Aprotic heterocyclic anion-based dual functionalized ionic liquid

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Aprotic heterocyclic anion-based dual functionalized ionic liquid solutions for efficient CO2 uptake: Quantum chemistry calculation and experimental research Junhai Wu, Bihong Lv, Xiaomin Wu, Zuoming Zhou, and Guohua Jing ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00420 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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ACS Sustainable Chemistry & Engineering

Aprotic heterocyclic anion-based dual functionalized ionic liquid solutions for efficient CO2 uptake: Quantum chemistry calculation and experimental research

Junhai Wu, Bihong Lv*, Xiaomin Wu, Zuoming Zhou, Guohua Jing

Department of Environmental Science & Engineering, College of Chemical Engineering, Huaqiao University, Jimei Road 668, Xiamen, Fujian 361021, China *E-mail address: [email protected]

ABSTRACT A series of novel dual functionalized ionic liquids (FILs) with the combinations of diethylenetriamine cation ([DETAH]+) and aprotic heterocyclic anion (AHA) are explored for efficient and reversible CO2 capture, which was distinguished from the traditional strategy by increasing the amino group number of ionic liquids. The structural features and the interactions between ions and CO2 were performed under DFT method, at B3LYP/6-311++G** level. The CO2 absorption loadings of [DETAH][Im], [DETAH][Py] and [DETAH][Tz] were 11.91, 11.36 and 10.10 mol CO2/ kg IL, respectively. After the 5th cycle, their regeneration efficiencies still kept above 90%. Based on the calculation and 13C NMR results, the reaction mechanism of CO2 capture into [DETAH][AHA] solutions were clarified. The amino groups of [DETAH]+ reacted with CO2 to produce [DETAH]+-carbamates, which guaranteed the high absorption rate. Meanwhile, [AHA]- could also equimolar react with CO2 to form carbamate, which subsequently hydrolyze into HCO3-/CO32-. Notably, the [AHA]- was 1

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protonated during the CO2 absorption process, and [AHA]-H could react with [DETAH]+-carbamates to produce [DETAH]+, which helped to recycle the active components of the absorbent and further increased the CO2 capacity and regenerability of the absorbent. The novel dual FILs present as an efficient candidate for CO2 capture. KEYWORDS: CO2 capture; ionic liquids; functionalized; quantum chemistry calculation; mechanism

INTRODUCTION The over emission of greenhouse gases especially CO2 has caused serious environmental problems, such as earth surface temperature rising, severe climatic disturbance.1 Hence, the development of carbon capture and sequestration (CCS) technologies become one of the most concern topics for mitigation of CO2 emissions.2 For the purpose of green and sustainable development, it is quite necessary to explore a cost-effective, low energy-consumption and available technique for CO2 capture.3-5 Ionic liquids (ILs) have gained more and more attention in recent years due to their unique properties such as negligible vapor pressure, low melting point, high thermal stability and high thermal conductivity.6-8 ILs can be designed as speciﬁc structures for certain properties and particular applications taking advantage of structure-property relationships.9 Among the numerous works, amine or amino acid functionalized ionic liquids (AFILs) were the most outstanding ones, which helped to achieve excellent performance on CO2 capture.10 How to enhance the capacity of the FILs has become a focus of interest. It was proved that increase the number of

2


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functional groups, especially amine or amino acid group, could directly enhance the absorption capacity of AFILs.11 The CO2 loadings of [emim][Gly] aqueous solution was found to fluctuate near 0.5 mol CO2/mol ILs as the water content changed, and the reaction mechanism could be ascribed to zwitterion mechanism.12 It was revealed that [aP4443][Gly] solution could absorb more than 1.20 mol CO2/kg ILs under 1.5 MPa CO2 pressure.13 Several ammonium ILs based on multiamino cations and inorganic acid were prepared, and the capacity of [TETAH][NO3] was 1.49 mol CO2/ mol ILs at 40 wt % aqueous solution.14 Lv et al,15-16 synthesized dual FILs of [APmim][Gly] and [APmim][Lys], and their CO2 capacity was up to 1.23 mol CO2/mol IL and 1.80 mol CO2/mol IL, respectively, which were much higher than that of the most existing dual functionalized ILs. Although AFILs exhibit great CO2 absorption capacity, they still suffer some difficulties in application.17 Limited by the steric hindrance effect of ILs, the number of amine groups can’t be increased infinitely.18 The formation of strong, pervasive hydrogen-bonded network between CO2 and amine group of AFILs increase the viscosity of the saturated solvent19, while the carbamate products also require relatively high energy for regeneration. Different with AFILs, the azoles derived aprotic heterocyclic anion (AHA)-based ILs show tunable physicochemical properties, especially their equimolar CO2 capture and efficient regeneration performance.20 Xu21 synthesized DBNH/DBUH cations and AHAs based ILs, and revealed that CO2 could directly equimolar react with AHAs to form carbamates in CO2 capture process. It was more efficient than that of alcoholamine, which has a CO2 loading of 0.5 mol CO2/mol amine according to 3



