Accepted Manuscript Title: Hydrophobic task-specific ionic liquids: Synthesis, properties and application for the capture of SO2 Author: Shidong Tian Yucui Hou Weize Wu Shuhang Ren Jianguo Qian PII: DOI: Reference:
S0304-3894(14)00494-4 http://dx.doi.org/doi:10.1016/j.jhazmat.2014.06.037 HAZMAT 16046
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
23-3-2014 8-6-2014 10-6-2014
Please cite this article as: S. Tian, Y. Hou, W. Wu, S. Ren, J. Qian, Hydrophobic task-specific ionic liquids: Synthesis, properties and application for the capture of SO2 , Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.06.037 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract:
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Highlights Capture of SO2 by hydrophobic task-specific ILs is proposed. [Et2NEmim][PF6] and [Et2NEmpyr][PF6] show excellent hydrophobicity.
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[Et2NEmim][PF6] can absorb 0.94 mol SO2 per mole IL (3% SO2).
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[Et2NEmim][PF6] is a promising absorbent for the capture of SO2.
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ABSTRACT The capture of SO2 by ionic liquids (ILs) has drawn much attention all over the world. However, ILs can absorb not only SO2 but also water from flue gas. The
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removal of water from ILs is necessary for reusing the absorbent. In order to reduce the energy costs of removing water, it would be helpful to weaken the interactions
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between ILs and water. In this work, two kinds of hydrophobic task-specific ILs,
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1-(2-diethyl-aminoethyl)-3-methylimidazolium hexafluorophosphate ([Et2NEmim] [PF6]) and 1-(2-diethyl-aminoethyl)-1-methylpyrrolidinium hexafluorophosphate
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([Et2NEmpyr][PF6]), were designed and synthesized. Thermal stability and physical properties of the ILs were studied. Furthermore, the application of the ILs for the
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capture of SO2 and the absorption mechanism were systematically investigated. It has been found that both of the ILs are immiscible with water, and [Et2NEmim][PF6] has
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much lower viscosity, much higher thermal stability and much higher SO2 absorption
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rate than [Et2NEmpyr][PF6]. [Et2NEmim][PF6] shows high SO2 absorption capacities up to 2.11 mol SO2 per mole IL (pure SO2) and 0.94 mol SO2 per mole IL (3% SO2)
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under hydrous conditions at 30 oC. The result suggests that [Et2NEmim][PF6] is a
promising recyclable absorbent for the capture of SO2. Keywords: Hydrophobic ionic liquid; SO2 capture; Synthesis; Property
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Hydrophobic task-specific ionic liquids: Synthesis, properties and application for the capture of SO2 Shidong Tiana, Yucui Houb, Weize Wua,*, Shuhang Rena, Jianguo Qiana
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a
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical
Technology, Beijing 100029, China; bDepartment of Chemistry, Taiyuan Normal University,
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Taiyuan 030031, China
*
Corresponding author; Email: [email protected], Tel./Fax: +86 10 64427603.
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ABSTRACT The capture of SO2 by ionic liquids (ILs) has drawn much attention all over the world. However, ILs can absorb not only SO2 but also water from flue gas. The
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removal of water from ILs is necessary for reusing the absorbent. In order to reduce the energy costs of removing water, it would be helpful to weaken the interactions
cr
between ILs and water. In this work, two kinds of hydrophobic task-specific ILs, 1-(2-diethyl-aminoethyl)-3-methylimidazolium hexafluorophosphate ([Et2NEmim]
us
[PF6]) and 1-(2-diethyl-aminoethyl)-1-methylpyrrolidinium hexafluorophosphate
an
([Et2NEmpyr][PF6]), were designed and synthesized. Thermal stability and physical properties of the ILs were studied. Furthermore, the application of the ILs for the
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capture of SO2 and the absorption mechanism were systematically investigated. It has been found that both of the ILs are immiscible with water, and [Et2NEmim][PF6] has
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much lower viscosity, much higher thermal stability and much higher SO2 absorption
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rate than [Et2NEmpyr][PF6]. [Et2NEmim][PF6] shows high SO2 absorption capacities up to 2.11 mol SO2 per mole IL (pure SO2) and 0.94 mol SO2 per mole IL (3% SO2)
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under hydrous conditions at 30 oC. The result suggests that [Et2NEmim][PF6] is a
promising recyclable absorbent for the capture of SO2. Keywords: Hydrophobic ionic liquid; SO 2 capture; Synthesis; Property
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1. Introduction In recent years, the emission of sulfur dioxide (SO2), which is mainly from fossil fuel combustion, has caused serious environmental and health issues. As a result,
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many technologies have been developed for the control of SO2 emission [1-3]. In particular, the capture of SO2 from flue gas by wet scrubbing using calcium-based
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absorbents is recognized as one of the most efficient ways. However, one problem is
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that calcium-based absorbents cannot be regenerated, which results in the continuous consumption of absorbents. Another problem is that there is a large amount of CaSO4
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produced during the process as a byproduct. Furthermore, as valuable chemical raw material, SO2 cannot be recovered from flue gas. Organic absorbents such as aqueous
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amines have also been applied for the capture of acidic gases (SO2, CO2 and H2S) [4-6]. However, the high volatility of amines causes waste of resources and secondary
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pollution. Therefore, the development of recyclable absorbents with low volatility and
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high capacity is of great significance.
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Ionic liquids (ILs) are considered as promising substitutes for traditional organic solvents in many fields due to their excellent properties, such as negligible vapor pressure, tunable structure and high gas solubility [7-9]. The captures of SO2 have been broadly studied by many research groups. Han et al. reported that 1,1,3,3-tetramethylguanidinium lactate ([TMG]L), which is the first task-specific IL, could absorb SO2 (8% by volume) with a high capacity up to 0.978 mole SO2 per
mole IL at 40 oC [10]. Riisager et al. reported that other guanidinium-based ILs, such as
[TMG][BF4],
[TMG][BTA],
[TMGB2][BTA],
[TMG][POBF4]
and
[TMG][PO2BF4], could also absorb large amounts of SO2 [11, 12]. Recently, Zhang et al. reported new guanidinium-based ILs with quite low viscosity for the capture of SO2 [13]. Besides guanidinium-based ILs, other types ILs, such as alkanolaminium-
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[14-16], imidazolium- [17-26], quaternary ammonium- [27] and quaternary phosphonium-based [24, 28, 29] ILs, caprolactam tetrabutyl ammonium bromide IL [30-32] and IL analogues (deep eutectic solvents) [33, 34] have also been developed
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for the capture of SO2. Simultaneously, theoretical researches on the absorption of SO2 by ILs have been carried out to further understand the absorption behavior and
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mechanism [35-37].
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Due to their high hydrophilicity, ILs can absorb water when exposed to flue gas [38], which usually contains 8% to 20% moisture (water vapor) [39]. In our previous
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work, the effect of water on the absorption of SO2 by [TMG]L was studied and it was found that [TMG]L could independently absorb SO2 and water from simulated flue
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gas simultaneously. The water absorbed by [TMG]L increased almost linearly over time when there was 7.3% water vapor in simulated flue gas [38]. Obviously, with the
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accumulation of water in IL, the SO2 absorption capacity would be decreased
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dramatically based on the total mass of absorbent. Therefore, the water absorbed by
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ILs should also be taken into account in the process of desulfurization. Currently, task-specific ionic liquids (TSILs) used for the capture of SO2 are
strongly hydrophilic and miscible with water. A reduction of the interactions between ILs and water could be very helpful in decreasing the energy costs of removing water. Additionally, the interactions between hydrophobic ILs and water are much weaker than that between hydrophilic ILs and water, which suggests that the removal of water from hydrophobic ILs will require less energy. ILs are composed of cation and anion, both of which can influence the hydrophilicity of ILs. Marrucho et al. overviewed the mutual solubilities of water–imidazolium-based ILs systems, and they indicated that the solubilities of water in ILs were primarily determined by the anion. The increase of alkyl chain length of
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cation could also decrease the water solubilities in ILs [40]. Mu et al. studied the kinetics and mechanisms of water absorption in ILs, and they also found that the anion played a major role on determining the hydrophilicity of ILs. Among the 18
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studied ILs, [Ac] based IL has the highest amount of water absorption, with the ranking of [Ac] > [Cl] > [Br] > [TFA] > [NO3] > [TFO] > [BF4] > [Tf2N] > [CHO] >
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[PF6] [41]. As a result, [PF6] is a better choice as the anion of hydrophobic IL. The
commonly studied hydrophobic IL [Bmim][PF6] has a saturated water solubility of
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2.67 wt% at 30 oC [42]. However, [Bmim][PF6] can only absorb SO2 by physical
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interactions, and it is not efficient for the capture of SO2 with low concentrations [43]. In order to obtain task-specific ILs that can absorb SO2 with low concentrations by
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chemical interactions, a functional group must be introduced to the cation of ILs. It has been reported that triethylamine (Et3N), which is immiscible with water, can
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chemically react with SO2 [44, 45]. Through this line of reasoning, ILs that have the
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anion of [PF6] and the cation with a functional group (Et2NE-) could simultaneously have a hydrophobic property and chemical interactions with SO2.
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In this work, two kinds of ILs with [PF6] anion, 1-(2-diethyl-aminoethyl)-3-
methylimidazolium
hexafluorophosphate
([Et2NEmim][PF6])
and
1-(2-diethyl-
aminoethyl)-1-methylpyrrolidinium hexafluorophosphate ([Et2NEmpyr][PF6]), were
designed and synthesized. It was found that these functional ILs were immiscible with water and could absorb SO2 with high absorption capacities. Thermal stability and physical properties of the ILs and application of the ILs for the capture of SO2 were systematically studied.
