Base-free oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid boosted by Ru-Na synergy in Ru/Na-Y zeolite catalysts

The catalytic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is an important reaction in the sustainable production of polymers from biomass resources. To advance the environmental sustainability of this process, it is crucial to develop a catalytic system that circumvents the need for liquid alkaline media, which have been conventionally employed in HMF-to-FDCA conversion. Herein, we developed an efficient catalytic system comprising Ru nanoparticles supported on Na-type zeolite Y (Ru/Na-Y) for the base-free oxidation of HMF to FDCA. Reducing the particle size of Ru significantly enhances the activity of the Ru/Na-Y catalysts. Specifically, the optimized Ru/Na-Y-300N-200H, featuring Ru nanoparticles with an average size of ∼2.5 nm, afforded nearly 100 % HMF conversion and ∼ 85 % FDCA yield under base-free aqueous-phase conditions at 100 °C and 5 MPa air pressure. Kinetic studies further evidenced that Ru/Na-Y-300N-200H could effectively catalyze the sequential oxidation of both alcohol and aldehyde groups during the HMF-to-FDCA conversion. Comprehensive characterizations and DFT calculations elucidated the enhanced interaction between Ru centers and Na sites in the Ru/Na-Y-300N-200H due to the smallest Ru particle size. The unique Ru-Na synergistic effect was thus realized, which could facilitate the activation of O 2 and the generation of O 2 •– radicals as active oxygen species, while simultaneously boosting the rate-determining step of 5-formyl-2-furancarboxylic acid (FFCA) oxidation to FDCA, resulting in a high FDCA yield. These findings unveil promising prospects for developing advanced metal-zeolite bifunctional catalysts, which could facilitate the base-free oxidation of biomass-derived HMF into FDCA.