Highly Efficient CO2 Capture by Imidazolium Ionic Liquids through a

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Highly Efficient CO2 Capture by Imidazolium Ionic Liquids through Reducing the Formation of Carbene-CO2 Complex Ke Mei, Xi He, Kaihong Chen, Xiuyuan Zhou, Haoran Li, and Congmin Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01001 • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 22, 2017

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Industrial & Engineering Chemistry Research

Highly Efficient CO2 Capture by Imidazolium Ionic Liquids through Reducing the Formation of Carbene-CO2 Complex Ke Mei,1 Xi He,1 Kaihong Chen,1 Xiuyuan Zhou,1 Haoran Li1 and Congmin Wang*1,2 1

Department of Chemistry, ZJU-NHU United R&D Center, Zhejiang University, Hangzhou

310027, China. 2

Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Zhejiang

University, Hangzhou 310027, China. KEYWORDS: Ionic Liquids; CO2 Capture; basicity; steric hindrance; carbene. ABSTRACT: A strategy for improving CO2 capture by imidazolium ionic liquids through reducing the formation of carbene-CO2 complex was reported. The carbene-CO2 complex content in CO2 capture by imidazolium ionic liquids was determined by a quantitative NMR method and the carbene-CO2 complex formation was decreased through reducing the basicity of the anion and enlarging the steric hindrance of the cation. Thus, both enhanced absorption capacity and improved desorption were achieved, where an ideal ionic liquid [Ipmim][Triz] exhibited a very high capacity of 0.21 g CO2 per g IL and excellent reversibility.

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1. INTRODUCTION CO2 is the main cause of global warming, thus, how to achieve highly efficient CO2 capture and storage (CCS)1, 2 becomes an urgent thesis. Ionic liquids (ILs) have been paid more and more attention for their excellent properties such as extremely low vapour pressure, wide liquid temperature range, excellent designable property, and so on,3-6 performing unique features compared to other absorbents.7 Imidazolium ILs are the representative type of IL in the field of CO2 capture and separations8, 9. Physical absorption has been known as the essential interaction between traditional ILs and CO2, which expresses an absorption capacity of no more than 0.03 mole CO2 per mole IL at normal pressure and temperature.10 Then, Davis and co-workers11 made use of aminefunctionalized IL to bring absorption capacity up to 0.5 mole CO2 per mole IL by introducing –NH2 substituent group to the imidazolium cation. Afterwards, chemical absorption between anion-functionalized ILs and CO2 comes into sight widely including sulfone12, acetate13-16,

27

, amino acid anions15,

17-19

and so on20,

21

. Other cation-

functionalized ILs such as containing –OH substituent group or dication have also made efficient absorption,22 and equimolar CO2 capture has been achieved in imidazolium ILs by combining superbases with imidazolium cation.23-25 It is well known that the acidic C2-H on imidazolium ring can be also removed by the basic anion to form carbenes.26 The existance of reactive carbene intermediate27 has been found in the process of CO2 capture by ILs composed of imidazolium cations and basic anions through single-crystal X-ray structures, where the absorption capacity of [Emim][OAC] is about 0.3 mole CO2 per mole IL under atmospheric pressure. Nheterocyclic carbene (NHC) could yield NHC-CO2 adducts by catching carbon dioxide28.


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Inesi29,30 confirmed the presence of NHC in electrochemically reduced imidazolium ILs by using cyclic voltammetry and investigated the stability and CO2 capture ability of NHC. Other forcible evidences31-33 in IR, NMR, Raman and DFT calculations have recently been proposed. The mechanism of carbene formation in ILs34 presents the researchers a puzzle. Brennecke and co-workers35 reported that there were two pathways in the interaction between Imidazolium ILs and CO2. One was that basic anions reacted with acidic CO2 directly. The other was the combination process of CO2 and carbene caused by removing the acidic C2-H in the imidazolium cations by basic anions. However, several disadvantages emerge in the pathway of carbene-CO2 reaction pathway, where neither high absorption capacity nor easy desorption36 can be achieved. What is more, the carbene intermediate has so high activity that it is unstable and can be easily affected by other substances.37 In this work, a strategy of tuning the structure in imidazolium ILs was put forward to weaken the carbene-CO2 pathway. A quantitative method to determine the content of carbene-CO2 complex using NMR method was firstly set up. Then the combination of carbene and CO2 was reduced through reducing the anion’s basicity and enlarging the cation’s steric hindrance. Absorption experiments, quantum chemical calculations, spectroscopic investigations and calorimetric data were carried out to demonstrate their effective effect on CO2 absorption. These imidazolium ILs were divided to two kinds of systems according to the kind of the ions. According to literature methods38,

39

, these functionalized ILs composed of

imidazolium cations with steric hindrance and basic anions were prepared by acid-base interaction between the corresponding imidazolium hydroxide in ethanol obtained by the


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anion-exchange method and 1,2,4-triazole, benzotriazole, tetrazole. The structures of these imidazolium ILs (Scheme 1) were confirmed by NMR and FTIR spectroscopy [see the Supporting Information].

