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1. Paired Electrolysis of 5-(Hydroxymethyl)Furfural (HMF) for Production of Higher-Valued Chemicals in Electrochemical Flow Cells

   https://doi.org/10.1149/MA2021-0227845mtgabs  

   Li, Wenzhen; Liu, Hengzhou; Lee, Ting-Han; Chen, Yifu; Cochran, Cochran (October 2021, 240th ECS Meeting)

   Electrocatalytic upgrading of biomass-derived feedstocks driven by renewable electricity offers a greener way to reduce the global carbon footprint associated with the production of value-added chemicals. Paired electrolysis is an emerging platform for cogenerating high-valued chemicals from both the cathode and anode, potentially powered by renewable electricity from wind or solar sources. By pairing with an anodic biomass oxidation upgrading reaction, the elimination of the sluggish and less valuable water oxidation increases flow cell productivity and efficiency. In this presentation, we report our research progress on paired electrolsysis of HMF to production of higher valued chemicals in electrochemical flow cells. We first prepared an oxide-derived Ag (OD-Ag) electrode with high activity and up to 98.2% selectivity for the ECH of 5-(hydroxymethyl)furfural (HMF) to 2,5-bis(hydroxymethyl)furan (BHMF), and such efficient conversion was achieved in a three-electrode flow cell. The excellent BHMF selectivity was maintained over a broad potential range with long-term operational stability. In HMF-to-BHMF paired with 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO)-mediated HMF-to-FDCA conversion, a markedly reduced cell voltage from ~7.5 V to ~2.0 V was observed by transferring the electrolysis from the H-type cell to the flow cell, corresponding to more than four-fold increase in energy efficiency in operation at 10 mA. A combined faradaic efficiency of 163% was obtained to BHMF and FDCA. Alternatively, the anodic hydrogen oxidation reaction on platinum further reduced the cell voltage to only ~0.85 V at 10 mA. Next, we have demonstrated membrane electrode assembly (MEA)-based flow cells for the paired electrolysis of 5-(hydroxymethyl)furfural (HMF) paired electrolysis to bis(hydroxymethyl)furan (BHMF) and 2,5-furandicarboxylic acid (FDCA). In this work, the oxygen evolution reaction (OER) was substituted by TEMPO-mediated HMF oxidation, dropping the cell voltage was from 1.4 V to 0.7 V at a current density of 1.0 mA cm−2. A minimized cell voltage of ~1.5 V for a continuous 24 h co-electrolysis of HMF was then achieved at the current density of 2 mA cm−2(constant current of 10 mA), leading to the highest combined faradaic efficiency (FE) of 139% for HMF-to-BHMF and HMF-to-FDCA. A NiFe oxide catalyst on carbon cloth further replaced the anodic TEMPO mediator for HMF paired electrolysis in a pH-asymmetric flow cell. We envision renewable electrical energy can potentially drive the whole process, thus providing a sustainable avenue towards distributed, scalable, and energy-efficient electrosynthesis. 

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2. Paired electrolysis of 5-(hydroxymethyl)furfural in flow cells with a high-performance oxide-derived silver cathode

   https://doi.org/10.1039/D1GC00988E  

   Liu, Hengzhou; Lee, Ting-Han; Chen, Yifu; Cochran, Eric W.; Li, Wenzhen (January 2021, Green Chemistry)

   null (Ed.)

