Decreasing the Viscosity in CO2 Capture by Amino-Functionalized

Article pubs.acs.org/JPCB

Decreasing the Viscosity in CO2 Capture by Amino-Functionalized Ionic Liquids through the Formation of Intramolecular Hydrogen Bond Xiao Y. Luo,† Xi Fan,† Gui L. Shi,† Hao R. Li,† and Cong M. Wang*,†,‡ †

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

ABSTRACT: A strategy for decreasing the viscosity variation in the process of CO2 capture by amino-functionalized ionic liquids (ILs) through the formation of intramolecular hydrogen bond was reported. Different with the dramatic increase in viscosity during CO2 uptake by traditional amino-functionalized ILs, slight increase or even decrease in viscosity was achieved through introducing a N or O atom as hydrogen acceptor into amino-functionalized anion, which could stabilize the active hydrogen of produced carbamic acid. Quantum chemical calculations and spectroscopic investigations demonstrated that the formation of intramolecular hydrogen bond between introduced hydrogen acceptor and carbamic acid was the key to avoid the dramatic increase in viscosity during the capture of CO2 by these amino-functionalized ILs.

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and conformation of molecule.37−39 However, intermolecular hydrogen bond often causes inconvenience for amino-based ILs as CO2 absorbent because their viscosity would increase dramatically upon reaction with CO2 and then transform into gel or solid-like substance, which restricts the mass transfer and limits their application in carbon capture.7,20,23,27,31,40 Maginn41 and Zhang42 used a molecular dynamics simulation method and explained that the dramatic increase in viscosity was due to the formation of strong and dense hydrogen-bonded networks between the zwitterion and dication species during CO2 captured by amino-tailored cation. To avoid the formation of strong hydrogen-bonded networks, an efficient method for improving CO2 capture was reported by non-amine-functionalized ILs,30,33,34,43−48 whose viscosity did not increase dramatically upon the uptake of CO2 because of lacking the formation of hydrogen bond. It is well-known that intramolecular hydrogen bond was another hydrogen bond mode that benefits the stability of organic molecules49,50 and biomolecule,51,52 determines the product ratios, 53 and influences physical and chemical characteristics,51,54−60 which could avoid the formation of hydrogen bonding network among IL molecular.61 Therefore, can we develop a new strategy for avoiding the dramatic increase or realizing the decrease in viscosity during CO2 capture by ILs making use of intramolecular hydrogen bond?

INTRODUCTION The development of more efficient, reversible, and economical processes for capturing CO2 from the burning of fossil fuels is critical for the reduction of the emission of greenhouse gas implicated in global warming. Currently, available technology for CO2 capture in industry is based on the chemisorption process by an aqueous alkanolamine solution1−3 with some advantages such as low cost, rapid kinetics, and high capacity. Unfortunately, this amino-based process has some intrinsic disadvantages, including high energy demand required to regenerate the liquid, solvent loss, and degradation.4−7 Ionic liquids (ILs) are promising in this regard because of their unique properties such as negligible vapor pressures, wide liquid ranges, high thermal stabilities, and virtually limitless chemical tunabilities.8−18 Taking a clue from the amino-based process, Davis and co-workers19 have reported the first example for CO2 chemisorption that employs an amino-functionalized IL. Subsequently, some other functionalized ILs, including sulfone,20 acetate,21−26 and amino acid anions,23,27−29 were reported for improving CO2 absorption. Although much effort has been made for enhancing carbon capture, including improving absorption capacity27,29−31 and decreasing energy consumption,32−34 their applications were limited because of the dramatic increase in viscosity during CO2 absorption.19,23,28,35 Here we report a novel solution to avoid the dramatic increase in viscosity for CO2 capture by designing the specific geometric construction of amino-functionalized IL through the formation of intramolecular hydrogen bond. Hydrogen bond plays an important role in some fields such as the self-assembly of chemistry,36 the synthesis of materials,36 © XXXX American Chemical Society

