Integration of plasma and electrocatalysis to synthesize cyclohexanone oxime under ambient conditions using air as a nitrogen source

Abstract

Direct fixation of N₂ to N-containing value-added chemicals is a promising pathway for sustainable chemical manufacturing. There is extensive demand for cyclohexanone oxime because it is the essential feedstock of Nylon 6. Currently, cyclohexanone oxime is synthesized under harsh conditions that consume a considerable amount of energy. Herein, we report a novel approach to synthesize cyclohexanone oxime by in situ NO₃⁻ generation from air under ambient conditions. This process was carried out through an integrated strategy including plasma-assisted air-to-NO_x and co-electrolysis of NO_x and cyclohexanone. A high rate of cyclohexanone oxime formation at 20.1 mg h⁻¹ cm⁻² and a corresponding faradaic efficiency (FE) of 51.4% was achieved over a Cu/TiO₂ catalyst, and the selectivity of cyclohexanone oxime was >99.9% on the basis of cyclohexanone. The C–N bond formation mechanism was examined by in situ experiments and theoretical calculations, which showed that cyclohexanone oxime forms through the reaction between an NH₂OH intermediate and cyclohexanone.

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Introduction

Dinitrogen (N₂) constitutes approximately 80% of the air, and is an inexhaustible nitrogen source that can be used to produce N-containing chemicals with a wide range of applications.^1,2 However, because of the ultra-inert N≡N triple bonds, the kinetics are sluggish and the thermodynamics are limited for the conversion of N₂.^3–5 Over the past few decades, researchers were mainly focused on the fixation of N₂ to NH₃ as a feedstock that would be further processed into various organonitrogen chemicals through reductive amination, oxidative cyanation, and ammoxidation.^6–8 Industrial NH₃ production through the traditional Haber–Bosch process consumes large amounts of energy and emits high levels of CO₂, and therefore cannot meet the demands for carbon neutrality or sustainability goals.^9–11 As an alternative route, the direct utilization of N₂ to construct C–N bonds under mild conditions is a promising strategy for the production of value-added chemicals and the decoupling of chemical manufacturing from fossil fuel energy, but it is a challenge.^12–14

There is great interest in the use of the electrochemical N₂ reduction reaction (NRR) to produce NH₃ because of the utilization of renewable electricity and protons directly from water.^15–17 The electrochemical construction of the C–N bond via the coupling of CO₂ and N₂ has also been considered to produce some organonitrogen chemicals, such as urea.^18,19 However, the reported reactions are very limited, and they are hindered by low reactivity and selectivity due to low N₂ solubility, difficult N₂ activation, and undesired hydrogen evolution reaction (HER).

Based on the occurrence of lightning in nature, air plasma oxidation offers an effective method for N₂ activation. Air can be converted into reactive NO_x or NO_x⁻, which can be used as a N source in electrolysis.^20,21 The C–N coupling would be achieved by the in situ generation of nucleophilic N-containing intermediates followed by their coupling with carbon substrates or intermediates as the electrophile.^22,23

Cyclohexanone oxime is the essential feedstock of Nylon 6, with worldwide annual demand of approximately 10 million tons and an approximate global market size of USD 25 billion, and is currently produced through the coupling between cyclohexanone and NH₂OH intermediates from unsustainable (NH₃OH)(NH₄)SO₄ (Scheme 1).^24–26 Therefore, optimization of N sources for NH₂OH intermediates is urgently needed for cyclohexanone oxime production. Researchers have recently explored the synthesis of cyclohexanone oxime using in situ-generated H₂O₂ to produce NH₂OH intermediates from NH₃.⁸ Furthermore, the direct use of N₂via air plasma oxidation integrated with electroreduction may result in the formation of cyclohexanone oxime under ambient conditions.

Scheme 1 Schematic diagram of different production routes for cyclohexanone oxime.

Herein, we report the first work to efficiently synthesize cyclohexanone oxime using air as the nitrogen source by combining plasma-assisted air-to-NO_x and co-electrolysis of NO_x and cyclohexanone. A faradaic efficiency (FE) of 51.4% was achieved for cyclohexanone oxime with a corresponding formation rate of 20.1 mg h⁻¹ cm⁻² and remarkable catalyst (Cu/TiO₂) recyclability. The superior performance can be attributed to Cu sites supported on TiO₂ with a stable oxidation state. In addition, the reaction pathway was studied based on in situ experiments and theoretical calculations.

