A Pickering-Emulsion-Droplet-Integrated Electrode for the Continuous-Flow Electrosynthesis of Oximes

Physical Chemistry Energy Chemistry Chemistry Catalysis Chemical Engineering Materials Chemistry
Pickering emulsion electrosynthesis oximes continuous flow electrocatalysis interfacial reaction industrial application

Cyclohexanone oxime is a critical intermediate in the production of nylon-6, with global nylon-6 production projected to reach 8.9 million tons by 2024, leading to an increasing demand for cyclohexanone oxime. Traditional methods for synthesizing cyclohexanone oxime primarily involve the reaction of hydroxylamine (NH2OH) with cyclohexanone. However, this approach faces several issues, such as the explosive nature of hydroxylamine, the use of corrosive acids, and the generation of low-value by-products like ammonium sulfate. Additionally, another industrial method involves the ammoxidation of cyclohexanone using hydrogen peroxide (H2O2), but this process is hindered by the high cost and low stability of H2O2. Therefore, developing a sustainable and efficient method for synthesizing cyclohexanone oxime is of great significance.

In recent years, the electrochemical synthesis of cyclohexanone oxime has gained attention. This method utilizes nitrogen oxides (NOx) to react with cyclohexanone, avoiding many of the problems associated with traditional approaches. However, this process still faces challenges such as high mass transport resistance in biphasic reactions and the competitive hydrogenation of hydroxylamine, resulting in low Faradaic efficiency (FE) and production rates. To address these issues, researchers have designed a Pickering-emulsion-droplet-integrated electrode aimed at improving the efficiency of continuous-flow electrochemical synthesis of cyclohexanone oxime.

Source of the Paper

This research was conducted by a collaborative team from Shanxi University, the National Synchrotron Radiation Research Center, Hunan University, and other institutions. The primary authors include Feifan Zhang, Qi-Yuan Fan, Yu-Cheng Huang, and others. The paper was published in April 2025 in the journal Nature Synthesis, titled “A Pickering-emulsion-droplet-integrated electrode for the continuous-flow electrosynthesis of oximes.”

Research Process

1. Design and Preparation of the Pickering-Emulsion-Droplet-Integrated Electrode

The researchers first designed a Pickering-emulsion-droplet-integrated electrode by combining conductive polypyrrole and amphiphilic silver particles to construct an oil-in-water Pickering emulsion system. The silver particles served not only as electrocatalysts but also as emulsifiers, ensuring the stable dispersion of emulsion droplets at the biphasic interface. By adjusting the content of polypyrrole, the researchers optimized the surface wettability of the catalyst, enabling the formation of an ideal localized microenvironment at the oil-water interface.

2. Experimental Validation of the Electrochemical Synthesis of Cyclohexanone Oxime

To validate the effectiveness of the system, the researchers conducted constant-current electrolysis experiments in an H-type electrolytic cell. The electrolyte consisted of a 0.5 M Na2CO3 and 0.5 M NaNO2 aqueous solution mixed with a 0.1 M cyclohexanone in cyclohexane solution at a 1:1 volume ratio. By uniformly dispersing the emulsifier (1 mg/mL) in this solution, the researchers prepared Pickering emulsion droplets with an average particle size of approximately 117 μm. The experimental results showed that at a current density of 20 mA/cm², the Faradaic efficiency of cyclohexanone oxime reached 80.7%, with a production rate of 0.15 mmol/h/cm², significantly higher than that of fixed electrochemical reaction systems (FEC) and fluidized electrochemical reaction systems (FLUEC).

3. In-Depth Study of the Reaction Mechanism

To further understand how the Pickering emulsion system enhances reaction efficiency, the researchers analyzed the reaction pathway using in situ attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR). The results indicated that the orientational ordering of water molecules and incomplete hydrogen bonding at the emulsion droplet interface enhanced the coupling process between cyclohexanone and hydroxylamine, thereby improving reaction efficiency. Additionally, Raman spectroscopy analysis revealed that the structure of interfacial water played a critical role in the catalytic process, particularly the presence of 2-coordinated hydrogen-bonded water (2-HB·H2O), which promoted the adsorption and reaction of cyclohexanone.