zwitterionic mechanism. Wang et al.22 used diverse azoles to tune the stability and CO2 absorption enthalpy of AHA ILs derived from quaternary phosphonium cation. In that case the CO2 absorption capacity of [P66614][Triz] reached 1.0 mol CO2/ mol IL, which had no significant loss even after 25 times absorption/desorption cycles. Onscik et al.23 used lower-mass DMEDAH and DMAPAH cations in combination with diverse AHAs, and their CO2 capacities were more than 20 wt%. Especially, the captured CO2 could be almost completely released upon slight heating at 50 oC under N2. Although these AHA-based ILs showed good CO2 uptake performance, their low CO2 reaction rate and capacity were far from ideal. On the whole, increasing the amine group number is an efficient strategy to enhance the CO2 capacity of AFILs. However, the stable structure of carbamate products is inconducive to the regeneration. On the other side, the carbamate products of CO2 capture into AHA-based ILs are easier for desorption, while their absorption performance need to be further enhanced for application. Thus, a novel dual FILs of amine functional cation coupled with aprotic heterocyclic anion were proposed for efficient and reversible CO2 uptake. Diethylenetriamine (DETA) derived [DETAH]+ was chosen as ideal cation due to its long alkyl chain length and high number of amino groups, which were both favorable for improving the absorption capacity of ILs.24 Meanwhile, imidazole, pyrazole, triazole derived AHAs were chosen as the qualified anions for the novel ILs, which were expected to further promote the performance of the solvents, especially their CO2 regenerability. The physicochemical properties of these novel ILs ([DETAH][AHA]) and the interaction between ions and 4


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CO2 were explored by using quantum chemistry calculations under the B3LYP/6-311++G** level. Subsequently, the CO2 capture performance of these FILs were carried out, and the species of the solution during CO2 capture were analyzed by 13C

NMR. Based on the calculation and experimental results, reaction mechanism of

CO2 capture into these novel [DETAH][AHA] could be clarified.

EXPERIMENTAL SECTION Chemicals Diethylenetriamine (DETA) was purchased from Chengdu Xiya Chemical Reagent Co., Ltd., China. Imidazole, pyrazole, triazole were purchased from Shanghai Aladdin Industrial Co., Ltd China. Ethanol was purchased from Shanghai Sinopharm Chemical Regent Co., Ltd. CO2 was supplied by Fujian Nanan Chenggong Gas Co., Ltd. D2O was provided by J&K Scientific, Ltd. All chemicals were purchased in highest purity without further purification. The configuration of chemicals was presented in Table 1. Table 1. Configuration of chemicals for synthesizing ILs. Structure H N H 2N

NH2

Abbreviation

Name

DETA

Diethylenetriamine

Im

Imidazole

Py

Pyrazole

Tz

1, 2, 4-Triazole

HN N

NH N

NH N

N

5



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Synthesis of [DETAH][AHA] The novel functionalized ILs of [DETAH][Im], [DETAH][Py] and [DETAH][Tz] ILs were synthesized by directly acid-base neutralization reaction. Take [DETAH][Im], for instance, about 0.0125 mol of DETA and equimolar imidazole were firstly mixed in 30 mL of the ethanol-water solvent (4:1, V/V) in a beaker. After stirred for 24 h, the compound was transferred to a round-bottom flask and concentrated under 60 oC in rotary evaporator until even. Finally, the product was diluted with distilled water in the volumetric flask (25 mL), and its concentration reached 0.5 mol/L. CO2 absorption/desorption performance measurement The experimental apparatus used for CO2 absorption and desorption had been reported in our previous work.25 The pure CO2 gas stream with a flow rate of 60 mL/min was supplied to a bubbling absorption bottle in water bath at 40 oC, The CO2 absorption rate of the solvent was calculated from the difference value between the inlet and outlet CO2 flow rates of the reactor. The CO2 absorption capacity of the solution calculated by an integral expression of the absorption rate to the absorption time. The relevant formulas are shown as follows:

R

Qin  Qout  PactT0    22.4 1000  P0Tact 

(1)

t

L

 Rdt

(2)

0

cVM w

where R is the absorption rate of CO2 (mol/ min); Qin and Qout are the inlet and outlet flow rate of CO2 (mL CO2/ min), respectively; P0 and Pact refer to the 6


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atmosphere pressures under the standard and actual states (kPa), respectively; T0 and Tact refer to the temperatures under the standard and actual states (K), respectively; L is the CO2 absorption loading of IL solution (mol CO2/ kg IL); t is the time of the absorption (min); c is the concentration of IL solution (mol/ L); V is the total volume of IL solution (L); and Mw is the molecule weight of IL (kg/mol). After absorption, the flask filled with CO2-saturated IL solution was regenerated in an oil bath at 120 oC continued for 90 min. Besides, the CO2 residuals loading of the solution at different moment were measured by using acid hydrolysis method (Figure S1). The regeneration efficiency was calculated by following formula:



Ln 100% L0

(3)

where η is the regeneration efficiency of absorbent (%); L0 and Ln are the CO2 absorption loading of fresh absorbent and absorbent after nth regeneration (mol CO2/ kg IL), respectively. Viscosity and pH The viscosity (mPa·s) of the IL solutions was determined by using an Ubbelohde viscometer (capillary inner diameter of 0.5−0.6 mm), setting at different temperatures. The pH values of the IL solutions were measured by a pH meter (FE20) made by Mettler Toledo Co., Ltd., Shanghai, China. 13C

NMR analysis 0.2 mL of IL solutions at different CO2 loading during the absorption and

desorption process and 0.4 mL of D2O for the deuterium lock were injected into a series of 5 mm diameter NMR tubes. The samples were prepared for 7


