Optimization of Co/Mn/Br-Catalyzed Oxidation of 5-Hydroxymethylfurfural to Enhance 2,5-Furandicarboxylic Acid Yield and Minimize Substrate Burning

2,5-Furandicarboxylic acid (FDCA) is a valuable nonphthalate biomass-based plastic precursor with the potential to replace terephthalic acid (TPA) in a variety of polymer applications. In this work, the Co/Mn/Br-catalyzed semicontinuous oxidation of 5-hydroxymethylfurfural (HMF) to FDCA has been demonstrated at temperatures lower than those employed in the traditional Mid-Century (MC) process. As HMF is more susceptible to side reactions (e.g., the overoxidation to CO and CO2), lower temperatures compared to the MC process are typically used to prevent substrate burning. However, lower temperatures afford a much reduced FDCA yield compared to that of TPA in p-xylene oxidation. Therefore, optimization of other operating variables such as catalyst composition, water concentration in the acetic acid solvent, and pressure is essential to maximize FDCA yield. Using such optimization, we show that the FDCA yield can be enhanced to 90% at a 1/0.015/0.5 molar ratio of Co, Mn, and Br, 7% (v/v) water, 30 bar (CO2/O-2 = 1/1, mol/mol), and 180 degrees C, the highest value reported for HMF oxidation using Co/Mn/Br catalyst. The use of Zr(IV) as co-catalyst facilitates FDCA formation but only at lower temperatures (120-160 degrees C) where the FDCA yield is compromised. These findings broaden the scope of the application of the industrial MC catalytic process for FDCA production.