Solubility Properties and Absorption Mechanism Investigation of Dilute

Article pubs.acs.org/jced

Solubility Properties and Absorption Mechanism Investigation of Dilute SO2 in Propylene Glycol Monomethyl Ether + Dimethyl Sulfoxide System Xianshu Qiao, Fei Zhang, Feng Sha, Huihu Shi, and Jianbin Zhang* College of Chemical Engineering, Inner Mongolia University of Technology, Hohhot 010051, China ABSTRACT: In this work, the propylene glycol monomethyl ether (PGME) + dimethyl sulfoxide (DMSO) system was used in absorption process of SO2, and the gas−liquid equilibrium (GLE) data of SO2 in the PGME + DMSO system were determined at T = 298.15, 303.15, and 308.15 K and p = 123.15 kPa. The experimental data are used to develop SO2 solubility model, which shows a potential application for SO2 removal. After that, Henry’s law constants (HLCs) were calculated by fitting the linear slope of solubility, and thermodynamic parameters were computed on the basis of the correlation of HLCs. The regeneration experimental results showed that 98.5% SO2 can be desorbed at 333.15 K and N2 bubbling in 12 min, and no significant loss of absorption capacity for absorption solvents. Additionally, FTIR, UV−vis, and 1H NMR spectra were recorded to discuss the interaction between SO2 and the binary systems of PGME + DMSO, and the results indicated that intermolecular hydrogen bonding association among PGME, DMSO, and SO2 was formed.

1. INTRODUCTION Overuse of fossil fuels results in the large emission of SO2,1,2 which caused the increasingly serious climate situation and some environmental problems related to public health and human productive activities.3−5 So, there is no time to lose to control the emissions of SO2 by the development of low-carbon economy by effective measures.6 Absorption method by organic solvents is considered as quite efficient and is widely used in industry for the mitigation of SO2 emission from coal and diesel fuel power plants due to large absorption capacity and low cost.7−9 Thus, it was highly desirable to develop green, economical, and reversible absorbents. In recent years, various organic solvents, including amines10,11 and ionic liquids (ILs)12,13 were under investigation. However, amines present various inherent defects of high energy cost and degradation of amines for SO2 capture.14 Although ILs show strong solubility and selectivity to SO2 when they are used to capture SO2, the high viscosity and cost restrict ILs’ practical industrial applications.15,16 An alternate approach is scrubbing using polar organic solvents, and the physical solvents can be regenerated by changing operational conditions, which lead to lower energy consumption, so that it is a desirable solvent to remove SO2. As new desulfurizers, alcohols and their derivatives were good choices because of high efficiency, economic, environmental protection, and absence of byproducts for industrial application.17,18 As an alcohol derivative, propylene glycol monomethyl ether (PGME) was used as scrubbing liquid to remove SO2 from exhaust air and gas streams of production plants due to its excellent characteristics, including suitable vapor pressure, less toxicity, low melting point, suitable viscosity, and high chemical stability.19,20 Considering the low volatility and good affinity to © XXXX American Chemical Society

SO2, DMSO was added to PGME to form the binary system of PGME + DMSO and to boost SO2 capture capacity; meanwhile, the PGME + DMSO system overcomes the disadvantage of pure DMSO’s relatively high freezing point of 18 °C.21,22 Hence, the system of PGME + DMSO showed a potential application in SO2 removal, and it was of significance to study the absorption capacity and absorption mechanism of the binary system for SO2 absorption. In our previous work,23 the researches of physical properties for the system had been reported, including density and viscosity at different temperatures. As a continuation report, in this work the solubility were determined by equilibrium experiment for dilute SO2 in PGME + DMSO binary system at T = 298.15, 303.15, 308.15 K and p = 123.15 kPa. The absorption capacity of different alcohol and alcohol derivatives was evaluated by comparing the value of the Henry’s law, and the result showed PGME presented stronger absorption ability than other alcohol and alcohol derivatives. The thermodynamical parameters of dissolving SO2 in the PGME + DMSO binary system were obtained from the gas−liquid equilibrium (GLE) data. Regeneration studies were conducted on the basis of the regeneration data. Additionally, the intermolecular interaction among PGME, DMSO, and SO2 was also studied from FTIR, UV−vis, and 1H NMR spectral results.