Received: October 28, 2015 Revised: January 23, 2016

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DOI: 10.1021/acs.jpcb.5b10553 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

was obtained from [P66614][Br] using the anion-exchange resin method, and equimolar N-acetylanthranilic acid was added to the ethanol solution of [P66614][OH]. Then the mixture was stirred at room temperature for 24 h and then applying the vacuum-rotary evaporation procedure to remove the most of ethanol. All ILs were dried under vacuum at 60 °C for 24 h to reduce possible traces of water. The structures of these ILs were confirmed by NMR and IR spectroscopy, 1H and 13C NMR spectra were recorded on a Bruker spectrometer (500 MHz) in deuterated reagent using tetramethylsilane as the standard, FTIR spectra were obtained using a Nicolet 470 FT-IR spectrometer, and no impurities were found by the NMR method. Furthermore, the water content of these ILs was determined by Karl Fisher titration, which was lower than 0.5 wt %. Absorption of CO2. In a typical absorption of CO2, CO2 of atmosphere pressure went through the calcium chloride anhydrous dryer to remove the trace of water and then bubbled through about 0.7 g of IL in a plastic container with an inner diameter of 10 mm, where the flow rate of CO2 was 60 mL min−1. The plastic container was partly immersed in a metal block at desired temperature. The amount of CO2 captured was obtained at regular intervals by an electronic balance with an accuracy of ±0.1 mg. The IR absorption of captured CO2 with variation temperature was monitored by in situ FTIR spectroscopy (Bruker Optik GmbH, Matrix-MX). The ILs to be measured were processed by bubbling CO2 at 60 °C to equilibration, and the stack plot of the in situ FTIR spectra was collected from 60 to 20 °C. Viscosity Measurement. The viscosities of all samples were measured by a Brookfield DV-II+ pro viscometer under the set temperature. In order to evaluate the effect of captured CO2 on viscosity, the viscosity of IL with quantity CO2 was also measured. Furthermore, the effect of residual H2O in samples was investigated to check the reliability of our conclusion. Computational Section. All calculations were performed using the GAUSSIAN03 programs package. For each set of calculations, we calculated geometry optimization for each free anion, the free CO2 molecule, and anion−CO2 complexes at the B3LYP/6-31G++(d, p) level. The harmonic vibrational spectra of anion−CO2 complexes were obtained based on the B3LYP/6-31G++(d, p) frequency.

In this work, we present a method for decreasing the viscosity of amino-functionalized after CO2 capture through intramolecular hydrogen bond. Herein, the essence of our strategy is introducing a hydrogen acceptor such as N or O atom into amino-functionalized anion (structures shown in Scheme 1) to participate in the formation of intramolecular Scheme 1. Structures of Amino-Functionalized ILs with N or O Atom as Hydrogen Acceptor

hydrogen bond with the H of carbamic acid produced from the reaction with CO2. Two kinds of hydrogen acceptors such as N or O atom were introduced to amino-functionalized anion to form intramolecular hydrogen bond. Furthermore, different amines with carboxylate acid at the para- and ortho-positions were selected and compared to discuss the importance of the formation of intramolecular hydrogen bond during the capture of CO2. Thus, several amino-functionalized ILs including [P66614][2NH2-NC], [P66614][4NH2-NC], [P66614][4NH2-BA], [P66614][Ac-Gly], [P66614][Me-Gly], and [P66614][Ac-PhO] were prepared and applied for CO2 capture. The results showed that the viscosity of these amino-functionalized ILs increased slightly or even decreased during CO2 capture. Spectroscopic investigations and quantum chemical calculations demonstrated that the decrease of viscosity was originated from the formation of intramolecular hydrogen bond between hydrogen acceptor and the H of carbamic acid.

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EXPERIMENTAL SECTION Materials and Synthesis. Trihexyl(tetradecyl)phosphonium bromide ([P66614][Br], 98%), 6-aminopyridine3-carboxylic acid (6NH2-NCH, 98%), and 2-aminopyridine-3carboxylic acid (2NH2-NCH, 98%) were purchased from J&K Chemical Company Limited. p-Aminobenzoic acid (4NH2BAH, 99%), sarcosine (Me-Gly, 99%), N-acetylglycine (Ac-Gly, 98%), and N-acetylanthranilic acid (Ac-PhOH, 98%) were purchased from Sinopharm Chemical Reagent Company. An anion-exchange resin [Dowex Monosphere 550A (OH)] was obtained from Dow Chemical Company. All chemicals were used as received unless otherwise stated. These aminofunctionalized IL with hydrogen acceptor were prepared by neutralization of equimolar amine containing carboxylate acid or phenol with an ethanol solution of trihexyl(tetradecyl)phosphonium hydroxide, which was obtained from trihexyl(tetradecyl)phosphonium bromide by the anion-exchange method15,62 according to the literature method.9 For example, in a typical experiment, a solution of [P66614][OH] in ethanol

