Limited Number of Active Sites Strategy for Improving SO2 Capture by

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Limited number of active sites strategy for improving SO2 capture by ionic liquids with fluorinated acetylacetonate anio Guokai Cui, Ning Zhao, Yanan Li, Huiyong Wang, Yuling Zhao, Zhiyong Li, and Jianji Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b01551 • Publication Date (Web): 16 Jul 2017 Downloaded from http://pubs.acs.org on July 20, 2017

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ACS Sustainable Chemistry & Engineering

Limited number of active sites strategy for improving SO2 capture by ionic liquids with fluorinated acetylacetonate anion Guokai Cui,* Ning Zhao, Yanan Li, Huiyong Wang, Yuling Zhao, Zhiyong Li, and Jianji Wang* Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, China. KEYWORDS: Active site; ionic liquid; SO2; acetylacetone; C···S interaction CORRESPONDING AUTHORS: [email protected], [email protected]

ABSTRACT: SO2 capture is highly important because this acid gas can react with moisture in the atmosphere to produce acid rain, a kind of atmospheric pollution. In this contribution, we show how SO2 can be efficiently absorbed by a new class of fluorinated acetylacetonate ionic liquids (FILs) with limited number of active sites in the anions. The absorption of SO2 by these functionalized FILs is investigated under different partial pressures and temperatures, and a high SO2 capacity up to 4.27 and 1.82 mole SO2 per mole IL can be achieved under 1 and 0.1 bar, respectively, compared with 1.43 and 0.24 mole SO2 per mole [TFSI]-based FIL ([TFSI] = bis(trifluoromethylsulfonyl)imide anion). From a combined study of quantum chemical

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calculations, FT-IR and NMR analysis, it is found that the high SO2 absorption capacities by fluorinated acetylacetonate task-specific FILs can be ascribed to the multiple-site interactions between SO2 and limited number of active sites in the anions. Furthermore, the FILs can be easily regenerated and the SO2 absorption process could be recycled.

■ INTRODUCTION

From burning of fossil fuels, sulfur dioxide (SO2) is released into atmosphere and can thereby react with moisture to produce acid rain-a kind of atmospheric pollution. Thus, SO2 capture is highly important, and many kinds of flue gas desulfurization (FGD) techniques have been developed to remove this toxic gas. Actually, ammonia scrubbing and limestone scrubbing have been developed and used as conventional FGD removal techniques. However, the disadvantages such as the production of a large amount of wastewater and low-grade byproducts during these processes should not be ignored.1-4 Ionic liquids (ILs), a kind of liquid materials with a lot of outstanding properties like negligible vapor pressure, wide liquid temperature range, non-flammability, high thermal stability and virtually limitless chemical tenability, have been found in such fields as separation science, energy production, chemical synthesis, and gas capture.5-12 It is reported that acid gases like SO2, CO2 and H2S have a high solubility in conventional ILs, including fluorinated ionic liquids

(FILs)

with

hexafluorophosphate

([PF6]),

(tetrafluoroborate)

([BF4])

and

bis(trifluoromethylsulfonyl)imide ([TFSI]) anions through physical interaction. For the first time, Brennecke

et

al.13

investigated

CO2

dissolution

in

1-butyl-3-methylimidazolium

hexafluorophosphate ([Bmim][PF6]), and they found that CO2 dissolves in the IL while the IL does not dissolve in CO2. Maginn and Brennecke et al.14 determined the solubility of SO2 in 2 ACS Paragon Plus Environment

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[TFSI]-based FILs. Riisager et al.15 showed that conventional FILs can reversibly absorb only 1.33-1.60 mole SO2 per mole IL under ambient conditions through physical interaction. Jalili et al.16-17 measured H2S solubility in kinds of imidazolium-based FILs containing [PF6], [BF4] or [TFSI] anion. The fluorine atoms in these fluorinated anions disperse the negative charges on the anions due to their strong electron-withdrawing property, resulting in the weak interaction between these anions and acid gases. It is well-known that active sites in functionalized ILs play a key role in efficient gas capture, and it is necessary to active the potential sites for improving gas capture.18 In recent years, lots of ILs with various functional groups have been developed to capture SO219-20 and CO2,21-23 including ILs based on amino cations,24-28 ether-containing cations,29-32 aprotic heterocyclic anions,33-37 cyano-containing anions,38-39 carboxylate anions,40-42 and phenolate anions.43-44 The application of these functionalized ILs opens a new way to achieving high gas capture capacity and rapid absorption kinetics through the reactions between the acid gas and active sites in ions. The active sites in these ILs are mainly focused on the electron-negative O and N atoms. However, why [TFSI] anion containing four O atoms and one N atom can only have low SO2 capacity through weak interactions? We believe that this may be due to the number of these potential sites. Such potential sites in one anion share the negative charge, and more potential sites in the anion may lead to reduced reactivity and low gas capacity. For example, Wang et al.35, 39 showed that SO2 capacity by phosphonium ILs with pyrrolate ([Pyro]), imidazolate ([Im]) and tetrazolate ([Tetz]) anions was 2.52, 4.80 and 3.72 mole SO2 per mole IL, respectively. Thereby, more potential sites do not mean more active sites. How to activate the potential sites to be the active sites is great of importance. One strategy is not to design too many potential sites (O or N atoms) in a given anion, which means that limited number of potential



