2,5-Furandicarboxylic acid (FDCA) has received increasing attention as a near-market platform chemical that can potentially replace terephthalic acid in the production of commercial and high-performance polymers, such as polyethylene terephthalate. FDCA can be obtained from the oxidation of 5-hydroxymethylfurfural (HMF), which is produced from the dehydration of C-6 monosaccharides obtained from cellulosic biomass. Recently, various heterogeneous Ni- and Co-based electrocatalysts were reported that can efficiently oxidize HMF to FDCA. The actual catalytically active species of these catalysts are most likely NiOOH and CoOOH or species related to NiOOH and CoOOH. However, the intrinsic catalytic properties of NiOOH and CoOOH for HMF oxidation have yet to be carefully and systematically investigated. In this study, we prepared thin and thick sets of NiOOH, CoOOH, and FeOOH films having comparable numbers of metal sites to systematically and methodically compare the intrinsic catalytic activity of these materials for HMF oxidation in a 0.1 M KOH (pH 13) solution. Our investigation revealed that they have distinctively different catalytic abilities for HMF oxidation. The use of extremely thin MOOH films containing limited numbers of catalytic sites allowed us to resolve anodic currents that were generated from HMF oxidation by different oxidation pathways. By comparing the voltammetric results of thin and thick films, the effect of the film thickness on the current generated by different oxidation pathways could be observed. The thick set of MOOH films was also used to compare the performances of these films for constant potential HMF oxidation and product analysis. The work herein contributes to a better understanding of the mechanisms of HMF oxidation on Ni-, Co-, and Fe-containing heterogeneous electrocatalysts whose surfaces are covered by their hydroxide and oxyhydroxide phases.

2,5-Furandicarboxylic acid (FDCA) is a key near-market platform chemical that can potentially replace terephthalic acid in various polyesters such as polyethylene terephthalate (PET). FDCA can be obtained from oxidation of 5-hydroxymethylfurfural (HMF), which can be derived from cellulosic biomass through isomerization and dehydration of hexoses. In this study, electrochemical oxidation of HMF to FDCA is demonstrated using Cu, one of the cheapest transition metals, as the catalytic anode. The oxidized Cu surface is not catalytic for water oxidation, which is the major reaction competing with HMF oxidation in aqueous media. Therefore, a wide potential window to oxidize HMF without inducing water oxidation was available, enabling high Faradaic efficiencies for FDCA production. Cu was prepared as nanocrystalline and bulk electrodes by electrodeposition, and key differences in their surface oxidation and electrochemical HMF oxidation were investigated. The oxide and hydroxide layers formed on the nanocrystalline electrode appeared to have an intrinsically different catalytic ability for HMF oxidation from those formed on the bulk electrode. Both the HMF conversion and FDCA production by the nanocrystalline electrode were nearly perfectly proportional to the amount of charge passed with no significant accumulation of any intermediate oxidation product during the course of HMF oxidation. After the stoichiometric amount of charge was passed, the nanocrystalline electrode achieved a FDCA yield of 96.4%. In contrast, the bulk electrode accumulated a significant amount of 5-formyl-2-furancarboxylic acid (FFCA) during HMF oxidation and achieved an FDCA yield of 80.8%. The morphology and composition of the oxide and hydroxide layers formed on the nanocrystalline and bulk electrodes were investigated systematically before and after HMF oxidation.

Solid contact ion-selective electrodes (ISEs) typically have an intermediate layer between the ion-selective membrane and the underlying solid electron conductor that is designed to reduce the irreproducibility and instability of the measured electromotive force (emf). Nevertheless, the electrode-to-electrode reproducibility of the emf of current solid contact ISEs is widely considered to be unsatisfactory. To address this problem, we report here a new method of constructing this intermediate layer based on the lipophilic redox buffer consisting of the Co(III) and Co(II) complexes of 1,10-phenanthroline ([Co(phen)3](3+/2+)) paired with tetrakis(pentafluorophenyl)borate as counterion. The resulting electrodes exhibit emf values with an electrode-to-electrode standard deviation as low as 1.7 mV after conditioning of freshly prepared electrodes for 1 h. While many prior examples of solid contact ISEs also used intermediate layers that contained redox active species, the selection of a balanced ratio of the reduced and oxidized species has typically been difficult and was often ignored, contributing to the emf irreproducibility. The ease of the control of the [Co(phen)3](3+)/[Co(phen)3](2+) ratio explains the high emf reproducibility, as confirmed by the emf decrease of 58 mV per 10-fold increase in the ratio of the reduced and oxidized redox buffer species. Use of a gold electrode modified with a self-assembled 1-hexanethiol monolayer as underlying electron conductor suppresses the formation of a water layer and results in an electrode-to-electrode standard deviation of E° of 1.0 mV after 2 weeks of exposure to KCl solution.

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.

Recent studies investigating the electrocatalytic properties of nitroxyl radicals (e.g. 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)) for electrochemical alcohol oxidation have demonstrated that the thermodynamic equilibrium oxidation potential and the steric environments unique to each nitroxyl radical have significant implications for their electrocatalytic abilities for alcohol oxidation. However, as the catalytic cycle of the nitroxyl radical involves not only the production and use of the oxoammonium cation for alcohol oxidation but also the regeneration of the oxoammonium cation from the hydroxylamine, the mechanism and rate of the regeneration of the oxoammonium cation critically affect the overall rate of electrochemical nitroxyl radical-mediated alcohol oxidation. Currently, a detailed understanding of this important aspect is lacking. Herein, the factors that affect the regeneration pathways of the oxoammonium cation are systematically investigated, and the impact that the regeneration pathways have on the overall catalytic activity for electrochemical alcohol oxidation is demonstrated. In this study, 4-acetamido TEMPO and 2-azaadamantane-N-oxyl, which are on the opposite ends of the spectrum of nitroxyl radical derivatives in terms of thermodynamic driving force and steric effects, are used as example nitroxyl radicals. As such, the results and discussion provided in this study can be used as comprehensive guidelines to understand the catalytic activity and regeneration pathways of any TEMPO derivative or polycyclic nitroxyl radical during electrochemical alcohol oxidation.

No abstract