Specific Heat Capacities of Two Functional Ionic Liquids and Two

Article pubs.acs.org/jced

Specific Heat Capacities of Two Functional Ionic Liquids and Two Functional Deep Eutectic Solvents for the Absorption of SO2 Kai Zhang,† Haomin Li,‡ Shuhang Ren,† Weize Wu,*,† and Yuyun Bao† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Health Science Center, Peking University, Beijing 100191, China

‡

S Supporting Information *

ABSTRACT: The specific heat capacities (Cp) of four absorbents (1,1,3,3-tetramethlylguanidinium lactate, monoethanolammonium lactate, betaine-ethylene glycol, and L-carnitineethylene glycol), which can chemically absorb sulfur dioxide (SO2) with low concentrations, were measured by a Calvet BT 2.15 calorimeter in a temperature range of 318.15 K−363.15 K. The measured specific heat capacities as a function of temperature were correlated with a second-order empirical polynomial equation. The result indicates that the equation can accurately fit the measured specific heat capacities. All the measured specific heat capacities increase with increasing temperature. 1,1,3,3-Tetramethlylguanidinium lactate has the lowest specific heat capacity. The measured specific heat capacities of the four absorbents are lower than that of water, which is beneficial to the desulfurization process.

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INTRODUCTION The removal and recovery of sulfur dioxide (SO2) has become more and more important, as it is a hazardous gas if released to the environment, but also a useful chemical if used properly.1 In the past decade, functional ionic liquids (ILs), which have considerable solubility of low-concentration SO2,2 were proposed as SO2 absorbents instead of limestone, which is most widely used in industrial flue gas desulfurization, due to the unique physicochemical properties of the ILs, such as low vapor pressures, wide liquid temperature range, nonflammability, high thermal stability, and excellent solvation capacity.3−5 Han et al. reported the first functional IL 1,1,3,3tetramethlylguanidinium lactate (TMGL) to efficiently absorb SO2, and the result showed that the mole ratio of SO2 to IL could reach 0.98 at 40 °C with 8% SO2 in N2.6 Later, Wu et al. investigated a series of stable functional ILs based on lactate as anion for capturing low-concentration SO2, and all the investigated functional ILs exhibited good performance on the absorption of low-concentration SO2.7 Among them, the functional ILs, monoethanolammonium lactate (MEAL) and TMGL, are more promising for large-scale application on SO2 removal because of their low cost, environmentally friendly property, and high solubility of low-concentration SO2. Recently a new type of solvent, named deep eutectic solvent (DES), which shares similar characteristics and properties as ILs, has also gained attention for SO2 removal.8,9 DESs are typically mixtures combined by a hydrogen-bond acceptor (HBA) such as quaternary ammonium salts and a hydrogenbond donor (HBD) such as glycerol.10 The DESs which can capture low-concentration SO2 are called functional DESs. In our previous work, we developed two functional DESs: betaine (Bet) and L-carnitine (L-car) as HBA and ethylene glycol (EG) as HBD with nHBA/nHBD = 1:3, and both of them exhibited good performance on capturing SO2 with low concentrations.11 © 2017 American Chemical Society

Their properties, such as nontoxicity, easy preparation, and no byproduct in the synthesis of Bet-EG and L-car-EG DESs, also make them promising for wide applications. On the basis of the above information, two functional ILs, TMGL and MEAL, and two functional DESs, Bet-EG and Lcar-EG, have potential to be used for industrial SO2 absorption. As we know, the physicochemical properties of functional ILs and functional DESs are necessary for the design and operation of the absorption processes. Among the physicochemical properties, specific heat capacity (Cp) is an important property, which allows calculating various temperature-dependent thermodynamic properties, such as enthalpy, entropy, and Gibbs energy.12 Moreover, the specific heat capacities of absorbents are significant to many theoretical and engineering calculations. Specific heat capacities of numerous ILs and DESs have been reported in the literature,13−18 but most of them are normal ILs and normal DESs, which cannot capture low-concentration SO2 efficiently.19 This work measured the specific heat capacities of two functional ILs (TMGL and MEAL) and two functional DESs (Bet-EG and L-car-EG), shown in Figure 1, in a temperature range of 318.15 K−363.15 K, and the measurements were correlated with a second-order empirical polynomial equation.