13C

NMR


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analysis by 13C Nuclear magnetic resonance (NMR, Bruker AVIII500MHz) at 298 K. Method of quantum chemistry calculation The molecular configurations and corresponding thermodynamics parameters of ion pairs were obtained on the basis of density functional theory (DFT).26 The calculation was performed at the B3LYP/6-311++G** level using the Gaussian 09 package, including geometry optimization, natural bond orbital (NBO) analysis, interaction energy calculations. The partial charges and electrostatic potential surfaces (EPS) of ion pairs were performed by the population analysis, which were visualized in the Gauss View 09 interface. Besides, the activation barrier energies were calculated using the DMOL3 module included in the Accelrys Material Studio 7.0 software package. The interaction energy (△E), enthalpy energy change (△H), activation barrier (△Eact) were obtained by following formulas: △E = 2625.5 × [Ecation-anion - (Ecation + Eanion)]

(4)

△H/△Eact = 2625.5 × (Eproduct - Ereactant)

(5)

Where Ecation-anion, Ecation and Eanion are the energies of ion pair, isolated cation and anion, respectively; Eproduct and Ereactant are the energies of reactant and product. All the data were obtained after basis set superposition error (BSSE)-corrected and zero point energy (ZPE)-corrected, expressed in arbitrary unit (au).

RESULTS AND DISCUSSION Optimization of the AHA-based dual FILs Theoretically, DETA is a strong base, while azoles are aprotic nitrogen heterocycles. There were two primary amines and one secondary amine on DETA, 8


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and each of them have possibility to react with azoles through proton transfer by acid-base neutralization. The enthalpy changes (△H) of the acid-base neutralization reaction between DETA and diverse azoles were calculated (Table S1). The △H values of these reactions were all less than -20 kJ·mol-1, which indicated that the reactions were spontaneous and provided the theoretical foundation for the directly synthesis of FILs. Similar results were also found in Oncsik’s work23, they found that the azoles totally deprotonated after acid-base neutralization, which indicated the proton was transferred to the amines. For getting insight into the impact of various AHAs to ion pairs of FILs, the optimized geometries, partial NBO charges in nitrogen atoms and electrostatic potential surfaces (EPS) of ion pairs of the novel FILs were calculated. Took [DETAH][Im] for example, the results are shown in Figure 1. When Im reacted with the primary amine group of DETA, the partial charge on the nitrogen atoms of the protonated primary amine group, unprotonated primary and secondary amine group on [DETAH]+ were -0.856 e, -0.839 e and -0.680 e, respectively (Figure 1a). When Im reacted with the secondary amine group of DETA, the partial charge on the nitrogen atom of the protonated secondary amine group and others two unprotonated primary amine groups were -0.700 e, -0.844 e and -0.844 e, respectively (Figure 1b). However, the partial charge of the deprotonated nitrogen active site on [Im]- under this two conditions were only -0.566 e and -0.571 e, respectively. A higher value of NBO charges meant a higher electronegativity of nitrogen atom, which indicated a stronger interaction between CO2 and the active nitrogen sites.27 Meanwhile, the EPS maps of 9



the ion pairs also showed that the negative charge mainly concentrated on nitrogen atoms as indicated by red color (Figure 1c and Figure 1d). Nevertheless, the NBO charges on the interaction N sites of imidazole and DETA became more negative when the proton transferred (Figure S2). Besides, the charge density of protonated amine group on DETA dropped as indicated by blue color. Those were all the evidences of the charge transfer between cation and anion, and the protonated amine group couldn’t interact with CO2 any more. Based on the above results, it could be deduced that the amine groups especially primary amine groups on [DETAH]+ still dominated the CO2 absorption, and [Im]- played an inferior role. The similar results were observed in [DETAH][Py] and [DETAH][Tz] pairs as well (Figure S3 and Figure S4).

10


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Figure 1. The optimized structures of [DETAH]P[Im] (a), [DETAH]s[Im] (b) and representation of partial charges of nitrogen from the NBO analysis, Electrostatic potential map for [DETAH]P[Im] (c), [DETAH]s[Im] (d). Gray, blue and white represent carbon, nitrogen, and hydrogen, respectively. Scale: -0.08-0.08; isovalue: 0.001 The ion pair configurations have influence on the basicity and viscosity of [DETAH][AHA], which would further affect their CO2 uptake performance.28 Hence, the cation-anion bond length in the interaction N site and the interaction energy of cation-anion (△E) were calculated, and the related data is listed in Table 2. Table 2. Hydrogen bond length and interaction energy of corresponding FILs. [DETAH][Im] bond

[DETAH][Py]

[DETAH][Tz]

standard primary

secondary

primary

secondary

primary

secondary

1.999

1.985

1.951

1.930

1.911

1.901

Azole N···H

1.026

1.030

1.028

1.034

1.034

1.038

△E (kJ·mol-1)

-567.932

-532.488

-568.176

-546.172

-516.299

N···H

[DETAH][Py] > [DETAH][Tz], which was opposite to the hydrogen bond length of N···H. Since there was a positive correlation between the N-H chemical bond lengths and the basicity of FIL solutions,29 The basicity of these three [DETAH][AHA] was forecasted follow the order of [DETAH][Im] > [DETAH][Py] > [DETAH][Tz]. The strong interaction between IL pairs might lead to a high structure stability of ILs. Thus, the thermal stability of [DETAH][AHA] might be adverse to the negative value of the interaction energy between cation and anion, which follow the order of [DETAH][Py] < [DETAH][Im] < [DETAH][Tz]. The interaction energies (△E) of these [DETAH][AHA] are also shown in Table 2. It could found that the negative of △E decreased along the anion series of [Py]-, [Im]-, [Tz]- for the same cation. The more negative the interaction energy, the stronger the interaction between cation and anion.30 Thus, it would be easy to form a coupled structure, which could efficiently reduce the interaction between IL pairs, resulting in a low viscosity. Therefore, it could be speculated that the viscosity of these three [DETAH][AHA] under the same conditions should follow the order of [DETAH][Py] < [DETAH][Im] < [DETAH][Tz]. Characteristics of the cation/anion-CO2 interactions The interactions between the cation and anion with CO2 were also of great importance, for which the functional groups on the cation and anion both could react with CO2. Firstly, the interactions of anion-CO2 pairs were calculated, and the results are shown in Figure 2. It was seen that the whole atoms on their optimized structures 12


