Nuclear Chemistry and Electrochemistry: Superoxide Ion

Chapter 14

Downloaded by UNIV OF LEEDS on November 2, 2014 | http://pubs.acs.org Publication Date: July 25, 2002 | doi: 10.1021/bk-2002-0818.ch014

Nuclear Chemistry and Electrochemistry: Superoxide Ion Electrochemistry in Ionic Liquids M. L. Leonard, M. C. Kittle, I. M. AlNashef, M. A. Matthews*, and J. W. Weidner Department of Chemical Engineering, University of South Carolina, Columbia, SC 29208

We have demonstrated that superoxide ion can be generated electrochemically in room-temperature ionic-liquid solvents. Similar superoxide ion chemistry has previously been demonstrated in volatile and environmentally-suspect aprotic solvents such as dimethyl formamide and acetonitrile. However, ionic liquids are non-volatile and should minimize the problems of secondary solvent waste. It is proposed that the resultant superoxide ion can be used to perform low-temperatureoxidation of wastes. Low-temperature oxidation of waste solvents can provide a much-needed alternative to high temperature waste incinerators, whose use is greatly complicated by regulatory requirements and locating suitable sites.

Introduction Superoxide ion chemistry. Sawyer and co-workers (1-3) pioneered work on superoxide ion (O2*)» particularly the direct electrochemical reduction of dissolved oxygen gas in aprotic solvents to form 0 *" according to the following reaction 2

178

© 2002 American Chemical Society

In Ionic Liquids; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

179 e

0 +e->0 2

(1)

2

A comprehensive review of superoxide ion chemistry is given by Sawyer et al. (4). Sawyer in the early 1980's investigated this chemistry as a means of destroying organic solvent wastes (3,5). Superoxide ion can be formed directly from solvation of K 0 in aprotic solvents, or electrochemically via direct cathodic reduction of dioxygen (typically E=-1.0V vs, SCE) (23). 0 *" is a strong nucleophile and is rapidly reduced in water to 0 and hydroperoxide: 2

2

2

2 0 · " + H 0 -> 0 + HOCr + H O 2

2

(2)

2

For this reason, generation and utilization of 0 *" must be done in aprotic solvents. Acetonitrile (MeCN), dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) are commonly used. To impart sufficient electrical conductivity, a supporting electrolyte such as tetraethyl ammonium perchlorate (TEAP) is dissolved in the solvent. Reagents must be scrupulously dried, but i f properly prepared and stored, are stable for extended times.


2

The superoxide ion can degrade polychlorinated aromatics and pCBs to bicarbonates and chlorides (2,3,6). The mechanism of destruction is thought to be a nucleophilic reaction of 0 * with the chlorine-carbon bond, as is illustrated for the reaction involving hexachlorobenzene (6): _

2

c c i + ο · - -> [ c c i ο / ι -> c c i ο · + c r 6

6

2

6

6

6

5

2

(3) c c i ο · + o - -» c ci o + o + cr 6

5

2

2

6

4

2

2

(4) 2

c c i o + io o " -> 3 c o " + 2 o + 4 c r 6

4

2

2

2

6

2

(5) 2

3 C 0 - + 3 0 + 3 H 0 -> 6 H C 0 ' + 3/2 0 2

6

2

2

3

2

(6) The nucleophilic addition of superoxide ion in reaction [3], as well as the reactions [4] and [5], must take place in an anhydrous environment. Kalu and White (6) have determined that PCBs were not completely destroyed in a flowthrough reactor due to slow kinetics. In any case, however, the present state of knowledge requires use of volatile aprotic organic solvents, which suggests a serious problem with atmospheric solvent emissions and generation of secondary


180 solvent wastes. Casadei et al (7) utilized the methods of Sawyer et al (1-4) to generate stable solutions of 0 *" in M e C N using T E A P as the electrolyte. The objective of this work is to replace aprotic solvents with ionic liquids as the medium for conducting superoxide ion chemistry. Room Temperature Ionic Liquids (RTELs) are stable mixtures of an organic cation/anion salt with an inorganic salt (8-12). They are directly related to more familiar high temperature molten salts that are used, for example, as heat transfer media.