2. EXPERIMENTAL SECTION 2.1. Materials. The pure N2 (≥99.9%) and certified standard mixed gas of SO2 in N2 (0.1 vol % SO2 and 99.9 Received: July 22, 2016 Accepted: May 17, 2017

A

DOI: 10.1021/acs.jced.6b00660 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Description of Chemistry Used in Present Work chemical name

source

basis of purity (%)

purification methoda

mixed SO2 + N2 gas pure N2 dimethyl sulfoxide propylene glycol monomethyl ether double distilled water sodium thiosulfate (Na2S2O3) iodine (I2)

Beijing Gas Company Beijing Gas Company Sinopharm Chemical Reagent Co., Ltd., China Sinopharm Chemical Reagent Co., Ltd., China Inner Mongolia University of Technology Tianjin Chemical Reagent Corporation Tianjin Chemical Reagent Corporation

1% 99.9% ≥99.0% ≥0.98% ≤0.1 mS cm−1 (conductivity, 25 °C) ≥99.9 wt % ≥99.5 wt %

none none desiccation and degasification desiccationc and degasificationd degasification none none

a

Molecular sieve type 4A and Ultrasound.

known volume of standard I2 solution in which experimental temperature was controlled at 0.01 K using a circulation water bath. In the liquid phase, the relative uncertainty of SO2 was found at 0.6%. The mass fraction of samples was weighed by a Sartorius BS 224S analytical balance, in which the standard uncertainty of mass fraction was found at 0.0001. 2.3. Absorption and Desorption Experiments. Absorption and desorption experiments were carried out to investigate the adsorption and regeneration processes of PGME (1) + DMSO (2) system for SO2, and 5 g of mixture (w1 = 50%) was employed to conduct absorption and desorption experiments. SO2 adsorption experiments were carried out in a 25 mL washing bottle, and the systems were exposed to SO2 until the quality of solvent did not observably increase (about 40 min) under a constant gas flow rate, which was adjusted by a Mass flow meter (SY-9312), and every 4 min recorded the quality of solvent and SO2. SO2 desorption experiment was performed by bubbling 80 mL·min−1 N2 gas and plunged into a 333.15 K water bath, and the qualities of SO2 release were recorded every 2 min with an analytical balance. 2.4. Spectral Analyses. Spectrometric experiments (UV− vis, FTIR, and 1H NMR) were conducted to analyze the SO2 absorption mechanism. UV−vis spectrometer (UV 2450 PC Shimadzu) was used to conduct UV−vis experiments with a resolution of 1 nm in the region of 190−400 nm. A Nicolet Nexus 670 FTIR spectrometer was used with a typical thin film method from 400 to 4000 cm−1 with 1 cm−1. 1H NMR experiments were performed using a 500 MHz Bruker Avance III spectrometer with external references. Samples and external references (d6-DMSO or CDCl3) were injected into capillary tubes (25 cm × 0.9 mm) and NMR tubes (17.8 cm × 5 mm), respectively.

vol % N2) were purchased from Beijing Gas Company. PGME and DMSO were purchased from Sinopharm Chemical Reagent Company. Sodium thiosulfate (≥99.9%, Na2S2O3) and iodine (≥99.5%, I2) of iodometry were obtained from Tianjin Chemical Reagent Company. Table 1 provides a summary of chemicals used in this work. 2.2. Solubility Measurements. The determination apparatus for SO2 absorption was used in this work and is shown in Figure 1, and the experimental process and data

Figure 1. Sketch of the experimental apparatus: (1) jacketed vessel; (2) cold trap; (3) thermostatic bath; (4) gas circulatory pump; (5) flue gas analyzer; (6) regulating valve; (7) thermometer; (8) pressure meter; (9) SO2/N2 gas cylinder; (10) buffer; (11) absorption apparatus; and (12) liquid circulatory pump.

treatment method were identical to the previous works.24 First, about 400 mL of solvent with a certain concentration of PGME was added into the jacketed vessel; a CS501SYC thermostatic bath with a Beckmann thermometer was adjusted to keep at a constant temperature of jacketed vessel with an accurate thermometer from Fuqiang Meter Factory, and the standard uncertainty of temperature was 0.01 K. Then, about 3000 mL of certified standard gas SO2 in N2 was bubbled into the experimental system to reach a certain total pressure, which was determined by a pressure gauge from Fuqiang Meter Factory, and the standard uncertainty of total pressure was evaluated to be 0.10 kPa. Finally, the gas of systems were recycled about 20 min by a gas recycle pump to achieve gas−liquid equilibrium. In addition, the SO2 concentrations in the gas phase were determined. On the basis of the above operation, a series of GLE data were determined. The gas phase SO2 concentration was measured using a Testo 350 flue gas analyzer (Testo Company, Germany), and the gas partial pressure of the solvent is not considered in the experience. The relative uncertainty of gas phase SO 2 concentration was evaluated to be 5%. Once equilibrium was reached, SO2 concentration in the liquid phase was measured through adding a known volume of solution absorbing SO2 to a