Results and discussion

As shown in Fig. 1A, as a proof-of-concept, we synthesized cyclohexanone oxime through the coupling of plasma air activation and NO_x electroreduction with cyclohexanone (Fig. S1†). A model air mixture of N₂/O₂ (v/v = 4/1) was used as the nitrogen source. The reaction between N₂ and O₂ activated by plasma produced NO_x, which was absorbed by 1 mol L⁻¹ NaOH aqueous solution, and the as-obtained solution was directly used as an electrolyte for further electrosynthesis (Fig. S2 and S3†). This process holds the potential to block the interference of trace H₂O and CO₂ from real air. The concentration of NO₃⁻ and NO₂⁻ was quantified by UV-Vis spectroscopy measurements (Fig. S4†). Fig. 1B shows that the yield of NO_x linearly increased over the plasma activation time, in which the concentration of NO₂⁻ was approximately 2 times that of NO₃⁻. A high total NO_x concentration of approximately 1 mol L⁻¹ was achieved after 400 min of plasma activation.

Fig. 1 The integration system for cyclohexanone oxime synthesis. (A) Schematic illustration of the synthesis of cyclohexanone oxime with air as nitrogen source. (B) The concentration of NO_x⁻ species under air activation. (C) GC-MS and (D) ¹H NMR, ¹³C NMR, and ¹⁵N NMR detection of cyclohexanone oxime.

In the following electrosynthesis process, the electroreduction of NO₃⁻ with cyclohexanone was initially verified as a model reaction in a flow cell with 1 mol L⁻¹ NaNO₃ and 50 mmol L⁻¹ cyclohexanone aqueous solution as the catholyte, and 1 mol L⁻¹ NaOH as the anolyte. Copper-based catalysts were selected due to their exceptional performance in nitrate reduction, as described in recent reports.^27–31 Cu/TiO₂ catalysts loaded onto porous carbon paper served as the cathode, and Ni foam was used as the anode.

The obtained catalysts contained 0.3, 0.6, and 0.9 wt% Cu, as determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). We denoted these catalysts as 0.3% Cu/TiO₂, 0.6% Cu/TiO₂, and 0.9% Cu/TiO₂ for clarity. H₂ was the only gas product, and cyclohexanone oxime was the only organic product during electrolysis quantified by gas chromatography (GC). Colorimetric methods were used to study the aqueous byproducts, including NO₂⁻ and NH₃ (Fig. S4 and S5†). We also identified cyclohexanone oxime via GC-mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR) (Fig. S6–S10†). As shown in Fig. 1C, the molecular weight of 113.08354 provided by high-resolution GC-MS indicated that the molecular formula of the product is C₆H₁₁NO.

Using model air with ¹⁵N₂ gas and Na¹⁵NO₃ as the reactant, the molecular ion peak of the product at 114.08057 and 114.08041 matched the calculated weight of 114.08055 for C₆H₁₁¹⁵NO, which verified that the origin of nitrogen in cyclohexanone oxime was NO₃⁻in situ generated from air. Moreover, all the peaks in the ¹H NMR, ¹³C NMR, and ¹⁵N NMR (conducted with the ¹⁵N-labeled product) spectra were well matched with the standard samples (Fig. 1D and S9†). These results confirmed the formation of cyclohexanone oxime, with NO₃⁻ as the nitrogen source.

Fig. 2A displays the linear sweep voltammetry (LSV) results for 0.6% Cu/TiO₂ in electrolyte with and without NaNO₃ plus cyclohexanone substrates. After the addition of the substrates, the sharpness of the increase in the current density (j) was greater than that of the initial NaOH electrolyte, which demonstrated that the electrosynthesis of cyclohexanone was more kinetically favorable than the HER. Electrochemical performance tests were carried out in the range of −1.0 V to −2.0 V versus Ag/AgCl, and the electrochemical performance of cyclohexanone oxime formation on Cu/TiO₂ is shown in Fig. 2B and C. The reactions under all the applied potentials presented a combined FE of approximately 100% for all products. A maximum cyclohexanone oxime FE of 50.0% and a corresponding formation rate of 20.1 mg h⁻¹ cm⁻² was achieved at −1.8 V over 0.6% Cu/TiO₂.

