Enhanced Electrohydrogenation of Cis,Cis-Muconic Acid to Trans-3-Hexenedioic Acid Using Hybrid Metal/Carbon Fiber Cathodes

Abstract

Organic electrosynthesis is a long-known technique for diverse product development and efficient synthesis. With the recent rise of biomass as an alternative carbon and energy source, electrochemistry is further gaining popularity due to its inherently green and environmentally benign operating conditions. Electrochemical hydrogenation (ECH) reactions are particularly attractive for biomass conversion due to their ability to abstract hydrogen from water, tolerate biogenic impurities, and utilize Earth-abundant base metal electrodes. Moreover, the rate and selectivity of the reaction can be tubed by controlling the electrode material, operating potential/current density, and reactor design. However, the implementation of this decarbonized technology for chemical production is still hampered by low productivity and, as a result, unfavorable economics. cis,cis-Muconic acid (ccMA), a bioderived C6 diunsaturated diacid platform molecule, can be converted into commodity chemicals like caprolactam, adipic acid (AA), and terephthalic acid, with AA being of particular interest for the synthesis of nylon 6,6.1 Apart from various drop-in chemicals, ccMA can also be electrochemically upgraded to trans-3-hexenedioic acid (t3HDA), a potential bio-advantaged substitute for AA as the additional C=C bond in t3HDA can be leveraged to synthesize performance-advantaged polyamides.2,3 In this work, we investigated the electrohydrogenation of ccMA to t3HDA over Pb cathodes. We demonstrate that the reaction follows a radical-based pathway involving sequential electron transfer and hydrogen addition in solution to selectively produce t3HDA (100% selectivity). Next, we contacted the cathode with carbon felts (CF) to increase productivity. CF are often used in fuel cells and H2O2 electrolyzers due to their excellent conductivity, porosity for enhanced turbulence, and ability to tune the gas-liquid-solid three-phase interface. Following similar principles, we placed a CF (3mm. thickness) in contact with the Pb cathode to lower the charge transfer resistance (~3Ω) at the interface. The surface functionalities of carbon allowed us to further tune the hydrophobicity/hydrophilicity of our hybrid electrode. Molecular dynamics (MD) calculations provided critical insights into the role of surface functionalities and guided the optimal functionalization of CF’s surface. Combining reactor configuration with fine-tuning of the electrode’s microenvironment, we were able to achieve a 50x increase in t3HDA productivity reaching up to 20 g/cm2/day at a current density of 200 mA/cm2. We further replaced the kinetically sluggish oxygen-evolution reaction with the glucose oxidation reaction at the anode, allowing us to operate the electrolyzer at a lower cell potential.Overall, ECH represents a valuable alternative to thermochemical transformations for biomass upgrading. This work provides critical insights into reactor modifications for the scaled-up synthesis of bio-derived performance monomers via hybrid microbial electrosynthesis (HMES).