RESULTS AND DISCUSSION

CO2 Capture and Viscosity Measurement. Table 1 also shows the effect of different amino-functionalized ILs on CO2 capacity. It was seen that the effect of the anion on CO2 capacity was significant. Among them, for [P66614][Ac-Gly], the molar ratio of the IL to CO2 is low due to the electronwithdrawing effect of the acetyl group.33 We prepared [P66614][Ac-PhO] simultaneously, which exhibits a high capacity of 1.2 mol of CO2 per mole of IL because of the presence of two kinds of interactions between N and O atoms in the anion with acid CO2, which was verified by NMR and IR methods. For example, it was seen in Figure S8 that two new signals in the 13C NMR spectra at 159.7 and 166.6 ppm produced after the absorption of CO2, which can be attributed to carbonate and carbamic carbon, respectively. The IR spectra showed two new peaks appeared at 1698 and 1616 cm−1, which can be assigned to two kinds of CO2 captured by the amine and oxygen atoms, respectively. Furthermore, the effect of water content on the viscosity by these ILs was also investigated. It B

DOI: 10.1021/acs.jpcb.5b10553 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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viscosities in [P66614][Ac-Gly] and [P66614][Ac-PhO] after CO2 capture reduced about 18% and 5%, respectively (Table 1). In contrast, the viscosity of [P66614][Me-Gly], which could not form intramolecular hydrogen bond after CO2 capture, increased more than 130-fold after CO2 capture under the same conditions. The significant difference of the viscosity change between methyl- and acyl-substituted amine in the process of CO2 capture is shown in Figure 2, which made this

Table 1. Effect of Different Amino-Functionalized ILs on the Viscosity and CO2 Capacity during the Capture of CO2 viscositya (cP) anion of IL [4NH2BA] [6NH2NC] [2NH2NC] [Me-Gly] [Ac-Gly] [Ac-PhO]

IL

IL-CO2

viscosity variation (n-fold)

CO2 capacityb (mol/mol IL)

18460

>992200

>53.7

0.80

10300

40130

3.9

0.78

840

895

1.1

0.56

312 1169 2690

41280 964 2565

132.5 0.82 0.95

0.90 0.49 1.20

The viscosity was measured at 30 °C by a Brookfield DV-II+ pro viscometer. bCO2 absorption was carried at 30 °C.

a

was seen in Table S1 that the effect of 2 wt % water on viscosity is weak. The effects of different amino-functionalized ILs on the viscosity variation during CO2 capture process were investigated, which are listed in Table 1. It was seen that the viscosities of [P66614][2NH2-NC] and [P66614][6NH2-NC] increased to 1.1- and 3.9-fold after CO2 capture, respectively. Clearly, compared with traditional amino-functionalized ILs, the viscosity increase during CO2 capture by [P66614][2NH2NC] is very low. However, the viscosity of [P66614][4NH2-BA], which without N atom as hydrogen acceptor, increased more than 53-fold after CO2 capture because of the formation of strong intermolecular hydrogen bonding networks.41,42 The effect of CO2 loading on viscosity was also investigated in detail. Figure 1 shows a small variation in viscosity for

Figure 2. Effect of CO2 loading on the viscosity of aminofunctionalized ILs containing an O atom as hydrogen acceptor; the viscosity was measured at 30 °C by a Brookfield DV-II+ pro viscometer. ▲, [P66614][Me-Gly]; ●, [P66614][Ac-Gly]; ▼, [P66614][AcPhO].

same point a little more visually. Obviously, there was a significant difference in the viscosity variation between methyland acyl-substituted amino acid-based ILs with an increasingly CO2 loading. The captured CO2 could be easily removed by bubbling N2 at 80 °C. Figure 3 shows five cycles for CO2 absorption and

Figure 1. Effect of CO2 loading on the viscosity of aminofunctionalized ILs containing N atom as hydrogen acceptor. The viscosity was measured at 30 °C by a Brookfield DV-II+ pro viscometer. ▲, [P66614][4NH2-BA]; ●, [P66614][6NH2-NC]; ▼, [P66614][2NH2-NC].