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were coplanar. The CO2 angle of anion-CO2 pairs were all nearly 130o and followed the order of [Tz]- > [Py]- > [Im]-, which were different with the linear angle of CO2. The covalent bond length of N-C on [AHA]--CO2 followed the order of [Tz]- > [Py]- > [Im]-. Generally, the C-N bond length as well the CO2 angle of [AHA]--CO2 were the criteria for evaluating the molecular interaction intensity. The shorter N-C bond length and the smaller angle of ∠O=C=O, the stronger interaction between CO2 and [AHA]-.31 The interaction energy of these three [AHA]--CO2 also confirmed this conclusion. It could find that [Im]--CO2 has the most negative value of interaction energy (-75.98 kJ·mol-1), indicating the strongest molecular interaction between [Im]and CO2.

Figure 2. The related thermodynamic parameters and interaction energies of the optimized geometries of the anion-CO2 pairs. Gray, blue, white and red represent carbon, nitrogen, hydrogen and oxygen, respectively. ((a) [Im]--CO2 pair, (b) [Py]--CO2 pair, and (c) [Tz]--CO2 pair) It had been reported that CO2 could react with [AHA]- to form carbamates directly in the pure FILs22, and the reaction was given as follows: [AHA]- + CO2 ⇌ [AHA]-COO-

(6) 13



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In this work, these three [DETAH][AHA] FILs were dissolved into aqueous solution to form the novel solvent for CO2 capture. Different from the pure solution, [AHA]- is easy to hydrolyze in the water, which would promote the CO2 absorption. The reaction could be expressed as follows: [AHA]- + H2O ⇌ [AHA]-H + OHOH- + CO2 ⇌ HCO3-

(7) (8)

To figure out how the AHAs reacted with CO2, the enthalpy changes (ΔH) of CO2 capture into these three [AHA]- though the above two reaction pathways were calculated, and the results are shown in Figure 3.

Figure 3. B3LYP/6-311++G** comparison of anion participating reactions of CO2 with [Im]- (black), [Py]- (red), and [Tz]-(blue). Enthalpy in kJ·mol-1. The anion with zero energy was the beginning point. It could be found that the ΔH values of the reactions to produce carbamate were negative, which meant an exothermic reaction. Nevertheless, the ΔH values of [AHA]- hydrolysis reaction were positive while that of CO2 hydrolysis reaction were negative, which meant that CO2 14


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reacted with [AHA]- according to the hydrolysis pathway had to overcome a certain barrier. Finally, the ΔH values of the former reaction pathway were lower than that of the latter pathway, which indicated that the formation of carbamate products is a more active process. Besides, the ΔH values of CO2 capture into these three [AHA]- were similar with each other and followed the order of [Im]- > [Py]- > [Tz]-. Since the [AHA]- exhibited equimolar CO2 capture, the novel FILs were expected to own a higher CO2 loading than that of traditional amine, whose capacity was about 0.5 mol CO2/ mol amine according to zwitterion mechanism. In our previous work24, the calculation of CO2 capture into amine FILs had been investigated. Herein, the interactions between [DETAH]+ and CO2 just only briefly explored in the present work (Figure S5). Based on the zwitterion mechanism, the ΔH values of CO2 reacted with the primary (p) amine and secondary (s) amine on [DETAH]+ was given as follows: RP-NH2 + CO2 ⇌ RP-NH2+COO- ;△H= -5.86 kJ·mol-1

(9)

RP-NH2+COO- + DETA ⇌ RP-NHCOO- + DETAH+ ;△H= -74.46 kJ·mol-1 (10) RS-NH + CO2 ⇌ RP-NH+COO- ;△H= -3.73 kJ·mol-1

(11)

RS-NH+COO- + DETA ⇌ RS-NCOO- + DETAH+;△H= -72.90 kJ·mol-1

(12)

It could be found that the carbamate formation reactions were all exothermic, and the △H value of zwitterion was significantly lower than that of carbamate products, indicating that the zwitterion was a transitory species during the absorption. The total ΔH values of primary and secondary amines on [DETAH]+ reacted with CO2 were -80.32 and -76.63 kJ·mol-1, respectively. A higher negative value of ΔH, a stronger 15



interaction between amine and CO2.32 It seemed that the primary amino group on [DATAH]+ would have prior and faster reaction with CO2 compared with the secondary amino group on [DATAH]+. Based on the above results, there were [DETAH]+ derived carbamates (RNCOO-), [AHA]- derived carbamates and carbonates presenting in the CO2 saturated solution of [DETAH][AHA] solution. The desorption of the latter two products were easier than RNCOO-. Thus, RNCOO- were the main products of the system, the regenerabiliy of the [DETAH][AHA] was depended on the decomposition of RNCOO- products. The activation barrier of the decomposition of the RNCOOproducts using different proton donors were explored as shown in Figure 4. 250