Scheme 1. Structures of the anions and cations in imidazolium ionic liquids.

2. EXPERIMENTAL SECTION 2.1 Calculation method All calculations were employed in this work by the GAUSSIAN03 programs package. Geometry optimization of each set of each IL, the free CO2 molecule, and each IL-CO2 complex was calculated at the B3LYP/6-31++G (d,p) level with BSSE (Basis Set Superposition Error) correction.

2.2 Materials and general methods All chemicals used in this work were purchased from commercial and used without further purification unless otherwise stated. 1,2,4-Triazole (Triz), 1H-Benzotriazole (Bentriz), Tetrazole (Tetz) were purchased from Aladdin Ind. Co., Ltd. N2 (99.9%), CO2 (99.9%) were purchased from Hangzhou Jingong Special Gas Co., Ltd. 1H NMR and 13C NMR spectra were recorded on


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a Bruker spectrometer (400 MHz or 500 MHz) in DMSO with tetramethylsilane as the standard. FT-IR spectra were obtained using a Bio-Rad Excalibur FTS-3000 spectrometer. The water contents of these ILs were determined through Karl Fisher titrations (Jie Dao V10, China).

2.3 The synthesis of the ILs These imidazolium ILs were prepared by the acid-base neutralization between azole and an ethanol solution of [Emim]OH, [Ipmim]OH, [Me-Pe-Im]OH, or [Ip-Pe-Im]OH, which were obtained from [R-Im]X using an anion-exchange resin. In a typical preparation of [Emim]Br, [Emim]Br was prepared according to the literature40-43 with a slight modification. Degassed bromoethane (86.04 g, 0.790 mol, Avocado) and the redistilled N-methyl imidazolium (49.86 g, 0.607 mol, Aldrich) was mixed with constant stirring under inert atmosphere conditions. The reflux of mixture was carried out at 40 °C for 3 h. Then, it was cooled down to room temperature by being placed statically. To remove product from the mixture, it was washed by adding ethyl acetate to the solution. This was dried under a vacuum at 25 °C for 10 h after the filtration and washing. Finally, [Emim]Br as a white solid was obtained, which should be stored under an inert atmosphere. In a typical preparation of [Emim][Triz], the mixture was produced by mixing equimolar triazole and [Emim]OH solution in ethanol. The mixture stayed at room temperature with constant stirring for 12 h. Subsequently, ethanol and water were removed by rotary evaporation at elevated temperature of 40 oC under reduced pressure. [Emim][Triz] obtained was dried in high vacuum for 1 day at 50 oC. The structures of these imidazolium ILs were verified by NMR and IR spectroscopy with no impurities observed.

2.4 CO2 absorption and desorption


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In a typical absorption of CO2, CO2 at atmospheric pressure was bubbled through about 1 g IL in a glass container at a flow rate of about 60 ml min-1. The glass container about 5ml was placed on the heating apparatus set at 30 oC. The amount of CO2 absorbed was acquired by calculating the mass difference at regular intervals by an electronic balance with an accuracy of ±0.0001g. As for a typical desorption of CO2, the captured CO2 was released by bubbling N2 of a flow rate of about 60 ml min-1 through the IL in a glass container. The glass container was partly immersed in an oil bath at 80 oC. The regeneration of IL was achieved by desorption of CO2, which was determined at regular intervals by an electronic balance with an accuracy of ±0.0001g.

3. RESULTS AND DISCUSSIONS 3.1 Quantitative determination of the carbene-CO2 complex ratio by NMR method It is known that there is carbene-CO2 complex formed in the reaction between a typical IL [Emim][Triz] with CO2. It is speculated that there are two pathways (anion-CO2 and cation-CO2) in chemisorption. To further explore the CO2 absorption process in details, NMR method was employed to investigate the structure change in the process of CO2 capture. Figure 1 shows the structure variation of fresh IL [Emim][Triz] and the reacted IL with CO2 by NMR method. A quantitative way of calculating the carbene-CO2 complex ratio was proposed. We obtained the content of carbene-CO2 complex formed in the combination process of ILs and CO2 by calculating the new peak integral area ratio of imidazolium cations in 1H NMR spectra after the CO2 absorption as equation (1).