2. Experimental
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2.1. Materials SO2 (99.95%) and N2 (99.999%) were obtained from Beijing Haipu Gases Co., Ltd. (Beijing, China). SO2 (3%) was prepared by mixing SO2 and N2 together in a 40
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dm3 high pressure cylinder. N-methylimidazole was obtained from Leadership Chemical Co., Ltd. (Shandong, China) and it was distilled before use.
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1-Methylpyrrolidine (98%), potassium hexafluorophosphate (99%), sodium hydroxide
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(98%) and 2-chloro-N,N-diethylethylamine hydrochloride (99%) were supplied by Aladdin Chemical Co., Ltd. (Shanghai, China). Analytical reagent acetonitrile and
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dichloromethane were obtained from Beijing Tongguang Fine Chemical Co., Ltd. (Beijing, China). [Et2NEmim][PF6] and [Et2NEmpyr][PF6] used in this work were
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synthesized in our laboratory. The structures of [Et2NEmim][PF6] and [Et2NEmpyr] [PF6] (shown in Fig. 1) were confirmed by 1H NMR spectra (Bruker AM 600 MHz,
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CD3Cl). The water contents in pure ILs were determined by Karl Fischer titration, and
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they were found to be not more than 0.1 wt%. Detailed synthesis procedure and 1H
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NMR spectra of the ILs can be found in the supporting information. 2.2. Thermal stability of the ILs Thermal stability of the ILs was studied by determining the weight loss of the
ILs at temperatures ranging from 100 to 140 oC. In a typical experiment, about 2 g IL
was loaded in a test tube (15 × 150 mm). And then, N2 with a flow rate of 100 cm3/min was bubbled into the IL to remove the volatile compounds. Weight of the test tube and IL was measured at fixed intervals via an analytical balance (BS 224S, Sartorius) with a precision of 0.1 mg. The IL that remained in the test tube could be calculated by subtracting the weight of the test tube from the total weight of test tube and IL.
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2.3. Properties of the ILs Densities of the ILs were determined by a 5 cm3 pycnometer, which was calibrated by pure water before the experiment. A constant temperature water bath
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that was maintained within 0.1 oC by a temperature controller (Model A2, Beijing Changliu Scientific Instrument Co., Ltd., China) was used to control the experiment
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temperature. All of the measurements were carried out at least three times, and the
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average values were the final results. The uncertainty of density measurements was estimated to be ± 0.0002 g/cm3. Viscosities of the ILs were determined by
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gravitational capillary viscometers, which were supplied by Shanghai Shenyi Glass Instrument Co., Ltd. (Shanghai, China), in the same water bath. The temperature was
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controlled in the same way as mentioned above. Absolute viscosity (η) of the ILs was calculated from viscometer constants, running time of the liquid, and density of the
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liquid. The uncertainty of viscosity measurements was estimated to be ± 2 %.
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2.4. Absorption and desorption of SO2
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The absorption and desorption of SO2 were carried out at ambient pressure in a constant temperature water (oil) bath, which was controlled within 0.5 oC. Before absorption, about 2 g IL was loaded in a test tube (15 × 150 mm), and then the IL was treated with 100 cm3/min N2 at 80 oC to remove volatile compounds. The absorption
of pure SO2 was treated with 50 cm3/min SO2 and the absorption of simulated flue gas (3% SO2) was treated with 100 cm3/min mixed gas at desired temperatures. Weight increase of the test tube was measured at regular intervals by an analytical balance (BS 224S, Sartorius) with a precision of 0.1 mg, and then the solubility of SO2 in IL
could be calculated. Water might be released from the absorption test tube during the absorption of SO2 by IL with water. To capture the released water, downstream gas from the absorption test tube was bubbled through concentrated sulfuric acid in a 10
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glass tube. Before the absorption, the concentrated sulfuric acid was treated with pure SO2 or 3% SO2 until the weight of concentrated sulfuric acid kept constant. The water absorbed by the concentrated sulfuric acid was also considered when determining the
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solubility of SO2 in IL with water. The desorption of SO2 was carried out in a similar way by bubbling 100 cm3/min N2 to accelerate the release of SO2. The uncertainty in
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solubility measurements was estimated to be ± 4%.
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3. Results and discussion 3.1. Miscibility of ILs with water
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As expected, both of the ILs are experimentally immiscible with water. Solubilities of water in [Et2NEmim][PF6] and [Et2NEmpyr][PF6] are 5.4 wt% and 6.2
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3.2. Thermal stability of the ILs
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wt% at 30 oC, respectively, which indicates that they are hydrophobic.
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Fig. 2(a) and Fig. 2(b) show the weight losses of [Et2NEmim][PF6] and [Et2NEmpyr][PF6] at temperatures from 100 to 140 oC. As can be seen from the
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figures, [Et2NEmpyr][PF6] shows an obvious weight loss that increases dramatically with the increase of temperature. For example, [Et2NEmpyr][PF6] loses 0.14 wt%,
0.60 wt% and 2.85 wt% of its weight for 600 min at 100, 120 and 140 oC, respectively. [Et2NEmim][PF6] shows much lower weight loss than [Et2NEmpyr][PF6]. For
example, [Et2NEmim][PF6] only loses 0.12 wt% of its weight for 600 min even at 140 o
C. The above result suggests that [Et2NEmim][PF6] is much more stable than
[Et2NEmpyr][PF6]. The high thermal stability of [Et2NEmim][PF6] is an excellent property for its application. 3.3. Densities and viscosities of the ILs Densities and viscosities of [Et2NEmim][PF6] and [Et2NEmpyr][PF6] at 11
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temperatures ranging from 25 to 70 oC are listed in Table S1. The data of densities and viscosities as a function of temperature were fitted by the following equations
/ (g / cm3) A0 A1(T / K) A2(T / K)2
(1)
(2)
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/ mPa s B(T / K)0.5 exp(k / (T / K T0 / K))
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according to the literature [46].
where T is Kelvin temperature, ρ and η are density and viscosity of the ILs,
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respectively, and A0, A1, A2, B, k, T0 are fitting coefficients. The fitting coefficients for
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densities and viscosities of the ILs are presented in Table S2. The illustrations of densities and viscosities of the ILs as a function of temperature are shown in Fig. 3(a)
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and Fig. 3(b), respectively. As can be seen from the figures, [Et2NEmim][PF6] has a slight higher density but much lower viscosity than [Et2NEmpyr][PF6]. Densities of
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[Et2NEmim][PF6] and [Et2NEmpyr][PF6] decrease with the increase of temperature.
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For example, density of [Et2NEmim][PF6] decreases from 1.3041 to 1.2664 g/cm3 when the temperature increases from 25 oC to 70 oC. Viscosities of [Et2NEmim][PF6]
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and [Et2NEmpyr][PF6] also show the same trends as the change of densities, but they
are more sensitive to temperature. For example, viscosity of [Et2NEmim][PF6]
decreases dramatically from 600.0 to 50.5 mPa·s when the temperature increases from 25 oC to 70 oC.
3.4. Comparison of the ILs on the absorption of SO2 Fig. 4 shows the absorption of SO2 with a flow rate of 50 cm3/min by [Et2NEmim][PF6] and [Et2NEmpyr][PF6] at 60 oC. As can be seen from the figure, [Et2NEmim][PF6] shows much higher absorption rate than [Et2NEmpyr][PF6]. After
the absorption of SO2 by [Et2NEmim][PF6] for about 30 min, an absorption equilibrium can be reached. At absorption equilibrium, the mole ratio of SO2 to
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[Et2NEmim][PF6] is 1.11. Even after absorbing SO2 by [Et2NEmpyr][PF6] for 180 min, the absorption equilibrium still cannot be reached. The main difference between the absorption of SO2 by [Et2NEmim][PF6] and [Et2NEmpyr][PF6] is due to the fairly
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high viscosity of [Et2NEmpyr][PF6]. The high viscosity of IL hinders the mass transfer significantly when it is used for the capture of SO2. Compared with
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[Et2NEmpyr][PF6], [Et2NEmim][PF6] shows much lower viscosity, much higher thermal stability, and much higher SO2 absorption rate, making it a much better
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choice.
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3.5. Effect of temperature and SO2 partial pressure on the absorption of SO2 by [Et2NEmim][PF6]
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Temperature is one of the most important factors that influence the solubility of SO2 in ILs. Effect of temperature on the absorption of SO2 by [Et2NEmim][PF6] was
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studied, and the result is shown in Fig. 5. As shown in the figure, the solubility of SO2
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in the IL decreases with the increase of temperature. For example, the mole ratio of
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SO2 to [Et2NEmim][PF6] is 2.02 at 30 oC, while it is 1.11 at 60 oC. Effect of SO2 partial pressure on the absorption of SO2 by the IL has also been investigated, and the
result is shown in Fig. S3. The result shows that SO2 absorption capacity of the IL increases with an increase of SO2 partial pressure. This phenomenon is similar to the solubility of SO2 in other ILs [24, 28]. The above result suggests that the solubility of SO2 in IL is sensitive to the changes of temperature and pressure, so [Et2NEmim][PF6]
saturated with SO2 can be regenerated by increasing temperature and/or decreasing pressure. 3.6. Effect of functional group on the absorption of SO2 In order to investigate the role of functional group (Et2NE-) on the absorption
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capacity, a comparison between [Et2NEmim][PF6] and [Bmim][PF6] on the absorption of pure SO2 and 3% SO2 was studied, and the result is shown in Fig. 6. [Bmim][PF6] is normal IL, which can absorb SO2 only by physical interactions [43];
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[Et2NEmim][PF6] is functional IL, which can absorb SO2 by both chemical and physical interactions. For the contribution of chemical interactions between the
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functional group (Et2NE-) and SO2, the absorption capacity of [Et2NEmim][PF6] is much higher than that of [Bmim][PF6]. For the absorption of 3% SO2, [Bmim][PF6]
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can absorb a small amount of SO2, suggesting that the physical interactions between
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IL and SO2 is very weak. However, [Et2NEmim][PF6] shows much higher absorption capacity than [Bmim][PF6], which suggests that there are mainly chemical
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interactions between [Et2NEmim][PF6] and SO2 with low concentrations. As a result, the absorption capacity of IL for pure SO2 absorption results from both chemical and
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chemical interactions.