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sites is necessary. In other words, one should adequately decrease the number of sites, but increase their activity, thus resulting in the increased number of active sites. Therefore, development of alternative functional ILs with limited number of active sites that are able to achieve high capacity is always highly desired.

Chart 1. The structures of cation and fluorinated anions employed in this work. In this contribution, we have prepared several kinds of phosphonium FILs with fluorinated acetylacetonate as the task-specific anion and [TFSI] as the counterpart anion for SO2 capture (see Chart 1 for these ILs’ structures). It is found that compared with acetylacetone, fluorinated acetylacetone is easy to donate hydrogen proton from –CH2– and form FILs.45-48 Physical properties of these FILs with fluorinated acetylacetonate are also investigated. Absorption experiment, FT-IR and NMR investigations, as well as quantum chemical calculations have been used to show that the highly efficient and reversible SO2 capture are ascribed to the multiple-site interactions between SO2 and limited number of active sites in the anions. ■ EXPERIMENTAL METHOD

Materials.

Benzoyltrifluoroacetone

hexafluoroacetylacetone

(HFA),

(BTFA),

thenoyltrifluoroacetone

tributylphosphane

(P444),

bromoethane

(TTFA), and


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bis(trifluoromethane)sulfonimide lithium (LiTFSI) were purchased from J&K Scientific. SO2 (99.95 %) and N2 (99.9993 %) were purchased from Beijing Oxygen Plant Specialty Gases Institute Co., Ltd. Mixed gases with different SO2 partial pressure were prepared by mixing SO2 and N2. An anion-exchange resin (Amersep 900 OH) was acquired from Alfa Aesar. All chemicals were obtained in the highest purity grade possible, and were used without purification unless otherwise stated. Preparation of the fluorinated ILs. [P4442][Br] was prepared from P444 and bromoethane using the procedure reported previously.30 In a typical synthesis of fluorinated acetylacetonate IL [P4442][BTFA], equimolar BTFA was added to the [P4442][OH] solution in ethanol, which was prepared from [P4442][Br] by anion-exchange procedure.49-50 The mixture was stirred at room temperature for 24 h. Then, ethanol and water were distilled off at 60 °C under reduced pressure. Furthermore, a FIL with [TFSI] as the anion was also prepared form [P4442][Br] and LiTFSI, and washed with DI water for more than 20 times to remove LiBr. Thus obtained fluorinated ILs were dried under vacuum for 24 h at 60 °C to reduce trace of water. The exact purity of the prepared FILs was measured by 1H NMR method,51 and 99%, 97%, 97%, and 99% (in mole percent) were found for [P4442][BTFA], [P4442][TTFA], [P4442][HFA], and [P4442][TFSI], respectively. Characterization of the ILs. 1H NMR and

13

C NMR spectra were determined on a Bruker

spectrometer (400 MHz) in DMSO-d6 or CDCl3 with tetramethylsilane (TMS) as the standard. FT-IR spectra were recorded on a Nicolet 470 FT-IR spectrometer. The structures of these ILs were confirmed by NMR and IR spectroscopy. NMR and FT-IR spectra measurements were also performed to track SO2 binding and release. The bromide content in these ILs was determined by using a Br− selective electrode (Shanghai Precision & Scientific Instrument Co. Ltd) coupled 5 ACS Paragon Plus Environment


