Adipic acid, a crucial building block for polyamides and polyesters like nylon-66, is currently produced via a thermo-catalytic oxidation process using nitric acid. This method, however, relies on corrosive nitric acid and generates significant amounts of nitrous oxide (N₂O), a potent greenhouse gas. The high environmental impact and safety concerns associated with this process necessitate the development of sustainable alternatives. Electrocatalytic oxidation offers a promising pathway, converting cyclohexanone (a component of ketone-alcohol oil, KA oil) to adipic acid under mild conditions. Previous research has explored electrocatalytic approaches using various catalysts, but these have been limited by low reaction rates and adipic acid yields. A major challenge stems from the low solubility of cyclohexanone in aqueous electrolytes, hindering mass transfer and catalyst surface adsorption. Inspired by ligand modification strategies used to enhance the reactivity of immiscible gases like CO₂ and N₂, this study aims to improve the electrocatalytic oxidation of cyclohexanone by modifying the catalyst surface to enhance cyclohexanone enrichment. The incorporation of SDS, a surfactant, into the Ni(OH)₂ catalyst is hypothesized to increase the hydrophobicity, thereby improving cyclohexanone adsorption and subsequently boosting adipic acid productivity and faradaic efficiency.

Prior research has demonstrated the feasibility of electrocatalytic oxidation of cyclohexanone to adipic acid, albeit with limited success. Lyalin and Petrosyan (2004) reported a NiOOH catalyst achieving a 52% yield at 6 mA cm⁻². More recent work by the authors demonstrated a manganese-doped cobalt oxyhydroxide catalyst with a 64.2% yield. However, these methods suffer from low reaction rates and yields, generally below 70%. The low solubility of cyclohexanone in aqueous solutions significantly impedes mass transfer to the catalyst surface, limiting the reaction kinetics. Existing literature highlights the effectiveness of ligand modification strategies in improving the electrocatalytic conversion of immiscible substrates, primarily gases such as CO₂ and N₂. Modifying electrode surfaces with hydrophobic ligands increases the concentration of the reactant near the catalyst surface, enhancing reactivity. These studies inspired the current work, which explores the application of a ligand modification strategy to enhance the electrocatalytic conversion of a liquid, immiscible substrate, namely cyclohexanone, in an aqueous solution.

The researchers synthesized an SDS-modified α-Ni(OH)₂ catalyst (Ni(OH)₂-SDS) on a Ni foam substrate via a hydrothermal method. Pure Ni(OH)₂ was synthesized using a similar method without SDS. The structural characteristics of the synthesized catalysts were extensively analyzed using techniques like X-ray diffraction (XRD), scanning electron microscopy (SEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) coupled with energy-dispersive X-ray spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and contact angle measurements. These analyses confirmed the successful intercalation of SDS into the Ni(OH)₂ interlayers. Electrocatalytic oxidation of cyclohexanone was evaluated using linear sweep voltammetry (LSV), chronoamperometry (CA), and normal pulse voltammetry (NPV). The reaction products were quantified using high-performance liquid chromatography (HPLC), and the faradaic efficiency (FE) was calculated. The catalyst stability was assessed through long-term electrolysis. The mechanism of cyclohexanone enrichment was investigated using quartz crystal microbalance (QCM) and coarse-grained molecular dynamics (CGMD) simulations. Density functional theory (DFT) calculations were also employed to study the adsorption and release of cyclohexanone. Finally, a membrane-free flow electrolyzer was constructed to evaluate the practical applicability of the system. Isotope labeling experiments (using D₂O and H₂¹⁸O) and experiments with cyclohexane-1,2-dione as a substrate provided further insights into the reaction pathway. The effects of varying surfactant alkyl chain length (C₄, C₈, C₁₂, C₁₆) were also investigated by synthesizing various Ni(OH)₂-Cn catalysts. The universality of the ligand modification strategy was examined using various substrates (both miscible and immiscible) and other layered materials like Co(OH)₂ and different layered double hydroxides (LDHs).

The SDS-modified Ni(OH)₂ catalyst (Ni(OH)₂-SDS) demonstrated significantly enhanced electrocatalytic activity towards the oxidation of cyclohexanone to adipic acid compared to the unmodified Ni(OH)₂. The Ni(OH)₂-SDS catalyst exhibited a 3.6-fold increase in adipic acid productivity (90 µmol cm⁻² h⁻¹ at 1.5 V vs RHE) and a much higher faradaic efficiency (FE) of 93% at 1.5 V vs RHE, compared to 56% for pure Ni(OH)₂. The enhanced performance is attributed to the facilitated enrichment of cyclohexanone molecules at the edge of the Ni(OH)₂-SDS catalyst, as evidenced by QCM, NPV, and CGMD simulations. The CGMD simulations revealed that SDS facilitates the diffusion of cyclohexanone molecules to the catalyst surface, with cyclohexanone molecules predominantly accumulating at the edges of the Ni(OH)₂-SDS nanosheets. DFT calculations supported the experimental findings, showing a higher adsorption energy of cyclohexanone on Ni(OH)₂-SDS than in water, facilitating substrate adsorption. The catalyst showed good stability over multiple reaction cycles, maintaining its activity and morphology. The reaction mechanism involves the initial oxidation of cyclohexanol (if present in the KA oil) to cyclohexanone, followed by base-catalyzed keto-enol tautomerism and subsequent oxidation steps, yielding adipic acid. The isotope labeling experiments confirmed the involvement of keto-enol tautomerism. A membrane-free flow electrolyzer utilizing the Ni(OH)₂-SDS catalyst achieved a high adipic acid productivity of 4.7 mmol and H₂ production of 8.0 L at 0.8 A (30 mA cm⁻²) over 24 h. The ligand modification strategy was demonstrated to be versatile, effectively enhancing the electrocatalytic oxidation of various immiscible aldehydes and ketones, as well as applicable to other layered materials.

The findings of this study directly address the limitations of the current industrial adipic acid production method by providing a sustainable, electrocatalytic alternative. The significant enhancement in adipic acid productivity and faradaic efficiency using the SDS-modified Ni(OH)₂ catalyst demonstrates the effectiveness of the ligand modification strategy in overcoming mass transfer limitations associated with immiscible substrates. The versatility of this strategy, as demonstrated by its applicability to a range of immiscible compounds and different layered materials, highlights its broad potential. The successful implementation of the electrocatalytic reaction in a membrane-free flow electrolyzer showcases its potential for practical application. The results have implications for the development of green chemical processes, offering a significant step toward sustainable production of adipic acid while simultaneously generating renewable hydrogen.

This research presents a promising approach for the sustainable synthesis of adipic acid coupled with hydrogen production. The SDS modification strategy significantly improves the electrocatalytic activity of Ni(OH)₂, enhancing adipic acid productivity and faradaic efficiency. The method's versatility and successful demonstration in a flow electrolyzer showcase its potential for industrial applications. Future work could focus on optimizing the catalyst design, exploring different ligands, and improving the process for industrial-scale production and product separation. Investigating the economic viability and conducting a life-cycle assessment are also crucial next steps.

While the study demonstrates significant progress in electrocatalytic adipic acid production, several limitations exist. The study primarily focuses on cyclohexanone, and further investigation is needed to optimize the process for other substrates. The membrane-free electrolyzer design, although practical, could be further improved for enhanced efficiency and scalability. Detailed economic analysis and life-cycle assessment are needed to fully evaluate the industrial feasibility of this technology. The long-term stability of the catalyst under industrial conditions requires further evaluation. Finally, while product separation is addressed conceptually, detailed investigation and optimization of an efficient separation process are still needed.