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physical interactions while that for diluted SO2 absorption mainly results from
The above result also can be supported by the viscosity change of IL when
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absorbing pure SO2 and 3% SO2. For the absorption of pure SO2, the viscosity of SO2-saturated [Et2NEmim][PF6] (2.03 mol SO2 per mol IL) is 274.6 mPa·s at 30 oC,
which is even lower than that of pure IL (411.5 mPa·s). For the absorption of 3% SO2,
the viscosity of IL with SO2 (0.356 mol SO2 per mol IL) is 1426.8 mPa·s, which is much higher than that of pure IL, and the liquidity of IL becomes worse when more SO2 is absorbed. The reason for the different absorption behaviors between pure SO2 and 3% SO2 is that: the physically absorbed SO2 can serve as a solvent, which decreases the interactions between the cation and anion of IL, especially the Coulombic interactions, and decreases the viscosity of IL significantly; while the chemically absorbed SO2 leads to the formation of strong chemical bonds, which
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increases the viscosity of IL obviously. This phenomenon is similar to the finding on the absorption of SO2 or CO2 by ILs reported by others [47-51]. Therefore, [Et2NEmim][PF6] can absorb SO2 by physical and chemical interactions
concentrations.
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3.7. Effect of water on the absorption of SO2 by [Et2NEmim][PF6]
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simultaneously, and the chemical interactions is predominant at low SO2
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There is a substantial amount of moisture in flue gas, so it is almost impossible to use the IL under anhydrous conditions. However, it was reported that some ILs, such
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as [Bmim][Ac], could not be regenerated when they were used for the capture of SO2 under hydrous conditions [22]. Therefore, effect of water on the absorption of SO2 by
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IL must be investigated. The absorption of pure SO2 and simulated flue gas (3% SO2) by [Et2NEmim][PF6] and [Et2NEmim][PF6] + 20 wt% H2O at 30 oC were studied.
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Fig. 7(a) and Fig. 7(b) show the effect of water on the absorption of pure SO2
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and 3% SO2 by [Et2NEmim][PF6], respectively. As shown in the figures, the
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absorption rates increase obviously both for the absorption of pure SO2 and 3% SO2 under hydrous conditions. Water molecules play the roles of decreasing the viscosity of IL. For example, the viscosity of pure [Et2NEmim][PF6] is 411.5 mPa·s and that of
water-saturated [Et2NEmim][PF6] is 68.4 mPa·s at 30 oC. Therefore, the decrease of the viscosity of IL could improve the mass transfer during the absorption of SO2,
which accelerates the absorption rate. It also can be seen from Fig. 7(a) and Fig. 7(b) that, the effect of water on the absorption of 3% SO2 is more significant than that on the absorption of pure SO2. For the absorption of pure SO2, the absorption equilibrium can be reached under both hydrous and anhydrous conditions. For the absorption of 3% SO2, the absorption
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equilibrium can be reached under hydrous conditions while it cannot under anhydrous conditions. The reason is that the chemically absorbed SO2 results in an extremely high viscosity of IL under anhydrous conditions. Fortunately, the mass transfer can be
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improved significantly when there is 20 wt% water in the IL, and the absorption equilibrium can be reached in 90 min. [Et2NEmim][PF6] shows high absorption
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capacity up to 0.94 mol SO2 per mole IL at 30 oC in the presence of water. It should
be mentioned that, SO2 also can be dissolved in water. According to the result
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reported by Rabe and Harris, the solubility of 3% SO2 in water is 0.0033 g SO2/g H2O
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at 30 oC [52]. Because some amount of water reacts with SO2 to form H2SO3, the SO2 dissolved in water is less than 0.002 g, which is about 0.5% of the total absorbed SO2.
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Therefore, the SO2 dissolved in water can only make a minor contribution to the total absorbed SO2.
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3.8. Desorption of SO2 and reuse of [Et2NEmim][PF6]
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The absorption of SO2 by [Et2NEmim][PF6] and [Et2NEmim][PF6] + 20 wt%
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H2O were carried out with 50 cm3/min SO2 at 30 oC for 60 min. The desorption of SO2 from [Et2NEmim][PF6] and [Et2NEmim][PF6] + 20 wt% H2O were carried out at
80 oC with the sweeping of 100 cm3/min N2, and the results are shown in Fig. 8(a) and Fig. 8(b), respectively. As can be seen from the figures, SO2 (SO2 + H2O) can be
released from [Et2NEmim][PF6] ([Et2NEmim][PF6] + 20 wt% H2O) easily. The reuses
of [Et2NEmim][PF6] and [Et2NEmim][PF6] + 20 wt% H2O were studied for five cycles. During the absorption/desorption cycles, the absorption of SO2 by
[Et2NEmim][PF6] and [Et2NEmim][PF6] + 20 wt% H2O lasted for 60 min and the desorption of SO2 from [Et2NEmim][PF6] and [Et2NEmim][PF6] + 20 wt% lasted for 60 min and 240 min, respectively. Fig. 8(c) and Fig. 8(d) show that there are no obvious losses of SO2 absorption capacities of the absorbents. For example, the mole
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ratios of SO2 to [Et2NEmim][PF6] are 2.11, 2.16, 2.13, 2.12 and 2.12 for the five cycles when [Et2NEmim][PF6] + 20 wt% H2O was used as the absorbent. The result suggests that the absorption of SO2 by [Et2NEmim][PF6] is highly reversible under
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both conditions, hydrous and anhydrous. 3.9. The absorption mechanism
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The functional group of the ILs is Et2NE-, which has the same structure of Et3N.
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Therefore, we proposed that the absorption of SO2 by the ILs shows a similar mechanism to that of the absorption of SO2 by Et3N. Under anhydrous conditions,
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SO2 reacts with Et3N to form a sulfur dioxide addition compound Et3N·SO2 [44]. When Et3N absorbs one mole of water, SO2 reacts with Et3N to form [Et3NH]+HSO3−
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[45]. In order to verify the absorption mechanism, FT-IR (KBr), 1H NMR (DMSO-d6) spectra of [Et2NEmim][PF6] and water-saturated [Et2NEmim][PF6] (30 oC) before and
d
after SO2 absorption were studied, and the results are shown in Fig. 9.
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As can be seen from Fig. 9(a), for the absorption of SO2 by the IL without water,
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new absorption band at 968.5 cm−1 was found, which can be assigned to S–O stretches. The FT-IR study confirms the existence of chemical interactions between the IL and SO2. The 1H NMR spectra of [Et2NEmim][PF6] before and after SO2 absorption (Fig. 9(c)) shows that the typical peaks of H at the positions of a, b, c and d move downfield from 0.86, 2.47，2.71, 4.18 ppm to 0.93, 2.59, 2.85, 4.27 ppm,
respectively, which suggests that the functional group (Et2NE-) on the cation reacts
with SO2 through chemical interactions. As can be seen from Fig. 9(b), for the absorption of SO2 by water-saturated [Et2NEmim][PF6], the absorption band assigned to S–O stretches was also found. The 1H NMR spectra of water-saturated IL before and after SO2 absorption (Fig. 9(d)) shows that the typical peak of water disappeared after SO2 absorption. And the typical peaks of H at the positions of a, b, c and d also 17
Page 17 of 35
show obvious chemical shifts. This phenomenon suggests that water reacts with SO2 to form H2SO3, and then H2SO3 reacts with the functional group (Et2NE-) to form [ILH]+HSO3−. An acid-base analysis was also performed to explain the absorption
ip t
mechanism in the presence of water. The amine value, which is often expressed as the equivalent KOH (in mg) to the content of basic nitrogen in one gram sample [53], is
cr
used to evaluate the basicity of [Et2NEmim][PF6]. The amine value of
[Et2NEmim][PF6] is determined by the titration of [Et2NEmim][PF6] with 0.1 M
us
perchloric acid in glacial acetic acid [54]. It was found that the amine value of
an
[Et2NEmim][PF6] is 175.3 mg KOH/g IL (1.02 mol KOH per mol IL). Based on the above analysis, the IL shows strong basicity, so it can react with the acid H2SO3 and combine with H+ to form [ILH]+HSO3−. The mole ratio of absorbed SO2 to
M
[Et2NEmim][PF6] is 0.94 at 30 oC when there is water in the IL, which is very close to
d
1.0. The result also suggests that the IL can react with SO2 in a 1:1 stoichiometry
te
under hydrous conditions. Therefore, based on the above results, the absorption
Ac ce p
mechanism is proposed as shown in scheme 1. In the absence of water:
[Et 2 NEmim][PF 6] . SO 2
[Et 2NEmim][PF 6] + SO 2
In the presence of water:
N
H3C
N
+
d
PF 6
c
N
H HSO 3 +
H 2SO 3
CH3
N
+ N
+ N
H3C
PF 6 CH3
b H3C
H3C
a
Scheme 1．Proposed mechanism of the absorption of SO2 by the IL.
