Title: Organic Electrochemistry: Expanding the Scope of Paired Reactions
Abstract Paired electrochemical reactions allow the optimization of both atom and energy economy of oxidation and reduction reactions. While many paired electrochemical reactions take advantage of perfectly matched reactions at the anode and cathode, this matching of substrates is not necessary. In constant current electrolysis, the potential at both electrodes adjusts to the substrates in solution. In principle, any oxidation reaction can be paired with any reduction reaction. Various oxidation reactions conducted on the anodic side of the electrolysis were paired with the generation and use of hydrogen gas at the cathode, showing the generality of the anodic process in a paired electrolysis and how the auxiliary reaction required for the oxidation could be used to generate a substrate for a non‐electrolysis reaction. This is combined with variations on the cathodic side of the electrolysis to complete the picture and illustrate how oxidation and reduction reactions can be combined. more »« less
Paired electrolysis has been emerged as an electricity-powered platform for converting biorewable feedstock to higher-valued chemicals at both the cathode and anode. In this presentation, we explored paired electrolyzers of different architectures with remarkable performance and stability. We first designed three-electrode flow electrolyzers to pair electrocatalytic hydrogenation of 5-(hydroxymethyl)furfural (HMF) on oxide-derived silver electrode and TEMPO-mediated HMF oxidation on carbon cloth. The paired flow cell achieved a combined faradaic efficiency of 163% to desired 2,5-bis(hydroxymethyl)furan (BHMF) and 2,5-furandicarboxylic acid (FDCA) with a cell potential of ~1.7 V, at the constant current of 10 mA. When the anodic reaction was replaced by hydrogen oxidation, a minimized cell voltage of only ~0.9 V was achieved. We then assmbled a membrane electrode assembly (MEA)-based two-electrode flow cell, which realized a minimized cell potential of only ~1.5 V for a continuous 24 hours paired electrolysis of HMF. Finally, a pH asymmetric architecture was designed to match the optimum reaction conditions and to replace HMF oxidation on a NiFeOOH anode catalyst without a redox mediator. Our recent development of electrochemical-chemical combined reactors for furanic compounds conversion will also be briefly presented.
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.
Li, Wenzhen; Chadderdon, Xiaotong; Chadderdon, David; Liu, Hengzhou
(, 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.
Liu, Yifan; Kiss, István Z
(, Journal of The Electrochemical Society)
We investigate the emergence of current oscillations of a bipolar electrode (BPE) in coupled anode/cathode reaction under potentiostatic condition. In a traditional three-electrode setup, the nickel dissolution in sulfuric acid requires a minimum amount of IR ohmic drop, and thus series resistance for the oscillations to occur. In this paper, it is shown that in bipolar setup, when the nickel electrodissolution on the anodic side is coupled to hydrogen ion reduction on the cathodic side, spontaneous current oscillations can occur. An electrochemical analysis of the dynamics shows that the required circuit potential for the oscillations can be predicted from estimating the overpotentials needed for the anodic and cathodic reactions, the driving electrode, and the ohmic drop in the electrolyte. The dynamics and range of oscillations can be tuned by different concentrations of electrolyte, on both the anodic and the cathodic sides. In the considered example, the charge transfer resistance of the cathodic reaction can provide sufficient total resistance even when the solution resistance does not yield sufficient IR drop for the oscillations. Our findings have the potential to promote further studies of the collective behavior of electrochemical reactions using multielectrode arrays in bipolar electrode setups.
Shang, Xiao; Liu, Xuan; Sun, Yujie
(, Green Chemistry)
Direct electrochemical halogenation has appeared as an appealing approach in synthesizing organic halides in which inexpensive inorganic halide sources are employed and electrical power is the sole driving force. However, the intrinsic characteristics of direct electrochemical halogenation limit its reaction scope. Herein, we report an on-site halogenation strategy utilizing halogen gas produced from halide electrolysis while the halogenation reaction takes place in a reactor spatially isolated from the electrochemical cell. Such a flexible approach is able to successfully halogenate substrates bearing oxidatively labile functionalities, which are challenging for direct electrochemical halogenation. In addition, low-polar organic solvents, redox-active metal catalysts, and variable temperature conditions, inconvenient for direct electrochemical reactions, could be readily employed for our on-site halogenation. Hence, a wide range of substrates including arenes, heteroarenes, alkenes, alkynes, and ketones all exhibit excellent halogenation yields. Moreover, the simultaneously generated H 2 at the cathode during halide electrolysis can also be utilized for on-site hydrogenation. Such a strategy of paired halogenation/hydrogenation maximizes the atom economy and energy efficiency of halide electrolysis. Taking advantage of the on-site production of halogen and H 2 gases using portable halide electrolysis but not being suffered from electrolyte separation and restricted reaction conditions, our approach of flexible halogenation coupled with hydrogenation enables green and scalable synthesis of organic halides and value-added products.
Wu, Tiandi, and Moeller, Kevin D. Organic Electrochemistry: Expanding the Scope of Paired Reactions. Angewandte Chemie International Edition 60.23 Web. doi:10.1002/anie.202100193.
Wu, Tiandi, & Moeller, Kevin D. Organic Electrochemistry: Expanding the Scope of Paired Reactions. Angewandte Chemie International Edition, 60 (23). https://doi.org/10.1002/anie.202100193
Wu, Tiandi, and Moeller, Kevin D.
"Organic Electrochemistry: Expanding the Scope of Paired Reactions". Angewandte Chemie International Edition 60 (23). Country unknown/Code not available: Wiley Blackwell (John Wiley & Sons). https://doi.org/10.1002/anie.202100193.https://par.nsf.gov/biblio/10225910.
@article{osti_10225910,
place = {Country unknown/Code not available},
title = {Organic Electrochemistry: Expanding the Scope of Paired Reactions},
url = {https://par.nsf.gov/biblio/10225910},
DOI = {10.1002/anie.202100193},
abstractNote = {Abstract Paired electrochemical reactions allow the optimization of both atom and energy economy of oxidation and reduction reactions. While many paired electrochemical reactions take advantage of perfectly matched reactions at the anode and cathode, this matching of substrates is not necessary. In constant current electrolysis, the potential at both electrodes adjusts to the substrates in solution. In principle, any oxidation reaction can be paired with any reduction reaction. Various oxidation reactions conducted on the anodic side of the electrolysis were paired with the generation and use of hydrogen gas at the cathode, showing the generality of the anodic process in a paired electrolysis and how the auxiliary reaction required for the oxidation could be used to generate a substrate for a non‐electrolysis reaction. This is combined with variations on the cathodic side of the electrolysis to complete the picture and illustrate how oxidation and reduction reactions can be combined.},
journal = {Angewandte Chemie International Edition},
volume = {60},
number = {23},
publisher = {Wiley Blackwell (John Wiley & Sons)},
author = {Wu, Tiandi and Moeller, Kevin D.},
}
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