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

Abstract

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.

Advances in Electrochemical Modification Strategies of 5‐Hydroxymethylfurfural

“…examined the efficacy of 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO) and 4‐acetamido‐TEMPO (ACT) as examples of these organic molecule catalysts (Figure 4). [2] By using TEMPO or ACT, the researchers were able to conduct the selective oxidation of HMF to FDCA in borate buffer (pH 10) with a carbon felt electrode, obtaining a 93.8 % FDCA conversion and FE for the TEMPO‐catalyzed system and a 92.6 % conversion and FE for ACT at 1.6 V constant potential vs. RHE at room temperature with 20 m m HMF present. They reasoned, however, that ACT was a more suitable catalyst for large‐scale HMF oxidation given the faster turnover of ACT and, therefore, higher current densities and shorter time scales for HMF oxidation to FDCA.…”

“…As promising environmentally benign alternatives to the standard methods, approaches based on the electrocatalytic oxidation and reduction of furanic compounds have gained significant attention as they offer advantages for electrocatalytic furanic transformations in biomass conversion [26] . Specifically, these advantages include (1) use of H 2 O as the proton source instead of costly H 2 gas, [27] (2) operation at ambient temperatures and pressures, (3) control of product selectivity and reaction rate via a chosen potential (or current) applied to the electrode, [2,28] (4) simple and rapid characterization of molecule reactivity by voltammetric methods (e. g., cyclic voltammetry), and (5) concurrent generation of oxidation and reduction products in a pair electrolysis system. For instance, research studies have reported the use of Au, Pt, and Ni‐based transition metals as electrocatalysts for the electrochemical oxidation of HMF [1a,28,29] .…”

“…During the last few decades, fossil fuel resources (e. g., coal, crude oil, natural gas) have diminished significantly, while the global energy demands have increased due to the rapidly growing world population [1] . Due to the unfavorable environmental impacts of fossil fuels, [2] green approaches and chemical processes independent of fossil fuels have received increasing attention as new technologies to exploit clean and renewable energy sources [2,3] . In this context, the use of biomass as alternative chemicals and fuels is attractive as biomass is both accessible and renewable [2,4] and can give non‐fossil‐based carbon‐forms from CO 2 in the air [5]…”

Innovative Protocols in the Catalytic Oxidation of 5‐Hydroxymethylfurfural

“…[49] HMF, as a trapped antioxidant inside C-dot, could be oxidized into furan derivatives during the continuous electrolysis, and the ensuing furan derivatives serve as catalytic sites to drive the OER activity. Besides, TEMPO and 4-acetamido-TEMPO (ACT) have been compared and explored in electrochemical HMF oxidation in terms of stabilities, electrochemical properties, and electrocatalytic performance; [50] both media allowed the production of FDCA under mildly basic condition (pH 9-10) as compared to other heterogeneous catalysts that require more basic media. 95.9 % FDCA yield under optimum conditions [40 mm ACT at 1447 C passed (stoich.…”

2,5-Furandicarboxylic acid production via catalytic oxidation of 5-hydroxymethylfurfural: Catalysts, processes and reaction mechanism

“…The photogenerated holes oxidized TEMPO to TEMPO + , which was highly active to oxidize HMF to form final FDCA in alkaline media, whereas the photoexcited electrons were transferred to the Pt counter electrode by external bias to reduce water to H 2 . The high FE and yield for FDCA formation, up to 93% and 99%, respectively, indicated that TEMPO-mediated HMF oxidation was kinetically much faster than OER and was the dominant oxidation reaction, even in the pure electrocatalytic reaction [162]. The vital role of BiVO 4 is providing a suitable valence-band (VB) to generate holes to initiate TEMPO oxidation, demonstrated by the photocurrent onset potential was shifted to 0.32 V vs RHE while the electrochemical onset potential was 1.01 V vs RHE.…”