   Electrocatalytic upgrading of biomass-derived feedstocks driven by renewable electricity offers a greener way to reduce the global carbon footprint associated with the production of value-added chemicals. In this respect, a key strategy is the electrocatalytic hydrogenation (ECH) reaction, which is typically paired with the anodic oxygen evolution reaction (OER) with sluggish kinetics, producing O 2 with little value. Here we prepared an oxide-derived Ag (OD-Ag) electrode with high activity and up to 98.2% selectivity for the ECH of 5-(hydroxymethyl)furfural (HMF) to 2,5-bis(hydroxymethyl)furan (BHMF), and such efficient conversion was achieved in a three-electrode flow cell. The excellent BHMF selectivity was maintained over a broad potential range with long-term operational stability. We then considered the oxidation of HMF to 2,5-furandicarboxylic acid (FDCA) and hydrogen (to water) as more efficient and productive alternatives to the OER. In HMF-to-BHMF paired with 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO)-mediated HMF-to-FDCA conversion, a markedly reduced cell voltage from ∼7.5 V to ∼2.0 V was observed by transferring the electrolysis from the H-type cell to the flow cell, corresponding to more than four-fold increase in energy efficiency in operation at 10 mA. A combined faradaic efficiency of 163% was obtained for BHMF and FDCA. Alternatively, the anodic hydrogen oxidation reaction on platinum further reduced the cell voltage to only ∼0.85 V at 10 mA. These paired processes show the potential for integration of renewable electricity and carbon for green and economically feasible distributed chemical manufacturing. 

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3. Paired Electrochemical Hydrogenation and Oxidation of Biomass-Derived Furanic Compounds to Value-Added Monomers in the Single Electrolyzer

   https://doi.org/https://doi.org/10.1149/MA2020-02503758mtgabs  

   Liu, Hengzhou; Lee, Ting-Han; Cochran, Eric; Li, Wenzhen (November 2020, ECS Meeting Abstracts)

   null (Ed.)

   Organic electrosynthesis is emerging as a cost-effective and environmental-friendly chemical production strategy by utilizing renewable electricity. Paired electrolysis cogenerates valuable chemicals at both electrodes can optimize the energy efficiency and economic feasibility. We report pairing hydrogenation and oxidation of 5-(hydroxymethyl)furfural (HMF) or furfural to desired chemicals at a single electrolysis cell. Electrocatalytic hydrogenation of HMF to 2,5-bis(hydroxymethyl)furan (BHMF) and furfural to furfural alcohol (FA) with high selectivity of >90% can be operated at near-neutral pH on Ag-based and Pb-based catalysts, respectively. In addition, oxidizing HMF to 2,5-furandicarboxylic acid (FDCA) and furfural to furoic acid can both be realized at TEMPO mediated process by using carbon-based catalysts or at Ni-based catalyst in an alkaline medium. Taken together, HMF or furfural can be performed in a single electrolysis cell with a minimized cell voltage only around 1.6 V. Products selectivity and faradaic efficiency are highly related to the reaction conditions, including potential or current density, architectures of the reactor, type of catalysts. By optimizing the single flow reactor, a three-electrode system, two-electrode membrane assembly architecture, and pH-symmetric and pH-asymmetric structure can be designed to reduce the capital expense, minimize required energy, and simplify processing steps. Finally, a complete electrons economy can be achieved by pairing two electrochemical reactions, and the overall charge efficiency can attain over 170% without any crossover issue detected. As a result, the continuous cogeneration of high value-added BHMF or FA and FDCA or furoic acid can be performed in a single electrolyzer. 

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4. The production of valuable biopolymer precursors from fructose

   https://doi.org/10.1039/d0gc02315a  

   Liu, Xuan; Leong, Daniel Chee; Sun, Yujie (October 2020, Green Chemistry)

   null (Ed.)

   The increasing demand for green chemical products calls for the exploration of sustainable and renewable carbon resources beyond fossil-based materials, whose utilization inevitably results in environmental concerns. As such, biomass valorisation has attracted increasing attention because biomass is the most widely available and sustainable carbon source. Among the available biomass-derived platform chemicals, 5-hydroxymethylfurfural (HMF) has long been regarded as an attractive candidate for the production of numerous value-added products. Nevertheless, the poor stability, and difficult separation and purification of HMF from fructose dehydration significantly inhibit its large-scale application. Herein, we report a two-step process for the direct production of two biopolymer precursors, 2,5-furandicarboxylic acid (FDCA) and 2,5-bis(hydroxymethyl)furan (BHMF), from fructose, bypassing the isolation of HMF. FDCA and BHMF are much easier to separate and purify from the reaction mixture than HMF, and they both can replace petroleum-based counterparts in the syntheses of many industrially important polymers, ranging from polyesters to polyamides. Optimized fructose dehydration under microwave irradiation achieved a high HMF yield (83%) using a biphasic strategy. The subsequent electrocatalytic conversion of the resulting microwave reaction mixture allowed us to carry out either oxidation or reduction via readily tuning the electrochemical parameters to yield FDCA or BHMF, respectively. The integration of microwave irradiation and electrocatalysis in a flow electrolyzer enabled the direct conversion of readily available fructose to highly valuable FDCA and BHMF without the expensive and challenging step of HMF isolation, suggesting an economically attractive approach for upgrading carbohydrates. 

