Efficient and Energy-Saving CO2 Capture through the Entropic Effect

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Efficient and Energy-Saving CO2 Capture through the Entropic Effect Induced by the Intermolecular Hydrogen Bonding in AnionFunctionalized Ionic Liquids Xiao Y. Luo,†,§ Fang Ding,†,§ Wen J. Lin,†,§ Yu Q. Qi,†,§ Hao R. Li,†,‡,§ and Cong M. Wang*,† †

Department of Chemistry, ZJU-NHU United R&D Center and ‡Department of Chemical and Biological Engineering, State Key Laboratory of Chemical Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: A strategy for improving the capture of CO2 was developed through the entropic effect by tuning the geometric construction of anion-functionalized ionic liquids. Several kinds of anion-functionalized ionic liquids with the amino group at the para or ortho position were designed and applied for the capture of CO2, which indicates that the former exhibited both higher capacity and lower enthalpy, resulting in the efficient and energy-saving CO2 capture. Viscosity measurements, spectroscopic investigations, and quantum chemical calculations showed that such a unique behavior originated from the entropic effect, which was induced by the intermolecular hydrogen bonding in these ionic liquids. The entropic control for gas separation developed by this work provides an efficient strategy to both increased capacity and reduced enthalpy. SECTION: Environmental and Atmospheric Chemistry, Aerosol Processes, Geochemistry, and Astrochemistry

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the basicity15 and introducing an electron-withdrawing substituent on the anion,16 have been developed for reducing the enthalpy. However, the enthalpy is the main driving force in these ILs, which often leads to the reduced capacity because of the decreased interaction between these ILs and gas. Therefore, can we develop a novel method for improving gas capture with both high capacity and low enthalpy, resulting in the efficient and energy-saving gas absorption? It is well-known that the entropy is another important factor that governs the reaction or process. The entropic effect is very important in some fields including the self-assembly of chemistry,17 the synthesis of materials,18,19 and the control of chemical reaction.20 Particularly, the entropy plays significant roles in nature,21 such as α-helical22,23 and β-sheets24,25 of protein folding and nucleic acid folding. Recently, there is some discussion on the impact of entropy change for CO 2 absorption.11,26−29 For example, Carvalho et al.27 suggested that CO2 physical solubility in ILs is controlled by entropic effects, while Brennecke and co-workers26 disagreed that the solubility is controlled by entropy. However, little attention has been given to the gas chemisorption by the IL through the entropic effect. The question remains whether both high capacity and low enthalpy can be achieved via tuning the

ecently, there has been a strong scientific drive to minimize the emission of acid gas such as CO2 and SO2 from the burning of fossil fuels, which threatens economies, environments, and human health. Accordingly, the development of new materials and processes for the efficient, energysaving, and economical capture of these gases is highly desired. The unique properties of ionic liquids (ILs), which include extremely low vapor pressures, wide liquid ranges, high stabilities, and tunable properties, offer an opportunity to address this challenge.1−8 They have been widely used in the field of gas separation for some gases such as CO2, SO2, H2S, and BF3. Generally, the gas absorption by these ILs is mainly driven by the enthalpic effect.9−11 Here, we report a novel strategy for improving the capture of CO2 by several anionfunctionalized ILs through the entropic effect. We show how we can achieve both high capacity and low enthalpy via designing the specific geometric construction of anionfunctionalized ILs, thereby enabling an efficient and energysaving CO2 capture. Inspired by the low volatility of the IL, Davis and coworkers12 reported a pioneering work for chemical absorption of CO2 that employs an amino-functionalized IL. Subsequently, some anion-functionalized ILs13−16 including amino-acid-based ILs, acetate ILs, phenolate ILs, and azolate ILs, have been explored for the chemisorption of gas with improved capacity. Normally, the chemisorption has high absorption capacity along with high absorption enthalpy, which means difficult desorption as well as extensive energy consumption for regeneration. Recently, several methods, including reducing © 2014 American Chemical Society

