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Research Article pubs.acs.org/journal/ascecg

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 S Supporting Information *

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 mol SO2 per mol IL can be achieved under 1 and 0.1 bar, respectively, compared with 1.43 and 0.24 mol SO2 per mol [TFSI]based FIL ([TFSI] = bis(trifluoromethylsulfonyl)imide anion). From a combined study of quantum chemical 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. KEYWORDS: Active site, Ionic liquid, SO2, Acetylacetone, C···S interaction

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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, nonflammability, 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 whereas the IL does not dissolve in CO2. Maginn and Brennecke et al.14 determined the solubility of SO2 in [TFSI]based FILs. Riisager et al.15 showed that conventional FILs can © 2017 American Chemical Society

reversibly absorb only 1.33−1.60 mol SO2 per mol 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 anions40−42 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 postulate that this may be due to the Received: May 16, 2017 Revised: July 12, 2017 Published: July 16, 2017 7985

DOI: 10.1021/acssuschemeng.7b01551 ACS Sustainable Chem. Eng. 2017, 5, 7985−7992

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ACS Sustainable Chemistry & Engineering

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 13C 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 FTIR 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 with a saturated calomel electrode (Shanghai Precision & Scientific Instrument Co. Ltd.), and the results showed that bromide content was lower than 0.0005 mol 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 m 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 mol SO2 per mol 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 of 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.

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 mol SO2 per mol 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 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. In this contribution, we have prepared several kinds of phosphonium FILs with fluorinated acetylacetonate as the taskspecific anion and [TFSI] as the counterpart anion for SO2 capture (see Chart 1 for these ILs’ structures). It is found that Chart 1. Structures of Cation and Fluorinated Anions Employed in This Work

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.

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RESULTS AND DISCUSSION Physical Properties. Density, viscosity and thermal stability of these fluorinated acetylacetonate ILs were measured, and the results are summarized in Table 1. It can be seen that the density

Table 1. Density (ρ), Viscosity(η), Glass Transition Temperature (Tg) and Thermal Decomposition Temperature (Td) of the Fluorinated Acetylacetonate ILsa

EXPERIMENTAL METHOD

Materials. Benzoyltrifluoroacetone (BTFA), thenoyltrifluoroacetone (TTFA), hexafluoroacetylacetone (HFA), tributylphosphane (P444), bromoethane and 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

IL

ρ (g cm−3)

η (mPa s)

Tg (°C)

Td (°C)

[P4442][HFA] [P4442][BTFA] [P4442][TTFA]

1.1897 1.0734 1.1089

171.6 803.3 1116.4b

−80.3 −53.1 −80.4

212.0 270.5 282.0

Density and viscosity were measured at 20 °C. measured by Brookfield DV-II+ Pro viscometer.

a

b

Viscosity was

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 on density and viscosity in the range from 20 to 80 °C at atmospheric pressure are given in Tables S2 and S3, and the details of density and viscosity are herein discussed. 7986

DOI: 10.1021/acssuschemeng.7b01551 ACS Sustainable Chem. Eng. 2017, 5, 7985−7992

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ACS Sustainable Chemistry & Engineering Figure 1 shows the experimental density values within the temperature range from 20 to 80 °C at atmospheric pressure. It

Figure 1. Linear relationship between density and temperature for fluorinated acetylacetonate ILs. [P4442][HFA], □; [P4442][TTFA], △; [P4442][BTFA], ▽.

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) −3

where ρ (in g cm ) is the density, a and b are adjustable parameters, T (in K) is the temperature, and their values are included in Table S4. The fitting curve of the density with the temperature was also shown in Figure 1. The temperature dependence of viscosity was also investigated for these fluorinated acetylacetonate ILs in the temperature range from 20 to 80 °C, and the result is 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 on FIL viscosity is the Arrhenius equation: ln η = ln A − Ea /RT

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], □.

(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. The ln η versus 1/T for these fluorinated acetylacetonate ILs is plotted in Figure 2b, and the values of A and Ea are given in Table S5. According to Figure 2b, the densities of these FILs could be approximately fitted by the Arrhenius model in the mentioned temperature range. Among these FILs, the Ea value decreased in the order [P4442][TTFA] (51.6 kJ mol −1 ) > [P 4442][BTFA] (48.5 kJ mol −1) > [P4442][HFA] (37.7 kJ mol−1). 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 mol SO2 per mol IL at 20 °C and 1 atm, respectively, while the capacity by [P4442][HFA] was only 2.80 mol SO2 per mol 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 fluorine-

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], ◇.

