Controlling Selectivity in the Electrochemical Reduction of Acrylonitrile: Towards a Solar-Driven Nylon 6,6 Production Process

Abstract

Fossil fuel derived carbon dioxide (CO2) emissions have continuously increased over the past century and their impact in climate change is self-evident. The chemical industry is a large contributor to this increase, accounting for more than 5.5% of the global CO2 emissions. Implementing electrochemical routes for the production of large volume chemicals can allow for the direct integration of renewable energy sources into chemical manufacturing. In this way, solar- or wind-powered electrochemical processes could greatly contribute to the reduction of the carbon footprint of the chemical industry. Although inorganic electrosynthetic processes have been successfully implemented in industry at scale (i.e. chloro-alkali and Aluminum production), the implementation of electrochemical methods for the production of organic chemicals has been limited. Adiponitrile (ADN) is the largest-volume organic chemical product currently produced electrochemically (>1.5 million tons/year). ADN is a precursor used in Nylon 6,6 manufacturing and it can be obtained via the electrohydrodimerization of acrylonitrile (AN). Although the electrosynthesis of ADN is considered the most successful organic electrochemical process in industry, it still faces many challenges owing to its energy conversion efficiency and selectivity. Optimizing the composition of the electrolyte can result in significant performance improvements. The implementation of tetraalkylammonium (TAA) ions has been shown to increase the reaction selectivity towards adiponitrile, but there is limited understanding on their role in the mass transport and kinetic aspects of the transformation. In this study, we provide experimental insights into the effects of TAA ions in the selectivity and efficiency of the AN electrohydrodimerization reaction. TAA ions can limit the hydrogen evolution reaction by sterically hindering water molecules from approaching the electrode surface and locally increasing the concentration of organic reactants. These effects are believed to be responsible for the increase in selectivity towards the reduction of AN. In our study, we explored electrolytes containing TAA ions with varying lengths of the alkyl substituents (i.e., methyl, ethyl, butyl and hexyl) and assessed the effects of their molecular size, affinity towards AN, and concentration on the electrolyte conductivity and selectivity. Using a combination of cyclic voltammetry, electrochemical impedance spectroscopy, and chronoamperometry at a cadmium electrode, we showed that optimal performance is achieved for electrolytes containing tetrabutylammonium (TBA) ions. A maximum selectivity towards ADN of 80% was demonstrated for TBA concentrations ranging between 0.04 to 0.5 mM. Furthermore, we studied the effects of TAA ions on the ohmic and mass transport limitations under varying current densities. Our results suggest that (i) the local concentration of AN in the electrical double layer strongly depends on the nature of the TAA and the applied current density and (ii) that this concentration is important in determining the predominant reaction pathways and thus the product distribution between ADN and other electrochemical side products (e.g., propionitrile and 1,3,6 tricyanohexane). The results obtained in this study can be used to derive electrolyte design guidelines which will improve the efficiency of the electrochemical adiponitrile production process. Furthermore, understanding the effects of current density in the selectivity of the reaction allows for the design of operating protocols to directly drive electrochemical transformations with intermittent sources of energy such as solar power. These design guidelines can lead to highly efficient solar-ADN reactors and enable the clean manufacture of precursors for the polymer industry. The performance of proof-of-concept prototypes solar-AND devices will be discussed. Figure 1