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EXPERIMENTAL SECTION Chemicals. Aluminum oxide (99.99%), monoethanolamine (99%), 1,1,3,3-tetramethylguanidine (99%), and lactic acid

Special Issue: Memorial Issue in Honor of Ken Marsh Received: January 30, 2017 Accepted: July 21, 2017 Published: August 17, 2017 2708

DOI: 10.1021/acs.jced.7b00102 J. Chem. Eng. Data 2017, 62, 2708−2712

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previous work.6,7 According to the 1H NMR spectra and water contents of four absorbents, the mass fraction purities of four absorbents are estimated to 98.8% for Bet-EG, 98.5% for L-carEG, 98.0% for MEAL (including 20.6% of lactic acid monoethanolamide, which was calculated based on the NMR data in Figure S1(d), due to the deep removal of water), and 98.3% for TMGL. Specific Heat Capacity Measurement. A schematic diagram of Cp-measurement experimental system for microcalorimetry is shown in Figure 3. The microcalorimetry system

Figure 1. Chemical structures of the four absorbents.

(80−85%) were obtained from Aladdin Chemical Co., Ltd. (Shanghai, China). Betaine (99%) and L-carnitine (99%) were purchased from Yuanye Bio-Technology Co., Ltd., (Shanghai, China). Ethylene glycol (99%) was purchased from Sinopharm Chemical Reagent Co., Ltd., (Beijing, China). All the chemicals are listed in Table S1. Preparation and Purification of DESs and ILs. The functional ILs and functional DESs (nHBA/nHBD = 1:3) used in this work were synthesized following the method reported in the literature.7,11,20,21 All DESs were prepared by mixing HBA (including Bet and L-car) and EG, and then stirred at 60 °C for an hour. The MEAL and TMGL were synthesized by neutralization of equimolar monoethanolamine and 1,1,3,3tetramethylguanidine, with lactic acid in water. The synthesis apparatus of ionic liquid is shown in Figure 2. The purification

Figure 3. Schematic diagram of the Cp-measurement experimental system for microcalorimetry. 1, Calvet BT 2.15 calorimeter; 2, calorimeter controller; 3, data collection system.

was mainly composed of a calorimeter, a calorimeter controller, and a data collection system. The calorimeter was a Calvet BT 2.15 system, supplied by Setaram, France. The temperature of the calorimeter was controlled within ±0.01 K, and the thermal sensitivity of the calorimeter was 1 μW. The calorimeter had two cells, a reference cell and an equilibrium cell. Initially, the reference cell and equilibrium cell were empty (filled with nothing), and the experiment initial temperature and final temperature were set at Ti (30.8 °C) and Tf (100 °C), respectively. The experiment started when the system reached the heat flow baseline of the BT 2.15 calorimeter as zero for 2 h, and the heat flow curve of the equilibrium cell could be acquired by the data collection system. After that, the equilibrium cell was filled with a reference substance (the specific heat capacity of which had been known already), and the reference cell was still empty. Then, the experiment started with the same temperature program as the above process, and the heat flow curve of equilibrium cell + reference substance was obtained, shown in Figure 4. The specific heat capacity of the reference substance was calculated by the following eq 1.

Figure 2. Synthesis apparatus of ionic liquids: 1, constant temperature water bath; 2, three-necked flask; 3, constant pressure addition funnel.

of ILs was performed using the method of sweeping N2.22 When the mixture of IL and water was obtained, the water in the ILs was evaporated by a rotary evaporator, and then the ILs were swept by bubbled N2 at 100 °C to remove the water in the ILs. The water contents of functional ILs and DESs were measured by the Karl Fischer analysis, and the water contents in mass fraction are 0.11% for Bet-EG, 0.15% for L-car-EG, 0.25% for MEAL, and 0.29% for TMGL. The identity of the DESs and ILs synthesized in this work was performed using 1H NMR.23 The 1H NMR spectra of the synthesized DESs and ILs were recorded on a 400 MHz spectrometer (Bruke Avance III, Switzerland). The relaxation delay (D1) was set for 10 s, which can ensure that relaxation time is adequate between NMR scans. Since the purity of the used D2O was 99.9%, the chemical shifts of the absorbents were based on the H chemical shift of the remaining 0.1% H2O, which is around 4.7, as references. All the 1H NMR spectra are shown in the Supporting Information, Figure S1. The 1H NMR spectra of the two ILs are in accordance with those reported in the

Figure 4. Specific heat flow curve of equilibrium cell and equilibrium cell + sample. Ab, instantaneous power of equilibrium cell; As, instantaneous power of equilibrium cell + sample. 2709

