Chemical Engineering Journal 245 (2014) 367–369

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Chemical Engineering Journal

journal homepage: www.elsevier .com/locate /cej

Letter to the Editor

Comment on ‘‘Selective chemical separation ofcarbon dioxide by ether functionalized imi-dazolium cation based ionic liquids’’, by Pan-kaj Sharma, Soo-Hyun Choi, Sang-Do Park, Il-Hyun Baek, and Gil-Sun Lee (Chem. Eng. J., 2-012, 181–182, p. 834–841)

1385-8947/$ - see front matter � 2013 Published by Elsevier B.V.http://dx.doi.org/10.1016/j.cej.2013.08.051

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Fig. 1. Density of [C2Omim][Tf2N] as a function of temperature (this work, s),compared with the value reported by Sharma et al. (h) [1].

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ty (

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Fig. 2. Viscosity of [C2Omim][Tf2N] as a function of temperature (this work, s),compared with the values reported by Sharma et al. (pure, h; CO2-saturated, j) [1].

1. Introduction

Recently, Sharma et al. [1] published a paper in which theyclaimed one-to-one uptake of CO2 by ether functionalized ionic liq-uids (ILs). Ethers, taking advantage of a weak Lewis acid–baseinteraction, have been used extensively to increase the solubilityof solutes in supercritical CO2 [2,3]. However, there is no evidencein the literature of a reaction forming a covalent bond between CO2

and ether functionalities, which is what would be needed to ex-plain one-to-one uptake by ether-functionalization on the cationof ILs. Rather, the ether oxygen atom is capable of van der Waalsinteraction with CO2, with a strength that corresponds to physicaldissolution [4]. Experimental [5] and computational [6] efforts byCosta Gomes and coworkers indicate that the solubilities of CO2

in ester and ether-functionalized imidazolium ILs are on the sameorder of magnitude as in alkylimidazolium. In addition, oligo(eth-yleneglycol) functionalized imidazolium ILs absorb similaramounts of CO2 as alkylsubstituted analogues, while dissolvingless N2 and CH4 [7].

To verify the results presented by Sharma et al. [1], we synthe-sized 1-methyl-3-dimethylether-imidazolium bis(trifluoro-methyl)sulfonylamide (which is designated as [C2Omim][Tf2N]by Sharma et al. [1]). The physical properties of this IL do notmatch those presented by Sharma et al. [1] and the CO2 uptakeat room temperature and 0.922 bar is only 0.03 mole of CO2 permole of IL. This is indicative of physical absorption and is morethan an order of magnitude below the 1 mole CO2/mole of IL re-ported by Sharma et al. [1]. Moreover, the NMR and IR spectra pre-sented by Sharma et al. [1] do not match our results and clearlyindicate that the compound they investigated is not [C2Omim][Tf2-

N]. We conclude that ether-functionalized ILs do not pick up onemole of CO2 per mole of IL and the results presented by Sharmaet al. [1] are in error.

2. Experimental

[C2Omim][Tf2N] was synthesized using methods previously de-scribed in the literature [8,9] from freshly distilled 1-methylimid-azole (Sigma–Aldrich, 99%), bromodimethylether (Sigma–Aldrich,90% technical grade), and LiTf2N (3M-Fluorad, >98%).

3. Results

The [C2Omim][Tf2N] was a clear liquid at room temperature.Sharma et al. [1] reported a light brown liquid. Its density wasmeasured with an Anton-Paar MDA 4500 vibrating tube densitom-eter and the results are shown in Fig. 1. The water content was31 ppm, as determined by model Metrohm 831 Karl-Fischer coulo-

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Fig. 3. Solubility of CO2 in [C2Omim][Tf2N] as a function of pressure at 22 �C (thiswork, s), compared to the values reported by Sharma et al. (h) [1].

368 Letter to the Editor / Chemical Engineering Journal 245 (2014) 367–369

meter. Clearly, the density is much higher than that reported bySharma et al. [1]; this difference cannot be explained by differ-ences in water content. The viscosity of the [C2Omim][Tf2N](31 ppm water) was measured with an ATS Rheosystems Viscoan-alyzer (equipped with a cone-and-plate spindle) and the values areshown in Fig. 2. Clearly, the values reported by Sharma et al.(2.1 ppm water) [1] are much lower. Thus, we conclude that thephysical properties of the sample reported by Sharma et al. [1]are not consistent with the [C2Omim][Tf2N] synthesized in ourlaboratory.

Acetone-d6

Fig. 4. 13C NMR spectra of [C2Omim][Tf2N] synth

The solubility of CO2 (99.995% purity, Praxair) in the[C2Omim][Tf2N] was measured at room temperature using anapparatus that has been described elsewhere [10,11]. Ourvalues are shown in Fig. 3, in comparison to the values reportedby Sharma et al. [1]. We measure a maximum value of 0.03 moleof CO2 per mole of IL at 0.922 bar. This clearly represents physicalabsorption of CO2 and no evidence of chemical complexation. Ourresults indicate a Henry’s Law constant (P = Hx, where P is thepressure in bar, H is the Henry’s law constant in bar, and x is themole fraction solubility of the CO2 in the IL) of 45.4 bar at22 ± 1 �C, which is consistent with 42.8 atm (43.4 bar) at 40 �Cfor a similar ether-functionalized imidazolium IL, as reportedpreviously by Bara et al. [7].

