(2iy) Liquid Flow Fuel Cell with Modified Anode for Efficient Oxidation of 5-Hydroxymethylfurfural to Produce 2, 5-Furandicarboxylic Acid with Co-Generation of Electricity

With the increasingly serious environmental pollution and energy crisis, renewable resources such as biomass have attracted more and more attention in industries and academic community for production of fuels and platform chemicals. As one of the top biomass-derived platform chemicals, 5-hydroxymethylfurfural (HMF) that can be produced by dehydration of glucose or fructose is a versatile precursor for synthesis of fine chemicals, plastics, and liquid fuels.Especially, the oxidation product of HMF, i.e. 2,5-furandicarboxylic acid (FDCA), can be used as an important monomer instead of the petroleum-based plastic monomer terephthalic acid (PTA) for synthesis of bio-based plastics polyethylene 2,5-furandicarboxylate (PEF). FDCA has been listed as one of the twelve most promising value-added platform compounds by the U.S. Department of Energy. Various technologies have been developed for oxidation of HMF to produce FDCA, including chemical, electrochemical and biological approaches. At present, the chemical methods are relatively mature, but most of the processes still require harsh reaction conditions or expensive oxidants, and the catalysts are usually made of noble metals that are too costly to be used in industry. In the 1890s, researchers first realized the electrochemical conversion of HMF to FDCA in an H-type electrolytic cell using green electricity as energy input.Electrooxidation can avoid the introduction of external oxidants without using high temperature and high pressure conditions. The reaction process also can be easily controlled by adjusting the applied potential, selection of electrodes and electrocatalysts. However, electrochemical oxidation still requires input of external electricity. To date, researchers are still pursuing more energy-efficient and sustainable processes for producing FDCA. As a sustainable energy storage and conversion device, liquid flow fuel cell (LFFC) has been intensively studied and developed due to its characteristic of eco-friendly, high energy conversion efficiency and small size. Briefly speaking, a LFFC consists of a cathode and an anode, where the oxidation of fuels take place on the anode and reduction of oxidant take place on the cathode.

Due to the existence of overpotential, rational design and synthesis of anode catalysts with high activity, good stability and low cost are crucial. Metal catalysts such as gold, platinum, palladium, nickel, cobalt, copper, and their alloys have already been used as anodic catalyst for the oxidation of HMF. Platinum is an efficient noble metal used in electrochemical oxidation of HMF, but the oxidation product is mainly DFF, with a FDCA yield of less than 1%. Choi's group studied oxidation of HMF over platinum catalysts under acidic conditions and got the conclusion that Pt could just oxidize the alcohol groups of HMF along with the production of DFF instead of FDCA. Reported works also revealed that rhodium-based catalysts mainly resulted in the formation of DFF, no matter in acid or alkaline medium. Li et al. explored the route of electrochemical synthesis of FDCA on gold and palladium under alkaline conditions and achieved yield of higher than 80%, showing that the HMF conversion and FDCA yield were dependent on the potential and the synergistic effect of the gold and palladium bimetals. For the electrochemical oxidation of HMF, although noble metal-based electrocatalysts exhibit low HMF oxidation onset potentials, they can only provide low current densities and cannot rapidly oxidize HMF to target products. The process is always accompanied by formation of a large amount of by-products (humin). In addition, it is difficult to completely oxidize HMF to FDCA on a single precious metal. Therefore, researchers have gradually turned their attention to inexpensive and readily available non-noble metal catalysts. Transition metal nickel has good catalytic potential due to its unique 3d orbital electronic properties, and is regarded as an important candidate for HMF catalytic oxidation. Nickel oxides, hydroxides, phosphides, sulfides, borides, nitrides etc. have been used in electrochemical oxidation of HMF to obtain FDCA with high FDCA yield. Because of similar electronic structure with nickel, researchers also pay special attention to cobalt-based catalysts, while it seems that the catalytic performance is slightly lower than that of nickel-based catalysts. Besides, there are also some studies using other metals such as iron, copper, vanadium, and manganese. However, even if the oxidation of HMF can be achieved to some extent, the FDCA yield is unsatisfactory.