TS

245.88 202.59

200

191.31 Free energy, kJ/mol

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150

145.91

100

-

RNH+[Im] +CO2 -

RNH+[Py] +CO2

50 0

-

-

RNH+[Tz] +CO2

RNCOO +AiH

-50 RNH+DETA+CO2 -100

Reaction coordinate

Figure 4. Activation barriers of the regeneration reaction (AiH represents different proton donors including DETAH+, Im, Py, and Tz). During the desorption, all the species of [DETAH]+, Im, Py and Tz could react with RNCOO- products theoretically. It was seen that the activation barrier energies of the desorption reactions were 245.88, 202.59, 191.31 and 145.91 kJ/mol, respectively, 16


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which decreased along the series of [DETAH]+, Im, Py and Tz. The higher activation barrier, the harder of the CO2 desorption reaction to carry out.24 The energies significantly reduced when azoles participated in the reaction compared with that of [DETAH]+. The results indicated that Im, Py and Tz in the solution could promote the decomposition of RNCOO- products, which was favorable for improving the regenerability of the solution. Thus, the regeneration efficiencies of these [DETAH][AHA] solutions were predicted to follow the order of [DETAH][Tz] > [DETAH][Py] > [DETAH][Im]. Similarly, Im, Py and Tz in the solution could also promote the decomposition of RNCOO- products in the absorption process, which could help to regenerate the main absorbent of [DETAH]+ and further enhance the absorption loading of the solution. Altogether, these dual FILs ([DETAH][AHA]) were expected to be efficient and reversible for CO2 capture. Among them, the two amine groups on [DETAH]+ guarantee the high absorption rate of the solvent, while [AHA]- promote the absorption loading and regenerability of the solution. CO2 capture into AHA-based dual FILs solutions Subsequently,

three

different

AHA-based

dual

FILs

[DETAH][Im],

[DETAH][Py] and [DETAH][Tz] are synthesized for CO2 capture, and the results are shown in Table 3.

17



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Table 3. CO2 absorption loadings of the studied FILs compared with related literature data. ILs

T

Mw

oC

CO2 absorption capacity mol CO2/ mol IL

wt %

mol CO2/ kg IL

Concentration

Ref.

[DETAH][Tz]

40

172.23

1.74

44.5

10.10

0.5 mol/L

This work

[DETAH][Py]

40

171.25

1.95

50.1

11.39

0.5 mol/L

This work

[DETAH][Im]

40

171.25

2.04

52.4

11.91

0.5 mol/L

This work

[DETAH][Gly]

40

178.24

1.81

44.7

10.15

0.5 mol/L

This work

[DETAH][Lys]

40

249.36

2.13

37.6

8.54

0.5 mol/L

24

[apmIm][Lys]

50

257.36

1.80

26.7

6.71

0.5 mol/L

16

[aemIm][Br]

30

191.07

0.33

-

-

0.1 mol/L

33

[DMAPAH][Im]

22

170.26

0.75

21.0

4.41

Pure

23

[DMAPAH][Py]

22

170.26

0.79

22.6

4.64

Pure

23

[DMAPAH][Tz]

22

171.25

0.70

18.3

4.09

Pure

23

There were two amine functional groups left in [DETAH] cation of the ILs, which theoretically could absorb equimolar CO2 in aqueous solution based on the zwitterions mechanism. The CO2 loadings of [DETAH][AHA] solutions were found to be 1.74, 1.95 and 2.04 mol CO2/ mol IL, respectively. The results indicated that the AHAs could enhance the absorption of CO2 and contributed the extra CO2 absorption loading. Meanwhile, the pH values of 0.5 mol/L [DETAH][Im], [DETAH][Py] and [DETAH][Tz] aqueous solutions at 25 oC were 11.34, 11.24 and 10.02, respectively. The basicity of FIL was also coincident with the predicted results shown in the 18


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calculation. Since a higher basicity of FIL solutions meant a higher CO2 affinity, the capacity of these [DETAH][AHA] followed the order of [DETAH][Im] > [DETAH][Py] > [DETAH][Tz]. Besides, their absorption rate also present the similar tendency (Figure S6). For the sake of contrast, the CO2 loadings of other related literature data were listed in this table as well. The loading per unit mol of these [DETAH][AHA] were much higher than that of AHA-based ILs because of a higher number of amine functional groups in [DETAH]+ than that of [DMAPAH]+. Considering industrial application, the viscosity of an absorbent is another important factor besides their CO2 uptake performance.34 Herein, the viscosities of these [DETAH][AHA] solutions were measured at different temperatures, and the results are shown in Figure 5. As it depicted that the viscosities of the [DETAH][AHA] solutions all decreased as the temperature increased. The high temperature causes the molecules to move more rapidly, thereby leads to a lower interaction between ion pairs and reduces the viscosity of IL solutions. The viscosities followed the order of [DETAH][Tz] > [DETAH][Im] > [DETAH][Py], which also agreed with the quantum chemistry calculation. It could be found that [DETAH][Tz] had the highest viscosity, because there were three nitrogen atoms in [Tz]- leading to a more intensive N-H···N hydrogen-bonding network.19

19



o

30 C 1.0

viscosity, mPa·s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.8

o

o

50 C

60 C 1.024

0.822

0.797

0.772

0.667

0.657

0.636

0.6

o

40 C 1.015

0.964

Page 20 of 40

0.549

0.543

0.531

0.4 0.2 0.0

[DETAH][Py]

[DETAH][Im]

[DETAH][Tz]