Received: November 22, 2013 Accepted: January 3, 2014 Published: January 3, 2014 381

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AA]. To explain the difference in CO2 capacity between them, we calculated the Mulliken charge of the N atom in these ILs (Table 1). It can be seen that the Mulliken charge of the N

entropy by changing the geometric configuration of anionfunctionalized ILs to chemical absorption of CO2. Herein, we describe a strategy for improving the capture of CO2 by anion-functionalized ILs through the entropic effect. The essence of our strategy is making use of the entropic variation induced by different structures of the IL. Thus, we designed and prepared several kinds of anion-functionalized ILs including methylbenzolate-based ILs and nicotinate-based ILs with the amino group at the para and ortho positions (see Chart 1 for these ILs’ structures). Through a combination of

Table 1. Mulliken Charge of the N Atom in the Anion and the Absorption Enthalpies and Entropies of CO2 with [P66614][p-AA] and [P66614][o-AA]

Chart 1. Structures of the Anion in Anion-Functionalized ILs Used in This Work for CO2 Capture

anion

p-AA

o-AA

Mulliken charge of N atoma ΔG (kJ mol−1)b ΔH (kJ mol−1)c ΔS (J mol−1 K−1)c ΔS in tetraglyme (J mol−1 K−1)b

−0.529 −6.9 −41 −124 −179

−0.584 −1.0 −56 −183 −192

a

The Mulliken charge of N was obtained at the B3LYP/6-31++G(p,d) level. bThe Gibbs free energy was calculated by ΔG = −RT ln K at 30 °C using the 1:1 reaction model. cThe enthalpy and entropy were obtained using the van’t Hoff equation (Figures S2 and S3, Supporting Information).

atom in [p-AA] is −0.529, while that in [o-AA] is −0.584, which means that the interaction of CO2 with the latter is stronger and it would lead to higher absorption enthalpy. However, the increase of CO2 absorption capacity from a higher enthalpy ([P66614][o-AA]) to a lower one ([P66614][pAA]) is surprising because the enthalpy is usually a main driving force for CO2 capture. Actually, the capacity essentially depends on the Gibbs free energy (ΔG), which can be calculated by an isothermal equation (showed in Table 1). However, then, ΔG is regarded as two parts including enthalpy and entropy; the explanation is that CO2 capture by these ILs is not enthalpy-driven, and we should pay attention to the entropic effect. The effect of the thermodynamic data on CO2 capture by these ILs was investigated (Table 1), which was obtained from the van’t Hoff equation in a manner of 1:1 stoichiometry (Figures S2 and S3, Supporting Information).4 As shown in Table 1, the absorption enthalpy of CO2 with [P66614][p-AA] is −41 kJ mol−1, while that with [P66614][o-AA] is −56 kJ mol−1, indicating that the latter is more exothermic, which is consistent with the Mulliken charge of the N atom in these ILs. Thus, compared with [P66614][o-AA], [P66614][p-AA] exhibited both higher capacity and lower enthalpy, which is favorable for CO2 capture. It is emphasized that the IL [P66614][p-AA] exhibited lower absorption enthalpy in comparison with other functionalized ILs, resulting in the equimolar and energy-saving CO2 capture (Table S2, Supporting Information). For example, the absorption enthalpies of [MTBDH][Im] and [P66614][Triz] are −117 and −56 kJ mol−1, respectively, while that of [P66614][pAA] is only −41 kJ mol−1. The entropic effect on CO2 capture by these ILs can also be seen in Table 1. The entropy changes for CO2 reacting with [P66614][p-AA] and [P66614][o-AA] are −183 and −124 J mol−1 K−1, respectively, indicating that the different entropy changes in these ILs is the reason that leads to such a unique behavior. Then, what causes the different absorption entropy changes of CO2 with [P66614][p-AA] and [P66614][o-AA]? The structures of these ILs before and after the capture of CO2 were investigated by the DFT method at the B3LYP/631++G(p,d) level. Figure 2 shows the optimized structures of the anions of [p-AA] and [o-AA], indicating the difference of the structures, where one H of −NH2 in [o-AA] forms intramolecular hydrogen bonding with the O of carboxylate in