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with a saturated calomel electrode (Shanghai Precision & Scientific Instrument Co. Ltd), and the results showed that bromide content was lower than 0.0005 mole per kilogram. Density (ρ) and viscosity (η) were determined with an Anton Paar DMA 4500 M digital densitometer and an Anton Paar Lovis 2000 M microviscosimeter, respectively. Glass transition temperatures (Tg) and decomposition temperatures (Td) were measured with a DSC-60 meter from SHIMADZU and TGA 2100 series of TA Instrument with a heating rate of 10 °C min–1 under N2 atmosphere, respectively. Td was the temperatures at which the weight loss of a compound in an aluminum pan reached 10 wt % under N2 atmosphere. SO2 Capture and Release Experiments. In a typical absorption experiment, SO2 of atmospheric pressure was bubbled through about 1.0 g IL 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 circulation water bath with desirable temperature. The amount of SO2 absorbed was determined at regular intervals by an electronic balance with an accuracy of ± 0.1 mg. The standard deviation of the absorption loadings under 1.0 atm was 0.05 mole SO2 per mole IL. Desorption of SO2 from SO2-saturated IL solutions was carried out and monitored in an analogous way as for the described absorption method. The ILs were regenerated by heating or bubbling nitrogen through the IL. In a typical desorption of SO2, N2 of atmospheric pressure was bubbled through about 1.0 g SO2-saturated ILs in a glass container, which was partly immersed in a circulation oil bath at 80 °C, and the flow rate was about 60 ml min–1. The amount of SO2 release was determined at regular intervals by the electronic balance.


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■ RESULTS AND DISCUSSION

Physical properties. Density, viscosity, and thermal stability of these fluorinated acetylacetonate ILs were measured, and the results were summarized in Table 1. It can be seen that the density of [P4442][HFA] was the highest but its viscosity was the lowest. In addition, these FILs were all room temperature ionic liquids according to their definition. Furthermore, they exhibited a higher thermal stability in the range of 200−300 °C, and thermal decomposition temperature decreased in the anionic order [TTFA]>[BTFA]>[HFA]. The mutual solubility of water and these FILs was also determined at 20 °C, and the results indicate that these FILs were all hydrophobic (Table S1). The temperature dependent of density and viscosity in the range from 20 °C to 80 °C at atmospheric pressure were given in Table S2-S3, and the details of density and viscosity were discussed as follows. Table 1. Density (ρ), viscosity(η), glass transition temperature (Tg) and thermal decomposition temperature (Td) of the fluorinated acetylacetonate ILs.a ρ/g cm−3 η/mPa s Tg/°C Td/°C

IL

a

[P4442][HFA]

1.1897

171.6

-80.3

212.0

[P4442][BTFA]

1.0734

803.3

-53.1

270.5

[P4442][TTFA]

1.1089

1116.4 b -80.4

282.0

Density and viscosity were measured at 20 °C; b Viscosity was measured by Brookfield DV-

II+ Pro viscometer.



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Figure 1 shows the experimental density values within the temperature range from 20 °C to 80 °C at atmospheric pressure. It can be seen that density of the ILs had a good linear relationship with the temperature. Thus, temperature dependence of density could be expressed by the following equation: ρ = a + bT

(1)

where ρ (in g cm−3) is the density, a and b are adjustable parameters, T (in K) is the temperature, and their values were included in Table S4. The fitting curve of the density with the temperature was also shown in Figure 1.

Figure 1. Linear relationship between density and temperature for fluorinated acetylacetonate ILs. [P4442][HFA], □; [P4442][TTFA], △; [P4442][BTFA], ▽. The temperature dependence of viscosity was also investigated for these fluorinated acetylacetonate ILs in the temperature range from 20 °C to 80 °C, and the result was shown in Figure 2. It is noted from Figure 2a that the viscosities of these FILs increased with the decrease of temperature. The most commonly used equation to fit the temperature dependent of FIL viscosity is the Arrhenius equation:


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ln η = ln A − Ea/RT

(2)

where η (in mPa s) is the viscosity, A is an adjustable parameter, Ea (in kJ mol−1) is the activation energy, R is the universal gas constant (R = 8.314 J mol-1 K-1), and T (in K) is the temperature. ln η versus 1/T for these fluorinated acetylacetonate ILs was plotted in Figure 2b, and the values of A and Ea were given in Table S5. According to Figure 2b, the densities of these FILs could be approximately fitted by the Arrhenius model in the above mentioned temperature range. Among these FILs, the Ea value decreased in the order: [P4442][TTFA] (51.6 kJ mol−1) > [P4442][BTFA] (48.5 kJ mol−1) >[P4442][HFA] (37.7 kJ mol−1).

