Aerobic Oxidation of 5-Hydroxymethylfurfural to Dimethyl Furan-2,5- dicarboxylate over CoMn@NC Catalysts Using Atmospheric Oxygen

Abstract: Dimethyl furan-2,5-dicarboxylate (DMFDCA) is a valuable biomass-derived chemical that is an ideal alternative to fossil-derived terephthalic acid as a monomer for polymers. The one-step oxidation of 5-hydroxymethylfurfural (HMF) to DMFDCA is of practical significance. It not only shortens the reaction pathway but also avoids the separation process of intermediates; thus, reducing cost. In this work, non-noble bimetallic catalysts supported on N-doped porous carbon (CoMn@NC) were synthesized via a one-step co-pyrolysis procedure using different pyrolysis temperatures and proportions of metal precursors and additives. We employed the prepared CoMn@NC catalysts in the aerobic oxidation of HMF under mild reaction conditions to obtain DMFDCA. High-yield DMFDCA was obtained by screening the prepared catalysts and optimizing the reaction conditions, including the strength and amount of the base, as well as the reaction temperature. The optimized yield of DMFDCA was 85% over the Co3Mn2@NC-800 catalyst after 12 h at 50 °C using ambient-pressure oxygen. The physicochemical properties of the catalysts were determined using a variety of characterization techniques, the factors affecting the performance of each catalyst were investigated, and the relationship between the physicochemical properties and performance of the prepared catalysts was elucidated. A porous structure with a high surface area had a positive effect on mass transfer efficiency. Cobalt nanoparticles (NPs) and atomically dispersed Mn were coordinated to N-doped carbon to form M―Nx (where M = Co or Mn). Based on the Mott-Schottky effect, there was significant electron transfer between each metal and the N-doped carbon, additionally, the metal NPs supplied electrons to the carbon atoms. The electron-deficient metal site in the pyridinic N-rich carbon was beneficial for the activation of HMF and oxygen. The activation of oxygen produced reactive oxygen species (such as superoxide radical anions) to ensure high selectivity to DMFDCA through dehydrogenative oxidation of the hemiacetal intermediate and hydroxymethyl group of 5-hydroxymethyl-2-methyl-furoate. The existence of disordered and defective carbons increased the number of active sites. Subsequently, we performed a series of control experiments. Based on our current experimental results and previous studies, we propose a simple mechanism for the aerobic oxidation of HMF to DMFDCA. The catalyst was stable, its performance decreased slightly after two cycles, and it was tolerant to SCN− ions and resistant against N or S poisoning. Furthermore, the use of this catalytic system can be expanded to various substituted aromatic alcohols, such as benzyl alcohols with different substituents, furfuryl alcohol, and heterocyclic alcohols. Simultaneously, the product type was further extended from methyl esters to ethyl esters with a high yield when the substrate reacted with ethanol. In conclusion, this catalytic system can be applied in the production of carboxylic esters for polymers.