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Direct electrosynthesis of methylamine from carbon dioxide and nitrate

Abstract

The electrochemical reduction of carbon dioxide is an appealing technology that stores renewable electricity in the chemical form and has the potential to transform the way carbon fuels are utilized today. While there have been successes in the electrosynthesis of alkanes, alkenes and alcohols, access to organonitrogen molecules such as alkylamines remains largely beyond the reach of current electrocatalysis. Here we report the first electrochemical reaction that converts carbon dioxide and nitrate to methylamine in aqueous media under ambient conditions catalysed by a cobalt β-tetraaminophthalocyanine molecular catalyst supported on carbon nanotubes. The overall reaction, involving the transfer of 14 electrons and 15 protons to form each methylamine molecule, is an eight-step catalytic cascade process enabled by the coupling of two reactive intermediates near the catalyst surface. The key C–N bond-forming step is found to be the spillover of hydroxylamine from nitrate reduction and its subsequent condensation with formaldehyde from carbon dioxide reduction. This study provides a successful example of sustainable alkylamine synthesis from inorganic carbon and nitrogen wastes, which could contribute to greenhouse gas mitigation for a carbon-neutral future.

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Fig. 1: Cascade electrocatalytic synthesis of methylamine from CO2 and NO3.
Fig. 2: Electrocatalytic performance of CoPc-NH2/CNT for the co-reduction of CO2 and NO3.
Fig. 3: The proposed reaction pathway of the eight-step cascade electrosynthesis of methylamine from CO2 and NO3 catalysed by CoPc-NH2/CNT.

Data availability

The data supporting the findings of this study are available from the corresponding authors upon reasonable request.

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Acknowledgements

The work at Yale was supported by the US National Science Foundation (grant no. CHE-1651717). The work at Southern University of Science and Technology was supported by the National Science Foundation of China (grant no. 22075125). Y.W. acknowledges the Dox Fellowship from Yale University. H.W. acknowledges the Sloan Research Fellowship.

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Authors

Contributions

Y.W. and H.W. conceived the project and designed the experiments. Y.W., Z.J. and Z.L. performed the experiments and analysed the data with input from Y.L. and H.W. Y.W. and H.W. wrote the manuscript with input from all other authors. Y.L. and H.W. supervised the project.

Corresponding authors

Correspondence to Yongye Liang or Hailiang Wang.

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Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature Sustainability thanks Feng Jiao and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Liquid-phase and gas-phase product quantification.

UV-vis spectra and calibration curves (with linear fitting equations and R2 values) for colorimetric quantification of a,b, NO2, c,d, NH2OH and e,f, NH3. Typical gas chromatography diagrams from g, the flame ionization detector and h, the thermal conductivity detector, showing the presence of CO, CO2 and H2. Calibration curves for i, CO and j, H2.

Extended Data Fig. 2 Electrochemical NO3 reduction catalyzed by CoPc-NH2/CNT.

a, Potential-dependent product selectivity and current density of electrochemical NO3 reduction catalyzed by CoPc-NH2/CNT. 24 min electrolyses were carried out in an Ar-saturated aqueous solution containing 0.1 M PBS and 0.5 M KNO3. b, 1H NMR spectrum of the catholyte solution after electrochemical NO3 reduction at −0.94 V.

Extended Data Fig. 3 Identification of liquid-phase products from co-reduction in NMR spectra.

Typical a, 1H NMR and b, 13C NMR spectra of the catholyte solution after co-reduction stacked together with standard spectra of methylamine, N-methylhydroxylamine and formaldoxime dissolved in fresh CO2-saturated electrolyte solutions.

Extended Data Fig. 4 Further confirmation of methylamine formation and its C, N sources.

a, Standard mass spectrum of N-methylpivalamide from NIST Mass Spectral Library. b, 1H NMR spectrum of methylamine after electrolysis in 0.1 M PBS + 0.5 M KNO3 under 13CO2 for 30 min at −0.95 V vs RHE. c, 1H NMR spectrum of formaldoxime, N-methylhydroxylamine and methylamine after electrolysis in 0.1 M KHCO3 + 0.5 M K15NO3 under CO2 for 5 h at −0.91 V vs RHE. The green traces show the 1H NMR spectra of formaldoxime, N-methylhydroxylamine and methylamine from co-reduction of unlabeled CO2 and NO3.

Extended Data Fig. 5 Statistics of reaction rate and selectivity.

Potential-dependent a, jmethylamine and b, FEmethylamine values measured from 27 independent co-reduction electrolyses of CO2 and NO3 in 0.1 M KHCO3 + 0.5 M KNO3 electrolyte.

Extended Data Fig. 6 Determination of the loading of electrochemically active CoPc-NH2 on the electrode.

Integration of the one-electron reduction peak in the cyclic voltammogram to determine the loading of electrochemically active CoPc-NH2 molecules. Scan rate: 10 mV/s.

Extended Data Fig. 7 Identification of key N intermediates.

1H NMR spectra of catholyte solutions after reduction of a, CO2 in the presence of NH4HCO3 and b, CO in the presence of NH4OH. Stacked together are spectra of CH3OH and methylamine co-dissolved in 0.1 M KOH and electrolyte solution after co-reduction of CO and NO3. c, 1H NMR spectra of catholyte solutions after co-reduction of CO2 in 0.1 M PBS with different N sources: KNO3, NaNO2, NO and NH2OH. The peaks pertaining to methylamine are highlighted in green.

Extended Data Fig. 8 Identification of key C intermediates.

1H NMR spectra of catholyte solutions after co-reduction of KNO3 with different C sources in 0.1 M PBS: a, CO, b, HCHO and c, CH3OH. The peaks pertaining to methylamine are highlighted in green.

Extended Data Fig. 9 Electrochemical reducibility of formaldoxime and N-methylhydroxylamine to methylamine.

1H NMR spectra of catholyte solutions after electroreduction of a, formaldoxime and b, N-methylhydroxylamine in Ar-saturated 0.1 M PBS. The peaks pertaining to methylamine are highlighted in green.

Extended Data Fig. 10 Condensation between formaldehyde and hydroxylamine to form formaldoxime.

a, 1H NMR spectra of 25 mM HCHO dissolved in electrolyte solution before and after adding 50 mM NH2OH. Peaks pertaining to formaldoxime are highlighted in purple. b, 1H NMR spectrum of the catholyte solution after co-reduction mixed with 1 M NaHSO3. Stacked together are spectra of HCHO and formaldoxime mixed with 1 M NaHSO3. Only the reaction product between formaldoxime and NaHSO3 (~4.00 ppm) but no reaction product between HCHO and NaHSO3 (~4.39 ppm) can be found.

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Wu, Y., Jiang, Z., Lin, Z. et al. Direct electrosynthesis of methylamine from carbon dioxide and nitrate. Nat Sustain 4, 725–730 (2021). https://doi.org/10.1038/s41893-021-00705-7

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