Fig. 2 Electrochemistry study of cyclohexanone synthesis. (A) LSV curves of 0.6% Cu/TiO₂ catalysts in different electrolyte. Potential-dependent (B) FE and (C) formation rate of cyclohexanone oxime production on different Cu-loaded TiO₂ catalysts. (D) Chronoamperometry curves of 0.6% Cu/TiO₂ under optimal conditions for cyclohexanone oxime synthesis. (E) Recyclability of 0.6% Cu/TiO₂ electrocatalysts. (The dotted line shows the formation rate of cyclohexanone at 100% conversion.) (F) The formation rate of cyclohexanone oxime with different gas feed and plasma conditions.

At low potentials, NO₂⁻ was the main byproduct, which could be attributed to the hindered reductive hydrogenation of NO₃⁻. Once the potential increased so that it was more negative than −1.8 V, the enhancement of the HER and the further electroreduction of NH₂OH intermediates led to a decrease in the selectivity for cyclohexanone oxime (Fig. S11†). Intriguingly, the byproducts generated during the electrolysis process, specifically NH₃ and H₂, hold significant value as chemicals with substantial potential for further utilization. Under optimal conditions, the FE of NH₃ and H₂ were 21.6% and 16.4%, respectively. Similar trends were found for the Cu/TiO₂ catalysts with various Cu loadings. In addition, as shown in Fig. 2D, the corresponding chronoamperometry curve of 0.6% Cu/TiO₂ catalysts at −0.8 V remained stable during electrosynthesis.

We further studied the influence of substrate concentrations on the electrochemical performance by varying the molar concentrations of cyclohexanone and NaNO₃ in the electrolytes at the applied potential of −1.8 V vs. Ag/AgCl (Fig. S12†). This indicated that a higher cyclohexanone and NO₃⁻ concentration promoted the formation of cyclohexanone oxime. However, when the concentration of cyclohexanone was greater than 50 mmol L⁻¹, there was a significant decrease in the cyclohexanone oxime FE and formation rate due to the phase separation in the catholyte. The increase in the NO₃⁻ concentration up to 2 mol L⁻¹ strengthened the side reaction of NH₃ formation, which was unsuitable for the formation of NH₂OH intermediates and cyclohexanone oxime. Consequently, the optimized substrate concentration was found to be 50 mmol L⁻¹ cyclohexanone and 1 mol L⁻¹ NaNO₃ for cyclohexanone oxime production.

The TiO₂ support played a crucial role in the improvement of cyclohexanone oxime formation. Fig. S13† shows that the catalytic performance of the bulk TiO₂ was very poor. For comparison, carbon black was also used as a support for Cu loading, but the as-prepared Cu/carbon black catalyst exhibited a maximum cyclohexanone oxime FE of only 8.4% with a formation rate of 2.5 mg h⁻¹ cm⁻². Therefore, the optimized catalytic activity could be ascribed to the cooperation between Cu and the TiO₂ support.

The stability of the 0.6% Cu/TiO₂ catalyst was also studied at −1.8 V. Fig. 2E shows that there was no notable change in the cyclohexanone oxime FE or formation rate during 5 successive cycles, which was indicative of prominent recyclability and is important for practical applications.

After establishing the baseline of NO₃⁻ electrolysis activity, NO_x obtained from plasma air activation was fed together with cyclohexanone in the electrosynthesis process. Fig. 2F shows that the integrated system provided cyclohexanone oxime at a formation rate of 20.1 mg h⁻¹ cm⁻² using air as the nitrogen source. No cyclohexanone oxime was detected in the absence of either plasma, N₂, or O₂, indicating the necessity of air as the nitrogen source under plasma oxidation for cyclohexanone oxime production. It was therefore concluded that the two-step integration of plasma air activation with electrocatalytic reduction is practically feasible for highly efficient and selective nitrogen synthesis of cyclohexanone oxime under ambient conditions.

Structural and morphological measurements clarified that Cu nanoparticles were uniformly loaded on TiO₂ in the Cu/TiO₂ catalysts (Fig. S14–S22†). In situ X-ray absorption near-edge spectroscopy (XANES) was conducted to further investigate the electronic structure of the catalysts during cyclohexanone oxime synthesis. Fig. 3A shows that an initial 0.6% Cu/TiO₂ catalyst exhibited an absorption edge between that of Cu foil and Cu₂O samples, indicating that the oxidation state of Cu was between 0 and +1. We further acquired the Cu average oxidation state as a function of the Cu K-edge energy shift, and the value was 0.42 during the in situ measurements (Fig. 3B). The value did not change over electrolysis time, indicating the excellent stability of the Cu active sites during electrolysis. The results indicated that a stable slightly positive oxidation state for Cu was beneficial for the reaction. In Fig. S23 and S24,† no obvious changes were observed in the structure or morphology of the catalyst after electrolysis, further indicating the stability of the catalyst.