4. Conclusion In this work, two kinds of hydrophobic task-specific ILs, [Et2NEmim][PF6] and
18
Page 18 of 35
[Et2NEmpyr][PF6], were designed and synthesized. Thermal stability and physical properties of the ILs and application of the ILs for the capture of SO2 were systematically studied. The results show that [Et2NEmim][PF6] has much higher
ip t
thermal stability and much lower viscosity than [Et2NEmpyr][PF6], which suggests that [Et2NEmim][PF6] is a better choice for the capture of SO2. Compared with
cr
[Et2NEmpyr][PF6], the absorption of SO2 by [Et2NEmim][PF6] can be reached an
absorption equilibrium rapidly. The absorption of SO2 by [Et2NEmim][PF6] shows
us
that [Et2NEmim][PF6] has high SO2 absorption capacities up to 2.11 mol SO2 per mole IL (pure SO2) and 0.94 mol SO2 per mole IL (3% SO2) under hydrous conditions
an
at 30 oC. The reuses of [Et2NEmim][PF6] with or without water were tested for five
M
cycles, and no obvious losses of absorption capacities were found. The absorption mechanism depends on whether there is water in the IL. Under anhydrous conditions,
d
the IL can react with SO2 to form sulfur dioxide addition compounds IL·SO2. Under
Ac ce p
Acknowledgments
te
hydrous conditions, the IL can react with SO2 to form [ILH]+HSO3−.
The authors thank Prof. Zhenyu Liu and Prof. Qingya Liu for their help. The
project is financially supported by the Natural Science Foundation of China (No. 21176020 and 21306007) and the Research Fund for the Doctoral Program of Higher Education of China (No. 20130010120005).
19
Page 19 of 35
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ip t
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ip t
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25
Page 25 of 35
List of figure captions Fig. 1. The structures of [Et2NEmim][PF6] and [Et2NEmpyr][PF6]. Fig. 2. Weight of (a) [Et2NEmim][PF6] and (b) [Et2NEmpyr][PF6] as a function of time at different , 100 oC;
, 120 oC;
, 140 oC.
ip t
temperatures:
Fig. 3. (a) Densities and (b) viscosities of the ILs at temperatures from 25 to 70 oC: ●, , [Et2NEmpyr][PF6]. Solid lines are the results calculated by equations.
cr
[Et2NEmim][PF6];
Fig. 4. Absorption of SO2 with a flow rate of 50 cm3/min by the ILs at 60 oC: ●, [Et2NEmim][PF6];
us
, [Et2NEmpyr][PF6].
Fig. 5. Effect of temperature on the absorption of SO2 by [Et2NEmim][PF6]: ■, 30 oC; ●, 40 oC; ▲,
an
50 oC; ▼, 60 oC.
Fig. 6. Absorption of pure SO2 and 3% SO2 by [Et2NEmim][PF6] and [Bmim][PF6] at 30 oC: ■,
M
[Et2NEmim][PF6] (pure SO2); ●, [Bmim][PF6] (pure SO2);
, [Et2NEmim][PF6] (3% SO2);
,
[Bmim][PF6] (3% SO2). The flow rate of pure SO2 is 50 cm3/min and that of simulated flue gas
d
with 3% SO2 is 100 cm3/min.
te
Fig. 7. Effect of water on the absorption of (a) pure SO2 with a flow rate of 50 cm3/min and (b) 3% SO2 with a flow rate of 100 cm3/min at 30 oC: ●, [Et2NEmim][PF6];
Ac ce p
20 wt% H2O.
, [Et2NEmim][PF6] +
Fig. 8. The desorption of SO2 from (a) [Et2NEmim][PF6] and (b) [Et2NEmim][PF6] + 20 wt% H2O,
and the reuse of (c) [Et2NEmim][PF6] and (d) [Et2NEmim][PF6] + 20 wt% H2O. Fig. 9. FT-IR spectra of (a) [Et2NEmim][PF6] and (b) water-saturated [Et2NEmim][PF6] and 1H
NMR
spectra
of
(c)
[Et
2
NEmim][PF
6
]
and
(d)
water-
saturated [Et 2 NEmim][PF 6 ] before and after SO 2 absorption: A, IL; B, IL-SO 2 .
N
+ N
+
N
H3C
CH3 PF6
H3C
N
N
H3C
CH3 PF 6
H3C
[Et2NEmim][PF6]
[Et2NEmpyr][PF6]
27
Page 26 of 35
Ac ce p
te
d
M
an
us
cr
ip t
Fig. 1. The structures of [Et2NEmim][PF6] and [Et2NEmpyr][PF6].
28
Page 27 of 35
101
(a)
ip t
99 98
cr
Weight /%
100
96
0
100
200
300
400
101
600
M
(b)
100
d
99 98
te
Weight /%
500
an
t/min
us
97
Ac ce p
97 96
0
100
200
300
400
500
600
t/min
Fig. 2. Weight of (a) [Et2NEmim][PF6] and (b) [Et2NEmpyr][PF6] as a function of time at different
temperatures:
, 100 oC;
, 120 oC;
, 140 oC.
29
Page 28 of 35
20000
(a)
1.29 1.28
10000
5000
1.27 1.26 20
(b)
15000
/mPa·s
/(g/cm3)
1.30
30
40
50
60
0 20
70
o
30
40
50
ip t
1.31
o
60
70
Temperature / C
cr
Temperature / C
, [Et2NEmpyr][PF6]. Solid lines are the results calculated by equations.
Ac ce p
te
d
M
an
[Et2NEmim][PF6];
us
Fig. 3. (a) Densities and (b) viscosities of the ILs at temperatures from 25 to 70 oC: ●,
30
Page 29 of 35
1.0
ip t
0.8 0.6
cr
0.4 0.2 0.0
0
50
100
150
200
an
t/min
us
Mole ratio of SO2 to IL
1.2
Fig. 4. Absorption of SO2 with a flow rate of 50 cm3/min by the ILs at 60 oC: ●, [Et2NEmim][PF6];
Ac ce p
te
d
M
, [Et2NEmpyr][PF6].
31
Page 30 of 35
ip t
1.5
cr
1.0
0.5
0.0
0
10
20
30
40
50
60
an
t/min
us
Mole ratio of SO2 to IL
2.0
Fig. 5. Effect of temperature on the absorption of SO2 by [Et2NEmim][PF6]: ■, 30 oC; ●, 40 oC; ▲,
Ac ce p
te
d
M
50 oC; ▼, 60 oC.
32
Page 31 of 35
Mole ratio of SO2 to IL
2.0
ip t
1.5
cr
1.0
0.0
0
50
100
150
us
0.5
200
an
t/min
250
300
Fig. 6. Absorption of pure SO2 and 3% SO2 by [Et2NEmim][PF6] and [Bmim][PF6] at 30 oC: ■,
M
[Et2NEmim][PF6] (pure SO2); ●, [Bmim][PF6] (pure SO2);
, [Et2NEmim][PF6] (3% SO2);
,
Ac ce p
te
with 3% SO2 is 100 cm3/min.
d
[Bmim][PF6] (3% SO2). The flow rate of pure SO2 is 50 cm3/min and that of simulated flue gas
33
Page 32 of 35
1.5
1.0
0.5
0.0
0
10
20
30
40
50
0.6 0.4 0.2 0.0
60
(b)
0.8
0
50
100
150
t/min
200
250
300
cr
t/min
ip t
Mole ratio of SO2 to IL
Mole ratio of SO2 to IL
1.0
(a)
2.0
us
Fig. 7. Effect of water on the absorption of (a) pure SO2 with a flow rate of 50 cm3/min and (b) 3% SO2 with a flow rate of 100 cm3/min at 30 oC: ●, [Et2NEmim][PF6];
Ac ce p
te
d
M
an
20 wt% H2O.
, [Et2NEmim][PF6] +
34
Page 33 of 35
0.4
0.2
0.0
0
10
20
30
40
50
60
(b)
1.2 1.0 0.8 0.6 0.4 0.2 0.0
0
50
100
150
0.5
2
3
4
5
Cycles
2.0 (d)
1.5
an
1.0
1
250
300
us
Mole ratio of SO2 to IL
(c)
1.5
0.0
200
t/min
1.0
0.5
0.0
M
Mole ratio of SO2 to IL
t/min
2.0
ip t
0.6
1.4
cr
Weight of SO2 / g
Weight of SO2 and water / g
(a)
0.8
1
2
3
4
5
Cycles
d
Fig. 8. The desorption of SO2 from (a) [Et2NEmim][PF6] and (b) [Et2NEmim][PF6] + 20 wt% H2O,
Ac ce p
te
and the reuse of (c) [Et2NEmim][PF6] and (d) [Et2NEmim][PF6] + 20 wt% H2O.
35
Page 34 of 35
ip t cr us an M NMR
spectra
te
d
Fig. 9. FT-IR spectra of (a) [Et2NEmim][PF6] and (b) water-saturated [Et2NEmim][PF6] and 1H of
(c)
[Et
2
NEmim][PF
6
]
and
(d)
water-
Ac ce p
saturated [Et 2 NEmim][PF 6 ] before and after SO 2 absorption: A, IL; B, IL-SO 2 .