Figure 3. Five cycles of CO2 absorption and desorption using [P66614][Ac-PhO]. CO2 absorption was carried out at 30 °C (green solid cycle), and CO2 desorption was carried out at 80 °C via bubbling N2 (pink open cycle).

[P66614][2NH2-NC] with the increase of CO2 loading while the viscosity of [P66614][4NH2-BA] increased dramatically. It indicates that N atom as hydrogen acceptor plays an important role in the viscosity by these amino-functionalized ILs during CO2 the uptake process. In order to further indicate the importance of the presence of hydrogen acceptor on the viscosity, several other aminofunctionalized ILs with O atom as hydrogen acceptor including [P66614][Ac-Gly] and [P66614][Ac-PhO] were designed, prepared, and applied for CO2 capture. Simultaneously, we also synthesized [P66614][Me-Gly], which tethered a methyl group at the α-site of amino instead of acyl group. To our surprises, the

desorption by [P66614][Ac-PhO]. It was seen that the absorption can reach equilibrium quickly, which is different from a very slow process by traditional amino-functionalized IL.19,23 The results reveal that CO2 absorption into and release from [P66614][Ac-PhO] could be repeatedly cycled with high absorption capacity and low viscosity, indicating that CO2 capture process by [P66614][Ac-PhO] is reversible and easy to be operated. Mechanism Analysis. Clearly, these amino-functionalized ILs with hydrogen acceptor exhibit efficient, reversible, and rapid CO2 absorption, where the viscosity would reduce during CO2 capture, which is superior to traditional amino-functionC

DOI: 10.1021/acs.jpcb.5b10553 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B alized ILs (Table S2). Why do these amino-functionalized ILs with hydrogen acceptor such as N or O atom show so different behavior for CO2 capture? We believe that the difference in the viscosity variation of these ILs must originate from the difference in the structures of these amino-functionalized anion, where N or O atom as hydrogen acceptor might play an important role. Thus, the interactions between these amino-functionalized anion and CO2 were investigated by the DFT method at the B3LYP/6-31++G (p, d) level. The optimized structures shown in Figure 4 indicated the difference among these anion−CO2

Table 2. Peak of CO Stretching Vibration Calculated by Gaussian and Measured by FT-IR Spectrometer anion

[4NH2-BA]− CO2

[6NH2-NC]− CO2

[2NH2-NC]− CO2

calculation (cm−1) experiment (cm−1)

1727 1705

1727 1661

1723 1735

peak of CO stretching vibration was about 1725 ± 2 cm−1, but the experimental data of [4NH2-BA]−CO2 and [6NH2NC]−CO2 were smaller due to the formation of intermolecular hydrogen bond in these systems. It is not the case for the [2NH2-NC]−CO2 system, where an intramolecular hydrogen bond formed. Generally, the intramolecular hydrogen bond has a larger hydrogen bond energy than the intermolecular hydrogen bond; thus, the latter is affected by the temperature more obviously. For example, enhancing the temperature decreases the population of transient helical conformations of DNA.66 But breaking the intramolecular hydrogen bond needs much higher temperature or adding solvent.67 Thus, the different hydrogen bond mode was further verified by temperature-dependent FTIR spectroscopy because the IR absorption of CO which took part in intermolecular hydrogen bond would blue-shift when temperature increase due to the breakage of CO···H.68 Thus, the effect of temperature on the peak intensity of CO was investigated, which is shown in Figure 5. For [P66614]-

Figure 4. Optimized structures of the anion−CO2 complexes of amino-functionalized ILs after CO2 absorption.