3. RESULTS AND DISCUSSION 3.1. Effect of Absorbent Ratio for Henry’s Constant. The equilibrium partial pressures of SO2 were measured from the obtained ySO2, which was determined via the equation pSO /Pa = ySO × p 2

2

(1)

where ySO2 was the SO2 concentration in the gas phase in ppm, and p was the actual total pressure value when the dissolved and released SO2 arrived at the equilibrium. GLE data of dilute SO2 for the different PGME concentration in various PGME (1) + DMSO (2) mixtures were determined at pSO2 = 0−130 Pa and T = 298.15, 303.15, and 308.15 K. The results are listed in Tables 2−4 and are shown in Figures 2−4. pSO2 and bSO2 denote partial pressure of SO2 in the gas phase and the concentration of SO2 in the liquid phase when the dissolved and released SO2 arrived at the equilibrium, respectively. B



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Table 2. GLE Data for PGME (1) + DMSO (2) + SO2 (3) + N2 (4)a w1 (%)

bSO2/(mol kg−1)

pSO2/Pa

106ySO2

w1 (%)

bSO2 (mol kg−1)

pSO2/Pa

106ySO2

0 0 0 0 0 0 0 0 0 0 20 20 20 20 20 20 20 20 20 20 20 20 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 40 60 60 60

0.00773 0.01815 0.03041 0.04720 0.05948 0.08219 0.10124 0.11374 0.14036 0.14776 0.00760 0.01235 0.01812 0.02987 0.03850 0.04871 0.06436 0.07485 0.08523 0.09639 0.10639 0.11409 0.00279 0.00380 0.00525 0.00593 0.00951 0.01108 0.01311 0.01680 0.02044 0.02382 0.02838 0.03327 0.04007 0.04489 0.05006 0.06045 0.00371 0.00614 0.01262

8.61 20.2 28.8 41.4 51.2 69.8 82.6 91.5 113 124 10.2 16.3 22.3 32.4 42.8 54.0 71.9 80.5 92.8 105 116 126 7.65 10.0 13.8 15.4 21.9 24.8 29.1 33.7 39.3 49.6 58.7 69.7 82.1 90.3 102 121 10.7 18.1 34.6

70 163 233 334 413 561 667 738 920 1003 82 132 180 262 346 436 581 652 751 846 938 1021 62 81 112 125 177 201 235 273 318 402 475 566 662 731 828 976 87 147 280

60 60 60 60 60 60 60 60 60 70 70 70 70 70 70 70 70 70 70 80 80 80 80 80 80 80 80 80 80 80 80 100 100 100 100 100 100 100 100 100 100

0.01656 0.02076 0.02519 0.02690 0.02935 0.03345 0.04012 0.04432 0.04948 0.00323 0.00393 0.00666 0.01024 0.01626 0.01913 0.02376 0.02802 0.03333 0.03088 0.00226 0.00305 0.00438 0.00505 0.00781 0.00999 0.01167 0.01354 0.01460 0.01699 0.01888 0.02098 0.00159 0.00336 0.00465 0.00575 0.00771 0.00868 0.00991 0.01288 0.01354 0.01609

42.7 53.9 61.6 68.3 76.6 84.3 101 111 124 10.0 13.6 24.8 38.3 60.7 72.1 87.4 104 127 117 10.5 15.2 22.3 30.1 43.6 62.6 70.5 78.9 89.2 101 112 122 11.4 24.4 36.9 50.4 64.7 73.3 87.0 107 111 130

347 438 499 551 621 682 816 903 1003 81 110 201 311 492 584 709 843 1026 942 85 123 181 244 354 505 569 641 721 811 905 987 92 198 298 409 523 593 703 865 895 1053