viscosity measurement, absorption experiment, quantum chemical calculation, and spectroscopic investigation, we show that both high capacity and low enthalpy can be achieved for the capture of CO2 through the entropic effect, which was induced by intermolecular hydrogen bonding in these ILs, resulting in the efficient and energy-saving process for CO2 capture. These anion-functionalized ILs with the amino group at different positions were easily prepared by the acid−base neutralization between aminomethylbenzoic acid or aminonicotinic acid and a solution of trihexyl(tetradecyl)phosphonium hydroxide in ethanol, which was obtained by the anion-exchange method.30−32 The structures of these ILs were verified by NMR and IR spectroscopies (see the Supporting Information). The physical properties such as the viscosities of these ILs were investigated, indicating that the IL with the amino group at the para position exhibited higher viscosity. For example, the viscosity of [P66614][p-AA] is 2867 cPa at 40 °C, while that of [P66614][o-AA] is 252 cPa at 40 °C. The effect of different anion-functionalized ILs with the amino group on CO2 absorption was investigated (Figure 1). It was seen that CO2 absorption capacities by [P66614][p-AA] and [P66614][o-AA] at 30 °C were 0.94 and 0.60 mole of CO2 per mole of IL, respectively. Clearly, the CO2 capacity by [P66614][p-AA] is significantly higher than that by [P66614][o-

Figure 1. CO2 absorption by [P66614][o-AA] (pink) and [P66614][pAA] (green) at different temperatures. 382

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Figure 2. The optimized structure of o-AA (upper, left), o-AA-CO2 (upper, right), p-AA (bottom, left), and p-AA-CO2 (bottom, right).

the form of a six-membered ring, while two H’s of −NH2 in [pAA] are free and can form intermolecular hydrogen bonding. The difference in the structures can be reflected by the difference in the viscosities in these ILs, which was shown in Table S1 (Supporting Information). It is clear that the viscosity of [P66614][p-AA] is significantly higher than that of [P66614][oAA] because of the formation of intermolecular hydrogen bonding in the former. On the other hand, for [P66614][p-AA], the viscosity increased significantly because the O in carbamic acid would compete with the O of benzoate to form intermolecular hydrogen bonding with the H left in N−H after the capture of CO2.33,34 In contrast, the viscosity of [P66614][o-AA] decreased a little after CO2 capture due to the weak ability of N−H in forming intermolecular hydrogen bonding, where strong intramolecular hydrogen bonding formed between N−H and the O of benzoate. The difference in the formation of intermolecular hydrogen bonding was investigated by the IR method.35,36 It is seen in Figure 3 that a signal at 1661 cm−1 was produced after the capture of CO2 by [P66614][p-AA], which could be attributed to the CO stretching vibration in carbamic acid.37−39 However, for [P66614][o-AA], a new peak at 1735 cm−1 attributed to the CO stretching vibration appeared. As shown, the bond strength of CO in [o-AA]-CO2 is stronger than that of CO in [p-AA]-CO2 because of the formation of intermolecular hydrogen bonding in [p-AA]-CO2, resulting in the blue shift in IR spectra, which is consistent with the results on the structure of poly(N-isopropylacrylamide),40,41 where the IR signal of intermolecular hydrogen-bonded CO had a lower wavenumber than the free form of non-hydrogen-bonded CO. The formation of intermolecular hydrogen bonding was further demonstrated by means of temperature-dependent FTIR spectroscopy because intermolecular hydrogen bonding would be destroyed by increasing the temperature.42 The effect of the temperature on the peak intensity for these ILs was investigated, which is shown in Figure 4. For [P66614][p-AA]CO2, a peak at 1735 cm−1 decreases because of the breakage of CO···H−N, and a signal at 1650 cm−1 increases due to the re-formation of CO···H−N with cooling the temperature from 60 to 30 °C. However, for [P66614][o-AA]-CO2, the peak does not change with changing temperature, where no intermolecular hydrogen bonding formed in this system. Thus, it is indicated that the N−H left in [P66614][p-AA] is the source of larger absorption entropy than [P66614][o-AA] because the CO2 captured by [P66614][p-AA] can disturb the