Fig. 3 In situ study. (A) XANES spectra at Cu K-edge and (B) Cu oxidation state of 0.6% Cu/TiO₂ measured at different electrolysis times during cyclohexanone oxime synthesis. (C) Online DEMS graph of NO and NH₂OH intermediates. In situ FTIR spectra of electrochemical cyclohexanone synthesis conducted at prolonged electrolysis time on 0.6% Cu/TiO₂ in (D) H₂O and (E) D₂O solvent.

Further study was conducted to reveal the possible reaction pathway. Online differential electrochemical mass spectrometry (DEMS, Fig. S25†) was used to detect the m/z signals of 30 and 33, as shown in Fig. 3C, which corresponded to the NO and NH₂OH intermediates, respectively, during NO₃⁻ reduction over 0.6% Cu/TiO₂ catalyst. The weak signal of NH₂OH indicated that the generated NH₂OH spontaneously coupled with cyclohexanone to produce oxime products and could not remain in the bulk electrolyte. Signals of other byproducts were also detected, as shown in Fig. S26.†In situ Fourier transform infrared (FTIR) spectroscopy (Fig. S27†) also confirmed the possible involvement of NO and NH₂OH signals at 1442 and 1204 cm⁻¹ in Fig. 3D on the surface of the 0.6% Cu/TiO₂ catalyst during electrosynthesis.^32–34 Additionally, when using D₂O as the electrolyte solvent, an obvious peak at 1689 cm⁻¹ was observed, as shown in Fig. 3E, indicating the formation of C=N bonds in cyclohexanone oxime molecules that was not observed in H₂O solvent because of the interference of O–H bending (1645 cm⁻¹).

Systematic control experiments were performed to study the reaction pathways of the electrosynthesis of cyclohexanone oxime (Table S1†). No cyclohexanone oxime was detected when there was no cyclohexanone or NO₃⁻ in the electrolyte (entries 1–3). We also used NO₂⁻, NO, and NH₂OH as the feedstock, which were formerly identified as the intermediates (entries 4–6). All three entries produced cyclohexanone oxime products. In addition, the C–N coupling between cyclohexanone and NH₂OH was proved to be spontaneous, and occurred without electrolysis (entries 6–7).

The discernible fluctuations in the FE, notably when feeding with NO, were ascribable to the limited solubility of NO gas in aqueous solutions, leading to heightened hydrogen evolution side reactions. Notwithstanding these disparities in the FE, the rate of cyclohexanone oxime formation continued to exhibit a noteworthy level of consistency across all nitrogenous substrates. Therefore, the above results present the possible reaction pathways of NO₃⁻ to NH₂OH intermediates through NO₃⁻ → NO₂⁻ → NO → NH₂OH progression, followed by the nonelectrochemical spontaneous condensation of NH₂OH and cyclohexanone to produce cyclohexanone oxime.

Furthermore, density functional theory (DFT) calculations were conducted to study the reaction pathway (Fig. 4A and S28†). According to high-resolution transmission electron microscopy (HRTEM) images and X-ray diffraction (XRD) patterns, Cu(111) and Cu(111)-Cu₂O(110) heterostructures loaded on anatase phase TiO₂(101) were used as the catalyst models. A free energy diagram of NO₃⁻-to-NH₂OH is presented in Fig. 4B. NO₃⁻ was chemically absorbed on the catalyst to initially form with a decrease in free energy. Then, the deoxygenation and hydrogenation steps were continuously proceeded by H⁺/e⁻ transfer to form and NO*. The conversion of to NOOH* was endergonic and acted as the rate-determining step (RDS), which was consistent with the detection of NO₂⁻ byproducts during electrolysis. Subsequently, the NO* intermediate was gradually converted to NOH*, NHOH*, and finally to NH₂OH* under hydrogenation.

