Ind. Eng. Chem. Res. 2002, 41, 4475-4478
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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 (O2•-) 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-butyl3-methylimidazolium hexafluorophosphate ([bmim][HFP]), at atmospheric pressure. A highpressure 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 O2•- to O2. 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 (O2•-) in aprotic solvents. Furthermore, we investigate the activation of CO2 by superoxide ion as a possible route to the electrochemical production of chemicals. Holbrey and Seddon1 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 alkenes2,3 and oligomerization of butene.4-6 With regard to electrochemistry, 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 electrochemical syntheses have been attempted, including polymerization of arenes to form conducting polymers,10 polymerization of benzene to poly(p-phenylenes),11-13 oligomerization 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, nonaqueous, and high-temperature molten salt systems.21 Osteryoung 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 AlCl3. 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* To whom correspondence should be addressed. Phone: (803) 777-0556. E-mail:
[email protected]. Fax: (803) 777-8265.
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 O2 in dipolar aprotic solvents in the presence of CO2 gave a carboxylating reagent (O2•-/CO2) able to convert amines and different types of their derivatives into carbamates. Moreover, it is known that the electrochemically generated O2•- 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 electrochemical superoxide chemistry in two different solvent systems, either pressurized O2 + CO2 + an aprotic solvent or O2 + CO2 + a RTIL. We show that superoxide ion (O2•-) can be generated through electrochemistry in both solvent systems and that the presence of CO2 increases the reduction current and reduces the oxidation current. This suggests, compared to similar behavior in volatile aprotic solvents reported by other workers,24-27,30 the generation of a carboxylating reagent. These findings are the first step in using electrochemical oxidation for organic synthesis or destruction of pollutants in these environmentally friendly solvent systems. Experimental Section Conductivity and cyclic voltammetry tests were performed on the aprotic solvent acetonitrile (MeCN), using as a supporting electrolyte tetraethylammonium perchlorate (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 electrochemistry was performed using an EG&G M273 or EG&G
10.1021/ie010787h CCC: $22.00 © 2002 American Chemical Society Published on Web 02/02/2002
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Figure 1. Schematic diagram of the high-pressure cell.
263A potentiostat for experiments in MeCN and RTIL, respectively, controlled by computer and data acquisition software. For experiments conducted outside the high-pressure cell, the electrode configuration was a glassy carbon disk (BAS, 3 mm diameter) or platinum disk (BAS, 1.6 mm diameter) as a working electrode and a platinum mesh counter electrode (Aldrich). Standard calomel electrode (SCE) and a Ag/AgCl reference electrode (both Fisher Scientific) were used for experiments in MeCN and [bmim][HFP], respectively. For experiments conducted under pressure, a pressurized cell (Figure 1) was constructed from 1-in.diameter Swagelock cross tubing. A platinum wire (0.13 mm diameter) was used as a quasi reference electrode in the cross cell. The working and counter electrodes were Pt mesh of 0.5 and 1 cm2 surface area, respectively. Electrodes were sealed in the cross cell with the aid of Conax Teflon fittings. The MeCN and RTIL samples were sealed or handled under nitrogen sparge to prevent water contamination. The systems were sparged prior to electrochemical experiments with ultrahigh-purity nitrogen or oxygen that was filtered through a Drierite gas purification column. All [bmim][HFP] experiments were performed in a dry glovebox under an argon atmosphere. Results and Discussion Cyclic voltammograms in 0.1 M TEAP/MeCN at atmospheric pressure were first run under nitrogen sparge to provide a background. Reduction currents are positive throughout this paper. As shown in Figure 2, no current flowed, confirming that the solvent was stable under these conditions and contained no electroactive species. Figure 2 shows also that CO2 is not electrochemically active under the studied conditions. Quasi reversible cyclic voltammograms, characteristic of oxygen reduction and oxidation, were obtained after sparging with oxygen for 30 min (Figure 2). The superoxide ion showed a reduction peak at approximately -1.0 vs SCE, corresponding to the generation of super-
Figure 2. Cyclic voltammograms in MeCN (0.1 M TEAP), at atmospheric pressure: after sparging with (1) N2, (2) O2, (3) CO2, and (4) CO2 and O2 simultaneously. All scans used a glassy-carbon disk working electrode at a scan rate of 100 mV/s.
oxide ion in MeCN. Figure 2 shows that when excess CO2 is present, the O2 reduction peak current is enhanced by almost a factor of 2 and the oxidation peak is eliminated (curve 4). Sawyer et al.31 indicated that the increase in the peak reduction current is characteristic of a reaction in which O2 is regenerated to produce the net effect of a two-electron irreversible reduction of O2.