Figure 3. IR spectra of (a) [P66614][o-AA], (b) [P66614][p-AA], (c) the mixtures of [P66614][o-AA] with tetraglyme, and (d) [P66614][p-AA] with tetraglyme in a molar ratio of 1:1 before (black) and after (red) CO2 absorption.

Figure 4. The stack plot of the in situ FTIR spectra collected from 60 to 30 °C after CO2 capture by [P66614][o-AA] (left) and [P66614][pAA] (right) at 60 °C.

structural order of the system significantly by competing with the formation of intermolecular hydrogen bonding, which results in the backing up of the entropy, while for [P66614][oAA], the disturbance of the structural order of [P66614][o-AA] by the captured CO2 is weak. As we know, intermolecular hydrogen bonding would be broken by adding a solvent, which can compete in the formation of hydrogen bonding.43 Therefore, tetraglyme was selected as a destroyer to further investigate the effect of intermolecular hydrogen bonding on the absorption entropy (Table 1). Clearly, for [P66614][o-AA], the effect of tetraglyme on the absorption entropy is weak, and the absorption of CO stretching vibration remains at 1735 cm−1. However, for [P66614][p-AA], the effect is significant, where the entropy changed from −124 to −179 J mol−1 K−1. Here, the O in tetraglyme competes with the O in [P66614][p-AA] to form 383

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intermolecular hydrogen bonding with H−N; hence, intermolecular hydrogen bonding in [P66614][p-AA]-CO2 was broken, resulting in the decrease of the absorption entropy. Accordingly, the absorption of a CO stretching vibration in a solution of tetraglyme exhibits a blue shift from 1661 to 1724 cm−1 because of the enhancement of CO bond strength that was induced by the reduced intermolecular hydrogen bonding. On the basis of the above results, we conclude that the difference in the entropic effect in these ILs results in both higher capacity and lower enthalpy for [P66614][p-AA], which stems from the difference in the formation of intermolecular hydrogen bonding. Thus, two kinds of new nicotinic acid-based ILs with the amino group at the para and ortho positions such as [P66614][p-ANA] and [P66614][o-ANA] were designed, prepared, and applied in the capture of CO2 to further verify the entropic effect induced by intermolecular hydrogen bonding (see Chart 1 for their structures). Similarly, in the structures of [p-ANA] and [o-ANA], the two H’s in [p-ANA] are free, while the one H in [o-ANA] forms intramolecular hydrogen bonding with carboxylate in the ortho position. Compared with [P66614][o-ANA], [P66614][p-ANA] exhibited both higher capacity and lower enthalpy because of the entropic effect (Table S3, Supporting Information). The studies on the viscosity, IR, and variable-temperature IR show that intermolecular hydrogen bonding is the source of the difference in the entropy change, which is similar with the results in [P66614][p-AA] and [P66614][o-AA] (Figures S4−S7, Supporting Information). In summary, a new way for the efficient CO2 capture making use of entropic effects via altering the geometric construction of anion-functionalized ILs was developed. It was found that several anion-functionalized ILs with the amino group at the para position exhibited both higher capacity and lower enthalpy than the corresponding ILs with the amino group at the ortho position. Thus, an equimolar capacity with low enthalpy of −41 kJ mol−1 by [P66614][p-AA] was achieved, resulting in the efficient and energy-saving CO2 capture. To the best of our knowledge, considering both high capacity and low enthalpy, [P66614][p-AA] exhibited superior CO2 absorption performance than other functionalized ILs so far (Table S2, Supporting Information). Through a combination of absorption experiment, viscosity measurement, quantum chemical calculation, and spectroscopic investigation, the entropic effect is the source of the difference in absorption performance, which stems from intermolecular hydrogen bonding in these ILs. The method developed in this work allows us to have new insight into gas separation, which opens a door to capture other gas such as SO2 and H2S by ILs with both high-capacity and low-energy demand. We believe that this highly efficient and energy-saving process through the entropic effect is promising in the field of gas separation.

