Understanding Near-Electrode Microenvironments in Acrylonitrile Electrohydrodimerization

The industrial sector, particularly chemical manufacturing, is a significant contributor to global greenhouse gas emissions, with traditional processes primarily fueled by fossil fuels and operating under high temperatures and pressures. Introducing electrosynthesis at an industrial scale offers a promising avenue for integrating renewable electricity in chemical manufacturing, thus accelerating the decarbonization of large-scale chemical processes. This is especially pertinent in the production of Nylon 6,6, a key polymer reliant on adiponitrile (ADN) as a crucial intermediate. The dominant thermochemical production of ADN is not only energy-intensive but also employs hazardous reagents, such as HCN, highlighting the need for more sustainable production methods such as the electrochemical hydrodimerization of acrylonitrile (AN). Our research focuses on improving the performance and fundamentally understanding the electrochemical production of ADN from AN, one of the largest electro-organic reaction practiced in industry. In this reaction, the addition of tetraalkylammonium (TAA) salts as supporting electrolytes in moderate concentrations can enhance the solubility of organic reactants. By manipulating the molecular size and concentration of TAA ions, we observed an improvement in ADN selectivity and production efficiency, primarily influenced by the mass transport of organic reactants to the electrical double layer (EDL). 1,2 To better understand the local effects of TAA ions at the electrode/electrolyte interface, we combined an electrochemical flow cell with attenuated total reflection Fourier-transform infrared (FTIR) spectroscopy. This approach revealed that TAA ions significantly increase the local concentration of AN near the electrode, which is influenced by the applied cathodic potentials and correlates with improved ADN selectivity. Additionally, we investigated the reaction mechanism of ADN synthesis using kinetic isotope effect (KIE) studies and electron paramagnetic resonance (EPR) spectroscopy. Results from KIE studies suggest that hydrogen transfer to AN is a rate-determining step in ADN production. EPR spectroscopy further revealed the presence of alkyl radicals in the solution, suggesting that ADN production partially occurs through the coupling of free AN radicals in solution. These insights can help in the design of electrolytes and the understanding of molecular processes that govern the selectivity and efficiency of ADN production via AN electrohydrodimerization and can inform the development and optimization of other electro-organic manufacturing processes. References 1. Blanco, D. E., Dookhith, A. Z. & Modestino, M. A. Enhancing selectivity and efficiency in the electrochemical synthesis of adiponitrile. Reaction Chemistry & Engineering 4 , 8-16, doi:10.1039/c8re00262b (2019). 2. Blanco, D. E., Lee, B. & Modestino, M. A. Optimizing organic electrosynthesis through controlled voltage dosing and artificial intelligence. Proc Natl Acad Sci U S A 116 , 17683-17689, doi:10.1073/pnas.1909985116 (2019). Figure 1