Key Takeaways & Executive Findings
- •• • Achieved a Faradaic efficiency of 74.63% and a yield rate of 0.156 mmol h−1 cm−2 for cyclohexanone oxime (CHO) production via pulsed electrolysis on amorphous Bi2O3, outperforming potentiostatic conditions (55.43% FE on amorphous Bi) and offering a safer, cost-effective route compared to conventional hydroxylamine-based processes. • • DFT calculations identified *NOH→*NHOH as the favored pathway on amorphous Bi2O3 (ΔG = 0.75 eV) versus *NOH→*N on amorphous Bi (ΔG = −1.76 eV), directly correlating with the higher FE for CHO and providing a mechanistic basis for catalyst design. • • The kinetic barrier for *NH2OH→*NH2 is 0.86 eV, while *NH2OH desorption to NH2OH is barrierless, ensuring selective NH2OH release and subsequent coupling with cyclohexanone, a critical factor for industrial scalability. • • The ICOHP analysis of the N–O bond in *NOH on amorphous Bi2O3 (1.34) versus amorphous Bi (0.84) indicates a weakened N–O bond (bond length 1.354 Å vs. 1.398 Å), which promotes the selective hydrogenation pathway and underscores the importance of the Bi2O3 phase in steering selectivity.
Abstract
Cyclohexanone oxime (CHO) is a pivotal feedstock for nylon-6 production, yet conventional synthesis routes suffer from high explosion risks, harsh conditions, and costly catalysts. Here, we report an electrocatalytic approach for CHO synthesis via reductive coupling of cyclohexanone (CYC) with nitrite over commercially available Bi2O3. A two-stage pulsed electrolysis protocol is employed: the first stage prepares amorphous Bi2O3, while the second stage produces CHO with a Faradaic efficiency (FE) of 74.63% and a yield rate of 0.156 mmol h−1 cm−2. Mechanistic studies, combining experiments and density functional theory (DFT) calculations, reveal that on amorphous Bi2O3, the *NOH intermediate preferentially undergoes hydrogenation to *NHOH and then *NH2OH, rather than the *NOH→*N pathway leading to NH3. This selectivity is attributed to the higher integral crystal orbital Hamilton population (ICOHP) for the N–O bond in *NOH on amorphous Bi2O3 (1.34 vs. 0.84 on amorphous Bi), indicating a weakened N–O bond that facilitates hydrodeoxygenation. Transition state calculations show a kinetic barrier of 0.86 eV for *NH2OH→*NH2, while desorption of *NH2OH to NH2OH is barrierless, favoring NH2OH release. This work provides a sustainable, efficient alternative to conventional CHO production, addressing safety and cost concerns while achieving high selectivity.
1. Introduction
Conventional cyclohexanone oxime (CHO) production, essential for nylon-6 manufacturing, relies on energy-intensive and hazardous processes involving hydroxylamine (NH2OH) intermediates, which pose explosion risks and require expensive catalysts. The global demand for nylon-6, exceeding 8.9 million tons annually, underscores the urgent need for sustainable and safe alternatives. Electrocatalytic synthesis, powered by renewable electricity, offers a promising route by coupling nitrite reduction with cyclohexanone under mild conditions. However, the complex proton-coupled electron transfer in nitrite reduction often leads to unwanted byproducts like ammonia, necessitating highly selective catalysts to steer the reaction toward NH2OH.
Bismuth-based catalysts have shown exceptional selectivity for NH2OH production, yet their performance is hampered by the reduction of Bi2O3 to inactive metallic Bi under cathodic potentials. This study introduces a pulsed electrolysis strategy that dynamically regulates the catalyst's oxidation state, maintaining an amorphous Bi2O3 phase that favors the *NOH→*NHOH pathway over the competing *NOH→*N route. By achieving a Faradaic efficiency of 74.63% and a yield rate of 0.156 mmol h−1 cm−2, this approach directly addresses the selectivity bottleneck, offering a commercially viable and safer alternative to conventional CHO synthesis.
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Junchao Yu, Zichao Xi, Peng Jing, Xuan Xu, Baocang Liu, Jun Zhang (2026). Pulsed-Electrolysis-Induced Bi-Bi2O3 Transformation Switches the Reaction Pathway for Enhanced Cyclohexanone Oxime Production. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4093-8
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Frequently Asked Questions
What is the long-term stability of the amorphous Bi2O3 catalyst under pulsed electrolysis conditions, and how does it compare to potentiostatic operation?
The study demonstrates that pulsed electrolysis maintains the amorphous Bi2O3 phase, preventing reduction to inactive Bi0. While specific long-term stability data are not provided in the excerpt, the FE of 74.63% under pulsed conditions versus 55.43% on amorphous Bi under potentiostatic conditions indicates superior selectivity retention. Further chronoamperometry or cycling tests would be required to quantify degradation rates over extended operation.
How does the energy consumption and cost of this electrocatalytic route compare to the conventional industrial synthesis of cyclohexanone oxime?
The paper does not provide a full techno-economic analysis, but the use of commercially available Bi2O3 and mild conditions (ambient temperature and pressure) suggests lower capital and operational costs. The yield rate of 0.156 mmol h−1 cm−2 and FE of 74.63% indicate reasonable efficiency, but a detailed comparison of electricity costs, catalyst longevity, and separation expenses versus the conventional route is necessary for a definitive cost parity assessment.
What is the selectivity for CHO relative to other possible products, such as cyclohexanone reduction or nitrite reduction to ammonia?
The FE of 74.63% for CHO indicates that the majority of electrons are used for the desired product. The remaining current likely goes to competing reactions, such as hydrogen evolution or further reduction of NH2OH to NH3. The DFT calculations show that on amorphous Bi2O3, the *NH2OH desorption is barrierless, while further reduction to *NH2 has a barrier of 0.86 eV, suggesting that NH2OH release is kinetically favored. However, exact selectivity values for byproducts are not provided in the excerpt.
Can this pulsed electrolysis approach be scaled up to industrial current densities, and what are the mass transport limitations?
The reported yield rate of 0.156 mmol h−1 cm−2 is at a lab scale. Scaling up would require addressing mass transport of nitrite and cyclohexanone to the electrode surface, as well as managing heat dissipation. Pulsed electrolysis may help mitigate concentration polarization by allowing relaxation periods, but engineering challenges such as electrode design and cell configuration need to be optimized. The study does not provide data at higher current densities, so further research is needed to assess scalability.
What is the role of the amorphous nature of Bi2O3 in the selectivity, and can crystalline Bi2O3 achieve similar performance?
The amorphous Bi2O3 is crucial for the observed selectivity. DFT calculations show that amorphous Bi2O3 favors the *NOH→*NHOH pathway, whereas amorphous Bi favors *NOH→*N. The amorphous phase likely provides a higher density of active sites and unique electronic properties that stabilize the *NHOH intermediate. Crystalline Bi2O3 may have different surface energies and coordination environments, potentially altering the reaction pathway. The study does not compare crystalline Bi2O3, but it is implied that the amorphous phase is essential for high FE.
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