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Table 1. Relative Deviation (rel dev) of the Heat Flow at Different Temperatures T T/K = 318.15

T/K = 323.15

T/K = 328.15

T/K = 333.15

T/K = 338.15

T/K = 343.15

T/K = 348.15

T/K = 353.15

T/K = 358.15

T/K = 363.15

ΔA s As

0.018

0.013

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

ΔA r Ar

0.018

0.013

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

ΔAb Ab

0.018

0.013

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

rel dev

Cpr =

1 dQ 1 dQ /dt 1 K (A r − A b ) = = m r dT mr dT /dt mr dT /dt

RESULTS AND DISCUSSION First, the specific heat capacities of ethylene glycol at temperatures from 318.15 K to 363.15 K were measured to verify the reliability of the calorimeter used in this work. The heat flow and measured Cp curve of ethylene glycol are shown in Figure S2. The specific heat capacities of ethylene glycol measured in this experiment and reported in the literature24 are listed in Table 2. As can be seen in Table 2, the average

(1)

where Cpr is the specific heat capacity of reference substance, J· g−1·K−1; mr is the quantity of reference substance, g; Q is the heat transferred, J; T is the temperature of the calorimeter, K; t is operation time, s; K is thermal power constant which is depend on the measuring instrument; Ar is the instantaneous power of equilibrium cell + reference substance, J·s−1; Ab is the instantaneous power of equilibrium cell, J·s−1;. In this work, the reference substance was chosen as aluminum oxide. Then the equilibrium cell was washed, and a sample (IL or DES) was filled in the equilibrium cell, and the experiment was conducted using the same temperature program, and the specific heat capacity of the sample (IL or DES) was calculated by eq 2. Cps =

1 dQ 1 dQ /dt 1 K (A s − A b ) = = ms dT ms dT /dt ms dT /dt

Table 2. Specific Heat Capacities Cp of Ethylene Glycol Measured at Different Temperatures T under a Pressure P of 100.59 kPa, and Compared with Literature Dataa T/K 323.15 333.15 343.15 353.15 363.15

(2)

m r × (A s − A b ) × Cpr ms × (A r − Ab)

Cps

=

∂ ln Cps ∂mr + +

=

∂ms

Δms

∂ ln Cps ΔA r ∂ ln Cps ΔA s + ∂A r ∂A s As Ar ∂ ln Cps ∂Ab

ΔA b Ab Ar Ab

−1

−

± ± ± ± ±

2.4178 2.4921 2.5573 2.6248 2.6968

1.03 0.38 0.09 0.95 1.51

2.442 2.501 2.555 2.599 2.656

0.070 0.057 0.058 0.059 0.061

Cp = a0 + a1 × T + a 2 × T 2

ΔAb

Δms Δmr + + mr ms ⎛ ⎜ + ⎜ ⎜ ⎝

100 (|Cp−Cpb|)/Cp

absolute relative deviations between the specific heat capacities obtained in this work and those reported in the literature are 1.03%, 0.38%, 0.09%, 0.95%, and 1.51% at temperatures of 323.15 K, 333.15 K, 343.15 K, 353.15 K, and 363.15 K, respectively. Thus, the good agreement between Cp values of ethylene glycol measured in this work and those reported in the literature suggests the method used in this work to measure Cp is reliable. The heat flows and calculated Cp curves of TMGL, MEAL, Bet-EG, and L-car-EG are shown in Figure S3, Figure S4, Figure S5 and Figure S6, respectively. The measured specific heat capacities of the four absorbents were correlated as a function of temperature by a second-order empirical polynomial equation shown as the following eq 5.

(3)

∂ ln Cps

Δmr +

Cpb/J·g−1·K−1

Standard uncertainties u are u(T) = 0.01 K, and u(P) = 0.73 kPa. The uncertainty following the ± sign is standard uncertainty. bCp values of ethylene glycol are obtained from ref 24.

On the basis of eq 3, the uncertainties of measured Cp were estimated using the following eq 4. ΔCps

Cp/J·g−1·K−1

a

where Cps is the specific heat capacity of sample (IL or DES), J· g−1·K−1; ms is the quantity of sample (IL or DES), g; As is the instantaneous power of equilibrium cell + sample (IL or DES), J·s−1. Then, the specific heat capacity of the sample (IL or DES) can be calculated by eq 3, which was obtained by dividing eq 1 with eq 2. Cps =

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ΔA s As

1−

⎞ ⎟ ⎟ − 1 ⎟⎠

Ab As

+

where Cp is the specific heat capacity of the four absorbents measured in this work, J·g−1·K−1; T is the conducted temperature, K; and ai is the parameter. The parameters of the four absorbents in eq 5 are listed in Table 3, and the absolute relative deviation σ was calculated by eq 6.