O2 + e- f O2•-
(1)
2CO2 + 2O2•- f C2O62- + O2
(2)
In [bmim][HFP] (Figure 3), the negligible background current in the presence of nitrogen indicates that the solvent is stable under these conditions. The oxygen showed a reduction peak at approximately -0.85 V vs Ag/AgCl; this is consistent with that reported by AlNashef et al.23 The cyclic voltammograms for the reduc-
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Figure 3. Cyclic voltammograms in [bmim][HFP], at atmospheric pressure: (1) N2 background, (2) after sparging with CO2, (3) after sparging with O2, and (4) after sparging with O2 and CO2 simultaneously. All scans used a glassy-carbon disk working electrode at a scan rate of 100 mV/s.
Figure 5. (a) Cyclic voltammograms in the pressure cell in MeCN (0.1 M TEAP) at different O2 pressures. All scans used a 0.5 cm2 Pt mesh working electrode at a scan rate of 128 mV/s. (b) Effect of the O2 pressure on the cathodic peak current.
Figure 4. Cyclic voltammograms in MeCN (0.1 M TEAP), at atmospheric pressure: (1) N2 background, (2) O2 at glassy-carbon disk working electrode, and (3) O2 at platinum disk working electrode. The scan rate is 100 mV/s.
tion of O2 to O2•- in [bmim][HFP] in the absence and presence of CO2 are illustrated in Figure 3. The enhancement in the cathodic peak current and the decrease of the peak current for the reverse scan (oxidation) that results from the presence of CO2 indicate a facile reaction between O2•- and CO2 as indicated by eq 2. This suggests, compared to similar behavior in volatile aprotic solvents reported by other workers,24-27,30 the generation of a carboxylating reagent which can be used in the synthesis of organic carbonates, carbamates, aldehydes, and ketones. More experiments need to be conducted to identify the reduction products. Also, the reduction potential becomes more negative in both solvents, MeCN (0.1 M TEAP) and [bmim][HFP], with the presence of CO2. This is probably due to ohmic resistance caused by the increase in current. The shift in MeCN (0.1 M TEAP) is much larger than that in [bmim][HFP] because the currents are higher and the conductivity is lower. Figure 4 shows the effect of the electrode material on the reduction of O2 in MeCN (0.1 M TEAP) at atmospheric pressure. A similar behavior was reported by Sawyer et al.31 They showed that the peak separation
varies with solvent and with electrode material. They attributed this variation to the heterogeneous electrontransfer kinetics and surface reactions for the metal electrodes. This explains the difference in the cyclic voltammograms in MeCN (0.1 M TEAP) at atmospheric pressure in the high-pressure cell (Figure 5a), where Pt mesh is used as a working electrode, and in a beaker (Figure 2), where glassy carbon (BAS, 3 mm diameter) is used as a working electrode. Figure 5a shows cyclic voltammograms obtained in the pressure cell at several values of oxygen overpressure. Peak current (i.e., the rate of generation of superoxide ion) increases with applied O2 pressure because of the increase in O2 solubility and the corresponding increase in the availability of O2 at the electrode surface. Figure 5b shows that the cathodic peak current increases linearly with an increase of the O2 partial pressure, and this is consistent with Henry’s law. Figure 6 shows cyclic voltammograms obtained in the pressure cell at several values of CO2 overpressure while keeping the O2 pressure at 160 psig. Peak current increases with applied CO2 pressure, probably because of the formation of the peroxydicarbonate ion, C2O62-. The peak current in the reduction direction increases, but the peak current in the oxidation direction decreases. This is consistent with a two-step sequence in which superoxide is formed, followed by the irreversible formation of the peroxydicarbonate ion from CO2 and O2•- as suggested by Sawyer et al.31 and others.
