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1. 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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2. Local reactivity descriptors to decipher the electrochemical hydrogenation of unsaturated carboxylic acids

   https://doi.org/10.1039/D3GC02909C  

   Dell'Anna, Marco Nazareno; Gupta, Geet; Prabhu, Prathamesh T; Chu, Ting-Hung; Roling, Luke T; Tessonnier, Jean-Philippe (December 2023, Green Chemistry)

   The decarbonization of chemical manufacturing is a multifaceted challenge that requires technologies able to selectively convert CO2-sequestering feedstocks using renewable energy. The electrochemical conversion of biomass-derived platform chemicals is well-positioned to address this need. However, the electroactivity of biobased molecules that carry multiple redox centers remains challenging to predict and control. For instance, cis,cis-muconic acid, a conjugated dicarboxylic acid, is electrohydrogenated to trans-3-hexenedioic acid (t3HDA) with excellent yield and stereoselectivity while free energy calculations predict mixtures of 2- and 3-hexenedioic acids. To decipher this discrepancy, we studied the electrohydrogenation of C4 and C6 unsaturated acids, diacids, and their esters, and tied the observed product distributions to the electronic structure of the parent molecules. We show that the electrohydrogenation of the three isomers of muconic acid proceeds through a hydrogenating proton-coupled electron transfer (PCET) in the α position of the carboxylic acids and invariably yields t3HDA as the sole product. The selectivity can be explained by the electron-withdrawing effect of the carboxylic acid groups and the resulting perturbation of the local electron density that promotes the 2,5-hydrogenation over the thermodynamically-preferred 2,3-hydrogenation. This electronic perturbation is reflected in the computed Fukui indices, which can serve as local reactivity descriptors to predict product distributions not captured by calculated reaction thermochemistry. In addition to predicting the electroactivity of other unsaturated acids, this approach can provide insights into homogeneous electrochemical processes that may coexist with surface-mediated electrocatalytic transformations. 

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3. Cyclotetrabenzil-Based Porous Organic Polymers with High Carbon Dioxide Affinity

   https://doi.org/10.1055/a-1512-5753  

   Ashirov, Timur; Alrayyani, Maymounah; Song, Kyung-Seob; Miljanić, Ognjen Š.; Coskun, Ali (April 2021, Organic Materials)

   null (Ed.)

   Porous organic polymers (POPs) incorporating macrocyclic units have been investigated in recent years in an effort to transfer macrocycles' intrinsic host–guest properties onto the porous networks to achieve complex separations. In this regard, highly interesting building blocks are presented by the family of cyclotetrabenzoin macrocycles with rigid, well-defined, electron-deficient cavities. This macrocycle shows high affinity towards linear guest molecules such as carbon dioxide, thus offering an ideal building block for the synthesis of CO2-philic POPs. Herein, we report the synthesis of a POP through the condensation reaction between cyclotetrabenzil and 1,2,4,5-tetraaminobenzene under ionothermal conditions using the eutectic zinc chloride/sodium chloride/potassium chloride salt mixture at 250 °C. Notably, following the condensation reaction, the macrocycle favors three-dimensional (3D) growth rather than a two-dimensional one while retaining the cavity. The resulting polymer, named 3D-mPOP, showed a highly microporous structure with a BET surface area of 1142 m2 g−1 and a high carbon dioxide affinity with a binding enthalpy of 39 kJ mol−1. Moreover, 3D-mPOP showed very high selectivity for carbon dioxide in carbon dioxide/methane and carbon dioxide/nitrogen mixtures. 

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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. 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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