At T = 298.15 K and p = 123.15 kPa with standard uncertainty (u) and relative uncertainty (ur), w1 (%), bSO2 (mol kg−1), pSO2 (Pa) and 106ySO2 donates the mass fraction of PGME concentration, the solubility of SO2 in the liquid phase, the partial pressure of SO2 in the gas phase and the mole fraction of SO2 in the gas phase, respectively. Standard uncertainties u are u(w1) = 0.0001, u(T) = 0.01 K and u(p) = 0.10 kPa. Relative uncertainties ur are ur(ySO2) = 0.05 and ur(bSO2) = 0.006 (level of confidence = 0.68). a

where HLC (T, P) is Henry’s constant in terms of molality; and fgas is the gas phase fugacity of SO2 in Pa. Because of the relative low experimental pressure of SO2 in this work, the fugacity of SO2 was approximately simplified to be equilibrium pressure of SO2. Henry’s law reflects the linear dependence of gas molality in liquid phase with the equilibrium pressure at infinite dilution.25 Therefore, HLC studied was derived from linear slope by fitting equilibrium pressure with molality of SO2. The Henry’s constants of SO2 at T = 298.15−308.15 K are listed in Table 5. As shown in Table 5, the Henry’s law constants values decreased with the increasing DMSO concentration, which demonstrated the absorption capacity increased; meanwhile, the other HLC values was between the maximum value of pure PGME and the minimum value of pure DMSO at all

From Figures 2−4, GLE curves of PGME + DMSO for dilute SO2 absorption present high linear relationship, and the linear extrapolation curves approximately passed through the zero point for all temperatures and PGME ratio, which means that the dissolution of SO2 in the binary system of PGME + DMSO may be a physical processes. As well-known, Henry’s law can conveniently describe physical dissolution behavior of gas.25 Therefore, on the basis of the experimental GLE data, Henry’s law constants were obtained from eq 225−29 HLC (T , P) ≡ lim

bi → 0

PSO2 f gas ≈ bSO2 bSO2

(2) C



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Table 3. GLE data for PGME (1) + DMSO (2) + SO2 (3) + N2 (4)a w1 (%)

bSO2/(mol kg−1)

pSO2/Pa

106ySO2

w1 (%)

bSO2 (mol kg−1)

pSO2/Pa

106ySO2

0 0 0 0 0 0 0 0 0 0 0 0 0 20 20 20 20 20 20 20 20 20 20 20 20 40 40 40 40 40 40 40 40 40 40 40 40 40 60 60 60 60 60 60

0.00470 0.00717 0.01403 0.02109 0.03063 0.04063 0.05099 0.06255 0.07428 0.08657 0.09721 0.10547 0.11464 0.00380 0.00600 0.01665 0.02404 0.03086 0.03850 0.04553 0.06105 0.05261 0.06855 0.07540 0.08431 0.00203 0.00253 0.00478 0.00768 0.01118 0.01474 0.01814 0.02439 0.02770 0.03173 0.03650 0.04621 0.05156 0.00200 0.00323 0.00459 0.00622 0.00932 0.01324

6.32 10.3 17.8 26.4 35.6 46.8 56.3 68.0 81.3 95.3 108 116 126 6.56 12.5 27.8 38.9 49.9 57.9 68.7 94.2 78.5 101 114 127 3.33 6.68 14.5 22.1 30.7 39.3 45.1 61.3 69.4 76.2 86.3 109 124 8.43 11.4 17.7 24.7 35.6 47.8

51 83 143 213 287 379 455 550 657 769 871 941 1026 53 101 224 314 403 470 557 762 633 817 925 1029 27 54 117 179 248 317 365 496 560 615 699 886 1006 68 92 144 200 288 389

60 60 60 60 60 60 70 70 70 70 70 70 70 70 70 70 70 70 70 80 80 80 80 80 80 80 80 80 80 80 80 80 100 100 100 100 100 100 100 100 100 100 100 100

0.01759 0.02394 0.02905 0.03213 0.03497 0.03749 0.00221 0.00272 0.00398 0.00538 0.00712 0.00861 0.01005 0.01226 0.01612 0.01783 0.02004 0.02196 0.02441 0.00154 0.00245 0.00379 0.00482 0.00644 0.00712 0.00905 0.01057 0.01137 0.01319 0.01549 0.01799 0.01888 0.00116 0.00169 0.00310 0.00446 0.00489 0.00589 0.00673 0.00746 0.00944 0.01097 0.01194 0.01302

62.4 83.6 97.8 110 122 126 8.04 11.6 19.3 27.9 36.1 44.3 54.6 63.6 86.7 95.3 108 115 129 6.54 14.2 21.7 30.9 42.9 53.7 63.4 68.8 78.5 89.0 101 115 130 7.99 15.0 26.6 44.2 47.5 55.9 66.3 79.1 94.3 108 117 124