containing IL [P4442][TFSI] for which only 1.43 mol SO2 per mol 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 mol SO2 per mol IL could be captured by [P4442][BTFA] under 100% humidity SO2 whereas dry SO2 absorption capacity was 4.27 mol SO2 per mol IL. 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 mol SO2 per mol 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 7987

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ACS Sustainable Chemistry & Engineering Table 2. Effect of Partial Pressure on SO2 Absorption by the FILsa Capacity (mol SO2 per mol IL) IL

0.2% SO2

1% SO2

10% SO2

100% SO2

[P4442][TFSI] [P4442][HFA] [P4442][BTFA] [P4442][TTFA]

0.005 0.32 1.30 1.23

0.01 0.34 1.34 1.26

0.24 1.10 1.82 1.80

1.43 2.80 4.27 4.05

a SO2 absorption was carried out at 20 °C and 1 atm until the equilibrium was reached.

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 mol SO2 per mol 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 mol SO2 per mol 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. 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

Figure 5. Effect of temperature at 1 atm on SO2 capture by [P4442][BTFA]. 20 °C, □; 30 °C, △; 40 °C, ▽; 60 °C, ○; 80 °C, ◇.

increased from 20 to 30 and 40 °C, respectively, which indicates that 1−1.5 mol SO2 per mol 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. 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

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

multiple SO2 absorption−desorption cycles for [P4442][BTFA]. It was found that the high capacities and rapid rates of SO2 absorption by [P4442][BTFA] remained during these cycles. This result indicates that the SO2 absorption process by [P4442][BTFA] was highly reversible. The desorption of [P4442][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 [P4442][BTFA] was 0.30 and 0 at 80 and 100 °C, respectively (Figure S1), under N2 (60 mL min−1) for 30 min. 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

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. Moreover, the temperature dependence of the SO 2 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 mol SO2 per mol IL when the temperature 7988

DOI: 10.1021/acssuschemeng.7b01551 ACS Sustainable Chem. Eng. 2017, 5, 7985−7992

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leading to another efficient active site that could interact with SO2. Thus, limited number of active sites in the anion is crucial for obtaining high binding affinities for more SO2. 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/6-31+ +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

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. Figure 9. Optimized structures of [BTFA]·SO2 showing interactions between the closest SO2 molecules and the active sites in the anion. (a), ΔH= −89.4 kJ mol−1; (b) ΔH= −83.8 kJ mol−1; (c) ΔH= −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.

charged N and O atoms in the [TFSI] were predicted as 3.515 and 2.946 Å, 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 2.692 and 2.455 Å, 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. 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,

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 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. Furthermore, the energetics of the gas phase reaction between SO2 and the fluorinated acetylacetonate ILs were also calculated, and the results are 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 are shown in Figure 10a−c, whereas Figure 10d shows 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 complexes of [BTFA]−SO2 and [BTFA]−2SO2 were mainly chemical interactions (ΔH > 60 kJ mol−1), indicating that nearly 2 mol SO2 per mol IL could be achieved at low pressure. Furthermore, multiple-site interactions between the acidic SO2 and the

Figure 8. 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 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, whereas those 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, 7989

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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. 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 downfield from 167.0 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

Figure 10. Multiple-site interactions in the optimized structures based on the anion [BTFA] and SO2. (a), [BTFA]−SO2, ΔH= −89.4 kJ mol−1; (b) [BTFA]−2SO2, ΔH= −60.6 kJ mol−1; (c) [BTFA]−3SO2, ΔH= −56.9 kJ mol−1; (d) [BTFA]−4SO2, ΔH= −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.

negatively charged fluorinated anion resulted in the SO2 capacity of about 4 mol SO2 per mol IL under ambient conditions. Spectroscopic Investigations. 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

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.

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 downfield 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. According to the above results and the previous studies reported in the literature,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.

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

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CONCLUSIONS 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

[P4442][BTFA] before and after the 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 7990

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ACS Sustainable Chemistry & Engineering

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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.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01551. NMR and IR data of ionic liquids before and after the absorption of SO2, Tables S1−S7 and Figures S1 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*G. Cui. E-mail: [email protected]. *J. Wang. E-mail: [email protected]. ORCID

Guokai Cui: 0000-0002-7223-2869 Jianji Wang: 0000-0003-2417-4630 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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.

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REFERENCES

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DOI: 10.1021/acssuschemeng.7b01551 ACS Sustainable Chem. Eng. 2017, 5, 7985−7992

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DOI: 10.1021/acssuschemeng.7b01551 ACS Sustainable Chem. Eng. 2017, 5, 7985−7992