Ambient temperature catalyzed air-oxidation of 5-hydroxymethylfurfural via ternary metal and oxygen vacancies

Catalytic oxidation of biomass-derived 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA, an alternative bioplastic monomer to petroleum-derived terephthalic acid), has been identified as an important biomass conversion reaction in bio-based polyester industry. However, it is still challenging to acquire a high FDCA yield from the selective oxidation of HMF at low temperatures. Herein, a ternary metal-based catalyst was prepared by loading AuPdPt noble metal nanoparticles on the oxygen-rich vacancy titanium dioxide layer deposited on natural clay mineral kaolin nanotubes (HNTs), and the catalytic activity was examined for air-oxidation of HMF to FDCA in water at ambient temperature (30 °C). By adjusting the Au/Pd/Pt ratio, a 93.6% FDCA yield was achieved with the optimal Au_0.5Pd_0.2Pt_0.3/TiO₂@HNTs catalyst, which revealed an impressive FDCA formation rate of 67.58 mmol g^-1 h^-1 and an excellent TOF value of 17.54 h^-1 under normal air pressure at 30 °C, surpassing the performance of mono- and bimetallic-based catalysts. Theoretical calculation and catalytic performance study clarified the structure–activity relationship. It was found that the ternary metal and oxygen vacancies revealing synergistic enhancement of ambient temperature catalyzed HMF air-oxidation via electronic structure tuning and adsorption intensification. DFT and kinetics study demonstrated that the presence of ternary metal significantly improved the adsorption capacity of substrate and enhanced the rate-determining step of the key intermediate 5-hydroxymethyl-2-furanocarboxylic acid (HMFCA) oxidation when compared to mono- and bimetal. Additionally, the TiO₂@HNTs support with high oxygen vacancy concentration facilitated the adsorption of oxygen, synergistically working with the ternary metal to activate and low the energy barriers for the generation of superoxide radical, thus enhancing the FDCA formation. This work offers a novel strategy for designing ternary metal-based catalysts for low-energy catalytic oxidation reactions.