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Synthesis of 2,5-Furandicarboxylic Acid by the Electrocatalytic Oxidation

Bowen Xia , Bin Zhu , Jing Liu , Chunlin Chen , Jian Zhang

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (8) : 1661 -1677.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (8) : 1661-1677 DOI: 10.7536/PC210934
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Synthesis of 2,5-Furandicarboxylic Acid by the Electrocatalytic Oxidation
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Abstract

At the 75th session of the United Nations General Assembly, China made a solemn commitment to strive to achieve carbon peaking by 2030 and carbon neutrality by 2060. Biomass, mainly produced by photosynthesis, will play an important role in the dual carbon targets. The efficient conversion of biomass can yield a range of high-value chemicals to replace fossil-derived products. Among these biomasses, 2,5-furandicarboxylic acid (FDCA) can be used to replace petroleum-based terephthalic acid (TPA) in the synthesis of bio-based furan polyesters with better thermal stability and gas barrier properties due to its similar conjugated carbon ring and diacid structure to TPA, significantly reducing the heavy reliance on fossil resources in polyester industry. In addition, FDCA is widely applied to pharmaceuticals, fragrances and metal coordination chemistry, making it one of the twelve most promising bio-based platform compounds. FDCA is typically synthesized by catalytic oxidation of 5-hydroxymethyl furfural (HMF). Compared to conventional thermocatalytic methods that require precious metal catalysts, high temperature and pressure conditions, and chemical potential as the driving force, electrocatalytic oxidation uses electrode potential as the main driving force and is a novel synthesis method that is greener and more efficient. This review summarises and analyzes the noble metal, transition metal and non-metal catalysts used in the preparation of FDCA reactions by electrocatalytic oxidation, outlines the research on catalyst design and reaction mechanisms, and points out the challenges and opportunities for the development of this field.

Contents

1 Introduction

2 Noble metal catalysts

3 Transition metal catalysts

3.1 Mono metal catalysts

3.2 Binary metal catalysts

3.3 Multiple metal catalysts

4 Non-metal catalysts

5 Mechanism

6 Conclusion and outlook

Graphical abstract

Keywords

biomass upgrading / electrocatalysis / oxidation / 5-hydroxymethylfurfural / 2,5-furandicarboxylic acid

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Bowen Xia, Bin Zhu, Jing Liu, Chunlin Chen, Jian Zhang. Synthesis of 2,5-Furandicarboxylic Acid by the Electrocatalytic Oxidation. Progress in Chemistry, 2022, 34(8): 1661-1677 DOI:10.7536/PC210934

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1 Introduction

The development and utilization of fossil resources has brought a series of environmental problems, especially the carbon dioxide emissions far exceed the amount that nature can absorb and fix, resulting in carbon imbalance and serious greenhouse effect. Replacing high-carbon resources with low-carbon resources and replacing non-renewable resources with renewable resources are the main ideas to reduce carbon emissions. Among all kinds of renewable resources and clean energy on the earth, such as biomass, nuclear energy, wind energy, solar energy, hydropower and tide, biomass is the only carrier with the characteristics of both resources and energy[1⇓⇓~4]. Among them, 2,5-furandicarboxylic acid (FDCA), as one of the 12 representative bio-based platform molecules released by the US Department of Energy in 2004, has important uses in many fields (Fig. 1). In the field of polymerization, the global production of polyethylene terephthalate (PET) prepared from petroleum-based terephthalic acid (TPA) reached 78.2 million tons in 2020. FDCA can be used to synthesize biodegradable poly (ethylene furandicarboxylate) (PEF) due to its similar chemical structure to TPA. PEF has higher glass transition temperature, gas barrier properties and mechanical properties, which can greatly reduce the environmental pollution caused by refractory polymer materials and produce huge economic benefits.
FDCA can be prepared from 5-hydroxymethylfurfural (HMF), furfuryl alcohol, furfural, furoic acid, furan, diglycolic acid, galactaric acid and other raw materials. The HMF route is the simplest and mildest because it does not involve the cleavage or addition of C — C bonds. FDCA can be obtained by metered oxidation of HMF, but strong oxidants such as KMnO4 are generally used, resulting in low selectivity and large amounts of wastewater. The catalytic oxidation method can reduce the reaction energy barrier, greatly reduce the reaction temperature and pressure, and improve the yield and separation efficiency of FDCA. Although there are many reports on the catalytic oxidation of HMF to FDCA, there are still many unovercome problems. First of all, HMF molecule contains three kinds of oxygen-containing functional groups: hydroxymethyl, aldehyde and epoxy, and their oxidation potentials are different. Usually, strong base is used to reduce the oxidation potential difference between hydroxymethyl and aldehyde, so that the aldehyde group is converted into geminal diol and further oxidized[5,6]. However, in the strong base system, the aldehyde group is prone to acetalization to produce highly colored Humin and other by-products, and the highly active species produced on the surface of the catalyst may also attack furan oxygen or even C-C bonds to produce small molecules or ring-opening products[7,8]. Secondly, HMF catalytic oxidation usually uses O2 as oxidant, and the kinetic study shows that the surface oxygen-rich catalyst is conducive to the first step of alcohol dehydrogenation to aldehyde, so the higher the pressure of O2, the faster the formation of FDCA. In order to promote the transfer and diffusion of O2 at the three-phase interface, the reaction conditions of heating and pressurizing are usually adopted, and the pressure of O2 is as high as 2 MPa. However, the reaction conditions of heating and pressurizing are easy to bring potential safety hazards. Thirdly, the activation of molecular oxygen usually depends on highly active noble metal catalysis, so most of the HMF catalytic oxidation systems reported so far are based on Pt, Au, Pd, Ru and their alloy catalysts. Although the catalytic activity is high, the natural scarcity of noble metals leads to the rising research on substitution[9⇓⇓~12]. HMF electrocatalytic oxidation technology developed in recent years can greatly improve the oxidation efficiency of HMF by using electrons as a clean reagent. Electrocatalytic oxidation uses potential as the main driving force, and can accurately and conveniently control the selectivity and rate of the reaction by controlling the potential and current, which is simpler and more convenient than the traditional chemical method using chemical potential as the driving force[13,14]. In addition, electrocatalytic oxidation has the characteristics of wide substrate selection, mild reaction conditions, low energy dissipation and superior safety, and is also widely used in the fields of organic synthesis, pollutant degradation and energy conversion[15][16][17].
In 1983, Kawana et al. First reported the electrolysis of HMF. Eight years later, Grabowski et al. Successfully oxidized HMF to FDCA by electrocatalysis. In 1996, Skowronski et al. First reported the selective electrooxidation of HMF to 2,5-furandicarboxaldehyde (DFF) on Pt electrode[18][19][20]. However, in the following ten years, the progress of this research field was very slow. It was not until Vuyyuru et al. Reported the comparative study of thermocatalytic and electrocatalytic oxidation of HMF in 2012 that the preparation of FDCA by electrocatalytic oxidation of HMF returned to the researchers' field of vision (Fig. 2)[5]. Since then, in 2015, Cha and Choi used photoelectrocatalysis and electrocatalysis to activate the homogeneous selective oxidation catalyst 2,2,6,6-tetramethylpiperidine oxide (TEMPO) to indirectly catalyze the oxidation of HMF to FDCA, which greatly increased the attention of researchers to the electrocatalytic oxidation of HMF to FDCA[21].
In recent years, there are more and more reports on heterogeneous direct electrocatalytic oxidation without additional catalysts, which has become an important research direction. In this paper, the types of catalysts used in the electrocatalytic oxidation of HMF to FDCA and the exploration of the reaction mechanism are reviewed, in order to clarify the design principle of the catalyst and the mechanism of activity improvement.

