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

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-NOx and co-electrolysis of NOx 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.

The cathode electrode was prepared over a spray-dry method.First, 15 mg of as-prepared electrocatalysts, 1 mL of IPA, and 15 μL of Nafion ionomer solution (5 wt% in IPA) were mixed to prepare the electrocatalyst slurry, followed by ultrasonic treatment for 30 min to form a homogeneous ink.Then, 250 μL of the electrocatalyst slurry was slowly spread onto the carbon paper (TGP-H-60) and dried in the air at room temperature to achieve a uniform electrocatalyst loading of ~1.0 mg cm -2 .NF was ultrasonicated with acetone, IPA, and water for 30 min, respectively, and dried in air under room temperature before using as an anode electrode.Electrochemical study Electrochemical synthesis of oximes in a flow cell.Electrochemical studies were conducted in an electrochemical flow cell, which was composed of a cathodic chamber and an anodic chamber.An anion exchange member (FumasepFAA-3-PK-130) served to separate the cathodic and anodic chambers.An Ag/AgCl electrode (3.0 mol L -1 KCl) and NF were used as the reference electrode and counter electrode, respectively.The electrolysis was conducted through a CHI 660 electrochemical workstation.For performance studies, 5 ml of 1 mol L -1 NaNO 3 and 50 mmol L -1 cyclohexanone aqueous solution was used as catholyte and 1 mol L -1 NaOH was used as anolyte.The electrolyte was bubbled with Ar for 30 min under stirring before the electrochemical reaction was started.The surface area of the electrode was 1.4 cm -2 .When studying the influence of cyclohexanone to the performance, cyclohexanone of different concentration was added.When studying the concentration of NO 3 -, Na 2 SO 4 was used to keep the concentration of all the ions are equal in all studies.The synthesis of cyclohexanone oxime was conducted with 1 mol L -1 NO 3 -, 1 mol L -1 NaNO 3 and 1 mol L -1 NaOH.Product identifications and quantifications Cyclohexanone oxime.After the electrochemical reaction, 1 mL of catholyte was mixed with 1 mL of saturated NH 4 Cl solution to adjust the pH of the solution.Then, the solution was extracted with 10 mL of EA.Reactants and products in EA were identified by gas chromatography-mass spectrometry (GC-MS, Agilent 5977A, HP-5MS capillary column with 0.25 mm of diameter and 30 m of length), and high-resolution GC-MS (Thermo Fisher Scientific, Exactive GC).The analysis of 1 H, 13 C, and 15 N NMR was conducted on Bruker Avance Neo 700 with CDCl 3 as the solvent.In the 15 N NMR, the reference was CH 3 NO 2 + 10% CDCl 3 , which was calculated as 0 ppm.The quantitative analysis of the products in the extract liquor was conducted using gas chromatography (GC, Agilent 6820) equipped with a flame ionization detector (FID) and HP 5MS/HP-INNOWAX capillary column with 0.25 mm in diameter and 30 m in length.Dodecane was used as the internal standard to quantitative the yields of electrochemical reactions.The calibration curve was established by preparing and measuring the peak area ratio between cyclohexanone oxime and dodecane.Several standard EA solution samples with the known concentration of cyclohexanone oxime were used.The concentration of cyclohexanone oxime of unknown extract liquor samples was then calculated through the calibration curve.The Faradaic efficiency (FE) and yield rate of cyclohexanone oxime were calculated as follows: Where n is the electrons transferred in per mole reaction of electrosynthesis of cyclohexanone oxime, F is the Faraday constant, c is the concentration of the products, V is the volume of the electrolyte used in the reaction, Q is the total number of charges transferred in the reaction, M is the molecular mass of the product, S is the area of the electrode in the flow cell, and t is the reaction time.
The gaseous products were collected in gas bags and analyzed by gas chromatography (GC, HP 4890D) with Ar as the load gas.
Long-established colorimetric methods were utilized to quantify the concentration of NH 3 and NO 2 -in the current study with some modifications 1 .All the samples were taken for UV-vis absorption measurement on Perkin Elmer, Lambda 1050+ within 2 h after preparation.The salicylate method was used to quantify NH 3 in the catholyte, enabled by the transformation of NH 3 to indophenol blue through a chemical reaction among NH where n is the electrons transferred in pre mole reaction of generation of NH 3 , F is the Faraday constant, c is the concentration of the NH 3 , V is the volume of the electrolyte, and Q is the total number of charges transferred in the reaction.Griess test was used to quantify NO 2 -in the catholyte, in which azo compound with pink color could form through the reaction among NO 2 -, 4-aminobenzenesulfonamide, and N-(1-naphthyl) ethylenediamine.Solution α and β were prepared for further use.Solution α: 10 g L -1 of 4aminobenzenesulfonamide aqueous solution.Solution β: 1 g L -1 N-(1-naphthyl)ethylenediamine dihydrochloride aqueous solution.When qualifying the concentration of NO 2 -in an aqueous sample, 200 μL of solution α and 200 μL of solution β were added to the sample at a 10-minute interval.The fresh electrolyte was also used to prepare the blank sample for UV-vis measurement as the baseline.The calibration curve was established by preparing and measuring the absorbance at 540 nm of several standard solution samples with the known concentration of NO 2 -in the typical electrolyte of electrosynthesis of oximes.The concentration of NO 2 -of unknown samples was calculated through the absorbance at 540 nm and the calibration curve.The Faradaic efficiency (FE) of NO 2 -was calculated as follows: where n is the electrons transferred in per mole reaction of generation of NO 2 -, F is the Faraday constant, c is the concentration of the NO 2 -, V is the volume of the electrolyte, and Q is the total number of charges transferred in the reaction.