Figure 5. Viscosities of [DETAH][Im], [DETAH][Py], [DETAH][Tz] solution at different temperatures. (CILs: 0.5 mol/L) Take the [DETAH][Im] solution for example, the performance of CO2 capture was investigated as CO2 absorption process was carried out. The variation tendency of CO2 loadings, pH values and temperature are shown in Figure 6. The absorption rate significantly reduced as the reaction time increased until closed to zero, and the absorbent equilibrated within 60 min. The pH value of the [DETAH][Im] solution decreased from 11.39 to 7.20 when the acid gas of CO2 was continuously pass through to the absorbent. Moreover, it could be seen that the temperature of the solvent significantly increased from 23.7 oC to 33.3 oC at the initial stage, which was consistent with the calculations that the carbamate formation reaction was an exothermic process. Then, it gradually decreased to 26.5 oC at the later stage of the reaction, which was ascribed to an endothermic process. The experimental results revealed that the characteristic of CO2 capture into the AHA-based FIL solution was similar to that of the traditional aqueous amine solutions during CO2 absorption 20


Page 21 of 40

process.35 Exothermic

12

Endothermic

11

10.0

10

32 30

pH

o

7.5

34

Temperature, C

12.5

CO2 loading, mol CO2/ kg IL

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9

28

2.5

8

26

0.0

7

5.0

0

10

20

30 Time, min

40

50

24

60

Figure 6. Temperature and pH value changes of [DETAH][Im] during CO2 absorption. CO2 desorption from the saturated AHA-based dual FILs solutions As mentioned above, many works were carried out to increase the number of the amine functional groups to enhance the CO2 uptake performance of FILs, while an AHA-based dual functional FIL was proposed in this work. For the sake of contrast, some efficient amino acid-based FILs, such as [DETAH][Gly] and [DETAH][Lys] were also synthesized. The absorption and desorption comparison results are given in Figure 7. It was shown that pure [DETAH][Gly] and [DETAH][Lys] were in viscous gel state, while [DETAH][Im] still kept in proper liquid state at room temperature, which indicated that the viscosity of pure [DETAH][Im] was much lower than that of [DETAH][AA]. It was because the carboxyl group on amino acid containing extra O-atom led to strong hydrogen-bonding networks.23 The similar results were also observed in the other two [DETAH][AHA]. The results proved that the [AHA]- would 21



help to reduce the viscosity of the FILs.

Figure 7. Absorption/desorption performance of CO2 capture into [DETAH]+ based FILs. (Absorption: T: 40 oC; t: 60 min; desorption: T: 120 oC; t: 90 min) From the CO2 absorption/desorption performance, it was seen that the captured CO2 of these three FILs could be almost completely released under 120 oC continuing for 90 min. More remarkable, the CO2 molar absorption loadings of [DETAH][Im], [DETAH][Gly] and [DETAH][Lys] solutions reached 2.04, 1.81 and 2.13 mol CO2/ mol IL, respectively (Table 3). Since they owned the same cation, the molar absorption loading of [Im]- seemed to be higher than that of [Gly]- while close to that of [Lys]-. The results indicated that [Im]- could interact with more CO2 than single amine group and even approach to equimolar CO2 uptake mechanism. Due to its lower molecular weight, the CO2 mass-uptake loading of [DETAH][Im] (11.91 mol CO2/ kg IL) was much higher than that of [DETAH][Gly] and [DETAH][Lys] even the latter two owned a higher number of amine functional groups. Additionally, the reusability is an essential criterion for judging the potential use 22


Page 22 of 40

Page 23 of 40

of CO2 absorbent. Besides, improving thermal stability of the ILs helps them achieve good reversibility during CO2 uptake.36 From the TGA curves of these FILs (Figure S7), it can be seen that the AHAs can efficiently improve the thermal stability of chain alkyl amine DETA. Especially, [DETAH][Tz] presented the highest thermal stability among these FILs, which indicated the best regenerability. The five times absorption-desorption cycles of these [DETAH][AHA] are shown in Figure 8. 12 10

CO2 loading, mol CO2/kg IL

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8 6 4 2 0

[DETAH][Im]

[DETAH][Py]

[DETAH][Tz]

Figure 8. Absorption capacity of different [DETAH][AHA] solutions after cyclic regeneration. (Absorption: T: 40 oC; t: 60 min; desorption: T: 120 oC; t: 90 min) The

regeneration

efficiencies

of

[DETAH][Im],

[DETAH][Py],

and

[DETAH][Tz] after one regeneration cycle were 93.66%, 94.62%, and 97.93%, respectively, which was highly consistent with the calculation of the activation barrier energies. The FIL solutions changed from alkalinity to neutralization after saturation, and returned to alkalinity after regeneration. Thus, only slight difference existed between the pH values of fresh and 1st regenerated. Take [DETAH][Tz] as an example, pH value of the fresh, saturated and 1st regenerated one were 10.02, 7.01 23



and 9.95, respectively. As the regeneration cycle increased, the regeneration efficiencies of these solvents were all slightly reduced. After 5th cycle, their regeneration efficiencies still kept 91.66%, 91.54%, and 96.11%, respectively. The results indicated that all the AHA-based FILs present stable reusability, and the regeneration stability of triazole-based FILs were higher than that of diazole-based ILs. 13C

spectra of CO2 capture into [DETAH][Im] solutions In order to understand the mechanism of CO2 capture into absorbent, 13C NMR

was performed. The molecular structures and the type of carbon nuclei in these FILs-CO2 systems are shown in Scheme 1.