2 Precious metal catalyst

Similar to most heterogeneous thermocatalytic reactions, the electrocatalytic conversion process usually involves a surface reaction between the active site and the electrolyte interface. Therefore, certain noble metals used in thermochemical reactions and their corresponding supported catalysts also exhibit electrocatalytic activity. In 2012, Vuyyuru et al. Utilized a pure platinum sheet as the working electrode to perform the electrocatalytic HMF oxidation reaction in NaOH-NaClO4 aqueous solution[5]. Different from the thermal catalytic process of Pt/C under the same pH environment, the electrocatalysis has better DFF selectivity, and can form 18% DFF with 70% HMF conversion within 12 H. Zhou Chunmei et al. Of Nanjing University of Technology found that in neutral K2SO4 electrolyte, the oxidation peak potential of HMF hydroxymethyl group (—CH2OH) on graphene-supported Pt catalyst was 0.4 V lower than that of aldehyde group (— CHO), which indicated that —CH2OH was oxidized in preference to — CHO, and finally the DFF selectivity of 94.4% was obtained[22].
In 2014, Li Wenzhen et al., still working at Michigan Technological University, reported the preparation of FDCA in an anion exchange membrane (AEM) electrolytic flow cell[23]. They found that the selectivity of the product was highly dependent on the electrode potential and the surface composition of the catalyst. 5-Hydroxymethyl-2-furoic acid (HMFCA) was formed by the preferential oxidation of aldehyde groups on Au/C catalyst, and higher electrode potential was required for further oxidation. However, the oxidation potentials of aldehyde group and alcoholic hydroxyl group on Pd/C catalyst are very close (0.58 V and 0.55 V, respectively), so there are two competitive reaction paths. When the potential is low, the oxidation of aldehyde group on Pd/C is much slower than that on Au/C, and the oxidation rate of aldehyde group can be greatly increased by increasing the potential or alloying with Au. Compared with the monometallic catalyst, the Pd-Au alloy catalyst (Pd1Au2/C) can complete all the oxidation steps at a lower potential and achieve 83% FDCA yield within 1 H. Therefore, the pathway and depth of the HMF electrocatalytic conversion reaction can be controlled by precisely adjusting the potential and the surface composition of the catalyst. According to the catalytic properties of different metals, Kim et al. Of Yonsei University designed a three-dimensional multilayer composite electrode of nano-graphene oxide supported Au and Pd nanoparticles (NPs) by layer-by-layer assembly[24]. When Au is in the outer layer, HMF is rapidly oxidized to HMFCA and diffuses to the surface of the inner Pd nanoparticle, and Pd can catalyze the rapid conversion of HMFCA to FFCA to obtain the final FDCA (Fig. 3). Although the yield of FDCA on the three-dimensional composite electrode is low, this study shows the importance of the design of the catalyst structure to control the overall reactivity of the multi-step consecutive reaction.
In addition to Pt, Au and Pd, noble metal Ru also has good HMF electrocatalytic oxidation activity. Chen Yu et al. Of Shaanxi Normal University loaded polyethyleneimine-complexed ruthenium catalyst (Ru (Ⅲ) -PEI @ MWCNTs) on carboxyl-modified carbon nanotubes by self-assembly, and used it for alkaline electrocatalytic HMF oxidation reaction. After 27 H of stable electrolysis, the FDCA yield and faradaic efficiency (FE) reached 94%[25]. The catalyst can also be used as a cathode catalyst to carry out an electrocatalytic nitrogen reduction coupling reaction while oxidizing HMF. Recently, Wang et al. Reported two new catalysts, Ir-Co3O4 and Pt/Ni(OH)2[26][27]. Unlike the conventional thinking, Ir single atom and Pt nanoparticles do not play the role of main catalyst in these two catalysts, but only act as co-catalysts for Co3O4 and Ni(OH)2 to enhance the catalytic activity of the latter. The Ir single atom improves the adsorption performance of the conjugated C = C bond in the HMF structure on the Co3O4 main catalyst, and the Pt nanoparticle not only is the HMF adsorption site itself and can reduce the HMF adsorption energy, but also can reduce the energy required for Ni(OH)2 dehydrogenation, thereby accelerating the formation of the catalytic active site of Ni (OH) O. It can be seen that the catalytic activity of noble metals is not as good as that of transition metal oxygenates under the condition of external potential driving.
The high catalytic activity of noble metals enables them to efficiently activate molecular oxygen and oxidize alcohols and aldehydes to the corresponding acids under thermal catalytic conditions without external potential driving force. Therefore, in addition to injecting electric energy to drive HMF oxidation to generate target products, another mode of electrocatalytic HMF oxidation conducts the charges generated by aerobic oxidation of HMF through an external circuit, and outputs electric energy while generating FDCA, thus forming a fuel cell that generates high-value products. Zhang Haimin et al., Institute of Solid State Physics, Chinese Academy of Sciences, combined precious metals with transition metals to design a Pt/NiSx nanocatalyst (PtNiSx/CB) supported by carbon black as the anode catalyst of HMF fuel cell[28]. At the reaction temperature of 60 ° C, the current density of the fuel cell system is 6.8 mA·cm-2, and the discharge efficiency is up to 2.12 mW·cm-2. At the same time, HMF can be efficiently converted into FDCA, and the FE can reach 98%. This work not only contributes to the design of bifunctional electrocatalysts for fuel cells, but also provides a new idea for biomass high-value coupling energy production. It is worth pointing out that the fuel cell strategy requires the use of catalysts with appropriate activity to complete the efficient conversion of HMF to FDCA, otherwise the reaction will stay in the intermediate stage or be completely oxidized to CO2. For example, Pera-Titus et al. Used PtRu (1:1) alloy catalyst at 50 ℃ for 100 H, the conversion of HMF was only 54%, and the products were mainly DFF (selectivity 72%) and FFCA (selectivity 24%), with only a small amount of FDCA[29].
Table 1 is a summary of the catalytic performance parameters of reported noble metal catalysts.