502 677 792 890 992 1022 65 94 156 226 292 360 443 516 701 772 875 934 1043 53 115 176 251 346 436 515 556 637 721 822 934 1053 65 122 216 358 385 451 538 642 762 875 946 1009

At T = 303.15 K and p = 123.15 kPa with standard uncertainty (u) and relative uncertainty (ur), w1 (%), bSO2 (mol kg−1), pSO2 (Pa), and 106ySO2 donates the mass fraction of PGME concentration, the solubility of SO2 in the liquid phase, the partial pressure of SO2 in the gas phase and the mole fraction of SO2 in the gas phase, respectively. Standard uncertainties u are u(w1) = 0.0001, u(T) = 0.01 K and u(p) = 0.10 kPa. Relative uncertainties ur are ur(ySO2) = 0.05 and ur(bSO2) = 0.006 (level of confidence = 0.68). a

the pure DMSO (w1 = 0%) showed the maximum value of 0.0781 mol·kg−1 to dissolve SO2.30,31 In addition, the absorption capacity of SO2 in the system of PGME + DMSO were stronger than PEG300 + DMSO30 and TEG + DMSO31 systems at the same alcohol ratio. 3.2. Effects of the Temperature for GLE Data. GLE data of dilute SO2 in PGME + DMSO (w1 = 40%) at three temperatures were plotted in Figure 6. The results indicated that the solubility of SO2 in a constant concentration decreased when the temperature increased. It might be due to the thermal motion rate of SO2 molecule increased when the temperature

experimental temperatures, which indicated that the interaction PGME and DMSO is so weak that it did not present the obvious effect on SO2 absorption. Solubility of SO2 in various PGME (1) + DMSO (2) mixtures was shown in Figure 5 when the volume fraction of SO2 was set at ySO2 = 500 × 10−6 at T = 298.15 K in the gas phase. As shown in Figure 5, the solubility of SO2 in the binary system of PGME (1) + DMSO (2) increased when DMSO concentrations increased, and the solubility of SO2 presented a minimum value (0.0074 mol·kg−1) in pure PGME; meanwhile, D



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Table 4. GLE data for PGME (1) + DMSO (2) + SO2 (3) + N2 (4)a w1 (%)

bSO2/(mol kg−1)

pSO2/Pa

106ySO2

w1 (%)

bSO2 (mol kg−1)

pSO2/Pa

106ySO2

0 0 0 0 0 0 0 0 0 0 0 0 0 20 20 20 20 20 20 20 20 20 20 20 20 40 40 40 40 40 40 40 40 40 40 60 60 60 60

0.00264 0.00507 0.01225 0.01400 0.01870 0.02764 0.03492 0.04275 0.05196 0.06864 0.07524 0.08163 0.08789 0.00589 0.00881 0.01244 0.01776 0.02523 0.03116 0.03580 0.04013 0.04578 0.05143 0.05664 0.06210 0.00571 0.01014 0.01494 0.02067 0.02660 0.03230 0.03759 0.04418 0.04986 0.05294 0.00267 0.00398 0.00587 0.01052

4.67 12.2 18.8 25.5 32.3 43.0 55.8 66.3 79.4 103 113 119 128 13.1 19.3 27.2 36.2 50.2 61.9 70.2 79.6 91.9 103 114 126 14.4 24.8 32.7 46.9 62.6 78.3 94.0 109.3 122.9 130 7.66 17.2 25.3 41.3

38 99 153 207 261 349 450 537 642 835 913 961 1035 106 156 219 292 405 502 569 644 742 836 923 1017 116 201 264 379 507 636 762 885 996 1056 62 139 205 333

60 60 60 60 60 60 70 70 70 70 70 70 70 70 70 70 70 80 80 80 80 80 80 80 80 80 80 80 80 100 100 100 100 100 100 100 100 100 100

0.01481 0.01993 0.02574 0.02819 0.03166 0.03431 0.00334 0.00424 0.00592 0.00674 0.00996 0.01163 0.01330 0.01539 0.01704 0.01896 0.02062 0.00112 0.00285 0.00304 0.00481 0.00622 0.00791 0.00928 0.01106 0.01276 0.01421 0.01546 0.01702 0.00079 0.00135 0.00258 0.00319 0.00418 0.00530 0.00645 0.00799 0.00885 0.01040

