Controlling Selectivity in the Electrocatalytic Hydrogenation of Adiponitrile through Electrolyte Design

Organic hydrogenations are key steps in the production of numerous valuable chemicals. Their requirement for high temperature, pressure, and compressed hydrogen has motivated the interest to develop safer electrocatalytic hydrogenation (ECH) routes in benign aqueous electrolytes. However, faradaic efficiencies in organic ECH tend to be greatly limited by competition with the hydrogen evolution reaction and low reactant solubility, which hinders the implementation of these more sustainable hydrogenation routes. Using the hydrogenation of adiponitrile to hexamethylenediamine (HMDA), a monomer used in the production of nylon-6,6, as a case study, we investigate the effect of reactant concentration, temperature, pH, and organic cosolvents on the ECH of nitrile groups with Raney nickel electrodes. Higher reactant concentrations, alkaline electrolytes, and mild temperature (40 °C) are key conditions that enhance the hydrogenation of organic substrates against hydrogen evolution. A maximum faradaic efficiency of 92% toward HMDA was obtained in aqueous electrolytes at -60 mA cm-2. The addition of an organic cosolvent is subsequently studied to evaluate the effect of enhanced reactant solubility, achieving a 95% faradaic efficiency at the same current density with 30% methanol by volume in water. The insights gained from this study are relevant for the design of energy efficient organic ECH and can help accelerate the implementation of sustainable chemical manufacturing.