36
Page 35 of 35
S0304-3894(14)00494-4 http://dx.doi.org/doi:10.1016/j.jhazmat.2014.06.037 HAZMAT 16046
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
23-3-2014 8-6-2014 10-6-2014
Please cite this article as: S. Tian, Y. Hou, W. Wu, S. Ren, J. Qian, Hydrophobic task-specific ionic liquids: Synthesis, properties and application for the capture of SO2 , Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.06.037 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ac ce p
te
d
M
an
us
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Graphical Abstract:
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Page 1 of 35
Highlights Capture of SO2 by hydrophobic task-specific ILs is proposed. [Et2NEmim][PF6] and [Et2NEmpyr][PF6] show excellent hydrophobicity.
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[Et2NEmim][PF6] can absorb 0.94 mol SO2 per mole IL (3% SO2).
Ac ce p
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an
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[Et2NEmim][PF6] is a promising absorbent for the capture of SO2.
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Page 2 of 35
ABSTRACT The capture of SO2 by ionic liquids (ILs) has drawn much attention all over the world. However, ILs can absorb not only SO2 but also water from flue gas. The
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removal of water from ILs is necessary for reusing the absorbent. In order to reduce the energy costs of removing water, it would be helpful to weaken the interactions
cr
between ILs and water. In this work, two kinds of hydrophobic task-specific ILs,
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1-(2-diethyl-aminoethyl)-3-methylimidazolium hexafluorophosphate ([Et2NEmim] [PF6]) and 1-(2-diethyl-aminoethyl)-1-methylpyrrolidinium hexafluorophosphate
an
([Et2NEmpyr][PF6]), were designed and synthesized. Thermal stability and physical properties of the ILs were studied. Furthermore, the application of the ILs for the
M
capture of SO2 and the absorption mechanism were systematically investigated. It has been found that both of the ILs are immiscible with water, and [Et2NEmim][PF6] has
d
much lower viscosity, much higher thermal stability and much higher SO2 absorption
te
rate than [Et2NEmpyr][PF6]. [Et2NEmim][PF6] shows high SO2 absorption capacities up to 2.11 mol SO2 per mole IL (pure SO2) and 0.94 mol SO2 per mole IL (3% SO2)
Ac ce p
under hydrous conditions at 30 oC. The result suggests that [Et2NEmim][PF6] is a
promising recyclable absorbent for the capture of SO2. Keywords: Hydrophobic ionic liquid; SO2 capture; Synthesis; Property
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Page 3 of 35
Hydrophobic task-specific ionic liquids: Synthesis, properties and application for the capture of SO2 Shidong Tiana, Yucui Houb, Weize Wua,*, Shuhang Rena, Jianguo Qiana
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a
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical
Technology, Beijing 100029, China; bDepartment of Chemistry, Taiyuan Normal University,
Ac ce p
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Taiyuan 030031, China
*
Corresponding author; Email: [email protected], Tel./Fax: +86 10 64427603.
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Page 4 of 35
ABSTRACT The capture of SO2 by ionic liquids (ILs) has drawn much attention all over the world. However, ILs can absorb not only SO2 but also water from flue gas. The
ip t
removal of water from ILs is necessary for reusing the absorbent. In order to reduce the energy costs of removing water, it would be helpful to weaken the interactions
cr
between ILs and water. In this work, two kinds of hydrophobic task-specific ILs, 1-(2-diethyl-aminoethyl)-3-methylimidazolium hexafluorophosphate ([Et2NEmim]
us
[PF6]) and 1-(2-diethyl-aminoethyl)-1-methylpyrrolidinium hexafluorophosphate
an
([Et2NEmpyr][PF6]), were designed and synthesized. Thermal stability and physical properties of the ILs were studied. Furthermore, the application of the ILs for the
M
capture of SO2 and the absorption mechanism were systematically investigated. It has been found that both of the ILs are immiscible with water, and [Et2NEmim][PF6] has
d
much lower viscosity, much higher thermal stability and much higher SO2 absorption
te
rate than [Et2NEmpyr][PF6]. [Et2NEmim][PF6] shows high SO2 absorption capacities up to 2.11 mol SO2 per mole IL (pure SO2) and 0.94 mol SO2 per mole IL (3% SO2)
Ac ce p
under hydrous conditions at 30 oC. The result suggests that [Et2NEmim][PF6] is a
promising recyclable absorbent for the capture of SO2. Keywords: Hydrophobic ionic liquid; SO 2 capture; Synthesis; Property
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Page 5 of 35
1. Introduction In recent years, the emission of sulfur dioxide (SO2), which is mainly from fossil fuel combustion, has caused serious environmental and health issues. As a result,
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many technologies have been developed for the control of SO2 emission [1-3]. In particular, the capture of SO2 from flue gas by wet scrubbing using calcium-based
cr
absorbents is recognized as one of the most efficient ways. However, one problem is
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that calcium-based absorbents cannot be regenerated, which results in the continuous consumption of absorbents. Another problem is that there is a large amount of CaSO4
an
produced during the process as a byproduct. Furthermore, as valuable chemical raw material, SO2 cannot be recovered from flue gas. Organic absorbents such as aqueous
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amines have also been applied for the capture of acidic gases (SO2, CO2 and H2S) [4-6]. However, the high volatility of amines causes waste of resources and secondary
d
pollution. Therefore, the development of recyclable absorbents with low volatility and
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high capacity is of great significance.
Ac ce p
Ionic liquids (ILs) are considered as promising substitutes for traditional organic solvents in many fields due to their excellent properties, such as negligible vapor pressure, tunable structure and high gas solubility [7-9]. The captures of SO2 have been broadly studied by many research groups. Han et al. reported that 1,1,3,3-tetramethylguanidinium lactate ([TMG]L), which is the first task-specific IL, could absorb SO2 (8% by volume) with a high capacity up to 0.978 mole SO2 per
mole IL at 40 oC [10]. Riisager et al. reported that other guanidinium-based ILs, such as
[TMG][BF4],
[TMG][BTA],
[TMGB2][BTA],
[TMG][POBF4]
and
[TMG][PO2BF4], could also absorb large amounts of SO2 [11, 12]. Recently, Zhang et al. reported new guanidinium-based ILs with quite low viscosity for the capture of SO2 [13]. Besides guanidinium-based ILs, other types ILs, such as alkanolaminium-
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Page 6 of 35
[14-16], imidazolium- [17-26], quaternary ammonium- [27] and quaternary phosphonium-based [24, 28, 29] ILs, caprolactam tetrabutyl ammonium bromide IL [30-32] and IL analogues (deep eutectic solvents) [33, 34] have also been developed
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for the capture of SO2. Simultaneously, theoretical researches on the absorption of SO2 by ILs have been carried out to further understand the absorption behavior and
cr
mechanism [35-37].
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Due to their high hydrophilicity, ILs can absorb water when exposed to flue gas [38], which usually contains 8% to 20% moisture (water vapor) [39]. In our previous
an
work, the effect of water on the absorption of SO2 by [TMG]L was studied and it was found that [TMG]L could independently absorb SO2 and water from simulated flue
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gas simultaneously. The water absorbed by [TMG]L increased almost linearly over time when there was 7.3% water vapor in simulated flue gas [38]. Obviously, with the
d
accumulation of water in IL, the SO2 absorption capacity would be decreased
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dramatically based on the total mass of absorbent. Therefore, the water absorbed by
Ac ce p
ILs should also be taken into account in the process of desulfurization. Currently, task-specific ionic liquids (TSILs) used for the capture of SO2 are
strongly hydrophilic and miscible with water. A reduction of the interactions between ILs and water could be very helpful in decreasing the energy costs of removing water. Additionally, the interactions between hydrophobic ILs and water are much weaker than that between hydrophilic ILs and water, which suggests that the removal of water from hydrophobic ILs will require less energy. ILs are composed of cation and anion, both of which can influence the hydrophilicity of ILs. Marrucho et al. overviewed the mutual solubilities of water–imidazolium-based ILs systems, and they indicated that the solubilities of water in ILs were primarily determined by the anion. The increase of alkyl chain length of
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Page 7 of 35
cation could also decrease the water solubilities in ILs [40]. Mu et al. studied the kinetics and mechanisms of water absorption in ILs, and they also found that the anion played a major role on determining the hydrophilicity of ILs. Among the 18
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studied ILs, [Ac] based IL has the highest amount of water absorption, with the ranking of [Ac] > [Cl] > [Br] > [TFA] > [NO3] > [TFO] > [BF4] > [Tf2N] > [CHO] >
cr
[PF6] [41]. As a result, [PF6] is a better choice as the anion of hydrophobic IL. The
commonly studied hydrophobic IL [Bmim][PF6] has a saturated water solubility of
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2.67 wt% at 30 oC [42]. However, [Bmim][PF6] can only absorb SO2 by physical
an
interactions, and it is not efficient for the capture of SO2 with low concentrations [43]. In order to obtain task-specific ILs that can absorb SO2 with low concentrations by
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chemical interactions, a functional group must be introduced to the cation of ILs. It has been reported that triethylamine (Et3N), which is immiscible with water, can
d
chemically react with SO2 [44, 45]. Through this line of reasoning, ILs that have the
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anion of [PF6] and the cation with a functional group (Et2NE-) could simultaneously have a hydrophobic property and chemical interactions with SO2.
Ac ce p
In this work, two kinds of ILs with [PF6] anion, 1-(2-diethyl-aminoethyl)-3-
methylimidazolium
hexafluorophosphate
([Et2NEmim][PF6])
and
1-(2-diethyl-
aminoethyl)-1-methylpyrrolidinium hexafluorophosphate ([Et2NEmpyr][PF6]), were
designed and synthesized. It was found that these functional ILs were immiscible with water and could absorb SO2 with high absorption capacities. Thermal stability and physical properties of the ILs and application of the ILs for the capture of SO2 were systematically studied.