Plasma air oxidation
The air oxidation plasma reaction system was composed of a stainless-steel rotating gliding plasma jet reactor, a gas feeder, mass flow controllers, a DC power supply, and a current-limiting resistor.
The reactor had a cylindrical anode inserted into a cylindrical cathode, with a non-thermal plasma ignited in the cavity and expelled through a nozzle-designed outlet via the gas flow.The reactants used were a mixture of N 2 and O 2 (model air) to produce NO x products.3.0 mL of sample solution was added to the test tube, and then 100 μL of 5.0 M HCl solution was added.After shaking well and standing for 10 min, the concentration of NO 3 -was measured by UV-Vis in the wavelength range of 200 ~ 300 nm.The standard curve for NO 3 -determination was plotted with the difference of absorbance values at 220 nm and 275 nm as vertical coordinates and NO 3 -concentration as horizontal coordinates.The concentration of NO 2 -was as above-mentioned.

Isotope-labeled experiments
The isotope-labeled electrochemical synthesis of oximes was performed on the 0.6% Cu/TiO 2 electrode with 1 mol L -1 Na 15 NO 3 and 50 mmol L -1 cyclohexanone aqueous solution at -1.6 V versus Ag/AgCl for 1 h.The 15 N-cyclohexanone oxime was not commercially available.The home-made standard sample of 15 N-cyclohexanone oxime was synthesized by mixing 0.05 mol L -

DEMS experiments
Differential electrochemical mass spectrometry (DEMS) experiments were conducted on Shanghai Linglu QAS 100.A typically optimized electrolyte and 0.6% Cu/TiO 2 catalyst were used in the test.
Ar was used as the loaded gas in the whole experiment to avoid the influence of other reactions.

In situ FTIR experiments
In situ FTIR experiments were conducted on Thermo Scientific Thermo 8700.A typically optimized electrolyte and 0.6% Cu/TiO 2 catalyst were used in the experiment with Ar atmosphere.In the experiments in the D 2 O solvent, the gas path, electrochemical cell, and reference electrode were washed with D 2 O several times to remove H 2 O.The solution in the reference electrode was also changed to 3M KCl in D 2 O.

Density functional theory (DFT) calculations
Vienna ab initio simulation package (VASP) was used to Spin-polarized density functional theory (DFT) calculations. 2The projector augmented wave (PAW) pseudopotential for core electrons and the generalized gradient approximation (GGA) in the form of Perdew-Burke-Ernzerhof (PBE) for the exchange correlation potentials were adopted. 3,4 he c axis of 25 Å was stetted to avoid the interactions of catalysts between adjacent images.In addition, a cut-off energy of 450 eV was used.The atoms were fully relaxed until the energy convergence reached 0.0001 eV.Van der Waals (vdW) interaction was considered at the DFT-D3 level as proposed by Grimme.

Figure S6 .
Figure S6.(A) GC-MS detection of the products.MS graph of detected (B) cyclohexanone and (C) cyclohexanone.

Figure S9 .
Figure S9.Full graph of (A) 1 H NMR, (B) 13 C NMR, and (C) 15 N NMR of standard samples and extract liquor of catholyte.
3, salicylate, and hypochlorite.Solutions A, B, and C were prepared as follows.Solution A: 0.32 mol L -1 sodium hydroxide + 0.4 mol L -1 sodium salicylate aqueous solution.Solution B: 0.75 mol L -1 NaOH + NaClO (active chlorine: ~4.5%) aqueous solution.Solution C: 10 mg ml -1 C 5 H 4 FeN 6 Na 2 O 3 aqueous solutions.To quantify the concentration of NH 3 in the catholyte, 3 mL of the aqueous sample solution, 500 μL of Solution A, 50 μL of Solution B, and 50 μL of Solution C were sequentially added to a sample tube.The fresh electrolyte was used to prepare the blank sample for UV-vis measurement as the baseline.The calibration curve was established by preparing and measuring the absorbance at 675 nm of several standard solution samples with the known concentration of NH 4 + in the typical electrolyte of electrosynthesis of oximes.The concentration of NH 3 in unknown samples was calculated through the absorbance at 675 nm and the calibration curve.The FE of NH 3 was calculated as follows: 1 15 N-(NH 2 OH) 2 H 2 SO 4 with 0.05 mol L -1 cyclohexanone aqueous solution and stirring for 30 min.The CDCl 3 extract liquor of the solution was used as the standard sample after being detected by GC-MS.

Table S1 .
Control experiments to reveal the electrosynthesis pathway of cyclohexanone oxime.

Table S2 .
FE and formation rate of different oxime molecules.