3 Transition metal catalyst

In the aerobic thermal catalytic oxidation of HMF, noble metal catalysts are more efficient than transition metals in activating molecular oxygen, so the overall catalytic reaction performance is more excellent[12]. However, in the electrocatalytic mode, the catalytic performance of transition metals can be brought into full play due to the support of the potential driving force. In addition to the huge price advantage, more and more researchers are paying attention to the more abundant transition metal catalysts. The catalytic performance can be further improved by optimizing the composition and structure of transition metals. In this section, according to the number of active metal components in the catalyst, the transition metal catalysts are divided into monobasic, binary and multicomponent catalysts.

3.1 Monobasic transition metal catalyst

In 1991, Grabowski et al. First used transition metal compound NiOOH as an electrocatalytic electrode in HMF oxidation reaction, and obtained 71% FDCA yield[19]. Since then, the research field of transition metal electrocatalytic oxidation of HMF has been silent for 25 years. The situation of energy crisis and environmental pollution makes the research of hydrogen production by water electrolysis a new force. However, the slow rate of oxygen evolution reaction in the anode leads to the low efficiency of water electrolysis. In 2016, Sun Yujie et al. Of Utah State University in the United States replaced the anodic water oxidation reaction of electrolytic water with HMF electrocatalysis, which was coupled with the cathodic hydrogen evolution reaction to obtain high-yield FDCA and high-purity hydrogen at the same time, and greatly reduced the potential required for the reaction, thus starting the research boom of transition metal electrocatalysis HMF oxidation reaction[31⇓~33].
Ni-series catalysts were first studied. Metal nickel foam and nickel sheet have certain catalytic performance, but their performance is not good because of their small electrochemical active area and small number of exposed active sites, so they are usually used as current collectors. Sun et al. Prepared a three-dimensional hierarchical porous metal nickel electrode (hp-Ni) by electrodeposition, which was used for HMF electrocatalytic oxidation to obtain 100% conversion and about 98% FDCA yield. Although the construction of porous high specific surface area structure can greatly improve the electrocatalytic performance of nickel metal, the X-ray photoelectron spectroscopy (XPS) characterization of the catalyst after reaction shows that the metal surface will inevitably be oxidized into oxides or hydroxides or oxyhydroxides in alkaline electrolyte. Therefore, the preparation of Ni-based oxides or the design of high-valence Ni surface may be an effective way to construct high-performance Ni-based catalysts. In fact, their Ni2P nanoparticle array (Ni2P NPA/NF) and hierarchical porous Ni3S2 hierarchical porous particle (Ni3S2/NF) catalysts constructed by phosphating and sulfurizing nickel foam directly increased the specific surface area of nickel foam and the proportion of high-valence Ni to greatly improve the catalytic oxidation performance, and more Ni oxides appeared on the surface of the catalyst after a limited number of cycles[32][33].
In addition to the simple surface heat treatment of the nickel foam, nano-metallic nickel or nickel compounds can also be designed as catalysts for HMF electrocatalytic oxidation.Such as NiO, Ni/NiO, Ni(OH)2, NixB, Ni3N@C,Ni3N, S-Ni, N-NiMoO4, Ni2S3, NiSe@NiOX, Ni/CP, etc[34][35][36][37][38][39][40][41][42][43][44]. Schuhmann et al. Of Ruhr University in Germany first identified that high-valence Ni species were the necessary prerequisite for the oxidation of HMF by comparing the appearance of Ni2+/Ni3+ oxidation peaks on the NixB and nickel foam electrode when HMF was added[37]. Qi Wei et al., Institute of Metal Research, Chinese Academy of Sciences, used nickel sulfate as raw material to electrodeposit Ni nanosheets on carbon paper, and achieved efficient electrocatalytic conversion of HMF at a very low reaction potential[44]. The structural analysis of the catalyst after the reaction revealed that the Ni nanosheets transferred electrons to the carbon support and were easily oxidized to NiOx or Ni(OH)x by themselves, while the theoretical calculation showed that the high-valence Ni species contributed to the adsorption of HMF molecules. However, with the formation of oxidized Ni species, the conductivity of the catalyst decreases rapidly, resulting in a rapid decline in catalytic performance. The conductivity and catalytic stability can be improved by coating the Ni-based catalyst with carbon, and the carbon can also modulate the electronic state of Ni species and stabilize the morphology of the catalyst[38,40]. However, it is worth noting that the oxidation corrosion of carbon under high anodic potential is unavoidable, which will lead to the shedding of active metal components from the carbon substrate. The strategy of further enhancing charge transfer by introducing a second non-metallic component can also be achieved by non-carbon elements. Wang Yanqin et al. Of East China University of Science and Technology doped NiBx with different degrees of P.XPS characterization showed that appropriate P-doping changed the electronic structure and distribution of nickel surface, and the surface nickel hydroxide species increased. AC impedance spectroscopy (EIS) confirmed that P-doping enhanced the charge transport ability of the catalyst[45]. Han Dongxue et al. Of Guangzhou University obtained high catalytic performance with 99% yield of FDCA by designing a core-shell structure catalyst with conductive NiSe nanowires as the core and high-valence NiOx as the shell[43].
In addition, controlling the preparation of catalytic active crystal plane and improving the dispersion of active components are effective means to improve the catalytic performance[46][47]. Tilley et al. Of the University of New South Wales synthesized hexagonal nano-Ni dendrites using a face-centered cubic Au core as a seed, and found that extending the length of the Ni dendrites could effectively expose specific catalytic active crystal planes, thereby improving the catalytic oxidation activity of HMF[46]. However, they have not studied whether the Au core has catalytic or synergistic effects. Covalent organic frameworks (COF) or metal-organic frameworks (MOF) are novel methods for the preparation of highly dispersed metal electrocatalysts. However, the yield of FDCA was only 58% when Morris et al. At Virginia Tech used a nickel-modified COF film TpBpy-Ni @ FTO electrode, while the oxidation of FFCA to FDCA, which is generally considered to be easy, was the rate-determining step in this system[48]. Kornienko et al., University of Montreal, Canada, studied the catalytic performance of Ni-MOF-74 defect sites and found that its electrocatalytic activity was closely related to the four-coordinate Ni-O4 sites[49].