NHC - CO2 ratio =

S2 S1 + S 2


(1)

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Where the S1 is primary peak integral area which presents original IL, S2 is the new peak integral area after CO2 absorption. It could be seen in Figure 1 that there were synchronous new peaks in 1H NMR after CO2 absorption. The primary peak integral area for N-CH3 was 3.0, and the synchronous new peak integral area after CO2 absorption was 1.5, so newly-formed carbene-CO2 complex ratio was 33.3% according to equation (1). The reason for that was the combination of the C2 on the cation imidazolium ring and CO2 caused the variation of the structure of the cation. The new absorption peaks of ILs in

13

C NMR after the CO2

capture compared very well with the mechanism of two pathways in the interaction of imidazolium ILs and CO2. The affiliation of simultaneous new peak at 154.8ppm occurring in

13

C NMR (Figure S1) was CO2 attaching to C2 while 159.3ppm represented

CO2 attaching to N on the anion. It meant that not only the basic anion but also the cation could react with CO2 for [Emim][Triz].

Figure 1. 1H NMR spectra of [Emim][Triz] before (black) and after the capture of CO2 (red).

3.2 The design of imidazolium ILs with less carbene-CO2 pathway


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To improve CO2 absorption performance by imidazolium ILs with less carbene-CO2 pathway, we proposed two ways of prohibiting the basic anion snatching the C2-H during the reaction. First of all, the effect of the basicity of the anion on the carbene-CO2 complex formation was carried out. As Table 1 showed, it was obvious that the declining basicity contributed to the considerable decrease of carbene-CO2 complex content. For instance, the pKa of triazole was 13.9, while the pKa of bentriazole was 11.921. The carbene-CO2 complex ratio of [Emim][Bentriz] turned out to be 14.9%, with a gap of 18.4% from 33.3% of [Emim][Triz]. However, the absorption capacity of ILs took a lot reduction as the basicity of the anion decreased. The absorption capacity at 30 oC under atmosphere pressure of [Emim][Bentriz] was 0.17, with a gap of 0.48 from 0.65 of [Emim][Triz]. The absorption capacity was divided to two parts: carbene-CO2 pathway and non carbene-CO2 pathway. According to NMR analysis, the absorption capacity through carbene-CO2 pathway of [Emim][Bentriz] and [Emim][Triz] was 0.15 and 0.33 mole CO2 / mole IL, respectively, and their absorption capacity through non carbene-CO2 pathway was 0.02 and 0.32 mole CO2 / mole IL, respectively. If the anion became tetrazole (pKa=8.2), extreme low absorption capacity of 0.09 mole CO2 per mole IL was exhibited by [Emim][Tetz] with no carbene-CO2 complex formation. Even though the decrease of the basicity of the anion induced a decrease of the NHC-CO2 ratio, it also caused a decrease in absorption capacity, which was opposite to the original intention. Obviously, the anion triazole exhibited the best performance in absorption capacity among the ILs with the same cation [Emim].


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Table 1．．The effect of the anion with different basicity and cation’s steric hindrance of imidazolium ionic liquids on carbene-CO2 complex ratio and CO2 capacity Carbene-CO2

Absorption

Carbene-CO2

Non carbene-

complex ratio a/%

capacity b(c)

capacity e

CO2 capacity f

[Emim][Triz]

33.3

0.65 (0.160)

0.33

0.32

[Emim][Bentriz]

14.9

0.17 (0.033)

0.15

0.02

[Emim][Tetz]

0

0.09 (0.022)

0

0.09

[Ipmim][Triz]

17.8

0.72 (0.164)

0.18

0.54

[Me-Pe-Im][Triz]

15.5

0.75 (0.129)

0.16

0.59

[Ip-Pe-Im][Triz]

9.1

0.77 (0.120)

0.09

0.68

[Ipmim][Triz] d

16.7

0.91 (0.207)

0.17

0.74

Ionic liquids

a

The carbene-CO2 complex ratio of new absorption peak in 1H NMR.

b

The absorption

was carried out at 30 oC and 1.0 bar for 2h, mole CO2 per mole IL. c Absorption capacity on the basis of weight, g CO2 per g ionic liquid. d The absorption was carried out at 20 oC and 1.0 bar for 2h. e Absorption capacity through carbene-CO2 pathway, equal to carbeneCO2 complex ratio, mole CO2 per mole ionic liquid. f Absorption capacity through non carbene-CO2 pathway, equal to absorption capacity minus carbene-CO2 capacity, mole CO2 per mole ionic liquid. Based on practical purpose for CO2 absorption, we chose the triazole anion for its high absorption capacity to investigate the effect of the cation. As seen in Table 1, due to the enlarged cation's steric hindrance, the content of carbene-CO2 complex in the process of CO2 capture reduced effectively. For example, the new peak integral area ratio of imidazolium cations in

