Selective Biomass Valorization in Neutral Electrolyte by Lowering the O2 Activation Energy via Tandem Catalysis

Abstract

Conventional aerobic oxidation of biomass 5-hydroxymethylfurfural (HMF) encounters challenges due to the harsh conditions stemming from low solubility and high activation energy of O₂, which hinder the efficient production of 2,5-furandicarboxylic acid (FDCA). Here, we propose a tandem catalytic system that lowers the activation energy for O₂ to enable efficient and selective HMF oxidation to produce FDCA or 5-formyl-2-furoic acid (FFCA) under mild conditions. To overcome the high activation energy of O₂ in the thermocatalytic processes, an electrochemical O₂ activation process was employed to efficiently produce H₂O₂ as an oxidant to valorize HMF into either FDCA or FFCA. This tandem catalytic system achieved a cumulative FDCA yield of 2.21 mol g_Ru^–1, full HMF conversion (100%), and complete product selectivity (100%), with an average productivity of 13.19 mmol g_Ru^–1 h^–1 over an extended 170 h operation period. The FDCA productivity in this work exceeds that in most aerobic oxidations under mild conditions. Further investigation into the detailed reaction process demonstrated that OH from H₂O dissociation directly interacted with the aldehyde group in HMF to form a carboxylic acid group, while H₂O₂ serves as an oxidant to extract electrons from HMF or reaction intermediates. Furthermore, the efficient oxidation of structurally diverse substrates from EtOH to glycerol demonstrates the broad applicability of this tandem catalytic system.