Transition metal Co is also a common catalyst for electrocatalysis, and its common oxide Co3O4 has good catalytic performance in HMF oxidation reaction due to its good conductivity and high valence Co component. Our research group synthesized a defect-rich Co3O4 nanowire bifunctional electrocatalyst (CoNW/NF) through hydrothermal and high temperature calcination treatment, which can simultaneously carry out efficient HMF oxidation and hydrogen evolution reaction in a two-electrode system, and the coupling system can be driven by a 1.5 V dry cell, and the structure and performance of the catalyst remain good after five cycles of 30 H (Fig. 4)[50]. The high catalytic activity and stability of Co3O4 have also been confirmed by other researchers[26,51]. Low valence CoO also has certain catalytic performance, and Han Buxing et al. Can significantly increase the concentration of oxygen vacancies on CoO by doping a small amount of selenium (Se), and obtain a higher electrochemical active area and a lower charge transfer resistance[52]. Introducing lattice defects such as oxygen vacancies to improve catalyst performance is expected to become a universal strategy for catalyst design. Schuhmann et al. Screened a series of cobalt alloy (CoX; X = B, Si, P, Te, As) catalysts and found that the CoB electrocatalytic HMF oxidation onset potential was the lowest, and 94% FDCA yield could be achieved by electrolyzing HMF at 1.45 VRHE[53]. Although the morphology of the catalyst was well maintained after the reaction, the possibility of oxidation of the catalyst could not be ruled out. Sun Yujie et al. Previously studied the Co-P/CF catalyst prepared by electrodeposition, and found that Co and P on the surface of the catalyst would be oxidized to CoOx (or CoOOH) and POx after electrolytic HMF reaction[31]. Cha et al. Found that Co was oxidized and P was completely lost after the reaction of Co-P catalyst electrodeposited in aqueous phase[54]. Therefore, the real catalytic active phase in the final stable operation of this type of catalyst may be Co(OH)2 or CoOOH.
Our research group has recently prepared CoOOH nanosheet array electrodes for the oxidation of furan and its derivatives by electrodeposition and in situ oxidation, and found that CoOOH can selectively and efficiently oxidize aldehyde groups and hydroxyl groups to the corresponding carboxyl groups without destroying the furan ring, and the activation energy of the electrocatalytic reaction is much lower than that of noble metal catalysis[55]. Because aldehyde-free 2,5-furandimethanol (BHMF) has obvious advantages over HMF in terms of thermal and chemical stability, it is easier to store and transport on a large scale, while the electrolysis of BHMF using simple and readily available CoOOH as a bifunctional electrocatalyst can obtain 90.2% FDCA yield and ~ 100% hydrogen evolution FE, showing good industrial scale-up prospects and energy utilization efficiency (Fig. 5). Kang et al., University of Electronic Science and Technology of China, prepared Co(CO3)0.5(OH) by solvothermal method, and then oxidized it to CoOOH by anodic oxidation, which could obtain ~ 100% FDCA selectivity and FE for HMF oxidation[56]. In order to compare the catalytic activity of oxyhydroxides, Chio et al. At the University of Wisconsin prepared MOOH films (M = Ni, Co, Fe) on FTO substrates for electrocatalytic HMF oxidation, and found that CoOOH could catalyze HMF oxidation at a lower potential because the potential required for Co2+/Co3+ conversion was lower than that of Ni2+/Ni3+[57]. However, the activity of CoOOH is much lower than that of NiOOH at the same potential. This is mainly due to the slow kinetics of HMF oxidation on CoOOH and the fact that not all Co3+ sites in CoOOH are available for a reversible catalytic cycle. However, this does not mean that CoOOH can not catalyze HMF oxidation efficiently, and high FDCA yield and FE can also be obtained by rationally designing catalysts with high electrochemical active surfaces with abundant reversible catalytic active sites. For FeOOH, there is no catalytic activity until water oxidation occurs, but traces of FDCA can be detected when the potential exceeds 1.71 V vs RHE. This is also consistent with the lack of literature reports on single Fe catalysts, and Fe-based high-performance catalysts need to be studied and developed.
Although the oxygen evolution reaction and the HMF oxidation reaction become competitive reactions beyond a certain potential, it can be seen from the above Fe-based catalyst that the oxygen evolution reaction active catalyst does not necessarily have the HMF electrocatalytic oxidation activity, and vice versa, and even a wider HMF oxidation voltage window can be obtained. Generally speaking, the surface of oxidized Cu has poor oxygen evolution reaction performance, but Choi et al. Of the University of Wisconsin found that nanocrystalline copper foam (NCF) has excellent HMF electrocatalytic oxidation performance, with FDCA yield greater than 96.4% and FE greater than 95%[58]. This is due to the formation of an amorphous oxide/hydroxide layer with a thickness of about 30 μm on the surface of the NCF electrode, which greatly increases the specific surface area and the catalytic active Cu ions, which have a more flexible coordination environment and a stronger force with HMF. Cha et al. compared the HMF electrocatalytic oxidation performance of CuO and Cu(OH)2 nanowires, and found that due to the Cu atom density on the surface of CuO nanowires was three times higher than that of Cu(OH)2 nanowires, not only the voltage required for HMF oxidation was lower, but also the electron transfer rate was higher, and both the FDCA yield and FE could exceed 90%[59].
Although more and more transition metal compounds have been found to have alkaline HMF electrocatalytic oxidation activity, such as WO3/Ni, VN, etc., the aldehyde group of HMF is prone to acetalization in strong alkaline electrolyte to produce highly colored Humin and other by-products, so some researchers have begun to study HMF electrocatalytic oxidation under acidic conditions[60][61]. Moreover, FDCA has low solubility in acidic solution and can be precipitated naturally, thereby reducing the cost of product separation. Chio et al. At the University of Wisconsin used manganese oxide (MnOx) as an anode in sulfuric acid solution at pH = 1 to carry out the electrocatalytic conversion reaction of HMF at 60 ° C, and achieved 99% HMF conversion, however, the yield of FDCA was only 53.8%, and the by-product was 21% maleic acid[62]. The precipitation and purification of FDCA can be rapidly achieved by cooling the reaction to room temperature. Latsuzbaia et al. Of TNO in the Netherlands tried to use post-transition metal PbO2 as an acidic electrolysis electrode, but the · OH radical produced by it would completely mineralize HMF[63]. Therefore, the design, reactivity and selectivity control of HMF oxidation electrocatalysts in acidic systems still need to be further studied.
Another strategy to reduce costs is to increase the total utilization efficiency of input energy, producing high value-added products at both ends. Theoretically, the FE of the system can be close to 200% by coupling the anodic HMF oxidation with the cathodic hydrogen evolution reaction (HER), which can be achieved by many bifunctional catalysts mentioned above. In addition, the cathode HER can also be replaced by HMF reduction or reduction reactions of other organic molecules, constituting paired organic electrolysis reactions. Wang Jianguo et al. of Zhejiang University of Technology sprayed 3D hollow VN nanospheres and Pd/VN on both sides of the bipolar membrane, which were used as catalysts for electrocatalytic HMF oxidation (ECO) and hydrogenation (ECH) reactions, respectively, to generate FDCA and 2,5-bis (hydroxymethyl) -tetrahydrofuran (DHMTHF) at the anode and cathode, respectively, and realized the coupling of HMF oxidation and hydrogenation in pairs (Figure 6a)[61]. Sun Licheng et al., Royal Institute of Technology, Sweden, used NiBx as a bifunctional catalyst to hydrogenate p-nitrophenol (p-NP) to p-aminophenol at the cathode while carrying out HMF oxidation reaction at the anode, with both conversion efficiency and selectivity ≥ 99% (Fig. 6B)[64].
Table 2 is a summary of the catalytic performance parameters of reported single transition metal catalysts.