In this work, inspired by the working principles of electrochemical oxidation of HMF on the anode, and conversion of organic fuels to electricity by LFFC, we aimed at developing a novel reaction system to realize continuous oxidation of HMF to form FDCA under mild conditions with air (oxygen) as the oxidant and co-generation of electricity. Namely, LFFC was employed as a reactor for HMF oxidation, which can achieve easy control of the electron transfer kinetics by changing the anode catalyst, external resistance loading and cathodic electron carriers or catalysts, thus achieving easy control of product profile and FDCA yield. This work was focused on the development of potential anodic catalysts for oxidation of HMF, characterization of the developed anode, optimization of the process operational parameters and interpretation of the reaction mechanisms. The finding of this work thus can provide a novel technology for efficient production of FDCA from HMF under mild conditions.

Efficient oxidation of HMF to produce FDCA was achieved in a liquid flow fuel cell with co-generation of electricity at room temperature with air as the final oxidants. Nickel foam was modified by electrodeposition to improve its efficiency for anodic oxidation of HMF. Co-P@NF anode showed the best performance. Electrochemical tests indicated that electrodeposition modification greatly reduced the polarization resistance of NF while increased the electrochemical active area, thus significantly improving the electron transfer rate. The conversion of HMF to FDCA and power density of were significantly affected by the operating factors. 0.37 M (VO₂)₂SO₄ in 2 M H₂SO₄ was screened as the most efficient cathodic oxidant and electron carrier, achieving P_max of 38.9 mW/cm² at room temperature and 100% HMF conversion with 89.0-93.4% FDCA yield. Analysis of the kinetics and distribution of products revealed that the oxidation of HMF on the anode proceeded with HMFCA as an intermediate. By fed-batch operation, FDCA in the anolyte reached 81.8 g/L with 90% yield after feeding HMF for six batches. Mechanism analysis showed that the active species of Co-P@NF anode to achieve HMF oxidation actually was NiOOH rather than cobalt, while electrodeposition modification significantly increased the active sites of NF substrate. Therefore, efficient production of FDCA was achieved under mild conditions with oxygen(air) as final oxidant via the electron transport chains constructed by active nickel species and cathode electron carriers.

In this work, the developed novel reaction system is a versatile reaction system to conduct other oxidation reaction and convert chemical energy to electricity. The novelty and new finding of this work are described as the following aspects:

(1) We have developed a new reaction system to achieve efficient oxidation of HMF by air under mild condition through construction of an artificial "electron transport chain" to promote the efficiency of electron transfer from HMF to the final electron acceptor, air (oxygen). This reaction system not only converts HMF to FDCA but also co-generates electricity.

(2) We have employed a novel alkaline anolyte-acidic catholyte asymmetrical design to increase the chemical bias of the cell for increasing the driving force of electron migration. Such an asymmetrical design greatly improve the kinetics of electron transfer, avoiding using noble metal, for example Pt, as a cathode catalyst.

(3) We have developed an efficient anode by electrodeposition modification of nickel foam and elucidated the mechanisms. The process was simple but efficient to improve the kinetics of HMF oxidation.

What'more, FDCA as a promising bio-based chemicals has attracted great interest in recent years. It can be produced from biomass such as lignocellulose by several steps including hydrolysis of cellulose to obtain glucose, isomerization of glucose to form fructose, dehydration of fructose to form HMF and oxidation of HMF. Lignocellulose as the most abundant organic biomass is usually discarded as solid wastes. Therefore, the technology developing in this work can provide a promising way to treat and utilize lignocellulosic biomass for sustainable production of high value-added products.

Research Interests: Biomass conversion and high-valued chemicals production; 5-hydroxymethylfurfural; 2, 5-furandicarboxylic acid; liquid flow fuel cell