2. Experimental
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Page 8 of 35
2.1. Materials SO2 (99.95%) and N2 (99.999%) were obtained from Beijing Haipu Gases Co., Ltd. (Beijing, China). SO2 (3%) was prepared by mixing SO2 and N2 together in a 40
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dm3 high pressure cylinder. N-methylimidazole was obtained from Leadership Chemical Co., Ltd. (Shandong, China) and it was distilled before use.
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1-Methylpyrrolidine (98%), potassium hexafluorophosphate (99%), sodium hydroxide
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(98%) and 2-chloro-N,N-diethylethylamine hydrochloride (99%) were supplied by Aladdin Chemical Co., Ltd. (Shanghai, China). Analytical reagent acetonitrile and
an
dichloromethane were obtained from Beijing Tongguang Fine Chemical Co., Ltd. (Beijing, China). [Et2NEmim][PF6] and [Et2NEmpyr][PF6] used in this work were
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synthesized in our laboratory. The structures of [Et2NEmim][PF6] and [Et2NEmpyr] [PF6] (shown in Fig. 1) were confirmed by 1H NMR spectra (Bruker AM 600 MHz,
d
CD3Cl). The water contents in pure ILs were determined by Karl Fischer titration, and
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they were found to be not more than 0.1 wt%. Detailed synthesis procedure and 1H
Ac ce p
NMR spectra of the ILs can be found in the supporting information. 2.2. Thermal stability of the ILs Thermal stability of the ILs was studied by determining the weight loss of the
ILs at temperatures ranging from 100 to 140 oC. In a typical experiment, about 2 g IL
was loaded in a test tube (15 × 150 mm). And then, N2 with a flow rate of 100 cm3/min was bubbled into the IL to remove the volatile compounds. Weight of the test tube and IL was measured at fixed intervals via an analytical balance (BS 224S, Sartorius) with a precision of 0.1 mg. The IL that remained in the test tube could be calculated by subtracting the weight of the test tube from the total weight of test tube and IL.
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Page 9 of 35
2.3. Properties of the ILs Densities of the ILs were determined by a 5 cm3 pycnometer, which was calibrated by pure water before the experiment. A constant temperature water bath
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that was maintained within 0.1 oC by a temperature controller (Model A2, Beijing Changliu Scientific Instrument Co., Ltd., China) was used to control the experiment
cr
temperature. All of the measurements were carried out at least three times, and the
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average values were the final results. The uncertainty of density measurements was estimated to be ± 0.0002 g/cm3. Viscosities of the ILs were determined by
an
gravitational capillary viscometers, which were supplied by Shanghai Shenyi Glass Instrument Co., Ltd. (Shanghai, China), in the same water bath. The temperature was
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controlled in the same way as mentioned above. Absolute viscosity (η) of the ILs was calculated from viscometer constants, running time of the liquid, and density of the
d
liquid. The uncertainty of viscosity measurements was estimated to be ± 2 %.
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2.4. Absorption and desorption of SO2
Ac ce p
The absorption and desorption of SO2 were carried out at ambient pressure in a constant temperature water (oil) bath, which was controlled within 0.5 oC. Before absorption, about 2 g IL was loaded in a test tube (15 × 150 mm), and then the IL was treated with 100 cm3/min N2 at 80 oC to remove volatile compounds. The absorption
of pure SO2 was treated with 50 cm3/min SO2 and the absorption of simulated flue gas (3% SO2) was treated with 100 cm3/min mixed gas at desired temperatures. Weight increase of the test tube was measured at regular intervals by an analytical balance (BS 224S, Sartorius) with a precision of 0.1 mg, and then the solubility of SO2 in IL
could be calculated. Water might be released from the absorption test tube during the absorption of SO2 by IL with water. To capture the released water, downstream gas from the absorption test tube was bubbled through concentrated sulfuric acid in a 10
Page 10 of 35
glass tube. Before the absorption, the concentrated sulfuric acid was treated with pure SO2 or 3% SO2 until the weight of concentrated sulfuric acid kept constant. The water absorbed by the concentrated sulfuric acid was also considered when determining the
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solubility of SO2 in IL with water. The desorption of SO2 was carried out in a similar way by bubbling 100 cm3/min N2 to accelerate the release of SO2. The uncertainty in
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solubility measurements was estimated to be ± 4%.
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3. Results and discussion 3.1. Miscibility of ILs with water
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As expected, both of the ILs are experimentally immiscible with water. Solubilities of water in [Et2NEmim][PF6] and [Et2NEmpyr][PF6] are 5.4 wt% and 6.2
d
3.2. Thermal stability of the ILs
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wt% at 30 oC, respectively, which indicates that they are hydrophobic.
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Fig. 2(a) and Fig. 2(b) show the weight losses of [Et2NEmim][PF6] and [Et2NEmpyr][PF6] at temperatures from 100 to 140 oC. As can be seen from the
Ac ce p
figures, [Et2NEmpyr][PF6] shows an obvious weight loss that increases dramatically with the increase of temperature. For example, [Et2NEmpyr][PF6] loses 0.14 wt%,
0.60 wt% and 2.85 wt% of its weight for 600 min at 100, 120 and 140 oC, respectively. [Et2NEmim][PF6] shows much lower weight loss than [Et2NEmpyr][PF6]. For
example, [Et2NEmim][PF6] only loses 0.12 wt% of its weight for 600 min even at 140 o
C. The above result suggests that [Et2NEmim][PF6] is much more stable than
[Et2NEmpyr][PF6]. The high thermal stability of [Et2NEmim][PF6] is an excellent property for its application. 3.3. Densities and viscosities of the ILs Densities and viscosities of [Et2NEmim][PF6] and [Et2NEmpyr][PF6] at 11
Page 11 of 35
temperatures ranging from 25 to 70 oC are listed in Table S1. The data of densities and viscosities as a function of temperature were fitted by the following equations
/ (g / cm3) A0 A1(T / K) A2(T / K)2
(1)
(2)
cr
/ mPa s B(T / K)0.5 exp(k / (T / K T0 / K))
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according to the literature [46].
where T is Kelvin temperature, ρ and η are density and viscosity of the ILs,
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respectively, and A0, A1, A2, B, k, T0 are fitting coefficients. The fitting coefficients for
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densities and viscosities of the ILs are presented in Table S2. The illustrations of densities and viscosities of the ILs as a function of temperature are shown in Fig. 3(a)
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and Fig. 3(b), respectively. As can be seen from the figures, [Et2NEmim][PF6] has a slight higher density but much lower viscosity than [Et2NEmpyr][PF6]. Densities of
d
[Et2NEmim][PF6] and [Et2NEmpyr][PF6] decrease with the increase of temperature.
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For example, density of [Et2NEmim][PF6] decreases from 1.3041 to 1.2664 g/cm3 when the temperature increases from 25 oC to 70 oC. Viscosities of [Et2NEmim][PF6]
Ac ce p
and [Et2NEmpyr][PF6] also show the same trends as the change of densities, but they
are more sensitive to temperature. For example, viscosity of [Et2NEmim][PF6]
decreases dramatically from 600.0 to 50.5 mPa·s when the temperature increases from 25 oC to 70 oC.
3.4. Comparison of the ILs on the absorption of SO2 Fig. 4 shows the absorption of SO2 with a flow rate of 50 cm3/min by [Et2NEmim][PF6] and [Et2NEmpyr][PF6] at 60 oC. As can be seen from the figure, [Et2NEmim][PF6] shows much higher absorption rate than [Et2NEmpyr][PF6]. After
the absorption of SO2 by [Et2NEmim][PF6] for about 30 min, an absorption equilibrium can be reached. At absorption equilibrium, the mole ratio of SO2 to
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Page 12 of 35
[Et2NEmim][PF6] is 1.11. Even after absorbing SO2 by [Et2NEmpyr][PF6] for 180 min, the absorption equilibrium still cannot be reached. The main difference between the absorption of SO2 by [Et2NEmim][PF6] and [Et2NEmpyr][PF6] is due to the fairly
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high viscosity of [Et2NEmpyr][PF6]. The high viscosity of IL hinders the mass transfer significantly when it is used for the capture of SO2. Compared with
cr
[Et2NEmpyr][PF6], [Et2NEmim][PF6] shows much lower viscosity, much higher thermal stability, and much higher SO2 absorption rate, making it a much better
us
choice.