2

Holbrey and Seddon (13) have recently reviewed the applications of RTELs as substitute solvents in green chemistry, with an emphasis on organic synthesis. Early work on RTELs in electrochemistry focused on their use as electrolyte for advanced battery systems. More recently, a number of classical organic syntheses have been demonstrated using RTELs, including dimerization of alkenes (14,15) and oligomerization of butene (20-22), which utilize the acid catalyst properties of A1C1 . Neutral ionic liquids containing [BF ]' or [CuCl ]" are less reactive and do not promote polymerization of alkenes, and have been utilized in homogeneous catalytic hydrogénation of olefins (19, 20). Furthermore, RTILs may provide facile separations in either liquid-RTIL (21) or supercritical fluid/RTIL (22) extractions due to their low volatility and adjustable affinity for water and organic solvents. 3

4

2

With regard to electrochemistry, certain RTELs are electrically stable over a range of 2-4V and higher, are thermally stable, and are resistant to oxidation, including photooxidation in solar cells (23-25). Various electrochemical syntheses have been attempted, including polymerization of arenes to form conducting polymers (26), polymerization of benzene to poly (p-phenylenes) (27-29), oligomerization of anthracene (30), and preparation of silane polymer films (31). More fundamental studies on redox reaction kinetics and behavior in RTELs have been done for anthracene (32), methylanthracene (33), and other aromatics (34-36). It is clear that some RTELs can be used to support electrochemistry; as far as we are aware, however, there has been no research on supporting superoxide ion chemistry in these novel solvents. In this paper we show that superoxide ion can be generated via reaction f 1] in room temperature ionic liquid solvents (RTILs). Blanchard and Brennecke (37) showed that halogen-carbon compounds are soluble in RTILs. This finding offers promise that electrochemical oxidation of chlorinated solvents in ionic liquid media may be an environmentally acceptable route for destruction of these pollutants.


181


Experimental Cyclic voltammetry (CV) tests were performed on the aprotic solvent system tetraethylammonium perchlorate (TEAP, 0.1M) in acetonitrile (MeCN) and in the ionic liquid l-butyl-3-methylimidazolium hexafluorophosphate (BMIM-HFP). T E A P (GFS Chemicals) was dried overnight in a vacuum oven at 40°C, HPLC-grade M e C N (Fisher Scientific) was used as provided, and B M I M - H F P (SACHEM) was dried overnight in a vacuum oven at 50°C. The electrochemistry was performed using an E G & G 263A potentiostat/galvanostat controlled by computer and data acquisition software. The electrode configuration was a glassy carbon working (BAS, 3mm dia.) and a platinum mesh counter (Aldrich) using S C E and Ag/AgCl references (both Fisher Scientific) for the experiments in M e C N and B M I M - H F P , respectively. The M e C N sample was sealed or handled under nitrogen sparge to prevent water contamination, and all B M I M - H F P experiments were performed in a dry glove box under an argon atmosphere. The systems were sparged prior to electrochemical experiments with U H P nitrogen or oxygen fitted through a Drierite gas purification column (W.A. Hammond). Prior to superoxide ion generation, a nitrogen sparge was used while obtaining a reference voltammogram. Oxygen was then bubbled through the system for 30 minutes to allow sufficient solubilization. Between consecutive C V runs, oxygen was bubbled briefly to refresh the system with oxygen and to remove any concentration gradients. Nitrogen or oxygen sparging was discontinued during the C V data acquisition.