Figure 2. (a) Viscosity of fluorinated acetylacetonate ILs at different temperatures. (b) Arrhenius plot of viscosity for these FILs. [P4442][TTFA], △; [P4442][BTFA], ▽; [P4442][HFA], □. 9 ACS Paragon Plus Environment


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Absorption of SO2. SO2 absorption by fluorinated acetylacetonate ILs with different chemical structures was investigated (Figure 3). It was shown that the capacities of SO2 absorption by [P4442][BTFA] and [P4442][TTFA] were 4.27 and 4.05 mole SO2 per mole IL at 20 °C and 1 atm, respectively, while the capacity by [P4442][HFA] was only 2.80 mole SO2 per mole IL. The structural difference of [P4442][HFA] with [P4442][BTFA] and [P4442][TTFA] is that there is an aromatic ring in the anion of the latter ILs. Thus, the higher absorption capacities by [P4442][BTFA] and [P4442][TTFA] may be due to the π···SO2 interaction between the aromatic rings and SO2.37 Clearly, these task-specific ILs with fluorinated acetylacetonate anions such as [P4442][HFA] exhibited high SO2 absorption capacities compared with conventional fluorinecontaining IL [P4442][TFSI] for which only 1.43 mole SO2 per mole IL was captured through physical interaction.14-15, 52-53 We also investigated the effect of water on the capture of SO2 by these FILs at 20 °C and 1 atm (Table S6). It can be seen that when water was added into the FILs, the SO2 absorption capacities were increased, presumably because of the reaction between SO2 and water.35 For example, 4.43 mole SO2 per mole IL could be captured by [P4442][BTFA] under 100% humidity SO2 while dry SO2 absorption capacity was 4.27 mole SO2 per mole IL.


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Figure 3. SO2 absorption by fluorinated acetylacetonate ILs as a function of absorption time at 20 °C and 1 atm of SO2 (60 ml min–1). [P4442][BTFA], □; [P4442][TTFA], △; [P4442][HFA], ○; [P4442][TFSI], ◇. In addition, SO2 capacities in these FILs were also investigated under low partial pressure. It is noted from Table 2 that the capacities of SO2 absorption by [P4442][BTFA] and [P4442][TTFA] were 1.82 and 1.80 mole SO2 per mole of IL at 20 °C and 0.1 atm, indicating that high absorption capacity could be reached under low-concentration of SO2. Besides, the mole ratio of SO2 to [P4442][HFA] was 1.10 under the same condition, which was close to 5 times that by [P4442][TFSI]. Furthermore, the absorption at low SO2 partial pressure such as 1% and 0.2% SO2 by these FILs was also investigated (Table 2), and reduced absorption capacities were observed at low SO2 partial pressure. For example, the capacity of SO2 absorption by [P4442][BTFA] was reduced to 1.34 and 1.30 mole SO2 per mole IL when 1% and 0.2% SO2 was absorbed, respectively. In comparison, the absorption capacity by [P4442][TFSI] was much lower than that of the fluorinated acetylacetonate ILs: 0.01 and 0.005 mole SO2 per mole IL was captured under 1% and 0.2% SO2, respectively. Thus, compared with [TFSI] counterpart, these fluorinated acetylacetonate ILs could efficiently absorb SO2 at low partial pressure through chemical interaction. Table 2. The effect of partial pressure on SO2 absorption by the FILs.a IL

Capacity (mole SO2 per mole IL) 0.2% SO2 1% SO2 10% SO2 100% SO2

[P4442][TFSI]

0.005

0.01

0.24

1.43



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

a

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[P4442][HFA]

0.32

0.34

1.10

2.80

[P4442][BTFA]

1.30

1.34

1.82

4.27

[P4442][TTFA]

1.23

1.26

1.80

4.05

SO2 absorption was carried out at 20 °C and 1 atm until the equilibrium was reached. Furthermore, as an example, the effect of SO2 partial pressure on the SO2 absorption of

[P4442][BTFA] at 20 °C was carried out to understand the capture performance by the fluorinated acetylacetonate ILs. As shown in Figure 4, the SO2 absorption capacities decreased with the decrease of partial pressure. For example, the molar ratios of SO2 to [P4442][BTFA] decreased from 4.27 to 1.82, when SO2 partial pressure decreased from 1 to 0.1 atm. This phenomenon may be attributed to a significant decrease in the physical absorption of [P4442][BTFA] at low partial pressure. These results indicate that the SO2 captured by [P4442][BTFA] could be facilely stripped by bubbling N2 through the IL.