1

H NMR presented a significant decrease from 33.3% of


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[Emim][Triz] to 17.8% for [Ipmim][Triz]. However, the formation of carbene-CO2 complex couldn’t be avoided absolutely as steric hindrance increased continuously, where there were still 9.1% carbene-CO2 complex content for [Ip-Pe-Im][Triz].The improved absorption capacity of IL could be observed via introducing substituent with certain steric hindrance instead of conventional alkyl groups. For instance, the absorption capacity of [Ipmim][Triz] was 0.72 mole CO2 per mole IL with a gap of 0.07 than 0.65 of [Emim][Triz]. The absorption capacity through carbene-CO2 pathway and non carbeneCO2 pathway of [Ipmim][Triz] was 0.18 and 0.54 mole CO2 / mole IL, respectively, while that of [Emim][Triz] was 0.33 and 0.32 mole CO2 / mole IL, respectively. As shown in Table 1, an increase in cation’s steric hindrance induced a decrease in carbene-CO2 pathway capacity and an increase in non carbene-CO2 pathway capacity. However, if steric hindrance on the cation of the IL increased to a certain degree such as [Me-PeIm][Triz] to [Ip-Pe-Im][Triz], the growth extent of absorption capacity was not as obvious as [Emim][Triz] to [Ipmim][Triz]. Therefore, the effect of cation’s steric hindrance on CO2 absorption capacity was less than the anion’s basicity. [Ipmim][Triz] with appropriate basicity and steric hindrance indicated the best alternative among these imidazolium ILs, which was superior to other kinds of functionalized ILs (Table S1) with the mass uptakes of 0.21 g CO2 per g IL at 20 o

C due to its low carbene-CO2 complex ratio and small molecular weight.

3.3 CO2 absorption by [Ipmim][Triz] as an ideal absorbent The CO2 absorption capacity for a selected IL [Ipmim][Triz] was measured as a function of temperature from 20-70 oC (Figure S2). It was obvious that the captured CO2 was easily released as the temperature increased. The absorption capacity was 0.91 mole


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CO2 per mole IL at 20 oC, while the absorption capacity dropped down to 0.23 mole CO2 per mole IL with the elevated temperature of 70 oC. It meant that the easy desorption could be achieved by simply heating. Desorption process at 80 oC could be completed in 30 min with few desorption residue. Figure 2 showed the CO2 absorption and desorption recycling exhibited by [Ipmim][Triz]. After five absorption/desorption recycling, the CO2 absorption capacity remained about 0.16 g CO2 per g IL at 30 oC and ambient pressure, which exhibited excellent reversibility.

Figure 2. Five cycles of CO2 absorption-desorption by [Ipmim][Triz]. In addition, the specific structure variation in the absorption-desorption process was further investigated through

13

C NMR and FTIR spectroscopy. New peaks could be

easily investigated after CO2 absorption as Figure 3 showed. There were two new absorption peaks in 13C NMR (Figure 3a) at 154.7 ppm and 159.5 ppm, which represented C-COO- and N-COO-, respectively. Synchronous new peaks in 1H NMR spectroscopy represented the existance of two pathways. A new peak at 1614 cm-1 in IR (Figure 3b) was attributed to asymmetrical stretching vibration of C=O, which belonged to the obvious evidence for the existance of N-COO- in anion-CO2 pathway. After absolute


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desorption, the structure of ILs returned to the original sample, where the new absorption peaks in NMR and IR which represented the captured CO2 disappeared thoroughly.

Figure 3. (a)

13

C NMR and (b) FT-IR of [Ipmim][Triz] (black, fresh IL; red, after the

absorption of CO2; blue, after the desorption of CO2). Compared to [Emim][Triz], the desorption property for [Ipmim][Triz] seemed to be improved as the cation steric hindrance increased. It could be seen in Table S2 that the desorption residue at 60 oC under N2 for 5 min for [Ipmim][Triz] was 0.45 mole CO2 per mole IL with a residue ratio of 62.6%, which was much less than [Emim][Triz]. Figure S3 showed the CO2 desorption property and thermal stability of different cation's steric hindrance by TGA. Generally, the desorption temperature for [Emim][Triz] was from 60


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to 150 oC. Nevertheless, when it came to [Ipmim][Triz], the captured CO2 began to release at 50 oC, and the desorption of the captured CO2 kept up until the temperature raised to 100 oC. Thus, we could conclude that the release of CO2 was more facile for [Ipmim][Triz] than [Emim][Triz], which was originated from the deduction of carbeneCO2 pathway in the CO2 capture process.