ΔA r Ar

1−

Ab Ar

ΔA b Ab As Ab

(5)

Table 3. Parameters for Equation 5 of the Four Absorbents (4)

where, Δmr = 0.001, Δms = 0.001, and the relative deviations of heat flow at different temperatures are listed in Table 1. The estimated uncertainties for each data point are listed in the data tables. 2710

absorbents

a0

a1

a2/10−5

TMGL MEAL Bet-EG L-car-EG

−7.265 −2.639 −3.334 −3.026

0.0436 0.0193 0.0292 0.0261

−4.842 −1.598 −3.746 −3.149

DOI: 10.1021/acs.jced.7b00102 J. Chem. Eng. Data 2017, 62, 2708−2712

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|Cpcal − Cp| Cp

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Table 6. Measured Specific Heat Capacities Cp and Calculated Specific Heat Capacities Cpcal of Bet-EG under a Pressure P of 100.59 kPa, and Absolute Relative Deviations σa

100 (6)

where Cp is the specific heat capacity measured and the Cpcal is the specific heat capacity calculated by eq 5. The measured specific heat capacities Cp, calculated specific heat capacities Cpcal, and absolute relative deviation σ are listed in Table 4 and Table 5, Table 6, and Table 7 for functional ILs

T/K 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15 363.15

Table 4. Measured Specific Heat Capacities Cp and Calculated Specific Heat Capacities Cpcal of TMGL under a Pressure P of 100.59 kPa, and Absolute Relative Deviations σa T/K 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15 363.15

Cp/J·g−1·K−1

Cpcal/J·g−1·K−1

σ /%

± ± ± ± ± ± ± ± ± ±

1.726 1.783 1.844 1.903 1.959 2.013 2.064 2.108 2.154 2.198

0.116 0.391 0 0.052 0.204 0.348 0.389 0.190 0.046 0.272

1.724 1.790 1.844 1.902 1.955 2.006 2.056 2.104 2.155 2.204

0.067 0.055 0.045 0.047 0.048 0.050 0.051 0.052 0.054 0.055

318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15 363.15

Cpcal/J·g−1·K−1

σ/%

± ± ± ± ± ± ± ± ± ±

1.893 1.939 1.984 2.028 2.071 2.114 2.156 2.193 2.233 2.272

0.211 0.513 0.101 0 0.145 0.284 0.372 0.320 0 0.394

1.897 1.949 1.986 2.028 2.068 2.108 2.148 2.186 2.233 2.281

0.074 0.060 0.049 0.050 0.051 0.052 0.053 0.053 0.054 0.054

σ/%

± ± ± ± ± ± ± ± ± ±

2.172 2.198 2.222 2.244 2.265 2.284 2.301 2.314 2.327 2.338

0.046 0.272 0 0.044 0.044 0.131 0.174 0.173 0 0.128

2.173 2.204 2.222 2.245 2.264 2.281 2.297 2.310 2.327 2.341

0.085 0.068 0.055 0.055 0.056 0.057 0.057 0.057 0.058 0.058

Standard uncertainties u are u(T) = 0.01 K, and u(P) = 0.73 kPa. The uncertainty following the ± sign is standard uncertainty.

Table 7. Measured Specific Heat Capacities Cp and Calculated Specific Heat Capacities Cpcal of L-car-EG under a Pressure P of 100.59 kPa, and Absolute Relative Deviations σa T/K 318.15 323.15 328.15 333.15 338.15 343.15 348.15 353.15 358.15 363.15

Table 5. Measured Specific Heat Capacities Cp and Calculated Heat Capacities Cpcal of MEAL under a Pressure P of 100.59 kPa, and Absolute Relative Deviations σa Cp/J·g−1·K−1

Cpcal/J·g−1·K−1

a

a Standard uncertainties u are u(T) = 0.01 K, and u(P) = 0.73 kPa. The uncertainty following the ±sign is standard uncertainty.

