Electrosynthesis of the Nylon-6 Precursor from Nitrate and Cyclohexanone over a Rutile TiO2 Catalyst

Abstract

Electrocatalytic reduction of nitrate (NO₃⁻) to valuable organonitrogen compounds beyond ammonia is a promising strategy for mitigating the human-caused unbalance of the global nitrogen cycle. Herein, we present an electrochemical strategy for synthesizing cyclohexanone oxime (CHO), an important feedstock in nylon-6 production through hydrogenative coupling of NO₃⁻ and cyclohexanone (CYC) using a rutile titanium dioxide (R-TiO₂) catalyst under ambient conditions. The CHO productivity achieved 127.3 μmol cm⁻² h⁻¹ with a high Faradaic efficiency (FE) of 68.2% at a current density of 30 mA cm⁻². Moreover, the yield of CHO reached 98.2%. We demonstrated that the electrosynthesis of CHO operated through a tandem reaction mechanism involving the in situ generation of hydroxylamine (NH₂OH) from NO₃⁻ reduction, followed by a spontaneous nucleophilic addition–elimination reaction between NH₂OH and CYC. Additionally, we revealed that R-TiO₂ exhibited a superior scaling relation with a high NH₂OH generation rate and excellent CYC adsorption ability, which promoted CHO production. This electrochemical strategy was also effective for the synthesis of different oximes. Finally, we designed a coupling reaction system to realize the simultaneous production of CHO and CYC by combining cathodic NO₃⁻ reduction and anodic cyclohexane oxidation, demonstrating a greener and more economical approach.