H6O3 + 3O2 → 2C6H4O5 + 2H2O) has been reported and receiving much attention very recently. However, the mechanism of HMF oxidation catalyzed by MnO2 is uncertain. Herein, we proposed the reaction mechanism, through two oxidative dehydrogenations via lattice oxygen (Obri) to generate −CHO and HObri groups first; then, the H involved in the −CHO group was further oxidized by Obri to form −CO and −HObri, followed by the −HObri group inserting into the −CO group to form a carboxyl group (−COOH); the excess −HObri then releases the H by an O2-assisted mechanism, and at last, the consumed lattice oxygen was recovered by O2. Furthermore, we explored the structure–activity relationship of MnO2, finding that the active sites on the β-MnO2 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 (Ovac) 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 Ovac 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.

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