complexes. It was seen that the H of −COOH in [6NH2-NC]− CO2 and [2NH2-NC]−CO2 participated in the intramolecular hydrogen bond with the N of pyridine framework in the form of six-member ring, where the hydrogen bond length is 1.62 and 1.66 Å, respectively. However, the H of −COOH in [4NH2-BA]−CO2 is free, which would form an intermolecular hydrogen bond. Therefore, the formation of intramolecular hydrogen bond between the N in pyridine ring and the H in COOH for the [2NH2-NC]−CO2 complex led to the fact that the variation in viscosity is small, while the formation of intermolecular hydrogen bond in the [4NH 2-BA]−CO 2 complex resulted in the dramatic increase of viscosity. It was specially mentioned that the viscosity of [P66614][6NH2-NC] increased more than that of [P66614][2NH2-NC], which would be caused by the formation of intermolecular hydrogen bond in the former system. As shown in Figure 4, one H of N−H in the [6NH2-NC]−CO2 complex is free, which can form an intermolecular hydrogen bond with O in another molecule. As can be seen above, the more free the active hydrogen in the IL, the viscosity increases more dramatically. Similarly, the O atom of acyl group in [P66614][Ac-Gly] could participate intramolecular hydrogen bond formation (Figure S1), leading to different viscosity variation from [P66614][Me-Gly] in the process of CO2 capture. The formation of intramolecular and intermolecular hydrogen bonds was also investigated by the IR method. It was seen in Figure S2 that a new peak at 1740 cm−1 appeared after CO2 capture by [P66614][2NH2-NC], which was attributed to the CO stretching vibration in carbamic acid. However, for [P66614][4NH2-BA], a signal at 1698 cm−1 is assigned to the CO stretching vibration (Figure S3), which exhibited at lower wavenumber due to the formation of intermolecular hydrogen bond, which is agreeable with the reported results in poly(N-isopropylacrylamide).63,64 For [P66614][6NH2-NC], two signals at 1732 and 1665 cm−1 appeared after the capture of CO2 because intramolecular and intermolecular hydrogen bonds formed, respectively (Figure S4). The different hydrogen bond mode could be also got out of the comparison of the calculated and experimental data65 (Table 2). The calculated

Figure 5. Stack plot of the in situ FTIR spectra collected from 60 to 30 °C after CO2 capture by [P66614][6NH2-NC] (left) and [P66614][2NH2-NC] (right) at 60 °C.

[6NH2-NC]−CO2, a peak appeared at about 1730 cm−1 at 60 °C because of the breakage of intermolecular hydrogen bond CO···H, which decreased along with a signal increased at about 1650 cm−1 due to the re-formation of CO···H with cooling the temperature from 60 to 20 °C. However, the absorption of CO for [P66614][2NH2-NC]−CO2 complexes appearing at about 1730 cm−1 remained unchanged during temperature variation. There were similar phenomena for Ocontaining systems; as shown in Figure 6, the absorption CO in [P6614][Ac-Gly]−CO2 appeared at about 1730 cm−1, and two absorption of CO in the [P6614][Ac-PhO]−CO2 complex appeared at 1698 and 1616 cm−1, respectively, which did not generate any real change, while the absorption of CO in [P66614][Me-Gly]−CO2 complexes red-shift from 1734 to 1653 cm−1. Thus, we could regard [P66614][2NH2-NC], [P6614][AcGly], and [P6614][Ac-PhO] as the intramolecular hydrogen bonding system. Therefore, bringing hydrogen acceptor atom into amino-functionalized IL, which helps to form the D

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Figure 6. Stack plot of the in situ FTIR spectra collected from 60 to 20 °C after CO2 capture by [P66614][Ac-Gly] (left), [P66614][Me-Gly] (middle), and [P66614][Ac-PhO] (right) at 60 °C.

intramolecular hydrogen bond, led to the decrease in viscosity during CO2 capture.

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CONCLUSIONS We have developed a new method for avoiding the dramatic increase of viscosity in the process of CO2 absorption by amino-functionalized ILs through introducing a hydrogen acceptor, which could stabilize the active hydrogen produced from the captured CO2. The viscosity exhibited a slight increase or even decrease after CO2 capture, which would be interesting in carbon sequestration by amino-functionalized IL. Quantum chemical calculations and spectroscopic investigations indicated that the formation of an intramolecular hydrogen bond instead of a strong intermolecular hydrogen bonding networks made the viscosity of amino-functionalized IL−CO2 complex come down. We believe that this strategy through intramolecular hydrogen bond can provide a potential alternative for improving CO2 capture by amino-functionalized ILs, which maybe more suitable for industrial application.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.5b10553. NMR data, Tables S1−S3; Figures S1−S9 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (C.M.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the support of the National Key Basic Research Program of China (2015CB251401), the Natural Science Foundation of China (21322602, 21176205, J1210042), the Program for Zhejiang Leading Team of S&T innovation (2011R50007), and the Fundamental Research Funds of the Central Universities.

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REFERENCES

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DOI: 10.1021/acs.jpcb.5b10553 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

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DOI: 10.1021/acs.jpcb.5b10553 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcb.5b10553 J. Phys. Chem. B XXXX, XXX, XXX−XXX