Our 13C NMR spectrum (Varian INOVA 500) for [C2Omim][Tf2N]in deuterated acetone is shown in Fig. 4, where each of the carbonsis labeled and identified. This spectrum indicates some of the prob-lems with the assignments made by Sharma et al. [1], whose spec-trum for the CO2 saturated sample is reproduced in Fig. 5.Specifically, Sharma et al. [1] argue that a new peak appearedaround d = 200–210 ppm after CO2 absorption and they attributethis peak to a carbonyl carbon atom of the newly formed carbox-ylic group. Interestingly, acetone-d6 (the solvent used both bySharma et al. [1] and in this work for the 13C NMR spectra) isknown to have two peaks at 29.9 and 206.7 ppm in the 13C NMRspectrum. Thus, it is clear that the peak near 205 ppm that Sharmaet al. [1] attribute to a CO2-reacted carbonyl carbon is actually justan acetone-d6 peak, the solvent they used for the NMR scans.Moreover, the 13C NMR spectrum presented by Sharma et al. [1]does not show any peaks corresponding to the anion, [Tf2N], whichshould appear as a quartet around d = 115–125 (see Fig. 4 for 13C

Acetone-d6

esized in our lab, using Aceton-d6 as solvent.

Acetone d6

Fig. 5. 13C NMR spectra of CO2 absorbed [C2Omim][Tf2N], as reported by Sharmaet al. [1].

Fig. 6. (a) FTIR spectra of neat (green) and (b) CO2 absorbed [C2Omim][Tf2N] (blue)synthesized in our laboratory. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

Letter to the Editor / Chemical Engineering Journal 245 (2014) 367–369 369

NMR of the pure [C2Omim][Tf2N] sample that we obtained). Theproton NMR results also do not match:

Sharma 1H NMR (700 MHz), [D6] acetone, 25 �C): d = 3.27 (s,3H), 3.81 (s, 2H), 3.82 (s, 3H), 7.31 (s, 1H), 7.42 (s, 1H), 8.66(s, 1H);This work 1H NMR (500 MHz), [D6] acetone, 25 �C): d = 2.050(Acetone), 3.425 (s, 3H), 4.068 (s, 3H), 5.638 (s, 2H), 7.715 (s,1H), 7.780 (s, 1H), 9.092 (s, 1H);

It seems that the sample used by Sharma et al. [1] was not[C2Omim][Tf2N].

Finally, our in situ FTIR spectra for [C2Omim][Tf2N] before andafter CO2 absorption are shown in Fig. 6. The green scan is beforeCO2 is added and the blue is after the [C2Omim][Tf2N] has beensaturated with CO2 at 22 �C and 3.3 bar. The one and only change

is a peak forming around 2380 cm�1 which is well known to be theasymmetric stretching of physically absorbed CO2. Neither peaksat 1700 cm�1 (@C@Ostr) nor 1405 cm�1 (AOAH in planebend) ofcarboxylic acid are found, as claimed by Sharma et al. [1].

4. Conclusions

We conclude that ILs with ether-functionalized cations simplyabsorb CO2 physically. There is no evidence of a strong covalentinteraction that would result in the uptake of a mole of CO2 permole of IL, as claimed by Sharma et al. [1]. The Henry’s Law con-stant for physical dissolution of CO2 in [C2Omim][Tf2N] at 22 �Cis roughly 45.4 bar, which is consistent with similar ether-func-tionalized imidazolium ILs reported by Bara et al. [7]. Based on thiswork, we conclude that the results presented by Sharma et al. [1]are in error.

Acknowledgement

We acknowledge support from the University of Notre DameSustainable Energy Initiative for support of this work.

References

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(2003) 5391–5396.[5] Y. Deng, S. Morrissey, N. Gathergood, A.-M. Delort, P. Husson, M.F. Costa

Gomes, ChemSusChem 3 (2010) 377–385.[6] A.S. Pensado, A.A.H. Padua, M.F. Costa Gomes, J. Phys. Chem. B 115 (2011)

3942–3948.[7] J.E. Bara, C.J. Gabriel, S. Lessmann, T.K. Carlisle, A. Finotello, D.L. Gin, R.D. Noble,

Ind. Eng. Chem. Res. 46 (2007) 5380–5386.[8] R.P. Singh, S. Manandhar, J.M. Shreeve, Tetrahedron Lett. (2002).[9] M.J. Muldoon, S.N.V.K. Aki, J.L. Anderson, J.K. Dixon, J.F. Brennecke, J. Phys.

Chem. B 111 (2007) 9001–9009.[10] B.F. Goodrich, de J.C. la Fuente, B.E. Gurkan, Z.K. Lopez, E.A. Price, Y. Huang, J.F.

Brennecke, J. Phys. Chem. B 115 (2011) 9140–9150.[11] B.F. Goodrich, de J.C. la Fuente, B.E. Gurkan, D.J. Zadigian, E.A. Price, Y. Huang,

J.F. Brennecke, Ind. Eng. Chem. Res. 50 (2011) 111–118.

Samuel SeoMauricio Quiroz-Guzman

Thomas R. GohndroneJoan F. Brennecke⇑

Department of Chemical and Biomolecular Engineering,University of Notre Dame, Notre Dame, IN 46556, United States

⇑ Tel.: +1 574 631 5847.E-mail address: jfb@nd.edu (J.F. Brennecke)

Available online 30 August 2013