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3.5. Effect of temperature and SO2 partial pressure on the absorption of SO2 by [Et2NEmim][PF6]
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Temperature is one of the most important factors that influence the solubility of SO2 in ILs. Effect of temperature on the absorption of SO2 by [Et2NEmim][PF6] was
d
studied, and the result is shown in Fig. 5. As shown in the figure, the solubility of SO2
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in the IL decreases with the increase of temperature. For example, the mole ratio of
Ac ce p
SO2 to [Et2NEmim][PF6] is 2.02 at 30 oC, while it is 1.11 at 60 oC. Effect of SO2 partial pressure on the absorption of SO2 by the IL has also been investigated, and the
result is shown in Fig. S3. The result shows that SO2 absorption capacity of the IL increases with an increase of SO2 partial pressure. This phenomenon is similar to the solubility of SO2 in other ILs [24, 28]. The above result suggests that the solubility of SO2 in IL is sensitive to the changes of temperature and pressure, so [Et2NEmim][PF6]
saturated with SO2 can be regenerated by increasing temperature and/or decreasing pressure. 3.6. Effect of functional group on the absorption of SO2 In order to investigate the role of functional group (Et2NE-) on the absorption
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Page 13 of 35
capacity, a comparison between [Et2NEmim][PF6] and [Bmim][PF6] on the absorption of pure SO2 and 3% SO2 was studied, and the result is shown in Fig. 6. [Bmim][PF6] is normal IL, which can absorb SO2 only by physical interactions [43];
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[Et2NEmim][PF6] is functional IL, which can absorb SO2 by both chemical and physical interactions. For the contribution of chemical interactions between the
cr
functional group (Et2NE-) and SO2, the absorption capacity of [Et2NEmim][PF6] is much higher than that of [Bmim][PF6]. For the absorption of 3% SO2, [Bmim][PF6]
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can absorb a small amount of SO2, suggesting that the physical interactions between
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IL and SO2 is very weak. However, [Et2NEmim][PF6] shows much higher absorption capacity than [Bmim][PF6], which suggests that there are mainly chemical
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interactions between [Et2NEmim][PF6] and SO2 with low concentrations. As a result, the absorption capacity of IL for pure SO2 absorption results from both chemical and
te
chemical interactions.
d
physical interactions while that for diluted SO2 absorption mainly results from
The above result also can be supported by the viscosity change of IL when
Ac ce p
absorbing pure SO2 and 3% SO2. For the absorption of pure SO2, the viscosity of SO2-saturated [Et2NEmim][PF6] (2.03 mol SO2 per mol IL) is 274.6 mPa·s at 30 oC,
which is even lower than that of pure IL (411.5 mPa·s). For the absorption of 3% SO2,
the viscosity of IL with SO2 (0.356 mol SO2 per mol IL) is 1426.8 mPa·s, which is much higher than that of pure IL, and the liquidity of IL becomes worse when more SO2 is absorbed. The reason for the different absorption behaviors between pure SO2 and 3% SO2 is that: the physically absorbed SO2 can serve as a solvent, which decreases the interactions between the cation and anion of IL, especially the Coulombic interactions, and decreases the viscosity of IL significantly; while the chemically absorbed SO2 leads to the formation of strong chemical bonds, which
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Page 14 of 35
increases the viscosity of IL obviously. This phenomenon is similar to the finding on the absorption of SO2 or CO2 by ILs reported by others [47-51]. Therefore, [Et2NEmim][PF6] can absorb SO2 by physical and chemical interactions
concentrations.
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3.7. Effect of water on the absorption of SO2 by [Et2NEmim][PF6]
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simultaneously, and the chemical interactions is predominant at low SO2
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There is a substantial amount of moisture in flue gas, so it is almost impossible to use the IL under anhydrous conditions. However, it was reported that some ILs, such
an
as [Bmim][Ac], could not be regenerated when they were used for the capture of SO2 under hydrous conditions [22]. Therefore, effect of water on the absorption of SO2 by
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IL must be investigated. The absorption of pure SO2 and simulated flue gas (3% SO2) by [Et2NEmim][PF6] and [Et2NEmim][PF6] + 20 wt% H2O at 30 oC were studied.
d
Fig. 7(a) and Fig. 7(b) show the effect of water on the absorption of pure SO2
te
and 3% SO2 by [Et2NEmim][PF6], respectively. As shown in the figures, the
Ac ce p
absorption rates increase obviously both for the absorption of pure SO2 and 3% SO2 under hydrous conditions. Water molecules play the roles of decreasing the viscosity of IL. For example, the viscosity of pure [Et2NEmim][PF6] is 411.5 mPa·s and that of
water-saturated [Et2NEmim][PF6] is 68.4 mPa·s at 30 oC. Therefore, the decrease of the viscosity of IL could improve the mass transfer during the absorption of SO2,
which accelerates the absorption rate. It also can be seen from Fig. 7(a) and Fig. 7(b) that, the effect of water on the absorption of 3% SO2 is more significant than that on the absorption of pure SO2. For the absorption of pure SO2, the absorption equilibrium can be reached under both hydrous and anhydrous conditions. For the absorption of 3% SO2, the absorption
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Page 15 of 35
equilibrium can be reached under hydrous conditions while it cannot under anhydrous conditions. The reason is that the chemically absorbed SO2 results in an extremely high viscosity of IL under anhydrous conditions. Fortunately, the mass transfer can be
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improved significantly when there is 20 wt% water in the IL, and the absorption equilibrium can be reached in 90 min. [Et2NEmim][PF6] shows high absorption
cr
capacity up to 0.94 mol SO2 per mole IL at 30 oC in the presence of water. It should
be mentioned that, SO2 also can be dissolved in water. According to the result
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reported by Rabe and Harris, the solubility of 3% SO2 in water is 0.0033 g SO2/g H2O
an
at 30 oC [52]. Because some amount of water reacts with SO2 to form H2SO3, the SO2 dissolved in water is less than 0.002 g, which is about 0.5% of the total absorbed SO2.
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Therefore, the SO2 dissolved in water can only make a minor contribution to the total absorbed SO2.
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3.8. Desorption of SO2 and reuse of [Et2NEmim][PF6]
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The absorption of SO2 by [Et2NEmim][PF6] and [Et2NEmim][PF6] + 20 wt%
Ac ce p
H2O were carried out with 50 cm3/min SO2 at 30 oC for 60 min. The desorption of SO2 from [Et2NEmim][PF6] and [Et2NEmim][PF6] + 20 wt% H2O were carried out at
80 oC with the sweeping of 100 cm3/min N2, and the results are shown in Fig. 8(a) and Fig. 8(b), respectively. As can be seen from the figures, SO2 (SO2 + H2O) can be
released from [Et2NEmim][PF6] ([Et2NEmim][PF6] + 20 wt% H2O) easily. The reuses
of [Et2NEmim][PF6] and [Et2NEmim][PF6] + 20 wt% H2O were studied for five cycles. During the absorption/desorption cycles, the absorption of SO2 by
[Et2NEmim][PF6] and [Et2NEmim][PF6] + 20 wt% H2O lasted for 60 min and the desorption of SO2 from [Et2NEmim][PF6] and [Et2NEmim][PF6] + 20 wt% lasted for 60 min and 240 min, respectively. Fig. 8(c) and Fig. 8(d) show that there are no obvious losses of SO2 absorption capacities of the absorbents. For example, the mole
16
Page 16 of 35
ratios of SO2 to [Et2NEmim][PF6] are 2.11, 2.16, 2.13, 2.12 and 2.12 for the five cycles when [Et2NEmim][PF6] + 20 wt% H2O was used as the absorbent. The result suggests that the absorption of SO2 by [Et2NEmim][PF6] is highly reversible under
ip t
both conditions, hydrous and anhydrous. 3.9. The absorption mechanism
cr
The functional group of the ILs is Et2NE-, which has the same structure of Et3N.
us
Therefore, we proposed that the absorption of SO2 by the ILs shows a similar mechanism to that of the absorption of SO2 by Et3N. Under anhydrous conditions,
an
SO2 reacts with Et3N to form a sulfur dioxide addition compound Et3N·SO2 [44]. When Et3N absorbs one mole of water, SO2 reacts with Et3N to form [Et3NH]+HSO3−
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[45]. In order to verify the absorption mechanism, FT-IR (KBr), 1H NMR (DMSO-d6) spectra of [Et2NEmim][PF6] and water-saturated [Et2NEmim][PF6] (30 oC) before and
d
after SO2 absorption were studied, and the results are shown in Fig. 9.
te
As can be seen from Fig. 9(a), for the absorption of SO2 by the IL without water,
Ac ce p
new absorption band at 968.5 cm−1 was found, which can be assigned to S–O stretches. The FT-IR study confirms the existence of chemical interactions between the IL and SO2. The 1H NMR spectra of [Et2NEmim][PF6] before and after SO2 absorption (Fig. 9(c)) shows that the typical peaks of H at the positions of a, b, c and d move downfield from 0.86, 2.47，2.71, 4.18 ppm to 0.93, 2.59, 2.85, 4.27 ppm,
respectively, which suggests that the functional group (Et2NE-) on the cation reacts
with SO2 through chemical interactions. As can be seen from Fig. 9(b), for the absorption of SO2 by water-saturated [Et2NEmim][PF6], the absorption band assigned to S–O stretches was also found. The 1H NMR spectra of water-saturated IL before and after SO2 absorption (Fig. 9(d)) shows that the typical peak of water disappeared after SO2 absorption. And the typical peaks of H at the positions of a, b, c and d also 17
Page 17 of 35
show obvious chemical shifts. This phenomenon suggests that water reacts with SO2 to form H2SO3, and then H2SO3 reacts with the functional group (Et2NE-) to form [ILH]+HSO3−. An acid-base analysis was also performed to explain the absorption
ip t
mechanism in the presence of water. The amine value, which is often expressed as the equivalent KOH (in mg) to the content of basic nitrogen in one gram sample [53], is
cr
used to evaluate the basicity of [Et2NEmim][PF6]. The amine value of
[Et2NEmim][PF6] is determined by the titration of [Et2NEmim][PF6] with 0.1 M
us
perchloric acid in glacial acetic acid [54]. It was found that the amine value of
an
[Et2NEmim][PF6] is 175.3 mg KOH/g IL (1.02 mol KOH per mol IL). Based on the above analysis, the IL shows strong basicity, so it can react with the acid H2SO3 and combine with H+ to form [ILH]+HSO3−. The mole ratio of absorbed SO2 to
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[Et2NEmim][PF6] is 0.94 at 30 oC when there is water in the IL, which is very close to
d
1.0. The result also suggests that the IL can react with SO2 in a 1:1 stoichiometry
te
under hydrous conditions. Therefore, based on the above results, the absorption
Ac ce p
mechanism is proposed as shown in scheme 1. In the absence of water:
[Et 2 NEmim][PF 6] . SO 2
[Et 2NEmim][PF 6] + SO 2
In the presence of water:
N
H3C
N
+
d
PF 6
c
N
H HSO 3 +
H 2SO 3
CH3
N
+ N
+ N
H3C
PF 6 CH3
b H3C
H3C
a
Scheme 1．Proposed mechanism of the absorption of SO2 by the IL.