3.2 Binary transition metal catalyst

It is a common idea in catalytic research to regulate the performance of catalysts by adding second components. For example, in the oxygen evolution reaction of water electrolysis, the presence of a small amount of Fe can greatly enhance the conductivity and OER electrocatalytic activity of NiOOH[68]. In 2018, Huber et Al. Of the University of Exeter first reported the performance of NiM (M = FE, Ga, Al) layered double hydroxide (LDH) nanosheets grown on carbon fiber paper to catalyze HMF to FDCA, and found that the performance of NiFe-LDH was better than that of NiGa-LDH, NiAl-LDH and Ni(OH)2, and 98% FDCA yield and 99.4% Fe could be obtained after 10 H of reaction[14]. For FE-based materials with low HMF electrocatalytic activity, Menezes et al., University of Berlin, used the structural order and high conductivity of intermetallic compounds (IMC) formed by Sn, Si and Fe to efficiently electrocatalyze the oxidation of HMF to FDCA, and Fe can reach more than 90%[69,70]. In situ X-ray absorption and Raman spectroscopy indicate that the FeIII species formed on the IMC surface may be the catalytically active phase under strong alkaline conditions.
Synergistic effect is one of the main contents of catalysis research. By comparing the catalytic activity of NiO, NixCo3-xO4 and Co3O4, Han Dongxue et al found that there was an obvious synergistic effect between Ni and Co, and increasing the content of Ni could reduce the Tafel slope and improve the reaction kinetics.The onset potential and charge transfer resistance decreased with the increase of Co content, and the morphology of the catalyst changed from nanosheets to nanowires with the increase of Co/Ni ratio. The 90% yield of FDCA was obtained by electrolysis on the optimal NiCo2O4 nanowires at a constant voltage of 1. 55 V vs RHE[71]. Kang et al. Of Sogang University in Korea found that the coordination number of Co atoms near the Ni atom was reduced due to the higher electronegativity of Ni than that of Co through in situ X-ray near-edge absorption spectroscopy (XANES), and the reduction ratio of Co3+ to Co2+ on the NiCo2O4 electrode was higher than that on the Co3O4 electrode in the HMF oxidation reaction, thus obtaining higher HMF oxidation performance[72]. However, the role of Ni3+/Ni2+ couple in HMF oxidation in NiCo2O4 has not been studied in depth. Fu Xianzhu et al. Of Shenzhen University found that although the initial potential and Tafel slope of MOF-based NiCo bimetallic catalyst are between those of monometallic Co and Ni catalysts,However, due to the strong electronic interaction between Ni and Co, the adsorption, desorption and electron transfer processes of oxygen-containing organic substrates are enhanced, and the conductivity and electrochemical active area are also higher, which make the activity (current density) of bimetallic catalysts much higher than that of monometallic catalysts[73]. Li Guangqin et al. of Sun Yat-sen University studied the modulation of Co, Mn and Fe doping on the activity of two-dimensional MOF catalyst (NiBDC), and found that Co doping is helpful to form high-valence Ni catalytic active species, thus improving the catalytic performance, while Mn and Fe doping is worse than undoped catalyst[74]. Zhejiang University Fu Jie et al. Prepared activated carbon-supported Cu, Ni monometallic and bimetallic electrocatalysts with different proportions by nitrate impregnation method, and the FDCA yields on Cu(OH)2, NiCu(OH)2 and Ni(OH)2 were 71.2%, 93.3% and 3.8%, respectively, after passing the same amount of electricity, indicating a strong synergistic effect between Cu and Ni, mainly because the addition of Ni improved the dispersion of Cu and the density of active sites[75]. Huang Hongwen et al. Of Hunan University modified ultrathin Ni(OH)2 nanosheets with amorphous Cu(OH)2, and found that a proper amount of Cu was helpful to form more Ni3+ active species, enhance the interaction between active sites and HMF and intermediates, and adjust the electronic structure of Ni, thus obtaining 91.2% FDCA yield[76].
Interface engineering is an effective means to optimize the electronic structure, regulate the adsorption behavior, and expose the active sites. The modification or doping of binary transition metal catalysts with nonmetal N, S, P can produce hybridization effect or construct heterogeneous interfaces, which can regulate the catalytic performance. Hou Yang et al of Zhejiang University found that the hybridization of N-doped MoO2 and Ni2S3 could construct a strong coupling interface, and the electrons shifted from Ni2S3 to MoO2 to improve the valence state of Ni, which enhanced HER and obtained a higher FDCA yield[77]. A similar phenomenon of electron migration from Ni to V2O3 was also observed on the Ni3N-V2O3 catalyst, resulting in good catalytic performance[78]. Li Feng et al. Of Beijing University of Chemical Technology prepared NiP-Al2O3/NF catalyst by reducing NiAl-LDH, whose Ni2P nanoparticles were highly dispersed and fully exposed active sites, and finally achieved 97.8% FDCA yield[79]. Fu Honggang et al. Of Heilongjiang University designed a carbon-coated MoO2-FeP heterojunction bifunctional electrocatalyst, and the interfacial electron migration from MoO2 to FeP was beneficial to the optimization of H2O and H* adsorption energy, while the accumulated holes on MoO2 were beneficial to the adsorption of HMF and promoted the oxidation of HMF, resulting in nearly 100% conversion and 98.6% FDCA selectivity[80].
The spatial position of atoms is usually closely related to the chemical coordination environment, and the change of coordination number caused by defects and vacancies will significantly change the molecular adsorption and catalytic activity. It is well known that there are two spatial configurations of tetrahedron Co2+ and octahedron Co3+ in Co3O4. Wang Shuangyin et al. Of Hunan University replaced the two sites with Zn2+ and Al3+ respectively, and the order of HMF electrocatalytic activity of spinel oxides was Co3O4>ZnCo2O4>>CoAl2O4. In-depth studies found that octahedral Co3+ was the catalytic active site for HMF oxidation, and tetrahedral Co2+ provided acidic adsorption sites to promote adsorption.Therefore, the introduction of Cu2+ with more Lewis acid sites than Co2+ to replace tetrahedral Co2+ can not only enhance the adsorption,It can also improve the exposure ratio of octahedral Co3+, thus improving the catalytic HMF electrooxidation performance of CuCo2O4 by 4 times compared with that of Co3O4 (Fig. 7)[81]. They also constructed a three-dimensional structure of NiO-Co3O4 defects on nickel foam etched by hydrochloric acid, and found that the abundant cation vacancies generated by the interface effect could control the electronic properties of Co and Ni atoms and improve the oxidation state of Ni, thus showing better catalytic activity than NiO and Co3O4[82]. Lemoine et al. Of Gakushuin University in Japan changed the coordination-saturated Co into an electron-deficient state through the electron-withdrawing induction effect of the copper fluorophosphate host, and the open-circuit voltage test showed that HMF could spontaneously adsorb on the surface of the catalyst, thereby improving the catalytic activity[83].
Table 3 is a summary of the catalytic performance parameters of reported binary transition metal catalysts.