T/K

Cp/J·g−1·K−1

Cp/J·g−1·K−1

Cpcal/J·g−1·K−1

σ/%

± ± ± ± ± ± ± ± ± ±

2.083 2.112 2.140 2.166 2.192 2.213 2.236 2.254 2.273 2.290

0.047 0.283 0.046 0.046 0.091 0.045 0.179 0.222 0 0.217

2.084 2.118 2.141 2.167 2.190 2.212 2.232 2.249 2.273 2.295

0.081 0.065 0.053 0.053 0.054 0.055 0.055 0.056 0.056 0.057

a Standard uncertainties u are u(T) = 0.01 K, and u(P) = 0.73 kPa. The uncertainty following the ± sign is standard uncertainty.

a

Standard uncertainties u are u(T) = 0.01 K, and u(P) = 0.73 kPa. The uncertainty following the ± sign is standard uncertainty.

and functional DESs, respectively. As shown in Tables 4−7, the absolute relative deviations σ obtained for all samples are very low. For instance, the absolute relative deviations σ for MEAL are 0.513%, 0%, 0.284%, 0.32%, and 0.394% at temperatures of 323.15, 333.15, 343.15, 353.15, and 363.15 K, respectively. The results demonstrate that the eq 5 can well describe the relationship between specific heat capacity and temperature. The measured specific heat capacities as a function of temperature are plotted in Figure 5. As shown in Figure 5, an increase in specific heat capacity with increasing temperature is observed for all the four samples, and TMGL functional IL has the lowest specific heat capacity. The specific heat capacities of two functional DESs are close to each other, as are the two functional ILs. For example, the specific heat capacities of BetEG and L-car-EG are 2.173 J·g−1·K−1 and 2.084 J·g−1·K−1, respectively, and the specific heat capacities of TMGL and MEAL are 1.724 J·g−1·K−1 and 1.897 J·g−1·K−1, respectively, at

Figure 5. Measured specific heat capacities of the four absorbents. ■, Bet-EG; ●, L-car-EG; ▲, MEAL; ▼, TMGL.

a temperature of 318.15 K. The close values for two functional DES and ILs may be due to the similarity of their molecular structure, respectively. All the four samples have of lower specific heat capacities compared with water.24 The smaller is the specific heat capacity, the lower is the amount of specific heat required to raise the temperature by the same amount. The temperature has a significant influence on SO2 solubility; that is, SO2 solubility decreases with increasing temperature;7 hence, low temperatures are favorable for the absorption process and high 2711