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Figure 6. Cyclic voltammograms in the pressure cell in MeCN (0.1 M TEAP) at different CO2 partial pressures, PO2 ) 174.7 psia. All scans used a 0.5 cm2 Pt mesh working electrode at a scan rate of 128 mV/s.
Conclusions This work has shown that the increased partial pressure of O2 in acetonitrile increases the reaction rate of the electrochemical generation of superoxide ion. The increase of the CO2 partial pressure in the high-pressure cell showed a similar effect on the reduction peak, but the oxidation peak decreased, indicating the formation of the carboxylating reagent (C2O62-). The high-pressure cell and quasi reference electrode system developed worked well for these studies. It was also shown that the electrochemically generated superoxide ion in the RTIL [bmim][HFP] is stable and reacts with CO2 to give what is believed to be a carboxylating reagent. More experiments need to be conducted to identify the reduction products. This finding, if proved, may lead to new routes for electrosynthesis in ionic liquid media. Acknowledgment The authors gratefully acknowledge the financial support from the U.S. Department of Energy Grant DE-FG07-98ER14923 and from NSF CTS-0086818. Literature Cited (1) Holbrey, J. D.; Seddon, K. R. Ionic Liquids. Clean Prod. Process. 1999, 99, 2071. (2) Chauvin, Y.; Commereuc, D.; Hirschaur, A.; Hugues, F.; Saussine, L. Process and Catalyst for the Dimerization or Codimerization of Olefins. French Patent 2,611,700, 1988. (3) Chauvin, Y.; Commereuc, D.; Hirschaur, A.; Hugues, F.; Saussine, L. Process and Catalyst for the Alkylation of Isoparaffins. French Patent 2,626,572, 1989. (4) Abdul-Sada, A. K.; Ambler, P. W.; Hodgson, P. K. G.; Seddon, K. R.; Stewart, J. J. Ionic Liquids. World Patent WO95/ 21871, 1995. (5) Abdul-Sada, A. K.; Atkins, M. P.; Ellis, B.; Hodgson, P. K. G.; Morgan, M. L. M.; Seddon, K. R. Alkylation Process. World Patent WO95/21806, 1995. (6) Ambler, P. W.; Hodgson, P. K. G.; Stewart, J. J. Butene Polymers. European Patent Application EP/0558187 A, 1996. (7) Bonhote, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundaram, K.; Gratzel, M. Hydrophobic, Highly Conductive Ambient Temperature Molten Salts. Inorg. Chem. 1996, 35, 1168. (8) Bonhote, P.; Dias, A. P. Hydrophobic Liquid Salts, the Preparation Thereof and Their Application in Electrochemistry. U.S. Patent 5,683,832, 1997. (9) Papageorgiou, N.; Athanassov, Y.; Armand, M.; Bonhote, P.; Pettersson, H.; Azam, A.; Gratzel, M. The Performance and Stability of Ambient Temperature Molten Salts for Solar Cell Applications. J. Electrochem. Soc. 1996, 143, 3009.
(10) Abuabdoun, I. I. Photoinitiated Cationic Polymerization by Imidazolium Salts. Abst. Pap. Am. Chem. Soc. 1989, 198, 96PMSE. (11) Kobryanskii, V. M.; Arnautov, S. A. Electrochemical Synthesis of Polyphynylene in an Ionic Liquid. Synth. Met. 1993, 55, 924. (12) Kobryanskii, V. M.; Arnautov, S. A.; Electrochemical Synthesis of Poly(p-phenylene) in an Ionic Liquid. Synth. Met. 1992, 193, 455. (13) Kobryanskii, V. M.; Arnautov, S. A. Electrochemical Synthesis of Poly(p-phenylene) in an Ionic Liquid. Vysokomol. Soedin, Ser. A 1993, 35, A611. (14) Hondrogiannis, G.; Lee, C. W.; Pagni, R. M.; Mammantov, G. Novel Photochemical Behavior on Anthracene in a RoomTemperature Molten