3.3 Multicomponent transition metal catalyst

On the basis of binary transition metal catalysts, it is possible to enhance the interaction between components by adding more components, so as to obtain better catalytic performance. The variety and structural diversity of LDH lamellar cations and interlayer anions make it an important platform for catalyst design. Yan Kai et al. Of Sun Yat-sen University designed two ternary transition metal electrocatalysts, NiCoFe-LDH and NiCoMn-LDH, based on NiCo-LDH[89][90]. Compared with the binary LDH catalyst, the addition of Fe makes NiCoFe-LDH show lower overpotential, smaller charge transfer resistance, larger double-layer capacitance value and better durability, with HMF conversion of 95.5% and FDCA yield of 84.9% in 1 H. Mn itself has good redox properties, so the yield of FDCA can reach 91. 7% after 2. 5 H of electrocatalytic oxidation of HMF by NiCoMn-LDH. In a word, the addition of Fe and Mn elements changes the electronic environment of Ni and Co, and the three transition metals will produce new synergistic effects, exposing more active sites or generating oxygen vacancies, thus promoting the catalytic reaction, which provides some ideas for the design of efficient electrocatalysts. However, the high onset potential (≥1.42 VRHE), the low applicable substrate concentration (1 – 5 mmol/L HMF), and the need to increase the reaction temperature to obtain high FDCA yield of these two catalysts indicate that there is still much room for structural optimization and performance improvement of these catalysts.
The electronic structure of the main active component can also be modulated by the addition of low or inactive components, thereby further improving the catalytic performance. Fu Yu of Northeastern University and Qi Wei of the Institute of Metals prepared CoNiFe-MOFs/NF electrode by hydrothermal method, and the yield of FDCA was up to 99.76% and FE was close to 100% by potentiostatic electrolysis of 1.4 VRHE[91]. XPS results show that the addition of Co and Fe causes an increase in the binding energy of Ni 2p, and the chemical shifts are + 0.36 eV (NiFe-MOFs), + 0.42 eV (CoNi-MOFs) and + 0.51 eV (CoNiFe-MOFs), respectively, which indicates that Co and Fe have a significant electronic modulation effect on Ni. In addition, NiOOH will be formed on the surface of MOF-based catalysts after a long time of electrooxidation reaction, so the final stable existence form and interaction of Co, Ni and Fe are worth further study. Zhao Guohua of Tongji University et al. Electrodeposited NiFe-LDH (CoFe @ NiFe) on hydrothermally grown CoFe-LDH, and found that its electrocatalytic performance was better than that of bimetallic LDH, because Ni3+ was the main catalytic active component, while the content of Ni3+ on CoFe @ NiFe was slightly higher than that on NiFe-LDH[92].
Defect engineering is another general strategy for catalyst design, where fabrication defects can manipulate the electronic structure and geometry of the catalyst. Wang Shuangyin et al. Have written a lot on defect catalysis. Recently, oxygen plasma was used to oxidize high-entropy LDHs containing up to eight metal elements into high-entropy oxides (HEOs) at low temperature, and the plasma treated (FeCrCoNiCu)3O4 five-component metal oxides (P-HEOs) were used as a representative to compare the HMF electrocatalytic oxidation performance with the sample prepared by the traditional reverse coprecipitation method at 1000 ℃ (C-HEOs)[93]. The results show that the oxygen plasma treatment can produce more oxygen vacancies and higher specific surface area, which makes the P-HEOs have higher HMF electrocatalytic activity, lower onset potential and faster reaction kinetics[94].
Improving the conductivity of the catalyst is one of the efficient strategies to improve the catalytic performance of LDH. Fu Xianzhu et al. Designed CuxS@NiCo-LDH core-shell structure nanoarray bifunctional electrocatalyst, which uses highly conductive CuxS as the core to help charge transfer from the substrate Cu to the catalytic active layer of NiCo-LDH, and the nanoarray structure constructs an open channel for mass transfer and exposes a large number of active sites[95]. In addition, Co can also modulate the electronic structure of Ni to produce more Ni3+, which is beneficial to the adsorption and oxidation of substrate and intermediate molecules. The synergy of the three aspects makes the catalyst show excellent electrocatalytic activity and high Faradaic efficiency, and the two-electrode coupled electrolysis system can simultaneously produce H2 and FDCA at a low voltage of only 1. 34 V. However, in the electrocatalytic reaction, the large loss of Cu in the CuxS does not lead to a significant decline in performance and serious damage to the structure, but it is still worthy of attention.
Table 4 is a summary of the catalytic performance parameters of the reported multicomponent transition metal catalysts.

4 Nonmetallic catalyst

There are few studies on electrochemical HMF oxidation using solid nonmetallic materials as catalysts. In 2019, Oschatz et al., Institute of Colloid and Interface, Max Planck Institute, Germany, prepared B and N co-doped porous carbon (BNC) by ball milling-carbonization for electrochemical HMF oxidation and N2 reduction reaction[96]. After potentiostatic electrolysis in 0.1 M KOH at 1.9 VRHE for 6 H, the HMF conversion over BNC-2 catalyst with high boron content was 71%, and the FDCA yield was 57%, which was much higher than that of nitrogen-doped porous carbon, while the carbon paper substrate had no catalytic effect on HMF. This is currently the only report of nonmetal direct electrocatalytic HMF oxidation.
TEMPO and its derivatives are the main nonmetallic catalysts for the electrocatalytic oxidation of HMF. Cha and Choi obtained 100% HMF conversion and 99% FDCA yield using conductive carbon felt as electrode and TEMPO as catalyst in boric acid buffer (pH = 9.2) at a potential of 1.54 V (vs RHE) for ~ 1.2 H[21]. Li Wenzhen et al. Used carbon felt as anode and 4-acetamido TEMPO (ACT) as catalyst to electrolyze in the same boric acid buffer at a potential of 0.7 ~ 0.9 V (vs Ag/AgCl), and obtained 97% FDCA yield and 98% FE after 72.2 C charge[97]. Chio et al. Recently used a glassy carbon electrode as the working electrode to compare the differences between TEMPO and ACT in terms of catalytic performance, stability, and reaction pathway[98]. It was found that ACT requires a higher potential to activate than TEMPO, but the catalytic reaction rate is higher at high potential. TEMPO evaporates easily, while ACT evaporates little at 40 ℃. ACT is more unstable at pH > 10. The main reason for these differences may be the electron-withdrawing effect of the 4-acetamide group on ACT.
To sum up, although the indirect catalytic oxidation of TEMPO and its derivative catalysts can obtain high FDCA selectivity and yield, due to its strong water solubility, a homogeneous system is formed in the catalytic reaction, which will bring new separation and purification problems[99,100].
Table 5 is a summary of the catalytic performance parameters of reported non-metallic catalysts.