Figure 4. The effect of SO2 partial pressure on SO2 capture by fluorinated acetylacetonate IL [P4442][BTFA] at 20 °C until equilibrium was reached


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Moreover, the temperature dependence of the SO2 absorption by [P4442][BTFA] at 1 atm was also investigated. It can be seen from Figure 5 that the capacity of SO2 absorption by [P4442][BTFA] significantly decreased with the increase of temperature. For instance, the capacity decreased from 4.27 to 3.27 and 2.72 mole SO2 per mole IL when the temperature increased from 20 to 30 and 40 °C, respectively, which indicates that 1~1.5 mole SO2 per mole IL was absorbed mainly through physical interaction. The molar ratio of SO2 to the IL decreased from 2.72 to 1.95 and 1.51, when the temperature increased from 40 to 60 and 80 °C, respectively. Clearly, these capacities were significantly greater than those at 1:1 stoichiometry, even at high temperature, which would be attributed to the chemical absorption. These results indicate that the absorbed SO2 could be facilely stripped by heating the IL.

Figure 5. The effect of temperature at 1 atm on SO2 capture by [P4442][BTFA]. 20 °C, □; 30 °C,

△; 40 °C, ▽; 60 °C, ○; 80 °C, ◇. Reversibility of SO2 absorption. The reversible absorption/desorption of SO2 is an important property, which directly affects the economics of gas absorption.35 Figure 6 shows the multiple SO2 absorption-desorption cycles for [P4442][BTFA]. It was found that the high capacities and rapid rates of SO2 absorption by [P4442][BTFA] were remained during these cycles. This result



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indicates that the SO2 absorption process by [P4442][BTFA] was highly reversible. The desorption of [P66614][BTFA] was not easy possibly because of the strong chemical interaction between SO2 and C=O group in the anion. However, the desorption efficiency could be increased through treatment of the SO2-saturated ILs at higher temperatures. For example, the molar ratio of SO2 to [P66614][BTFA] was 0.30 and 0 at 80 °C and 100 °C, respectively (Figure S1), under N2 (40 ml min−1) for 30 min.

Figure 6. SO2 absorption-desorption cycles for [P4442][BTFA]. Absorption (●): 20 °C and 1 atm of 100% SO2 (60 ml min–1); desorption (○): 80 °C and 1 atm of 100% N2 (60 ml min–1). Quantum Chemical Calculations. At the beginning, dispersion corrected density functional theory (DFT-D3(BJ)) calculation at the B3LYP/6-31++G(p,d) level54-56 was used to compare the interaction between SO2 and active sites in [TFSI] or [HFA]. As shown in Figure 7a-b, the distance between the positive charged S in the closest SO2 molecules and the negative charged N and O atoms in the [TFSI] were predicted as 3.515 and 2.946, respectively, indicating that the O···SO2 interaction was the main reason in physical absorption by [TFSI] which is in agreement with previous reports.57-58 As for [HFA]·SO2 (Figure 7c-d), the distances between the positive charged S in the closest SO2 molecules and the C and O atoms in the [HFA] were predicted as


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2.692 and 2.455, respectively, which were much shorter than the corresponding distances in [TFSI]·SO2. These results not only suggested the stronger interactions in [HFA]·SO2, but also predicted that O atom was the main active site and C atom could be acted as an active site in the anion.

Figure 7. Optimized structures of [TFSI]•SO2 (a-b) and [HFA]•SO2 (c-d) showing the interactions between S atoms in the closest SO2 molecules and the active sites in the anions. Note that the van der Waals radii59 of atoms are 1.70 Å for C (gray), 1.20 Å for H (white), 1.52 Å for O (red), 1.55 Å for N (blue), 1.80 Å for S (yellow), and 1.47 Å for F (green), respectively. To investigate the interactions between SO2 and the fluorinated acetylacetonate anions such as [BTFA], [TTFA], and [HFA], the natural bond orbital (NBO)60 charge distribution of O and C atoms in these anions were calculated using the Gaussian 09 program.61 As can be seen from Figure 8, the electron-withdrawing fluorine atoms shared the negative charge on the anions, thereby leading to the decrease in the NBO charge of O atoms on the anions. For example, the NBO charges of the O atoms in [BTFA] were –0.646 and –0.636, while that in [HFA] were decreased to both –0.619. In addition, the NBO charges of the C atoms in [HFA], [BTFA], and [TTFA] were –0.559, –0.545, and –0.539, respectively, leading to another efficient active site 15 ACS Paragon Plus Environment


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that could interact with SO2. Thus, limited number of active sites in the anion is crucial for obtaining high binding affinities for more SO2.