Results and Discussion Figures 1 shows cyclic voltammograms in: a) 0.1M T E A P / M e C N and b) B M I M - H F P . Reduction currents are positive through out this paper. The C V s were run with nitrogen and oxygen sparging. In M e C N , the presence of oxygen results in a faradic reduction and oxidation peaks at -1.00V and -0.72V vs. SCE, respectively. This C V is consistent with that obtained by Sawyer et al. (4) They concluded that the reduction peak is due to the generation of superoxide ion according to reaction [1] and the oxidation peak due to the reverse of reaction [1]. In contrast, the nitrogen baseline showed no such peak, which indicates that the solvent is stable under these conditions and the resulting current is due to the charging of the double layer. In B M I M - H F P , Figure l b , the presence of oxygen showed a reduction peak at approximately -0.86V and an oxidation peak at -0.54V vs. Ag/AgCl. Sawyer


182

e α β


a § υ

-0.5 -1.0 Potential vs. SCE (V)

0.0

-1.5

0.4

b) 0.3

oxygen —

•/

0.2

0.1

ι fi

0 "

f =

t

-0.1

nitrogen

-0.2 0.0

L

1

-0.5

-1.0

.. -1.5

Potential vs. Ag^AgCl (V)

Figure 1 Cyclic voltammogroms with nitrogen and oxygen sparging in a) 0.1M TEAP in MeCN and b) BMIM-HFP. All scans used a glassy carbon electrode at a scan rate of 100 mV/s. (Al Nashet et ai, "Electro ionic liquids, " reproduced by permission of the Electrochemical Society, Inc.)



183

-0.2

I

.

0.0

-0.5

^—.

-1.0

—1 -1.5

Potential vs. Ag/AgCl (V) Figure 2 Cyclic voltammograms for various scan rates (mV/s) in BMIM-HFP with oxygen. The working electrode was glassy carbon and the reference electrode Ag/AgCl. (Al Nashet et al, "Electrochemical generation of superoxide in room temperature ionic liquids, " reproduced by permission of the Electrochemical Society, Inc.)


184 et al. (4) showed that the solvent and electrode materials have significant effect on the reversibility and peak separation of the CVs. The reduction potential for Ο2/Ο2* couple shifts to more negative values as the solvating properties of the solvent decrease. The variation in the peak potential for 0 / 0 " in M e C N and B M I M - H F P is small enough that the peaks seen in the two solvents are consistent with reaction [1]. The scans performed solely in nitrogen showed no such peak. It should be noted that two different scales are used in Figure 1, so the current density for the N / M e C N system is approximately the same as that for N / B M I M - H F P system. Therefore, the two solvents have comparable electrochemical stability. 2

2

2


2

It is clear from Figure 1 that the current density for the 0 / M e C N system is much higher, by more than an order of magnitude, than that in the 0 / B M I M HFP system. This may be due to the difference in the solubility of 0 in the two solvents. Unfortunately, no data for the solubility of 0 in B M I M - H F P is available in the literature. Cyclic voltammograms were also run on B M I M - H F P for several scan rates, 9, 16, 25, 36, 49, 64, 81, and 100 mV/s. Four of these are shown in Figure 2. The cathodic peak current is proportional to the square root of the sweep rate, which indicates a diffusion-controlled process. This observation is consistent with reaction [1]. 2

2

2

2

Conclusions Preliminary experiments with the RTIL, l-butyl-3-methylimidazolium hexafluoro-phosphate (BMIM-HFP), showed promise that this solvent was capable of supporting the electrochemical generation of superoxide ion. This finding may lead to new routes for electrochemical oxidation of chlorinated compounds in ionic liquid media.

Acknowledgments The authors gratefully acknowledge the financial support from the U.S. Department of Energy grant DE-FG07-98ER14923 and from N S F C T S 0086818. M . Kittle was supported by N S F DMR-9732227, which funds the Research Experience for Undergraduates program in the Department of Chemical Engineering. * Author to whom correspondence should be addressed. Phone: (803) 7770556. E-mail: [email protected]. Fax:(803) 777-8265


185

References 1.

2.


3. 4. 5. 6.

7. 8. 9. 10. 11. 12.

13. 14.

15.

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Nuclear Chemistry and Electrochemistry: Superoxide Ion

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