5 Mechanism of Electrocatalytic Oxidation of HMF

The study of catalytic reaction mechanism mainly includes the identification of catalyst active sites, the derivation of reaction paths and the determination of elementary reaction rates. Thermal catalytic oxidation of HMF has been discussed in depth in these three aspects, but the mechanism of electrocatalytic oxidation mainly focuses on the derivation of reaction paths, and the active sites and structure-activity relationship are mentioned in the report of novel catalysts, while the elementary reaction kinetics is less involved[6,102⇓ ~104]. Using methanol, ethanol, benzyl alcohol and benzylamine as substrates, Shalom et al. Of Ben-Gurion University in Israel proved that the C-H bond cleavage process was involved in the rate-limiting step of the oxidation reaction by means of kinetic isotope experiments, and that the oxidation rate of substrate molecules was linearly related to the C-H bond dissociation energy[88]. This study did not directly use HMF as the substrate for kinetic studies, but the catalyst has been applied to the electrocatalytic oxidation of HMF, which may inspire future kinetic studies. At present, the "electrochemical-chemical" oxidation pathway mediated by metal redox couples or TEMPO+ is widely used to explain the active source of electrocatalytic HMF oxidation, that is, the external circuit drives the electrode surface to produce high-valence species, and the high-valence species carry out nucleophilic dehydrogenation oxidation of the reactant molecules through chemical oxidation[37,57,98,105,106]. However, Fu Xianzhu et al. Recently found that there were two redox couples, Co2+/Co3+ and Co3+/Co4+, on the CoOxHy catalyst prepared by electrodeposition, and their different electrode potentials led to the selectivity of the oxidation reaction. Co3+ could only oxidize aldehyde groups, while Co4+ could oxidize aldehyde groups and hydroxyl groups, while NiOOH had no such selectivity[107]. In addition, metal catalysts usually undergo surface reconstruction in electrocatalytic reactions, and the actual catalytic active sites or active species are different from those in the electrolyte, which depends on the use of dynamic characterization techniques for in-depth analysis[74]. Therefore, there is still room to study the intrinsic activity source of HMF electrocatalytic oxidation.
On the other hand, since the oxidation potentials of the —CH2OH and — CHO groups on the furan ring are very close (0.55 and 0.58 VRHE on Pd/C electrode, respectively), there are two possible reaction paths for HMF oxidation (Fig. 8)[23]. Wang et al. Found that there were two reaction paths for the electrocatalytic oxidation of HMF by Ni3N, but the relative content of HMFCA and DFF was closely related to the alkali concentration of the electrolyte. The concentration of HMFCA was much higher than that of DFF in 1 mol/L KOH electrolyte, while the concentration of DFF increased significantly in 0.1 mol/L KOH electrolyte[39]. Nam et al., Seoul National University, Korea, found that the electrocatalytic oxidation of HMF by NiO nanoparticles was easier than that of DFF in CO2 saturated 0.5 mol/L KHCO3 electrolyte (pH = 7.2), and the oxidation reaction rate of hydroxyl group was faster than that of aldehyde group[34]. This indicates that the OH- plays an important role in the electrocatalytic conversion of HMF, the high concentration of OH- in the bulk accelerates the hydration of aldehyde groups, and the high concentration of OH- adsorbed on the surface promotes the activation of C — H and O — H[106]. In fact, there is a similar pH dependence in the thermocatalytic reaction[108]. In addition, the surface chemical properties of the catalyst can affect the adsorption behavior and reaction rate of the two groups, and seriously affect the formation and transformation of intermediate products. In the existing literature on electrocatalytic HMF oxidation, a large number of studies have been based on the concentration changes of HMF, reaction intermediates (DFF, HMFCA and FFCA) and target product FDCA at different times or under different input charges to reverse the catalytic reaction pathway. In fact, due to the instability or high reactivity of aldehyde groups in the electrolyte environment, there is a huge gap between the chromatographic results and the real situation, and the derived reaction path is still questionable.
In Situ spectroscopies such as SFG, IR, UV-Vis, Raman, X-ray absorption spectroscopy and electrochemical impedance spectroscopy were used to study the surface structure of the catalysts, the adsorption and desorption of furan molecules on the catalyst surface and the reaction rules at different voltages.It is helpful to identify the real active sites of electrocatalytic HMF oxidation and clarify the reaction mechanism[38][46,49][49,109,110][39][81]. Second-order nonlinear SFG has become the most powerful tool for the study of interfacial reactions because of its unique interfacial selectivity and sensitivity, and is not disturbed by bulk solution. Wang et al. Used in situ SFG to study the reaction path of HMF on Ni3N@C electrocatalyst (electrolyte 1 mol/L KOH)[38]. Only HMF signal can be detected at potentials below 1.25 V, indicating that HMF is not oxidized; When the potential reached 1.35 V, HMF began to oxidize, and the signal peak of HMFCA was detected. After the potential was raised to 1.45 V, the characteristic peak of FDCA began to appear. The potentiostatic electrolysis experiment at 1.45 V showed that there was only the characteristic peak of HMF at first, and the characteristic peak of HMFCA began to appear with the extension of electrolysis time, and reached the strongest at 35 min. After that, the signal of the intermediate product gradually disappeared, and the characteristic peak of FDCA continued to increase and reached a constant at 90 min. HPLC analysis was consistent with the in situ SFG results, confirming the HMFCA reaction pathway on this catalyst (Fig. 9).
Surface-enhanced Raman spectroscopy (SERS) is also an efficient tool to study the surface and interface characteristics, intermolecular interactions and surface molecular adsorption behavior, with high surface selectivity. Kornienko et al., University of Montreal, Canada, carried out a series of in situ SERS studies of electrocatalytic HMF oxidation[49,109⇓ ~111]. In 2019, they first reported the in situ Raman study of HMF electrocatalytic oxidation using Au nanoparticles as catalyst and SERS substrate, and found that HMF was adsorbed on the Au surface through furan ring and hydroxyl group in 0.01 mol/L KOH electrolyte, and the surface alkoxy intermediate was formed after applying a bias of 1.0 ~ 1.4 V, and the final main product was DFF[110]. Because the catalytic activity of transition metal oxides is much higher than that of Au, but they lack surface Raman enhancement effect, they electrodeposited 1 ~ 2 nm transition metals (Ni, Co, Fe) on the electrodeposited dendritic Au plasmonic substrate to construct Au-transition metal heterojunction with SERS response (Fig. 10a). In situ Raman spectroscopy showed that the disordered Ni layer on the surface was transformed into NiOOH in 1 mol/L KOH, which could efficiently catalyze the oxidation of hydroxyl and aldehyde groups to FDCA. In 0. 01 mol/L potassium hydroxide, the formation of NiOOH was inhibited by the strong adsorption of hydroxyl groups of HMF with surface Ni(OH)2, while the aldehyde groups did not interact strongly with Ni(OH)2, resulting in the reaction staying in the DFF stage, and the oxidation of HMF was very slow even at high oxidation potential. In addition, NiFe and CoFe oxides with the highest oxygen evolution activity are not the optimal catalysts for HMF oxidation. This indicates that the alkali concentration, the physical and electronic structure of the catalyst, and the surface adsorption and interaction of the reacting molecules can all significantly affect the product selectivity.
Although many important information about the reaction pathway of HMF electrocatalytic oxidation can be obtained by in situ spectroscopy, the study of the reaction mechanism of HMF electrocatalytic oxidation is still in its infancy.It is necessary to obtain more key details with the help of advanced technologies such as isotope labeling, elementary reaction kinetics, and in situ transient spectroscopy. A comprehensive and in-depth understanding of the reaction mechanism will help to rationally design more efficient and stable HMF oxidation electrocatalysts and reaction systems[6,102][103,104][112].