salt. J. Am. Chem. Soc. 1993, 115, 9828. (15) Carlin, R. T.; Treulove, P. C.; Osteryoung, R. A. A SilaneBased Electroactive Film Prepared in an Imidazolium Chloroaluminate Molten Salt. J. Electrochem. Soc. 1994, 141, 1709. (16) Carlin, R. T.; Treulove, P. C.; Osteryoung, R. A. Electrochemical and Spectroscopic Study of Anthracine in a Mixed Lewis-Bronsted Acid Ambient Temperature Molten Salt System. Electrochem. Acta 1992, 37, 2615. (17) Lee, C.; Winston, T.; Unni, A.; Pagni, R. M. Mamantov, G. Photoinduced Electron-Transfer Chemistry of 9-Methylanthracene-Substrate as both Electron-Donor and Acceptor in the Presence of the 1-Ethyl-3-Methylimidazolium Ion. J. Am. Chem. Soc. 1996, 118, 4919. (18) Carter, M. T.; Osteryoung, R. A. Heterogeneous and Homogeneous Electron-Transfer reactions of Tetrathiafilvalene Ambient-Temperature-Temperature Chloroaluminate Molten Salt. J. Electrochem. Soc. 1982, 129, 560. (19) Cheek, G. T.; Osteryoung, R. A. Electrochemical and Spectroscopic Studies of 9, 10-Anthraquinone in a Room-Temperature Molten. J. Electrochem. Soc. 1982, 129, 2488. (20) Thapar, R.; Rajeshwar, K. Photoelectrochemical Oxidation of Aromatic Hydrocarbons and Decamethyl Ferrocene at the n-GaAs/Room-Temperature Molten Salt Electrolyte Interface. J. Electrochem. Soc. 1982, 129, 560. (21) Hoare, J. P. In Encylopedia of Electrochemistry of the Elements; Bard, A. J., Ed.; Marcel Dekker: New York, 1974; Vol. 2, pp 191-382. (22) Carter, M. T.; Hussey, C. L.; Strubinger, S. K. D.; Osteryoung, R. A. Electrochemical Reduction of Dioxygen in RoomTemperature Imidazolium Chloride-Aluminum Chloride Molten Salts. Inorg. Chem. 1991, 1149. (23) AlNashef, I. M.; Leonard, M. L.; Kittle, L. M.; Matthews, M. A.; Weidner, J. W. Electrochemical Generation of Superoxide in Room-Temperature Ionic Liquids. Electrochem. Solid-State Lett. 2001, 4 (11), D16. (24) Casadei, M. A.; Cesa, S.; Moracci, F. M.; Inesi, A.; Feroci, M. Activation of Carbon Dioxide by Electrogenerated Superoxide Ion: A New Carboxylating Reagent. J. Org. Chem. 1996, 61, 381. (25) Casadei, M. A.; Cesa, S.; Feroci, M.; Inesi, A.; Rossi, L.; Moracci, F. M. O2•-/CO2 System as Mild and Safe Carboxylating Reagent Synthesis of Organic Carbonates. Tetrahedron 1997, 53, 167. (26) Casadei, M. A.; Moracci, F. M.; Zappia, G.; Inesi, M.; Rossi, L. Electrogenerated Superoxide-Activated Carbon Dioxide: A New Mild And Safe Approach To Organic Carbamates J. Org. Chem. 1997, 62, 6754. (27) Casadei, M. A. Reactivity of the Electrogenerated O2•-/CO2 System Towards Alcohols. Eur. J. Org. Chem. 2001, 1689. (28) Singh, M.; Misra, R. A. Electrogenerated Superoxide Initiated Oxidation with Oxygen. A Convenient Method for the Conversion of Secondary Alcohols to Ketones. Synthesis 1989, 403. (29) Singh, M.; Singh, K. N.; Dwivedi, S.; Misra, R. A. Superoxide (O2•-) Initiated Oxidation of Primary Alcohols to Carboxylic Acids. Synthesis 1991, 291. (30) Roberts, J. L.; Calderwood, T. S.; Sawyer, T. D. Nucleophilic Oxygenation of Carbon Dioxide by Superoxide Ion in Aprotic Media to form C2O62- Species. J. Am. Chem. Soc. 1984, 106, 4667. (31) Sawyer, D. T.; Sobkowiak, A.; Roberts, J. L. Electrochemistry for Chemists, 2nd ed.; Wiley: New York, 1995.
Received for review September 21, 2001 Revised manuscript received December 4, 2001 Accepted December 13, 2001 IE010787H