Scheme 1. Molecular structures and type of carbon nuclei in these FILs-CO2 systems. There were two amine groups left in the cation of [DETAH][AHA] theoretically, which would impact the analysis of CO2 reaction with [AHA]-. To eliminate the disturbance of amines on [DETAH]+, an AHA-based FILs of [N1111][Im] that did not own amine functional group was characterized (Figure S8). It is founded that [Im]could react with CO2 to produce carbamate ([Im]-COO-), however, [Im]-COO- could completely hydrolyze to form HCO3-/CO32- and further enhance the hydrolysis of CO2. For the purpose of identifying the role of azoles played in the process of CO2 24


Page 24 of 40

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capture into FIL solutions, another

13C

NMR analysis was performed. CO2 was

continuously supplied to the DETA solution, and the absorption was shut up when only carbamates but no carbonates produced in the solution. The solution was divided into two parts, and one was evenly blended with Im. These two solutions were used for

13C

NMR analysis, and the results are compared in Figure 9. It was easily

observed that only carbamate products at δ=164.63 and δ=164.13 ppm appeared in DETA solution (9.30 mol CO2/ kg DETA). The peak at δ=161.03 ppm corresponding to the formation of HCO3-/CO32- appeared after adding Im. The results indicated that part of carbamates could decompose into carbonates by the promotion of azoles, which not only facilitate the decomposition of [DETAH]+ derived carbamate (RNCOO-), but also set [DETAH]+ (RNH) molecules free for interacting with more CO2 in aqueous solution.

Figure 9. 13C NMR spectra of DETA aqueous solution before (a) and after (b) adding 25



Im. (CDETA=0.5 mol/L, CIm=0.5 mol/L, La: absorption loading (mol CO2/ kg DETA)). Furthermore, [DETAH][Im] was taken as the example of [DETAH][AHA] for revealing the detailed mechanism. The

13C

NMR spectra of [DETAH][Im] solution

with different CO2 loadings during absorption and regeneration process were shown in Figure 10. In the fresh solvent, the peaks at δ=136.08 ppm and δ=121.79 ppm referred to C1 and C2, C3 in [Im]-. The signals at δ=39.81 ppm and δ=50.43 ppm were ascribed to the chemical equivalent C5, C8, and C6, C7 in [DETAH]+, respectively (Figure 10a). When CO2 loading of the solvent increased from 0 to 2.37 mol CO2/kg ILs, there was new slight signals appeared at δ=164.63 ppm, which corresponded to the carboxyl carbon of primary carbamate products from [DETAH]+ and [AHA]-. At a higher CO2 loading (5.04 mol CO2/kg ILs), beside the primary carbamate signal (δ=164.63 ppm), a new signal appeared at 164.13 ppm, which was assigned to the secondary carbamate products from [DETAH]+. As the absorption carried out, the CO2 loading further increased to 8.59 mol CO2/kg ILs, the peak at δ=164.63 ppm decreased while that at δ=164.13 ppm increased. Meanwhile, a new signal at δ=160.67 ppm was assigned to HCO3-/CO32- species. At this point, the temperature of the solution reached the highest level during the absorption (Figure 6). Continually, the intensity of the peak at δ=164.63 ppm decreased, which indicated the decomposition of carbamates. Interestingly, some signals corresponding to carbamates were shifted as a result of the presence of diverse protonated species.37 In the saturated solution (11.91 mol CO2/kg ILs), the signals of primary and secondary carbamate and HCO3-/CO32- all existed in this spectrum. Meanwhile, as CO2 26


Page 26 of 40

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absorption process carried out, the signal of C1 gradually shifted from δ=136.08 ppm to δ=135.46 ppm while the signals of C2 and C3 gradually shifted from δ=121.79 ppm to δ=121.12 ppm, which gave the evidence of [Im]- protonation. Besides, the singlets of C5-8 changed into some multiplets due to the influence of the carbamate products on the chemical shifts of carbon atom in [DETAH]+. Since the -COO- of carbamate was electron withdrawing group, the apparent electronic deshielding effect increased the density of surrounding electron clouds of carbon atom resulting in the decrease of their chemical shifts.38 The results indicated that the primary amine group of the [DEATH]+ was firstly reacted with CO2 to form carbamate, and the secondary amine group was followed during the first stage of the absorption. In the second stage, the carbamate products began to hydrolyze and the CO2 hydrolysis reaction enhanced, and these reactions were all ascribed to endothermic reactions.25 Furthermore, similar spectra were also presented in [DETAH][Py] and [DETAH][Tz] solutions (Figure S9 and Figure S10).

27



Figure 10. 13C NMR spectra of [DETAH][Im] solution during absorption (a) and desorption (b) process at different CO2 loadings. (La: absorption loading, Lb desorption loading (mol CO2/ kg IL)) Later, the saturated solution was recovered at 120 oC and atmospheric pressure. As the regeneration reaction went on, pH of the solution increased and the gas of CO2 gradually desorbed from the solution. In the spectrum of [DETAH][Im] solution 28


Page 28 of 40

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during the desorption (Figure 10b), the signals presented a reversible change with that of the absorption process. When the loading of the solution reduced from 11.91 to 3.73 mol CO2/kg ILs, the signal of HCO3-/CO32- at δ=160.26 ppm and the signal of secondary carbamate at δ=164.12 ppm were both significantly reduced until they disappeared. During the whole desorption process, the signal (δ=164.62 ppm) of primary carbamates was decreased gradually. Finally, the signals of C1-3 and C5-8 slightly shifted back to their original position, and all signals of CO2 products disappeared in the regional spectrum (δ=160-167 ppm), indicating that CO2 had been thoroughly desorbed from the solution and the absorbent almost regenerated. The results proved that the products of HCO3-/CO32- by CO2 hydrolysis were faster and easier to be decomposed than that of carbamates by [DETAH]+. Mechanism of CO2 capture into AHA-based dual FILs solutions The mechanism of CO2 capture into amine-based or AHA-based ILs had been clarified in the reported works.22,

