Nitroxyl Radical-Enhanced Proton-Coupled Electron Transfer for the Selective Oxidation of Biomass-Derived 5-Hydroxymethylfurfural.

The selective oxidation of biomass-derived 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA), a key monomer for degradable plastics, is crucial for biomass valorization and addressing plastic pollution. However, its efficiency is limited by high energy barriers and slow kinetics due to the complex multi-electron and multi-proton transfer steps. Herein, a MIL-100(Fe)/TEMPO/nitric acid catalyst system is developed to facilitate electron transfer in HMF oxidation. The catalyst system achieves 100% conversion of 3 wt% HMF in 16 h at 353 K and atmospheric pressure, with a 94% yield of total acid product. The cyclic voltammetry and in situ electron paramagnetic resonance (EPR) reveal that nitric acid promotes TEMPO oxidation to TEMPO⁺, facilitating electron transfer and increasing the oxidation rate. The Griess method, ferrocene cation probe, and in situ EPR confirm nitric oxide (NO) formation, which acts as an electron shuttle between oxygen and MIL-100(Fe), accelerating the Fe(III)/Fe(II) redox cycle. Hydrogen/deuterium kinetic isotope effect analysis supports a proton-coupled electron transfer (PCET) mechanism. This study demonstrates that nitric acid significantly enhances PCET, enabling rapid oxidation of HMF to FDCA under mild conditions.