Stabilities, Regeneration Pathways, and Electrocatalytic Properties of Nitroxyl Radicals for the Electrochemical Oxidation of 5-Hydroxymethylfurfural

2,5-Furandicarboxylic acid (FDCA) is a near market monomer that has been identified as a viable biomass derived replacement for petroleum-derived terephthalic acid in the synthesis of polyethylene terephthalate (PET). FDCA can be produced from the oxidation of 5-hydroxymethylfurfural (HMF), which is a versatile biomass intermediate produced from the dehydration of C-6 monosaccharides obtained from cellulosic biomass. In this study, we comparatively investigated the use of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 4-acetamido-TEMPO (ACT) for electrochemical HMF oxidation to FDCA. The distinct advantage of TEMPO- and ACT-mediated electrochemical oxidation of HMF is that they can efficiently achieve HMF oxidation in mildly basic conditions (pH 9-10), while other heterogeneous catalysts typically require the use of more basic media. Since HMF oxidation in a strongly basic condition increases the chance to form humins, which are difficult to separate from FDCA and decrease the commercial viability of FDCA and FDCA-derived products, TEMPO- and ACT-mediated HMF oxidation may offer a critical advantage for producing commercial-grade FDCA. In this study, the stabilities, electrochemical properties, and electrocatalytic performances of TEMPO and ACT, which has been identified as a less expensive alternative to TEMPO, were comparatively examined for electrochemical HMF oxidation. Through investigating the effect of pH, applied potential, and ratio of nitroxyl radical to HMF in solution on HMF oxidation, two different regeneration pathways of TEMPO and ACT in the catalytic cycle and the factors that affect their regeneration pathways were identified. The stability and catalytic activity of TEMPO and ACT for electrochemical HMF oxidation at an elevated temperature were also studied. On the basis of this investigation, optimal electrochemical conditions to efficiently oxidize a concentrated HMF solution (100 mM), which is relevant to large-scale electrochemical FDCA production, were identified.