Electrochemical Studies of Imidazolium Carboxylate

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Tetramethylammonium oxalate was obtained from SACHEM and dried in vacuo at 80 o ... In the work in BMIM Ac, a silver pseudoreference electrode was used; ...
ECS Transactions, 64 (4) 161-169 (2014) 10.1149/06404.0161ecst ©The Electrochemical Society

Electrochemical Studies of Imidazolium Carboxylate Adducts in a Room-Temperature Ionic Liquid

G. T. Cheeka, D. F. Roeperb , W. Pearsona, and W. E. O'Gradyb a

United States Naval Academy, Annapolis MD 21402 b Excet, Inc., Springfield VA 22151

The electrochemical reduction of carbon dioxide has been studied in the room-temperature ionic liquid 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate ( BMPY TfO ). CO2 reduction occurs at the negative solvent limit of BMPY TfO; however, addition of 1-ethyl-3methylimidazolium tetrafluoroborate (EMI BF4) to the solution results in an increase of the reduction current for the EMI cation, which occurs at a more positive potential than does CO2 reduction. These results indicate that the reduction product of EMI cation, probably a carbene, can catalyze CO2 reduction in BMPY TfO. Electrochemical responses for imidazolium carboxylates are similar to those for CO2 / EMI BF4 mixtures, supporting the carbene / CO2 interaction.

Introduction Recent concern about increasing carbon dioxide concentration in the atmosphere has prompted considerable research into ways of converting CO2 into useful compounds (1). In general, reduction of CO2 is necessary to form the desired products, and catalysis of this step is evidently very important in an effective and economical conversion process. Recent research has involved the use of transition metals as catalysts for CO2 reduction (2). Another possible route for this process is reaction of CO2 with carbenes (3,4), which can be formed from imidazolium cations (5). Such an approach has been taken in aqueous media (6); however, the voltammetric characteristics of CO2 and imidazolium cations are not completely observable in the limited potential range of aqueous solutions. This paper reports recent results in this regard, carried out in a room-temperature ionic liquid.

Experimental All electrochemical experiments were carried out in a Vacuum Atmospheres glovebox. Cyclic voltammograms were obtained on a PAR 283 potentiostat using PowerSuite software. Potentials are reported with respect to a Ag/AgCl reference electrode [0.1M 1-ethyl-3-methylimidazolium chloride (EMIC) in EMI BF4]. 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMI BF4) and 1-

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ECS Transactions, 64 (4) 161-169 (2014)

butyl-1-methylpyrrolidinium trifluoromethylsulfonate (BMPY TfO) were prepared by literature methods (7, 8). Tetramethylammonium oxalate was obtained from SACHEM and dried in vacuo at 80oC. BMPY OH was prepared by reaction of NaOH to BMPY Cl (stoichiometric amounts, each in methanol, followed by filtration of NaCl product. BMPY formate preparation involved BMPY OH addition to formic acid in methanol. CO2 concentration in BMPYTfO was determined gravimetrically by finding the mass differences for the cell before and after charging with CO2. 1,3-dimethylimidazolium-2-carboxylate was prepared by reaction of 1methylimidazole with dimethylcarbonate for 14 days at 80oC (9). It was found that the 4-isomer, 1,3-dimethylimidazolium-4-carboxylate, could be formed by heating for 24 hours at 120oC. Structures of each of these adducts were confirmed by X-ray diffraction structure determination (Bruker AXS Kappa APEX-II).

Results and Discussion Electrochemistry of CO2 in BMPY TfO

The electrochemical behavior of CO2 dissolved in BMPY TfO is shown in Figure 1. Reduction occurs in an irreversible process close to the potential limit of BMPY TfO, with some indication of product oxidation in the 0 to +1 V potential region. In an overall sense, reduction of CO2 in BMPY TfO is similar to that found in 1-butyl-3-methylimidazolium acetate (BMIM Ac) (10), in which CO2 reduction is also irreversible although it occurs at a much less negative potential. In the work in BMIM Ac, a silver pseudoreference electrode was used; however, the potential limit for BMIM Ac reduction is observed at -2.0V, similar to that found using a Ag/AgCl reference electrode. With this in mind, the reduction of CO2 in BMIM Ac ( -1.7 V vs Ag ) occurs much more readily than in BMPY TfO ( -2.7 V vs Ag/AgCl ). The BMIM ring apparently complexes the CO2 reduction product as it is formed, allowing CO2 to be reduced at a less negative potential than in BMPY TfO. Such behavior has been observed in the reduction of nitroaromatics in EMI BF4 (11). The BMPY cation is an aliphatic system and evidently does not interact significantly with CO2 reduction products. In nonaqueous media, formate and oxalate are commonly observed as products ( 1 ), according to the following two pathways :

CO2 + 1 e- → 2 CO2 -

-.