of water. 1H NMR and 13C NMR spectra were recorded on a Bruker spectrometer (500 MHz) in DMSO-d6 with tetramethylsilane as the standard. FT-IR spectra were recorded on a Bruker Vector 22 FT-IR spectrometer, and in situ IR spectra were recorded on a Bruker MATRIX-MF. The ILs were prepared by neutralizing trihexyl(tetradecyl)phosphonium hydroxide ([P66614][OH]) and the acid with the amino group such as p-AA according to literature methods.16,30 A solution of [P66614][OH] in ethanol was prepared from [P66614]Br using the anion-exchange resin method. Equimolar p-AA was added to the [P66614][OH] solution in ethanol. The mixture was then stirred at room temperature for 12 h. Subsequently, ethanol and water were distilled off at 60 °C under reduced pressure. The obtained ILs was dried in high vacuum for 24 h at 60 °C. The structures of these ILs were confirmed by NMR and IR spectroscopy; no impurities were found by NMR. The water content of these ILs was determined with a Karl Fisher titration and was found to be less than 0.1 wt %. The residual bromide content of these basic ILs was determined by a semiquantitative Nessler cylinder method, which showed that the bromide content was lower than 0.15 wt %. In a typical absorption of CO2, CO2 of atmospheric pressure was bubbled through about 1.0 g of ILs in a glass container with an inner diameter of 10 mm, and the flow rate was about 60 mL min−1. The glass container was partly immersed in a metal heating block of desirable temperature. The amount of CO2 absorbed was determined at regular intervals by the electronic balance with an accuracy of ±0.1 mg. The absorption enthalpy and entropy for CO2 capture by these ILs were obtained from the van’t Hoff equation. The linear fitting between ln K and 1/T is shown in Figures S2−S5 (Supporting Information), where the enthalpy was obtained from the slope, while the entropy change was obtained from the intercept.

EXPERIMENTAL METHODS 2-Amino-3-methylbenzoic acid (o-AA), 4-amino-3-methylbenzoic acid (p-AA), 2-aminonicotinic acid (o-ANA), 6-aminonicotinic acid (p-ANA), and trihexyl(tetradecyl)phosphonium bromide ([P66614][Br]) were purchased from Aldrich. Tetraglyme was purchased from Aladdin Industrial Corporation. An anion-exchange resin [DOWEX MONOSPHERE 550A (OH)] was obtained from Dow Chemical Company. All chemicals were obtained in the highest purity grade possible and were used as received, unless otherwise stated. All ILs samples were dried under vacuum at 60 °C for 24 h to reduce possible traces

Notes

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ASSOCIATED CONTENT

S Supporting Information *

Experimental sections, describing the NMR and IR data of ILs; Tables S1−S3, showing the viscosities of the ILs, comparison of CO2 capture by different functionalized ILs, and Mulliken charge of the N atom in the anion and reaction enthalpies and entropies of CO2; and Figures S1−S8, showing 1H NMR spectra, the dependence of the equilibrium constants for reaction of CO2 with various species, stack plots of the FTIR spectra, and capacity versus time plots. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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*E-mail: [email protected]. The authors declare no competing financial interest. § E-mail: [email protected] (X.Y.L.). [email protected] (F.D.). [email protected] (W.J.L.). qiwei0571@163. com (Y.Q.Q.). [email protected] (H.R.L.).

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21176205, No. 21322602, No. J1210042), the Zhejiang Provincial Natural Science Foundation of China (R12BL06002), the Program for Zhejiang Leading 384

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Team of S&T innovation (2011R50007), and the Fundamental Research Funds of the Central Universities.

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