In Situ Analysis of the Effect of Tetraakylammonium Salts on the Selectivity and Efficiency of Acrylonitrile Electrohydrodimerization

Abstract

Traditional chemical manufacturing processes are primarily powered by fossil-fuel combustion, and often require operation at elevated temperatures and pressures. Introducing electrosynthesis as an alternative to thermal processes at industrial scale can allow direct integration of renewable electricity with chemical manufacturing, accelerating the decarbonization of large-scale chemical processes. However, there are major challenges that must be addressed before implementing organic electrosynthesis on an industrial level, such as the low solubility of organic reactants in aqueous electrolytes and the existence of multiple reaction pathways that lead to undesired byproducts. In this presentation, we will discuss our work on electrolyte design strategies to enhance the performance of adiponitrile (ADN) electrosynthesis. ADN is a large volume precursor of hexamethylenediamine, which is a monomer used in Nylon 6,6 manufacturing. The most commonly used method for ADN production, the thermochemical hydrocyanation of 1,3-butadiene, is energy intensive and uses hydrogen cyanide, a highly toxic reactant. Alternatively, ADN can be produced electrochemically via the hydrodimerization of acrylonitrile (AN), which is the largest and most successful industrial organic electrochemical process. In this reaction, the addition of tetraalkylammonium (TAA) salts as supporting electrolytes in large concentrations has been widely accepted to enhance the solubility of organic reactants. It has been hypothesized that TAA ions boost the selectivity towards ADN by increasing the AN concentration in the electrical double layer (EDL). Recent studies by our group explored the effect of the molecular size and concentration of TAA ions on the selectivity and efficiency of ADN production. Resulting trends suggest that the reaction is strongly limited by mass transport of organic reactants to the EDL at high current densities. Despite these advances, the local effects of TAA ions on the concentration of reactants and intermediates in the EDL remain unclear. Inspired by prior demonstrations, we developed a spectroelectrochemical method for quantitatively assessing the concentration of species at the electrode/electrolyte interface. This method relies on an electrochemical flow cell integrated with an attenuated total reflection (ATR) Fourier-transform infrared (FTIR) spectrometer to quantify concentrations of reactive species in the near-electrode region. Our results demonstrate that the presence of TAA ions increases the local concentration of AN at the electrode, and that applied potential increases TAA ion concentration. These results provide insights into the effects of supporting ions and potential in organic electrosynthesis selectivity.Figure caption: Schematic view of the enhancement of acrylonitrile (AN) concentration in the electrical double layer (EDL) by the addition of Tetrabutylammonium (TBA) ions to the aqueous electrolyte. Figure 1