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5. Electrocatalytic Conversion of Furanic Compounds for Production of Valuable Chemicals

   Li, Wenzhen; Chadderdon, Xiaotong; Chadderdon, David; Liu, Hengzhou (August 2021, 72nd International Society of Electrochemistry Meeting)

   Biomass is abundant, inexpensive and renewable, therefore, it is highly expected to play a significant role in our future energy and chemical landscapes. The US DOE has identified furanic compounds (furfural and 5-(hydroxymethyl)furfural (HMF)) as the top platform building-block chemicals that can be readily derived from biomass sources [1]. Electrocatalytic conversion of these furanic compounds is an emerging route for the sustainable production of valuable chemicals [2, 3]. In my presentation, I will first present our recent mechanistic study of electrochemical reduction of furfural through a combination of voltammetry, preparative electrolysis, thiol-electrode modifications, and kinetic isotope studies [4]. It is demonstrated that two distinct mechanisms are operable on metallic Cu electrodes in acidic electrolytes: (i) electrocatalytic hydrogenation (ECH) and (ii) direct electroreduction. The contributions of each mechanism to the observed product distribution are clarified by evaluating the requirement for direct chemical interactions with the electrode surface and the role of adsorbed hydrogen. Further analysis reveals that hydrogenation and hydrogenolysis products are generated by parallel ECH pathways. Understanding the underlying mechanisms enables the manipulation of furfural reduction by rationally tuning the electrode potential, electrolyte pH, and furfural concentration to promote selective formation of important bio-based polymer precursors and fuels. Next, I will present electrocatalytic conversion of HMF to two biobased monomers in an H-type electrochemical cell [5]. HMF reduction (hydrogenation) to 2,5-bis(hydroxymethyl)furan (BHMF) was achieved under mild electrolyte conditions and ambient temperature using a Ag/C cathode. Meanwhile, HMF oxidation to 2,5-furandicarboxylic acid (FDCA) with ~100% efficiency was facilitated under the same conditions by a homogeneous nitroxyl radical redox mediator, together with an inexpensive carbon felt anode. The selectivity and efficiency for Ag-catalyzed BHMF formation were sensitive to cathode potential, owing to competition from HMF hydrodimerization reactions and water reduction (hydrogen evolution). Moreover, the carbon support of Ag/C was active for HMF reduction and contributed to undesired dimer/oligomer formation at strongly cathodic potentials. As a result, high BHMF selectivity and efficiency were only achieved within a narrow potential range near –1.3 V. Fortunately, the selectivity of redox-mediated HMF oxidation was insensitive to anode potential, thus allowing HMF hydrogenation and oxidation half reactions to be performed together in a single cathode-potential-controlled cell. Electrocatalytic HMF conversion in a paired cell achieved high molar yields of BHMF and FDCA, and nearly doubled electron efficiency compared to the unpaired cell. Finally, I will briefly introduce our recent work on the development of a three-electrode flow cell with an oxide-derived Ag (OD-Ag) cathode and catbon felt anode for paring elecatalytic oxidation and reduction of HMF. The flow cell has a remarkable cell voltage reduction from ~7.5 V to ~2.0 V by transferring the electrolysis from the H-type cell to the flow cell [6]. This represents a more than fourfold increase in the energy efficiency at the 10 mA operation. A combined faradaic efficiency of 163% was obtained to BHMF and FDCA. Alternatively, the anodic hydrogen oxidation reaction on platinum further reduced the cell voltage to only ~0.85 V at 10 mA operation. These paired processes have shown potential for integrating renewable electricity and carbon for distributed chemical manufacturing in the future. 

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