Figure 8. The NBO charges of O and C atoms in the anions [HFA] (a), [BTFA] (b), and [TTFA] (c). C, gray; H, white; O, red; S, yellow; F, green. The interactions between SO2 and the fluorinated acetylacetonate anion were further investigated through the geometry and energy optimizations by DFT-D3(BJ) at the B3LYP/631++G(p,d) level, and each optimization was calculated for the free anion, free SO2 molecule, and each anion–SO2 complex. The optimized structures shown in Figure 9 reflected the interactions between the closest SO2 molecules and the active sites in the anion [BTFA]. It can be seen that the intermolecular distance between the S in the closest SO2 molecules and the O and C atoms in the [BTFA] were predicted to be 2.335, 2.399, and 2.603 Å, which corresponds to a reduction of approximately 29.7, 27.7 and 25.6% of the sum of the van der Waals radii of the two interacting atoms, respectively, indicating the O···SO2 and C···SO2 interactions and that the former were stronger than the later. Compared the optimized structures of [BTFA]·SO2 in Figure 9 with [HFA]·SO2 in Figure 7, the shorter intermolecular distances in the [BTFA]·SO2 suggested the stronger interactions between the S atoms in the closest SO2 molecules and the


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active sites in the [BTFA]. Furthermore, the energetics of the gas phase reaction between the active sites in the anion and the closest SO2 molecules were also calculated. For example, the calculated enthalpies of interactions between the closest SO2 molecules and active sites in the [BTFA] were −89.4, −83.8, and −81.2 kJ mol−1, respectively.

Figure 9. Optimized structures of [BTFA]·SO2 showing interactions between the closest SO2 molecules and the active sites in the anion. (a), ∆H= –89.4 kJ mol−1; (b) ∆H= –83.8 kJ mol−1; (c) ∆H= –81.2 kJ mol−1. Note that the van der Waals radii59 of atoms are 1.70 Å for C (gray), 1.20 Å for H (white), 1.52 Å for O (red), 1.55 Å for N (blue), 1.80 Å for S (yellow), and 1.47 Å for F (green), respectively. Furthermore, the energetics of the gas phase reaction between SO2 and the fluorinated acetylacetonate ILs were also calculated and shown in Figure 10 and Table S7. The calculated absorption enthalpies of SO2 for the [BTFA]−SO2, [BTFA]−2SO2, [BTFA]−3SO2, and [BTFA]−4SO2 complexes were −89.4, −60.6, −56.9, and −54.7 kJ mol−1, respectively. Two C=O···SO2 interactions62 and one C···SO2 interaction between anion and SO2 were shown in Figure 10a-c, while Figure 10d showed SO2···SO2 interaction29 as well as π···SO2 interaction37 between benzene plane in the [BTFA] and S in the SO2. During the SO2 absorption, the interactions between the SO2 and the fluorinated anion gradually changed from a chemical one to a physical one due to the decreased absorption enthalpy.63 Accordingly, the interactions in the 17 ACS Paragon Plus Environment


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complexes of [BTFA]−SO2 and [BTFA]−2SO2 were mainly chemical interactions (∆H >60 kJ mol−1), indicating that nearly 2 mole SO2 per mole IL could be achieved at low pressure. Furthermore, multiple-site interactions between the acidic SO2 and the negatively charged fluorinated anion resulted in the SO2 capacity of about 4 mole SO2 per mole IL under ambient conditions.

Figure 10. Multiple-site interactions in the optimized structures based on the anion [BTFA] and SO2. (a), [BTFA]−SO2, ∆H= –89.4 kJ mol−1; (b) [BTFA]−2SO2, ∆H= –60.6 kJ mol−1; (c) [BTFA]−3SO2, ∆H= –56.9 kJ mol−1; (d) [BTFA]−4SO2, ∆H= –54.7 kJ mol−1. Note that the van der Waals radii59 of atoms are 1.70 Å for C (gray), 1.20 Å for H (white), 1.52 Å for O (red), 1.55 Å for N (blue), 1.80 Å for S (yellow), and 1.47 Å for F (green), respectively. Spectroscopic Investigations. In order to understand the experimental results, FT-IR and NMR spectroscopy was used to investigate the interactions between SO2 and the fluorinated acetylacetonate ILs. Figure 11 showed the IR spectrum of [P4442][BTFA] before and after the 18 ACS Paragon Plus Environment

