Base free HMF oxidation over Ru-MnO2 catalysts revisited: Evidence of Mn leaching to Mn-FDCA complexation and its implications on catalyst performance

Abstract

2,5-furan dicarboxylic acid (FDCA) is a high-value bio-based chemical useful for the production of next-generation renewable polymers, epoxy resins, and surfactants. Herein, we show that the performance and stability of MnO2-based oxidation catalysts used in FDCA production are considerably influenced by process conditions (especially initial HMF conc. and the presence of base additives). Although base-free aerobic oxidation of HMF to FDCA is known to be highly effective over Ru-modified MnO2 catalysts, our experiments showed considerable leaching of the MnO2 phase under reaction conditions with the formation of insoluble Mn-FDCA complexes. The formation of the Mn-FDCA complex and Mn leaching was also supported by control experiments between FDCA and various Mn oxides under hydrothermal conditions and its structure was confirmed by single crystal XRD. Formation of Mn-FDCA and Mn leaching was also identified as the underlying reason for carbon balance deficit, catalyst mass loss, and deactivation observed upon reuse.