→ C2O4 +

2-

CO2

CO2 reduction

-.

oxalate -

CO2 + 2 e + H → HCO2 formate

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ECS Transactions, 64 (4) 161-169 (2014)

40

20

10

Current, uA

30

0

3

2

1

0

-1

-2

-10 -3

Potential, V vs Ag/AgCl

Figure 1. CO2 reduction in BMPY TfO at Pt, 100 mV/s. [CO2] = 0.13 M In an attempt to identify the CO2 reduction products, cyclic voltammograms of possible substances were taken for comparison with the voltammogram of CO2. Figure 2 shows the voltammetric behavior of tetramethylammonium oxalate in BMPY TfO. This oxalate product undergoes oxidation at +0.43 V, similar to the first oxidation process following CO2 reduction (Figure 1). On the second sweep, a small reduction peak at -2.30 V was observed, evidently corresponding to CO2 formed in the oxalate oxidation process during the initial sweep. The process corresponding to the oxidation process at +0.7V is not known at present. BMPY formate, prepared by reaction of BMPY OH with formic acid, was also investigated by cyclic voltammetry ( Figure 3 ). The oxidation of the formate anion involves an initial oxidation process at 0.00V, followed by a larger process at +1.00V. Somewhat similar results have been obtained in DMSO ( 12 ), although the scan was not taken beyond +1.1V in that case. It appears that formate is not produced in CO2 reduction in BMPY TfO, there being no corresponding oxidation peaks in Figure 1 following the CO2 reduction peak. This finding is not surprising, given that the BMPY cation is not a good proton donor, as would be required for formate production.

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ECS Transactions, 64 (4) 161-169 (2014)

5 4 3

1

Current, uA

2

0 -1 -2

3

2

1

0

-1

-3 -3

-2

Potential, V vs Ag/AgCl

Figure 2. CV of tetramethylammonium oxalate 30 mM, in BMPY TfO. 100 mV/s at Pt

6 4 2

-2

Current, uA

0

-4 -6 -8

3

2

1

0

-1

-2

-10 -3

Potential, V vs Ag/AgCl

Figure 3. CV of 70 mM BMPY formate in BMPY TfO, at Pt 100 mV/s. Scan starts at -1.50 V. First scan is blue curve.

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ECS Transactions, 64 (4) 161-169 (2014)

Effect of EMI cation on CO2 Reduction Pathway in BMPY TfO

If the reduction of carbon dioxide is to be a commercially viable avenue to CO2 removal, the effectiveness of various catalysts for this process must be evaluated. Carbenes generated by reduction of imidazolium cations have been shown to be useful catalysts (13), suggesting the use of commonly available ionic liquid imidazolium cations as catalyst precusors for CO2 reduction. Figure 4 shows the reduction of 0.13 M CO2 in BMPY TfO, as in Figure 1, as well as the behavior resulting from the addition of 0.08 M EMI BF4 to the system. Even though the concentration of EMI BF4 is considerably lower than that of CO2, a new large reduction process is observed at -2.3 V, completely replacing the original CO2 reduction process. The new peak potential corresponds to the reduction of the EMI cation, and the lack of the original CO2 reduction process implies that the EMI+ reduction product reacts rather completely with dissolved CO2. The following mechanism is consistent with these observations :

H

H

+ 1 N

N

H

H

H

H

e-

H

+ CO 2 N

N

N

H

H

- 1/2 H2

N

N

N

C

H

+ 1 e-

O

O

C O

O

(2) O

O C

O

165

C O

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ECS Transactions, 64 (4) 161-169 (2014)

60 50

(b)

30 20 10

Current, uA

40

0

(a) -10

3

2

1

0

-1

-2

-20 -3

Potential, V vs Ag/AgCl

Figure 4. Voltammograms of (a) 0.13 M CO2 (blue curve) (b) addition of 0.08 M EMI BF4 . In other words, the electrochemical reduction of the EMI cation forms the carbene after hydrogen elimination. The carbene then interacts with CO2 by forming an EMI CO2 adduct. Further, it can be seen from Figure 5 that removal of CO2 by N2 purge results in a considerably smaller reduction peak at -2.3 V as the system returns to the electrochemical behavior of only the EMI cation. The fact that the peak potential has not substantially shifted, and that its peak current is much larger in the presence of CO2, indicates that the CO2 reduction process in the presence of EMI BF4 is catalytic. The catalyst is presumably the carbene generated from the EMI cation in the initial reduction step. Further description of the complete mechanism requires further information about the electrochemical characteristics of the EMI CO2 adduct. This adduct can be prepared by literature methods (9) involving heating an imidazole with dimethylcarbonate. As outlined in the Experimental section, two isomers of such an adduct ( 2carboxy-1,3-dimethylimidazole and 4-carboxy-1,3-dimethylimidazole ) are obtained depending on synthesis temperature. The electrochemical behavior of these two adducts is shown in Figure 6, and it can be seen that their voltammograms are very similar, and that the reduction peak is close to that of CO2 in the presence of EMI BF4. These adducts are not exactly those involved in the CO2 / EMI BF4 reaction; however, the substitution of a methyl vs ethyl group is not expected to substantially affect their electrochemical behavior. At the peak potential for the CO2 / EMI BF4 system, the adduct can apparently undergo reduction to re-form the carbene and to produce a carbon dioxide radical anion. Coupling of two CO2 radical anions forms an oxalate dianion, and a small oxidative response at +0.4 V ( oxalate oxidation ) is seen in Figure 4. The carbene released in this process is now available to continue its reaction with CO2. From the