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absorption of SO2. Compared with the FT-IR spectrum of neat [P4442][BTFA], new bands at 1324 (vas, S=O), 1144 (vs, S=O) and 959 cm−1 (vS–O) were observed in the spectra of [P4442][BTFA]– SO2, indicating the physical and chemical absorption of SO2.35, 40 In addition, there were two peaks in the 1600-1700 cm−1 region of the fresh [P4442][BTFA]: one was associated with the carbonyl functionality and the other arose primarily from the –C=C– alkene functionality.48 The band at 1637 cm−1 could be assigned to the C=O vibration, and this peak was blue-shifted by 37 cm−1 to 1674 cm−1 in the spectra of SO2-saturated [P4442][BTFA].62 Another peak at 1689 cm−1 related to the partial double bond emerged in the spectrum of the fresh IL disappeared in the spectrum of SO2-saturated [P4442][BTFA]. These results suggest the presence of CO···SO2 interaction, and that the electron density of the partial double bond decreased due to the interaction between anion and SO2. Furthermore, new bands at 729 and 1034 cm−1 could be assigned to the C···S64 and π···S37 interaction between C and phenyl group in the anion and S in SO2, respectively.

Figure 11. Comparison of FT-IR spectrum of SO2-saturated [P4442][BTFA] (red line) and neat [P4442][BTFA] (gray area).



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In 13C NMR spectra, compared with the fresh [P4442][BTFA], the typical peaks of negatively charged C–O and C=O groups on the anion of [P4442][BTFA] moved down-field from 167.6 and 194.0 ppm to 174.8 and 198.8 ppm, respectively, indicating the strong O···SO2 interaction between SO2 and carboxyl group on the anion (Figure 12). In addition, after SO2 absorption, the typical peaks of phenyl group moved from 126.8, 128.0, 129.4 and 143.6 ppm to 128.9, 129.4, 133.5 and 139.2 ppm, respectively, which appeared as a result of the interaction between phenyl group and SO2.37 Furthermore, the quartet peaks of -CF3 on the anion [BTFA] slightly moved down-field from 117.0, 118.9, 121.0 and 122.9 ppm to 122.0, 123.8, 125.7 and 127.7 ppm, respectively, due to the chemical interactions between CO-C-CO sites and SO2 and the physical interaction between -CF3 and SO2.

Figure 12. Comparison of

13

C NMR spectrum of SO2-saturated [P4442][BTFA] (red) and neat

[P4442][BTFA] (blue). The measurements were performed at 20 °C in d6-DMSO. According to the above results and the previous studies reported in literatures,37,

62

a

plausible SO2 absorption mechanism by [P4442][BTFA] can be proposed (Figure 10), which shows the multiple-site interactions between SO2 and the fluorinated acetylacetonate anion. ■ CONCLUSIONS 20 ACS Paragon Plus Environment

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In summary, several kinds of fluorinated acetylacetonate ILs were prepared and characterized. It was found that density values of these functional ILs decreased linearly with temperature, and the temperature dependence of their viscosities could be described by Arrhenius equation. These FILs exhibited remarkable SO2 capacity and good desorption performance. Thus, highly efficient capture and excellent reversibility could be reached. Quantum chemical calculations and spectroscopic investigations showed that the high capacity was due to the multiple-site interactions between SO2 and the limited number of active sites in the anion. The high capture capacity for SO2, rapid absorption/desorption rate, and excellent reusability of the ILs are very attractive for practical applications in acidic gas absorption. ■ ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.00000000000. NMR and IR data of ionic liquids before and after the absorption of SO2, Tables S1−S7, and Figures S1 (PDF) ■ AUTHOR INFORMATION Corresponding Author * [email protected], [email protected] Notes The authors declare no competing financial interest.



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■ ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21403059, 21673068), the Science Foundation for Excellent Young Scholars of Henan Normal University (No. 15YQ002), and the 2016 Annual Plan for Young Backbone Teachers of Henan Normal University. The DFT calculation was supported by the High Performance Computing Center of Henan Normal University. ■ REFERENCES

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Limited number of active sites strategy for improving SO2 capture by ionic liquids with fluorinated acetylacetonate anion Guokai Cui,* Ning Zhao, Yanan Li, Huiyong Wang, Yuling Zhao, Zhiyong Li, and Jianji Wang* Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan 453007, China. Table of Contents Graphic and Synopsis:

ILs containing fluorinated acetylacetonate anions were prepared and used for the highly efficient capture of SO2 through multiple-site interactions.