3.4 Computational analysis on the effect of the basicity and steric hindrance Why could [Ipmim][Triz] exhibit the efficient absorption and desorption performance under the effect of anion’s basicity and cation’s steric hindrance? To settle the issue, theoretical calculations using the Gaussian 03 program were carried out to calculate the interaction enthalpies ∆H between the IL and CO2 with BSSE (Basis Set Superposition Error) correction. The optimized structures of IL-CO2 complexes were shown in Figure 4. The interaction enthalpy of [Emim][Tetz] was -20.6 kJ/mole, which decreased significantly from -36.2 kJ/mole of [Emim][Triz] due to the reduced basicity of the anion. It was also found that the N-C distance and O-C-O bond angles (Table S3) in IL-CO2 complexes were predicted to be increased as the basicity of the anion decreased, indicating the weaker interaction between the IL and CO2. Moreover, as the cation's steric hindrance increased, the absorption enthalpy of [Ipmim][Triz] was -37.8 kJ/mole with a gap of 1.6 kJ/mole compared to [Emim][Triz]. It meant that the binding between the anion of the IL with CO2 was strengthened by introducing a certain steric hindrance substituent to the cation. The interaction enthalpies between CO2 and [Emim][Triz] or [Ipmim][Triz] through carbene-CO2 pathway were also calculated via the Gaussian 03 program with BSSE correction, which were shown in Figure S4. According to the calculation results, the


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interaction enthalpies between CO2 and [Ipmim][Triz] through carbene-CO2 pathway was -51.4 kJ mol-1 with a gap of 4.0 kJ mol-1 than -55.4 kJ mol-1 of [Emim][Triz], indicating that the binding between [Ipmim][Triz] and CO2 was weaker than [Emim][Triz] through carbene-CO2 pathway because cation’s steric hindrance increased. Pleasantly surprising, the results of quantum chemical calculations accorded well with the phenomenon of absorption capacity.

Figure 4 Optimized structures (O, red; N, blue; C, gray ; H, white) of complexes of ionic liquids and CO2 at the B3LYP/6-31++G(d,p) level. (a) [Emim][Triz]–CO2, ∆H = -36.2 kJ mol-1; (b) [Emim][Bentriz]–CO2, ∆H = -32.2 kJ mol-1; (c) [Emim][Tetz]–CO2, ∆H = -20.6 kJ mol-1 ; (d) [Ipmim][Triz]–CO2, ∆H = -37.8 kJ mol-1; (e) [Me-Pe-Im][Triz]–CO2, ∆H = -38.1 kJ mol-1; (f) [Ip-Pe-Im][Triz]–CO2, ∆H = -39.2 kJ mol-1

4. CONCLUSIONS In summary, on the basis of quantitative method of calculating carbene-CO2 complex ratio, a strategy of tuning the anion’s basicity and the cation’s steric hindrance by


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imidazolium ILs was reported through reducing carbene-CO2 pathway. Firstly, the exploration of how the basicity of the anion could influence the ILs’ absorption property was put into effect. The decrease of the basicity of the anion induced a decrease of the NHC-CO2 ratio, but it caused the decrease in CO2 absorption capacity. Therefore, the triazole anion exhibiting the perfect absorption capacity was chosen to investigate the effect of cation. With the same triazole anion employed, a series of alkyl groups containing considerable steric hindrance were introduced to the imidazolium cation, where the [Ipmim][triz] showed the best capacity with mass uptakes of 0.21 g CO2 per g IL. Herein, the increase of cation’s steric hindrance and decrease of anion’ basicity could reduce the ratio of the carbene-CO2 complex according to spectroscopic investigations, the main effect on CO2 absorption capacity was due to the anion not the cation. We believe that this efficient and reversible absorption by imidazolium IL with appropriate basicity and steric hindrance provides an attractive alternative for carbon capture, which could be applied in the field of gas separation as well as other fields.

ASSOCIATED CONTENT Supporting Information. Supplementary tables and figures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Fax. +86-571-8827-3181. E-mail: [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGEMENT This work was supported by the National Key Basic Research Program of China (2015CB251401), the National Natural Science Foundation of China (21176205, 21322602), and the Fundamental Research Funds of the Central Universities.

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Highly Efficient CO2 Capture by Imidazolium Ionic Liquids through a

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