Fig. 4 Theoretical calculations. (A) Catalytic pathway from NO₃⁻ to NH₂OH on Cu/TiO₂ catalyst according to the optimized configurations with adsorbed intermediates. Cyan, blue, purple, red, and pink balls denote Ti, Cu, N, O, and H, respectively. (B) Gibbs free energy diagrams of NH₂OH generation from NO₃⁻ on different models. (C) Calculated free energy diagram for (C) cyclohexanone* and (D) cyclohexanone oxime* adsorption on different TiO₂-loaded Cu catalytic sites. (E) The proposed pathway of NH₂OH generation and cyclohexanone oxime production.

The high selectivity of >99.9% is an obvious advantage of cyclohexanone oxime production through an electrocatalysis strategy.^8,35,36 Interestingly, the reduction of cyclohexanone and cyclohexanone oxime were not detected as side reactions because no GC-MS peaks of cyclohexanol or cyclohexylamine appeared in Fig. S6A.† The reduction of excess NO_x substrates as well as a coherent base environment (pH 13.4) of the electrolyte hindered the reduction of organic molecules.¹² Additionally, DFT study revealed the limitation of reduction of cyclohexanone (Fig. 4C) and cyclohexanone oxime (Fig. 4D). The calculated free energy of the absorption of cyclohexanone and cyclohexanone oxime on Cu(111)-Cu₂O(110) models was found to be lower than that on the Cu(111) structure, which implies that the side reactions were hindered during efficient production of cyclohexanone oxime on Cu/TiO₂ catalysts. As discussed above, the pathway of cyclohexanone is presented in Fig. 4E. NO_x substrates from air oxidation produced NH₂OH through electrocatalysis, followed by its non-electrochemical coupling with cyclohexanone to produce oxime.

The universality of our method was further confirmed by successfully converting a range of aldehydes and ketones in addition to cyclohexanone (e.g., benzaldehyde, acetone, cyclopentanone, and diethyl ketone) into oximes with high FE and selectivity under optimal conditions (Table S2†). This highlights the broad applicability of the electrocatalytic strategy to synthesize oxime, and affirms our proposed reaction pathway.

Conclusions

We propose a sustainable strategy to efficiently synthesize value-added cyclohexanone oxime using air as a nitrogen source, which includes plasma-assisted air-to-NO_x and co-electrolysis of NO_x and cyclohexanone. The highest performance was provided by 0.6% Cu/TiO₂, which produced cyclohexanone at a yield rate of 20.1 mg h⁻¹ cm⁻² and an FE of 51.4%. The selectivity of cyclohexanone oxime was >99.9% on the basis of cyclohexanone. Detailed study indicated that the optimized catalytic activity was ascribed to Cu sites supported on TiO₂ with a stable oxidation state.

Further in situ characterizations combined with DFT calculations revealed the possible pathway of NO₃⁻ → NO₂⁻ → NO → NH₂OH → cyclohexanone oxime. Together with transformation routes reported by our group, including the production of phenol using lignin and selective phenol-to-cyclohexanone conversion, this work provides a rational pathway to the synthesis of cyclohexanone oxime with biomass, an air nitrogen source, and green solvent.^37–39 It will also inspire innovative routes to the carbon-natural chemical manufacturing of industrially important fine C–N chemicals from renewable carbon sources and nitrogen using clean energy.

Data availability

All experimental data is available in the ESI.†

Author contributions

S. H. J., X. F. S., and B. X. H. proposed the project, designed the experiments, and wrote the manuscript. S. H. J. performed all the experiments. X. X. T., L. M. W., and X. D. M. performed the analysis of the experimental data. L. B. Z., J. Q. F., L. X., and X. N. S. conducted portions of the characterization study. Q. G. Z. and X. C. K. participated in discussions. X. F. S. and B. X. H. supervised the entire project.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22002172, 21890761, and 22121002), Beijing Natural Science Foundation (J210020), CAS Project for Young Scientists in Basic Research (Grant No. YSBR-050), and the Photon Science Center for Carbon Neutrality. The X-ray absorption spectroscopy measurements were performed at Beamline 4B9A at the Beijing Synchrotron Radiation Facility (BSRF). We also thank Dr Jiang Yan, Dr Ying Wang, Ms. Ziwei Zhao, and Ms. Jun He for their support in the GC-MS measurements. The authors thank the staff at the Centre for Physiochemical Analysis & Measurement of ICCAS for material characterizations.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3sc02871b

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