Superoxide Electrochemistry in an Ionic Liquid Inas M. AlNashef, Matthew L. Leonard, Michael A. Matthews,* and John W. Weidner Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 29208 The superoxide ion (O 2 •- ) has been generated electrochemically from oxygen dissolved in two different solvent systems: (1) acetonitrile with tetraethylammonium perchlorate (TEAP) as the supporting electrolyte at elevated pressure and (2) in a room-temperature ionic liquid, 1-n-butyl- 3-methylimidazolium hexafluorophosphate ([bmim][HFP]), at atmospheric pressure. A high- pressure electrochemical cell with a quasi reference electrode was developed for elevated pressure tests. Increasing the partial pressure of oxygen in the first system increased the rate of superoxide generation because of the increased solubility of oxygen according to Henry’s law. The subsequent addition of gaseous carbon dioxide enhances the rate of oxygen reduction in both systems but inhibits the reverse (oxidation) reaction of O 2 •- to O 2 . This later observation is consistent with the irreversible formation of a peroxydicarbonate ion, as has been postulated by others. Introduction Selective oxidation of organic compounds is vital for manufacturing value-added chemical intermediates. Typically, such reactions are conducted at elevated temperatures using catalysts, with organic solvents as the reaction medium. Green chemistry and engineering calls for better, sustainable approaches to manufacture these intermediates. In this work, we explore a possible initial step in the organic synthesis by electrochemical means, namely, the generation of superoxide ion (O 2 •- ) in aprotic solvents. Furthermore, we investigate the activation of CO 2 by superoxide ion as a possible route to the electrochemical production of chemicals. Holbrey and Seddon 1 have recently reviewed the application of room-temperature ionic liquids (RTILs) as substitute solvents in Green chemistry, with the emphasis on organic synthesis. More recently, a number of classical organic syntheses have been demonstrated using RTILs, including dimerization of alkenes 2,3 and oligomerization of butene. 4-6 With regard to electro- chemistry, certain RTILs are electrochemically stable over a range of 2-4 V and higher, are thermally stable, and are resistant to oxidation. 7-9 Various electrochemi- cal syntheses have been attempted, including polymer- ization of arenes to form conducting polymers, 10 polym- erization of benzene to poly(p-phenylenes), 11-13 oligo- merization of anthracene, 14 and preparation of silane polymer films. 15 More fundamental studies on redox reaction kinetics and the behavior in RTILs have been done for anthracene, 16 methylanthracene, 17 and other aromatics. 18-20 The electrochemistry of dioxygen reduction has been the subject of numerous studies in aqueous, nonaque- ous, and high-temperature molten salt systems. 21 Os- teryoung et al. 22 showed that superoxide ion could be generated by the reduction of dioxygen in the RTIL 1-ethyl-3-methylimidazolium chloride mixed with AlCl 3 . The resulting superoxide ion was unstable and thus cannot be used as a reagent in subsequent reactions. AlNashef et al. 23 showed that the RTIL 1-n-butyl-3- methylimidazolium hexafluorophosphate ([bmim][HFP]) is capable of supporting the electrochemical generation of a stable superoxide ion. Previous studies have shown that the system obtained by the reduction of dioxygen in aprotic solvents and in the presence of carbon dioxide is able to carboxylate different types of substrates. 24-27 Casadei et al. 26 showed that the electrochemical reduction of O 2 in dipolar aprotic solvents in the presence of CO 2 gave a carboxy- lating reagent (O 2 •- /CO 2 ) able to convert amines and different types of their derivatives into carbamates. Moreover, it is known that the electrochemically gener- ated O 2 •- is able to convert primary and secondary alcohols into the corresponding carboxylic acids and ketones, respectively. 28,29 The objective of this work is to investigate electro- chemical superoxide chemistry in two different solvent systems, either pressurized O 2 + CO 2 + an aprotic sol- vent or O 2 + CO 2 + a RTIL. We show that superoxide ion (O 2 •- ) can be generated through electrochemistry in both solvent systems and that the presence of CO 2 increases the reduction current and reduces the oxida- tion current. This suggests, compared to similar behav- ior in volatile aprotic solvents reported by other workers, 24-27,30 the generation of a carboxylating re- agent. These findings are the first step in using elec- trochemical oxidation for organic synthesis or destruc- tion of pollutants in these environmentally friendly solvent systems. Experimental Section Conductivity and cyclic voltammetry tests were per- formed on the aprotic solvent acetonitrile (MeCN), using as a supporting electrolyte tetraethylammonium per- chlorate (TEAP, 0.1 M). The ionic liquid used was [bmim][HFP]; no supporting electrolyte is required. TEAP (GFS Chemicals) was dried overnight in a vacuum oven at 40 °C; HPLC-grade MeCN (Fisher Scientific) was used as provided, and [bmim][HFP] (SACHEM) was dried overnight in a vacuum oven at 50 °C. Further drying was accomplished by sparging with dry nitrogen prior to electrochemical experiments. The electrochem- istry was performed using an EG&G M273 or EG&G * To whom correspondence should be addressed. Phone: (803) 777-0556. E-mail: matthews@engr.sc.edu. Fax: (803) 777-8265. 10.1021/ie010787h CCC: $22.00 © xxxx American Chemical Society PAGE EST: 4 Published on Web 00/00/0000