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Cooperative electrocatalytic alcohol oxidation with electron-proton-transfer mediators

Nature volume 535, pages 406–410 (2016)Cite this article

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Abstract

The electrochemical oxidation of alcohols is a major focus of energy and chemical conversion efforts, with potential applications ranging from fuel cells to biomass utilization and fine-chemical synthesis1,2,3,4,5,6,7. Small-molecule electrocatalysts for processes of this type are promising targets for further development8, as demonstrated by recent advances in nickel catalysts for electrochemical production and oxidation of hydrogen9,10,11. Complexes with tethered amines that resemble the active site of hydrogenases12 have been shown both to catalyse hydrogen production (from protons and electrons) with rates far exceeding those of such enzymes11,13 and to mediate reversible electrocatalytic hydrogen production and oxidation with enzyme-like performance14. Progress in electrocatalytic alcohol oxidation has been more modest. Nickel complexes similar to those used for hydrogen oxidation have been shown to mediate efficient electrochemical oxidation of benzyl alcohol, with a turnover frequency of 2.1 per second. These compounds exhibit poor reactivity with ethanol and methanol, however15. Organic nitroxyls, such as TEMPO (2,2,6,6-tetramethyl-1-piperidine N-oxyl), are the most widely studied electrocatalysts for alcohol oxidation5,6,7,16,17,18,19. These catalysts exhibit good activity (1–2 turnovers per second) with a wide range of alcohols18 and have great promise for electro-organic synthesis7. Their use in energy-conversion applications, however, is limited by the high electrode potentials required to generate the reactive oxoammonium species. Here we report (2,2′-bipyridine)Cu/nitroxyl co-catalyst systems for electrochemical alcohol oxidation that proceed with much faster rates, while operating at an electrode potential a half-volt lower than that used for the TEMPO-only process. The (2,2′-bipyridine)Cu(II) and TEMPO redox partners exhibit cooperative reactivity and exploit the low-potential, proton-coupled TEMPO/TEMPOH redox process rather than the high-potential TEMPO/TEMPO+ process. The results show how electron-proton-transfer mediators, such as TEMPO, may be used in combination with first-row transition metals, such as copper, to achieve efficient two-electron electrochemical processes, thereby introducing a new concept for the development of non-precious-metal electrocatalysts.

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Figure 1: Molecular electrocatalysts and related homogeneous and enzymatic catalysts.
Figure 2: Cyclic voltammogram of a solution of (bpy)Cu(OTf) and TEMPO and spectrophotometric evidence for (bpy)Cu(II)-mediated oxidation of TEMPOH (and benzyl alcohol) under anaerobic conditions.
Figure 3: Electrochemical oxidation of benzyl alcohol with (bpy)Cu/TEMPO and TEMPO catalysts.
Figure 4: Electronic effects and comparative reactivity of different catalyst systems with different alcohols.

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Acknowledgements

Financial support for this project was provided by the Great Lakes Bioenergy Research Center (Department of Energy Biological and Environmental Research Office of Science DE-FC02-07ER64494).

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  1. Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, 53706, Wisconsin, USA

    Artavazd Badalyan & Shannon S. Stahl

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  1. Artavazd Badalyan
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  2. Shannon S. Stahl
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Contributions

S.S.S. conceived the idea for the Cu/nitroxyl electrocatalytic alcohol oxidation, and A.B. and S.S.S. collaborated to design the project. A.B. performed all experimental work and led the data interpretation and analysis, in consultation with S.S.S. A.B. and S.S.S. wrote the manuscript.

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Correspondence to Shannon S. Stahl.

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The authors declare no competing financial interests.

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This file contains Supplementary Text and Data, Supplementary Figures 1 – 23 and additional references (see Contents for more details). (PDF 10424 kb)

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Badalyan, A., Stahl, S. Cooperative electrocatalytic alcohol oxidation with electron-proton-transfer mediators. Nature 535, 406–410 (2016). https://doi.org/10.1038/nature18008

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