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Chart 1. The structures of cation and fluorinated anions employed in this work. 37x17mm (300 x 300 DPI)

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Figure 1. Linear relationship between density and temperature for fluorinated acetylacetonate ILs. [P4442][HFA], □; [P4442][TTFA], △; [P4442][BTFA], ▽. 50x34mm (300 x 300 DPI)



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Figure 2. (a) Viscosity of fluorinated acetylacetonate ILs at different temperatures. (b) Arrhenius plot of viscosity for these FILs. [P4442][TTFA], △; [P4442][BTFA], ▽; [P4442][HFA], □. 117x188mm (300 x 300 DPI)


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Figure 3. SO2 absorption by fluorinated acetylacetonate ILs as a function of absorption time at 20 °C and 1 atm of SO2 (60 ml min–1). [P4442][BTFA], □; [P4442][TTFA], △; [P4442][HFA], ○; [P4442][TFSI], ◇. 52x34mm (300 x 300 DPI)



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Figure 4. The effect of SO2 partial pressure on SO2 capture by fluorinated acetylacetonate IL [P4442][BTFA] at 20 °C until equilibrium was reached 53x37mm (300 x 300 DPI)


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Figure 5. The effect of temperature at 1 atm on SO2 capture by [P4442][BTFA]. 20 °C, □; 30 °C, △; 40 °C, ▽; 60 °C, ○; 80 °C, ◇. 51x34mm (300 x 300 DPI)



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Figure 6. SO2 absorption-desorption cycles for [P4442][BTFA]. Absorption (●): 20 °C and 1 atm of 100% SO2 (60 ml min–1); desorption (○): 80 °C and 1 atm of 100% N2 (60 ml min–1). 47x35mm (300 x 300 DPI)


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Figure 7. Optimized structures of [TFSI]•SO2 (a-b) and [HFA]•SO2 (c-d) showing the interactions between S atoms in the closest SO2 molecules and the active sites in the anions. Note that the van der Waals radii59 of atoms are 1.70 Å for C (gray), 1.20 Å for H (white), 1.52 Å for O (red), 1.55 Å for N (blue), 1.80 Å for S (yellow), and 1.47 Å for F (green), respectively. 57x40mm (300 x 300 DPI)



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Figure 8. The NBO charges of O and C atoms in the anions [HFA] (a), [BTFA] (b), and [TTFA] (c). C, gray; H, white; O, red; S, yellow; F, green. 53x34mm (300 x 300 DPI)


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Figure 9. Optimized structures of [BTFA]•SO2 showing interactions between the closest SO2 molecules and the active sites in the anion. (a), ∆H= –89.4 kJ mol−1; (b) ∆H= –83.8 kJ mol−1; (c) ∆H= –81.2 kJ mol−1. Note that the van der Waals radii59 of atoms are 1.70 Å for C (gray), 1.20 Å for H (white), 1.52 Å for O (red), 1.55 Å for N (blue), 1.80 Å for S (yellow), and 1.47 Å for F (green), respectively. 40x19mm (300 x 300 DPI)



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Figure 10. Multiple-site interactions in the optimized structures based on the anion [BTFA] and SO2. (a), [BTFA]−SO2, ∆H= –89.4 kJ mol−1; (b) [BTFA]−2SO2, ∆H= –60.6 kJ mol−1; (c) [BTFA]−3SO2, ∆H= –56.9 kJ mol−1; (d) [BTFA]−4SO2, ∆H= –54.7 kJ mol−1. Note that the van der Waals radii59 of atoms are 1.70 Å for C (gray), 1.20 Å for H (white), 1.52 Å for O (red), 1.55 Å for N (blue), 1.80 Å for S (yellow), and 1.47 Å for F (green), respectively. 92x117mm (300 x 300 DPI)


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Figure 11. Comparison of FT-IR spectrum of SO2-saturated [P4442][BTFA] (red line) and neat [P4442][BTFA] (gray area). 51x32mm (300 x 300 DPI)



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Figure 12. Comparison of 13C NMR spectrum of SO2-saturated [P4442][BTFA] (red) and neat [P4442][BTFA] (blue). The measurements were performed at 20 °C in d6-DMSO. 59x45mm (300 x 300 DPI)


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Table of Contents Graphic and Synopsis: ILs containing fluorinated acetylacetonate anions were prepared and used for the highly efficient capture of SO2 through multiple-site interactions. 46x30mm (300 x 300 DPI)


Limited Number of Active Sites Strategy for Improving SO2 Capture by

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