Energy Efficient Capture and Release of Carbon Dioxide in Tetraalkyl Phosphonium and Tetraalkyl Ammonium Ionic Liquids

Described herein is an electrochemical method for the energy efficient capture and release of CO₂ that employs electrochemically-generated thiolates as the CO₂ capture agents.  In N,N-dimethylformamide at glassy carbon electrodes the onset of CO₂ reduction precludes capture by electrochemically generated thiolates.  This is not the case in tetraalkyl phosphonium and tetraalkyl ammonium ionic liquids.  In these media saturated with CO₂, electrochemical reduction of disulfides to thiolates proceeds via a two-electron process with potential inversion.¹  One-electron reduction of the disulfide results in dissociative electron transfer (DET), the first step of which is the formation of a short-lived radical anion.²  The S–S σ bond of the radical anion is subsequently cleaved leading to a thiolate and a sulfur-based radical.²  Resultant sulfur radicals are reduced at potentials positive of the potential applied for reduction of the corresponding disulfide.  Thus, electrochemical thiolate generation is best formulated as a two-electron reduction with concomitant disulfide cleavage.³  In this presentation we will show that the potent, electrochemically generated sulfur nucleophiles capture CO₂ via a facile SN₂ reaction leading to formation of a thiocarbonate (Scheme 1).  We will further demonstrate that the mechanism for the release of CO₂ and regeneration of the disulfide is the one-electron Kolbe oxidation of the thiocarbonate followed by coupling of two sulfur radicals (Scheme 2).⁴  Cyclic voltammograms show that thiocarbonate oxidation occurs at potentials positive of thiolate oxidation.  Increases in the peak-to-peak separation between thiolate oxidation and thiocarbonate oxidation have been ameliorated by targeted modifications to the disulfide. 

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(3)  Hall, G. B.; Kottani, R.; Felton, G. A. N.; Yamamoto, T.; Evans, D. H.; Glass, R. S.; Lichtenberger, D. L.  J. Am. Chem. Soc. 2014, 136, 4012–4018.

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