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(6) Wu, W. Z.; Han, B. X.; Gao, H. X.; Liu, Z. M.; Jiang, T.; Huang, J. Desulfurization of Flue Gas: SO2 Absorption by an Ionic Liquid. Angew. Chem., Int. Ed. 2004, 43, 2415−2417. (7) Tian, S. D.; Hou, Y. C.; Wu, W. Z.; Ren, S. H.; Zhang, C. Absorption of SO2 by Thermal-stable Functional Ionic Liquids with Lactate Anion. RSC Adv. 2013, 3, 3572−3577. (8) Liu, B. Y.; Zhao, J. J.; Wei, F. X. Characterization of Caprolactam Based Eutectic Ionic Lquids and their Application in SO2 Absorption. J. Mol. Liq. 2013, 180, 19−25. (9) Sun, S. Y.; Niu, Y. X.; Xu, Q.; Sun, Z. C.; Wei, X. H. Efficient SO2 Absorptions by Four Kinds of Deep Eutectic Solvents Based on Choline Chloride. Ind. Eng. Chem. Res. 2015, 54, 8019−8024. (10) Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K.; Tambyrajah, V. Novel Solvent Properties of Choline Chloride/Urea Mixtures. Chem. Commun. 2003, 1, 70−71. (11) Zhang, K.; Ren, S. H.; Hou, Y. C.; Wu, W. Z. Efficient Absorption of SO2 with Low-partial Pressures by Environmentally benign Functional Deep Eutectic Solvents. J. Hazard. Mater. 2017, 324, 457−463. (12) Ahmadi, A.; Haghbakhsh, R.; Raeissi, S.; Hemmati, V. A Simple Group Contribution Correlation for the Prediction of Ionic Liquid Heat Capacities at Different Temperatures. Fluid Phase Equilib. 2015, 403, 95−103. (13) Leron, R. B.; Li, M. H. Molar Heat Capacities of Choline Chloride-based Deep Eutectic Solvents and their Binary Mixtures with Water. Thermochim. Acta 2012, 530, 52−57. (14) Paulechka, Y. U. Heat Capacity of Room-Temperature Ionic Liquids: A Critical Review. J. Phys. Chem. Ref. Data 2010, 39, 24−260. (15) Rocha, M. A. A.; Bastos, M.; Coutinho, J. A. P.; Santos, L. M. N. B. F. Heat Capacities at 298.15 K of the Extended [CnC1im][Ntf2] Ionic Liquid Series. J. Chem. Thermodyn. 2012, 53, 140−143. (16) Rocha, M. A. A.; Coutinho, J. A. P.; Santos, L. M. N. B. F. Evidence of Nanostructuration from the Heat Capacities of the 1,3Dialkylimidazolium Bis(trifluoromethylsulfonyl)imide Ionic Liquid Series. J. Chem. Phys. 2013, 139, 104502. (17) Yamamuro, O.; Minamimoto, Y.; Inamura, Y.; Hayashi, S.; Hamaguchi, H. O. Heat Capacity and Glass Transition of an Ionic Liquid 1-butyl-3-methylimidazolium Chloride. Chem. Phys. Lett. 2006, 423, 371−375. (18) Zábranský, M.; Ruzicka, V., Jr; Domalski, E. S. Heat Capacity of Liquids: Critical Review and Recommended Values. Supplement I. J. Phys. Chem. Ref. Data 2001, 30, 1199−1689. (19) Jin, M. M.; Hou, Y. C.; Wu, W. Z.; Ren, S. H.; Tian, S. D.; Xiao, L.; Lei, Z. G. Solubilities and Thermodynamic Properties of SO2 in Ionic Liquids. J. Phys. Chem. B 2011, 115, 6585−6591. (20) Yuan, X. L.; Zhang, S. J.; Lu, X. M. Hydroxyl Ammonium Ionic Liquids: Synthesis, Properties, and Solubility of SO2. J. Chem. Eng. Data 2007, 52, 596−599. (21) Gao, H. X.; Han, B. X.; Li, J. C.; Jiang, T.; Liu, Z. M.; Wu, W. Z.; Chang, Y. H.; Zhang, J. M. Preparation of Room-Temperature Ionic Liquids by Neutralization of 1,1,3,3-Tetramethylguanidine with Acids and Their Use as Media for Mannich Reaction. Synth. Commun. 2004, 36, 3083−3089. (22) Ren, S. H.; Hou, Y. C.; Wu, W. Z.; Liu, W. N. Purification of Ionic Liquids: Sweeping Solvents by Nitrogen. J. Chem. Eng. Data 2010, 55, 5074−5077. (23) Zhang, Q. G.; Wei, Y.; Sun, S. S.; Wang, C.; Yang, M.; Liu, Q. S.; Gao, Y. A. Study on Thermodynamic Properties of Ionic Liquid NButyl-3-methylpyridinium Bis(trifluoromethylsulfonyl)imide. J. Chem. Eng. Data 2012, 57, 2185−2190. (24) Yang, C. S.; Ma, P. S.; Xia, S. Q. Heat Capacity of Glycol Determined by Differential Scanning Calorimeter. Tianjin Daxue Xuebao 2003, 36, 192−196.

temperatures are favorable for the desorption process. Because of the lower specific heat capacities of the four absorbents compared with that of water, less energy is needed to change their temperature during cycles of absorption/desorption of SO2 by changing temperature.

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CONCLUSIONS The specific heat capacities of four absorbents (TMGL, MEAL, Bet-EG, and L-car-EG), which can chemically absorb SO2 with low concentrations, were measured in a temperature range of 318.15 K−363.15 K. The specific heat capacities of the the four absorbents were correlated with a second-order empirical polynomial equation with high accuracy. The specific heat capacities of the four absorbents are lower than that of water. And they increase with increasing temperature. Functional IL TMGL has the lowest specific heat capacity among the four absorbents. The low specific heat capacities of the four absorbents indicate that less energy is needed to change their temperature, which is beneficial to the absorption/desorption of SO2 by changing temperature.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00102. Detailed information about the chemicals used in the present study, the 1H NMR spectra of the four absorbents, and the curves of heat flow and measured Cp for EG, TMGL, MEAL, Bet-EG, and L-car-EG (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86 10 64427603. ORCID

Weize Wu: 0000-0002-0843-3359 Funding

The project is supported financially by the National Natural Science Foundation of China (No. 21176020 and No. 21306007), and the Long-Term Subsidy Mechanism from the Ministry of Finance and the Ministry of Education of PRC (BUCT). Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.jced.7b00102 J. Chem. Eng. Data 2017, 62, 2708−2712