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ECS Transactions, 64 (4) 161-169 (2014)

relative peak currents in Figure 4, the catalytic turnover for the carbene is approximately four, but this may be limited by the relatively low CO2 concentration. Further evidence for the involvement of the imidazolium carboxylate adduct in this reaction is provided by the presence of an oxidation peak at -0.4 V in the voltammograms of both the CO2 / EMI BF4 mixture in BMPY TfO (Figure 4), and the EMI CO2 adducts ( Figure 6 ). The assignment of this peak is presently uncertain; however, it is possible that it corresponds to oxidation of the carbene liberated upon reduction of the EMI CO2 adduct.

60

(a)

50

30 20 10

(b)

Current, uA

40

0 -10

3

2

1

0

-1

-2

-20 -3

Potential, V vs Ag/AgCl

Figure 5. Cyclic voltammograms of EMI BF4 in BMPY TfO, 100 mV/s at Pt. (a) 0.13 M CO2 and 0.08 M EMI BF4 (blue curve) (b) After removal of CO2 by nitrogen purge ( black curve )

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ECS Transactions, 64 (4) 161-169 (2014)

6

(a)

5

(b)

3 2 1

Current, uA

4

0 -1

2

1

0

-1

-2

-2 -3

Potential, V vs Ag/AgCl

Figure 6. Cyclic voltammograms of Imidzaolium : CO2 adducts, 100 mV/s at platinum. (a) 1,3-dimethylimidazolium-4-carboxylate, 44 mM (b) 1,3-dimethylimidazolium-2-carboxylate, 33 mM

Conclusions The electrochemical behavior of carbon dioxide in the room-temperature ionic liquid 1-butyl1-methylpyrrolidinium trifluoromethanesulfonate ( BMPY TfO ) has been studied at platinum. CO2 reduction occurs at the extreme negative potential limit of BMPY TfO, and the reduction product appears to be the one-electron reduction product, oxalate. It has been found that CO2 reduction is catalyzed by the addition of 1-ethyl-3-methylimidazolium tetrafluoroborate ( EMI BF4 ) to the solution of CO2 in BMPY TfO. Addition of EMI BF4 to BMPY TfO results in a reduction peak at -2.3 V vs Ag/AgCl due to reduction of the EMI cation, well before the BMPY TfO solvent limit. Addition of CO2 to this solution produces an increase in the EMI cation reduction peak with no reduction process observed at the usual CO2 reduction potential in BMPY TfO, providing evidence for the catalytic reduction of CO2 by the reduction product of the EMI cation. This product is presumably the carbene formed by reduction of the EMI cation, and the carbene then reacts with CO2 to form an adduct. Reduction of this adduct then reforms the carbene while providing the CO2 radical anion. Coupling of this radical anion forms the observed oxalate final product.

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ECS Transactions, 64 (4) 161-169 (2014)

References

(1) “Electrochemical CO2 Reduction on Metal Electrodes,” Y. Hori, Modern Aspects of Electrochemistry, Number 42, edited by C. Vayenas et al., Springer, New York, (2008). (2) D. C. Grills, Y. Matsubara, Y. Kuwahara, S. R. Golisz, D. A. Kurtz, and B. A. Mello, J. Phys. Chem. Lett. , 5, 2033 (2014). (3) B. Gorodetsky, T. Ramnial, N. R. Branda, and J. A. C. Clyburne, Chem . Commun. , 1972 (2004). (4) I. Tommasi and F. Sorrentino, Tetrahedron Letters, 47, 6453 (2006) . (5) A. J. Arduengo, R. L. Harlow, and M. Kline, J. Am. Chem. Soc., 113, 361 (1991). (6) A. B. Bocarsly, Q. D. Gibson, A. J. Morris, R. P. L’Esperance, Z. M. Detweiler, P. S. Lakkaraju, E. L. Zeitler, and T. W. Shaw, ACS Catalysis, 2, 1684 (2012). (7) G. P. Smith, A.S. Dworkin, R. M. Pagni, and S. P. Zingg, J. Am. Chem. Soc.. 111, 525 (1989). (8) L. Crowhurst, N. L. Lancaster, J. M. P. Arlandis, and T. Welton, J. Am. Chem. Soc., 126, 11549 (2004). (9)

C. Rijksen and R. D. Rogers J. Org. Chem., 73, 5582 (2008) for EMI CO2 preparation.

(10) L. E. Barrosse-Antle and R. G. Compton, Chem. Commun., 3744 (2009). (11) A. J. Fry, Journal of Electroanalytical Chemistry, 546, 35 (2003). (12) E. Jacobsen, J. L. Roberts, and D. T. Sawyer, J. Electroanalytical Chem., 16, 351 (1968). JACOBSEN*, JULIAN L ROBERTS, 6 (1968) 351-36o T

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Acknowledgments Acknowledgment is made to the Naval Research Laboratory and to the United States Naval Academy for their support of this research.

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