39-41

However, the dual functionalized ILs in the

present work were completely different and more complex. Based on the quantum chemistry calculation and experimental results, the interaction of the two functional groups and the mechanism of CO2 capture into [DETAH][AHA] could be clarified. As above mentioned, there were two amine functional groups and one azolate group in [DETAH][AHA], which all could chemically react with CO2. However, the NBO charge distribution and experimental results proved that CO2 was prior to react with the amine groups on [DETAH]+ (RNH). Based on the zwitterion mechanism, the reaction was shown as follows: 29



2RNH + CO2 ⇌ RNCOO- + RNH2+

Page 30 of 40

(13)

It was proved that the primary amine group of [DETAH]+ was firstly reacted with CO2 while the secondary amine group of [DETAH]+ was subsequently consumed. Meanwhile, CO2 also could directly react with [AHA]- to form carbamate, and this equimolar reaction helped to enhance the CO2 absorption loading of the solvent, as presented in Eq. (6). These carbamate formation reactions were both exothermic processes, thus, the temperature of the solvent increased during this stage. As the reaction carried on, the concentration of [DETAH][AHA] decreased while the CO2 loading of the solvent increased. Several reactions proceeded simultaneously, including the formation of the secondary carbamates products, the carbamates hydrolyzing, and CO2 hydrolysis to form HCO3-/CO32-. Among them, the hydrolysis of carbamates and CO2 were endothermic reactions, therefore, the temperature of the solution decreased in this process. The hydrolysis reactions of the species were expressed as follows: [AHA]- + H2O + CO2 ⇌ [AHA]-H + HCO3RNCOO- + H2O ⇌ RNH2+ + CO32-

(14) (15)

Among them, the [AHA]-H could react with [DETAH]+ derived carbamate (RNCOO-) and set amino group (RNH) free, which helped to recycle the main components of the absorbent and further increased the CO2 loading of the solution. The reaction was expressed as follows: RNCOO- + [AHA]-H + OH- ⇌ RNH + [AHA]- + HCO3-

30


(16)

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After that, the saturated solution was regenerated under 120 oC for 90 min. The desorption presented a reversible change with that of the absorption process. As above mentioned,

the

regenerability

of

[DETAH][AHA]

decomposition of RNCOO-. Based on the

13C

was

depended

on

the

NMR spectra, it could be found that

some of HCO3-/CO32- were heated to release CO2, while others were reacted with RNH2+ to form RNCOO-. Finally, the RNCOO- was decomposed to RNH and CO2 under thermolysis, and the solvent was regenerated. CO32- + 2H+ ⇌ H2O + CO2 (g)

(17)

HCO3- + H+ ⇌ H2O + CO2 (g)

(18)

2HCO3- + RNH2+ ⇌ RNCOO- + 2H2O + CO2 (g)

(19)

RNCOO- + H+ ⇌ RNH + CO2 (g)

(20)

Similar with the absorption process, the species of AHA in the solution could also react with RNCOO- to form RNH, which helped to recycle the main components of the absorbent and enhanced the regeneration of the solution. RNCOO- + [AHA]-H ⇌ RNH + [AHA]- + CO2 (g)

(21)

Take [DETAH][Im] for example, the detailed mechanism of CO2 capture was shown in Scheme 2. During the CO2 absorption process, [DETAH]+ reacted with CO2 to produce RNCOO-, which guaranteed the high absorption rate and CO2 loading of the absorbent because of its adequate amine functional groups. The [AHA]- could equimolar react with CO2 and finally produced [AHA]-H, which could enhance the CO2 absorption loading and promote the regeneration of RNCOO- in the solution. Generally, [DETAH][AHA] present as an effective absorbent for postcombustion CO2 31



capture.

Scheme 2. The CO2 absorption/desorption mechanism of [DETAH][Im] solution.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs. Experimental apparatus for CO2 residuals measurement using acid hydrolysis method; Electrostatic potential surface (EPS) maps of raw materials and the optimized ion pairs; Optimized configuration for products of [DETAH]+ interacting with CO2; TGA curves of AHA-based FILs and DETA; and full 13C NMR spectra of related FIL solutions before and after capturing CO2.

AUTHOR INFORMATION Corresponding Author 32


Page 32 of 40

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*Tel.: +86 86-592-6166216; Fax: +86-592-6162300; E-mail: [email protected] Postal address Department of Environmental Science & Engineering, Huaqiao University, Xiamen, Fujian, 361021, China. ORCID Bihong Lv: 0000-0003-1794-1109 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was sponsored by the National Natural Science Foundation of China (21808074 and 21876053), supported by Promotion Program for Young and Middle-aged Teacher in Science and Technology Research of Huaqiao University. This work was sponsored by the Natural Science Foundation of Fujian Province (2016J05038). We also thank the Instrumental Analysis Center of Huaqiao University for analysis support and the subsidized project for cultivating postgraduates innovative ability in scientific research of Huaqiao University.

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CO2 Gas phase Liquid phase

H+

R-NH

[Im]-

H+

+CO2

+H2O

Im participation +CO2

Im

R-NCOO-

HCO3-/CO32-

R-NH2+ H+

[Im-COO]-

Desorption

HCO3-/CO32-

Absorption

Synopsis This paper simulated and experimentally researched the CO2 uptake performance of aprotic heterocyclic anion (AHA)-based dual FILs. 40


Aprotic heterocyclic anion-based dual functionalized ionic liquid

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