4. Development and Optimization of the Continuous-Flow System

To achieve industrial-scale application, the researchers developed a continuous-flow Pickering-emulsion-droplet electrochemical reaction system. By immobilizing the emulsion droplets on a carbon felt electrode, they established an efficient charge transfer channel, enabling the continuous production of cyclohexanone oxime. This system achieved a Faradaic efficiency of 83.8% at a current density of 100 mA/cm², with a production rate of 0.78 mmol/h/cm², and demonstrated long-term operational stability (100 hours). Furthermore, the system eliminated the need for additional demulsification steps, simplifying the product collection process.

Research Findings

  1. Design of the Pickering-Emulsion-Droplet-Integrated Electrode: By combining conductive polypyrrole and amphiphilic silver particles, the researchers successfully constructed an oil-in-water Pickering emulsion system, providing an ideal localized microenvironment for the synthesis of cyclohexanone oxime.
  2. High Efficiency of the Electrochemical Synthesis of Cyclohexanone Oxime: The experimental results showed that at a current density of 20 mA/cm², the Faradaic efficiency of cyclohexanone oxime reached 80.7%, significantly higher than that of traditional electrochemical reaction systems.
  3. Revealing the Reaction Mechanism: Through in situ ATR-FTIR and Raman spectroscopy analysis, the researchers uncovered the critical role of interfacial water molecules in the catalytic process, particularly the presence of 2-coordinated hydrogen-bonded water, which promoted the adsorption and reaction of cyclohexanone.
  4. Successful Development of the Continuous-Flow System: The researchers developed a continuous-flow Pickering-emulsion-droplet electrochemical reaction system, which achieved a Faradaic efficiency of 83.8% at a current density of 100 mA/cm² and demonstrated long-term operational stability.

Research Conclusion

By designing a Pickering-emulsion-droplet-integrated electrode, this research successfully achieved the efficient continuous-flow electrochemical synthesis of cyclohexanone oxime. The system not only improved reaction efficiency and Faradaic efficiency but also simplified the product collection process, demonstrating significant potential for industrial application. Additionally, the study revealed the critical role of interfacial water molecules in the catalytic process, providing new insights for the future design of efficient electrocatalytic systems.

Research Highlights

  1. Novel Design of the Pickering-Emulsion-Droplet-Integrated Electrode: By combining conductive polypyrrole and amphiphilic silver particles, the researchers successfully constructed an efficient Pickering emulsion system, providing an ideal localized microenvironment for the synthesis of cyclohexanone oxime.
  2. Efficient Electrochemical Synthesis: At a current density of 20 mA/cm², the system achieved a Faradaic efficiency of 80.7% for cyclohexanone oxime, significantly higher than that of traditional electrochemical reaction systems.
  3. In-Depth Revealing of the Reaction Mechanism: Through in situ ATR-FTIR and Raman spectroscopy analysis, the researchers uncovered the critical role of interfacial water molecules in the catalytic process, offering new insights for the future design of efficient electrocatalytic systems.
  4. Successful Development of the Continuous-Flow System: The system achieved a Faradaic efficiency of 83.8% at a current density of 100 mA/cm² and demonstrated long-term operational stability, showcasing significant potential for industrial application.

Summary

By designing a Pickering-emulsion-droplet-integrated electrode, this research successfully achieved the efficient continuous-flow electrochemical synthesis of cyclohexanone oxime, not only improving reaction efficiency and Faradaic efficiency but also simplifying the product collection process, demonstrating significant potential for industrial application. Additionally, the study revealed the critical role of interfacial water molecules in the catalytic process, providing new insights for the future design of efficient electrocatalytic systems.