6 Conclusion and prospect

In general, the electrocatalytic oxidation of HMF to FDCA is a cleaner and safer green method than the thermal catalytic method. The external potential drive excites the catalytic performance of the catalyst more effectively than the thermal drive, so that the transition metal and its compounds with poor activity in the thermal catalytic reaction show high catalytic activity and obtain better thermodynamic and kinetic performance than thermal catalysis or photocatalysis[55]. Noble metal catalysts usually have low initial potential, but the limited current density makes it difficult to improve the product yield and selectivity, and it is easier to produce DFF. Among the transition metals, nickel-based catalysts have higher catalytic activity and better kinetics than cobalt-based catalysts, but cobalt-based catalysts usually have lower initial potential. The iron-based catalyst itself has very limited catalytic activity, but it can be used as a second component or cocatalyst to regulate the structure and performance of the main catalyst. The OER performance of copper-based catalyst is poor, so HMF oxidation can obtain higher FE; Non-metallic catalytic electrode combined with TEMPO homogeneous catalyst can obtain better HMF conversion and FDCA selectivity, but the separation of TEMPO catalyst and product will bring new problems. In this paper, the literature report frequency of all electrocatalyst active elements is counted, as shown in Figure 11. Ni and Co compounds are the two most concerned electrocatalysts in the electrocatalytic oxidation of HMF.
Although many catalysts with high activity and novel structures have been developed in the field of electrocatalytic HMF oxidation, there are still many challenges that have not been overcome. Firstly, there are many misunderstandings and unfair comparisons in the performance evaluation of electrocatalyst development, so it is urgent to unify the performance evaluation conditions and establish the evaluation criteria. For example, different electrolyte concentrations lead to great differences in conductivity, different pH values lead to different activities of hydroxyl and aldehyde groups, different properties of catalyst carriers or current collectors lead to interference in the activity evaluation of the catalyst itself, and the carbon balance of HMF changes due to degradation and self-polymerization without catalyst or electrification[113].
Second, how to avoid the use of high-concentration electrolytes, especially strong alkaline electrolytes, is still a long way to go. Because of the low conductivity of pure water, the addition of conductive electrolyte is usually a necessary condition for electrocatalysis, and alkaline electrolyte can also improve the formation rate, selectivity and yield of FDCA.However, the subsequent separation and purification will consume a lot of energy and increase the operation process, and the direct selection of neutral or acidic electrolyte will cover the active sites of the catalyst due to the precipitation of FDCA, resulting in serious product retention and reduced electrolysis efficiency. Third, the technology of electrocatalytic preparation of FDCA based on low reactant concentration is still a long way from practical application. Although more and more research results have been published, the reaction current of most systems is in the 10~200 mA·cm-2, and the reaction volume is in the order of several to tens of milliliters, even if the flow reaction is carried out, it is difficult to meet the requirements of large-scale production. Therefore, it is still necessary to invest a lot of energy to study low-cost and high-activity catalysts, and to study the mass transfer and transfer problems after the scale-up of the reaction system. Fourthly, the global research intensity on the preparation of FDCA by electrocatalytic oxidation is unbalanced in different regions, and the research groups are mainly concentrated in China, North America and Germany. Five of the top 10 research groups have published research results in China, while other regions have not paid enough attention to it (Figure 12). However, in the research focus area, most of the research on the preparation of FDCA by electrocatalytic oxidation is transformed from the research on electrolytic water, or directly attempts the performance of OER catalysts, lacking original, targeted and focused catalyst development. Fifthly, from the economic point of view, the cost of preparing FDCA with HMF as the substrate is still high due to the insufficient large-scale production of HMF. Although some studies have proposed the one-pot synthesis of FDCA from biomass carbohydrates such as glucose, fructose and cellulose, the one-step method faces many problems such as low yield and poor efficiency due to the different types of catalytic active sites required for sugar dehydration and FDCA formation. In recent years, Zhejiang Tangneng Co., Ltd., a state-level high-tech enterprise jointly incubated by our research group, has made great progress in the large-scale production of HMF, which is expected to bring new hope to the field of HMF oxidation to FDCA[114⇓⇓~117].

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[2021-09-01] http://www.nimte.ac.cn/news/progress/201901/t20190107_5225680.html
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[2021-09-01] http://www.nimte.ac.cn/news/progress/202105/t20210531_6050324.html

Funding

National Natural Science Foundation of China(22072170)

Zhejiang Provincial Natural Science Foundation of China(LY19B030003)

Ningbo Science and Technology Bureau(2018B10056)

Ningbo Science and Technology Bureau(2019B10096)

Chinese Academy of Sciences(QYZDB-SSW-JSC037)

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