4. Conclusion In this work, two kinds of hydrophobic task-specific ILs, [Et2NEmim][PF6] and
18
Page 18 of 35
[Et2NEmpyr][PF6], were designed and synthesized. Thermal stability and physical properties of the ILs and application of the ILs for the capture of SO2 were systematically studied. The results show that [Et2NEmim][PF6] has much higher
ip t
thermal stability and much lower viscosity than [Et2NEmpyr][PF6], which suggests that [Et2NEmim][PF6] is a better choice for the capture of SO2. Compared with
cr
[Et2NEmpyr][PF6], the absorption of SO2 by [Et2NEmim][PF6] can be reached an
absorption equilibrium rapidly. The absorption of SO2 by [Et2NEmim][PF6] shows
us
that [Et2NEmim][PF6] has high SO2 absorption capacities up to 2.11 mol SO2 per mole IL (pure SO2) and 0.94 mol SO2 per mole IL (3% SO2) under hydrous conditions
an
at 30 oC. The reuses of [Et2NEmim][PF6] with or without water were tested for five
M
cycles, and no obvious losses of absorption capacities were found. The absorption mechanism depends on whether there is water in the IL. Under anhydrous conditions,
d
the IL can react with SO2 to form sulfur dioxide addition compounds IL·SO2. Under
Ac ce p
Acknowledgments
te
hydrous conditions, the IL can react with SO2 to form [ILH]+HSO3−.
The authors thank Prof. Zhenyu Liu and Prof. Qingya Liu for their help. The
project is financially supported by the Natural Science Foundation of China (No. 21176020 and 21306007) and the Research Fund for the Doctoral Program of Higher Education of China (No. 20130010120005).
19
Page 19 of 35
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ip t
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cr
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ip t
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25
Page 25 of 35
List of figure captions Fig. 1. The structures of [Et2NEmim][PF6] and [Et2NEmpyr][PF6]. Fig. 2. Weight of (a) [Et2NEmim][PF6] and (b) [Et2NEmpyr][PF6] as a function of time at different , 100 oC;
, 120 oC;
, 140 oC.
ip t
temperatures:
Fig. 3. (a) Densities and (b) viscosities of the ILs at temperatures from 25 to 70 oC: ●, , [Et2NEmpyr][PF6]. Solid lines are the results calculated by equations.
cr
[Et2NEmim][PF6];
Fig. 4. Absorption of SO2 with a flow rate of 50 cm3/min by the ILs at 60 oC: ●, [Et2NEmim][PF6];
us
, [Et2NEmpyr][PF6].
Fig. 5. Effect of temperature on the absorption of SO2 by [Et2NEmim][PF6]: ■, 30 oC; ●, 40 oC; ▲,
an
50 oC; ▼, 60 oC.
Fig. 6. Absorption of pure SO2 and 3% SO2 by [Et2NEmim][PF6] and [Bmim][PF6] at 30 oC: ■,
M
[Et2NEmim][PF6] (pure SO2); ●, [Bmim][PF6] (pure SO2);
, [Et2NEmim][PF6] (3% SO2);
,
[Bmim][PF6] (3% SO2). The flow rate of pure SO2 is 50 cm3/min and that of simulated flue gas
d
with 3% SO2 is 100 cm3/min.
te
Fig. 7. Effect of water on the absorption of (a) pure SO2 with a flow rate of 50 cm3/min and (b) 3% SO2 with a flow rate of 100 cm3/min at 30 oC: ●, [Et2NEmim][PF6];
Ac ce p
20 wt% H2O.
, [Et2NEmim][PF6] +
Fig. 8. The desorption of SO2 from (a) [Et2NEmim][PF6] and (b) [Et2NEmim][PF6] + 20 wt% H2O,
and the reuse of (c) [Et2NEmim][PF6] and (d) [Et2NEmim][PF6] + 20 wt% H2O. Fig. 9. FT-IR spectra of (a) [Et2NEmim][PF6] and (b) water-saturated [Et2NEmim][PF6] and 1H
NMR
spectra
of
(c)
[Et
2
NEmim][PF
6
]
and
(d)
water-
saturated [Et 2 NEmim][PF 6 ] before and after SO 2 absorption: A, IL; B, IL-SO 2 .
N
+ N
+
N
H3C
CH3 PF6
H3C
N
N
H3C
CH3 PF 6
H3C
[Et2NEmim][PF6]
[Et2NEmpyr][PF6]
27
Page 26 of 35
Ac ce p
te
d
M
an
us
cr
ip t
Fig. 1. The structures of [Et2NEmim][PF6] and [Et2NEmpyr][PF6].
28
Page 27 of 35
101
(a)
ip t
99 98
cr
Weight /%
100
96
0
100
200
300
400
101
600
M
(b)
100
d
99 98
te
Weight /%
500
an
t/min
us
97
Ac ce p
97 96
0
100
200
300
400
500
600
t/min
Fig. 2. Weight of (a) [Et2NEmim][PF6] and (b) [Et2NEmpyr][PF6] as a function of time at different
temperatures:
, 100 oC;
, 120 oC;
, 140 oC.
29
Page 28 of 35
20000
(a)
1.29 1.28
10000
5000
1.27 1.26 20
(b)
15000
/mPa·s
/(g/cm3)
1.30
30
40
50
60
0 20
70
o
30
40
50
ip t
1.31
o
60
70
Temperature / C
cr
Temperature / C
, [Et2NEmpyr][PF6]. Solid lines are the results calculated by equations.
Ac ce p
te
d
M
an
[Et2NEmim][PF6];
us
Fig. 3. (a) Densities and (b) viscosities of the ILs at temperatures from 25 to 70 oC: ●,
30
Page 29 of 35
1.0
ip t
0.8 0.6
cr
0.4 0.2 0.0
0
50
100
150
200
an
t/min
us
Mole ratio of SO2 to IL
1.2
Fig. 4. Absorption of SO2 with a flow rate of 50 cm3/min by the ILs at 60 oC: ●, [Et2NEmim][PF6];
Ac ce p
te
d
M
, [Et2NEmpyr][PF6].
31
Page 30 of 35
ip t
1.5
cr
1.0
0.5
0.0
0
10
20
30
40
50
60
an
t/min
us
Mole ratio of SO2 to IL
2.0
Fig. 5. Effect of temperature on the absorption of SO2 by [Et2NEmim][PF6]: ■, 30 oC; ●, 40 oC; ▲,
Ac ce p
te
d
M
50 oC; ▼, 60 oC.
32
Page 31 of 35
Mole ratio of SO2 to IL
2.0
ip t
1.5
cr
1.0
0.0
0
50
100
150
us
0.5
200
an
t/min
250
300
Fig. 6. Absorption of pure SO2 and 3% SO2 by [Et2NEmim][PF6] and [Bmim][PF6] at 30 oC: ■,
M
[Et2NEmim][PF6] (pure SO2); ●, [Bmim][PF6] (pure SO2);
, [Et2NEmim][PF6] (3% SO2);
,
Ac ce p
te
with 3% SO2 is 100 cm3/min.
d
[Bmim][PF6] (3% SO2). The flow rate of pure SO2 is 50 cm3/min and that of simulated flue gas
33
Page 32 of 35
1.5
1.0
0.5
0.0
0
10
20
30
40
50
0.6 0.4 0.2 0.0
60
(b)
0.8
0
50
100
150
t/min
200
250
300
cr
t/min
ip t
Mole ratio of SO2 to IL
Mole ratio of SO2 to IL
1.0
(a)
2.0
us
Fig. 7. Effect of water on the absorption of (a) pure SO2 with a flow rate of 50 cm3/min and (b) 3% SO2 with a flow rate of 100 cm3/min at 30 oC: ●, [Et2NEmim][PF6];
Ac ce p
te
d
M
an
20 wt% H2O.
, [Et2NEmim][PF6] +
34
Page 33 of 35
0.4
0.2
0.0
0
10
20
30
40
50
60
(b)
1.2 1.0 0.8 0.6 0.4 0.2 0.0
0
50
100
150
0.5
2
3
4
5
Cycles
2.0 (d)
1.5
an
1.0
1
250
300
us
Mole ratio of SO2 to IL
(c)
1.5
0.0
200
t/min
1.0
0.5
0.0
M
Mole ratio of SO2 to IL
t/min
2.0
ip t
0.6
1.4
cr
Weight of SO2 / g
Weight of SO2 and water / g
(a)
0.8
1
2
3
4
5
Cycles
d
Fig. 8. The desorption of SO2 from (a) [Et2NEmim][PF6] and (b) [Et2NEmim][PF6] + 20 wt% H2O,
Ac ce p
te
and the reuse of (c) [Et2NEmim][PF6] and (d) [Et2NEmim][PF6] + 20 wt% H2O.
35
Page 34 of 35
ip t cr us an M NMR
spectra
te
d
Fig. 9. FT-IR spectra of (a) [Et2NEmim][PF6] and (b) water-saturated [Et2NEmim][PF6] and 1H of
(c)
[Et
2
NEmim][PF
6
]
and
(d)
water-
Ac ce p
saturated [Et 2 NEmim][PF 6 ] before and after SO 2 absorption: A, IL; B, IL-SO 2 .
36
Page 35 of 35