60.1 77.8 96.6 107 118.6 130 18.5 25.5 34.0 40.5 59.8 69.4 82.8 92.4 102 116 129 8.14 16.3 24.9 32.9 41.1 55.7 65.0 80.7 94.0 106 114 126 7.64 15.5 28.3 36.4 47.5 59.7 73.2 93.2 102 125

486 630 783 870 962 1052 150 206 276 329 482 562 668 749 824 942 1046 66 132 201 267 334 449 527 651 762 855 925 1025 62 126 229 295 384 484 589 756 821 1012

At T = 308.15 K and p = 123.15 kPa with standard uncertainty (u) and relative uncertainty (ur), w1 (%), bSO2 (mol kg−1), pSO2 (Pa), and 106ySO2 donates the mass fraction of PGME concentration, the solubility of SO2 in the liquid phase, the partial pressure of SO2 in the gas phase and the mole fraction of SO2 in the gas phase, respectively. Standard uncertainties u are u(w1) = 0.0001, u(T) = 0.01 K and u(p) = 0.10 kPa. Relative uncertainties ur are ur(ySO2) = 0.05 and ur(bSO2) = 0.006 (level of confidence = 0.68). a

alcohol was due to the hydrogen bond and charge-transfer, which demonstrated that the charge-transfer may be easily between the alcohol-containing methoxy group and SO2. That is to say, there may be more free of alcohol hydroxyl than other alcohol and derivatives in PGME due to the weekly intermolecular hydrogen bond, which indicated the hydrogen bond is easily between free alcohol hydroxyls in PGME and SO2.32 In summary, PGME was a good choice for SO2 removal because of the excellent characters of higher absorption capacity, low density, and viscosity of PGME. 3.4. Thermodynamical Properties. Owing to the absorption of SO2 in the binary system, Henry’s law constants (H′), Gibbs free energy (ΔsolGmo), enthalpy changes (ΔsolHmo), and entropy changes (ΔsolSmo) were obtained from the GLE data based on a data treatment method.36,37 The thermody-

increased, which was contributed to the SO2 molecule that was more easily escaped from solvent; meanwhile, the heating method was an efficient for the regeneration of solvents.32 3.3. Henry’s Constant for Different Alcohol and Alcohol Derivatives. Henry’s constant and density and viscosity of five pure alcohol and alcohol derivatives including EG, DEG, TEG, PEG 300, and PGME are listed in Table 6.33−35 From Table 6, the trend of the density and viscosity increased with the increasing absorption capacity for EG, DEG, TEG, and PEG 300, which could be clarified that the lager density and viscosity prevented the SO2 molecule escaping from solvent. However, the density and viscosity of PGME was lower than the other four alcohols, and the absorption capacity was stronger by comparing the values of Henry’s constants. According to the literatures,32,35 the dissolving of SO2 in E



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Table 5. HLC of SO2 Absorption in Various PGME (1) + DMSO (2) Mixtures at T = 298.15, 303.15, and 308.15 K and p = 123.15 kPa HLC (103 × Pa kg mol−1) w1 (%)

T/K = 298.15

T/K = 303.15

T/K = 308.15

0 20 40 60 70 80 100

0.781 1.07 1.97 2.45 3.82 6.06 8.29

1.08 1.47 2.34 3.34 5.44 6.80 9.97

1.40 1.97 2.51 3.67 6.25 7.52 11.9

Figure 2. GLE data fitting lines for PGME (1) + DMSO (2) + SO2 (3) + N2 (4) (w2 = 0%, 20%, 40%, 60%, 70%, 80% and 100%) at T = 298.15 K and p = 123.15 kPa. □, 100%; ○, 80%; △, 70%; ☆, 60%; +, 40%; ×, 20%; and ◇, 0%.

Figure 5. Solubility of SO2 in various PGME (1) + DMSO (2) mixtures when SO2 volume fraction in the gas phase was designed to ySO2 = 500 × 10−6 at 298.15 K.


Figure 6. Solubility curves of dilute sulfur dioxide in w1 = 40% PGME (1) + DMSO (2) at temperatures ranging from 298.15 to 308.15 K and a constant pressure of 123.15 kPa. □, 298.15 K; ○, 303.15 K; and △, 308.15 K.

indicated that it is a spontaneous process. The values of ΔsolGom increased with the increasing mass fractions of DMSO or increasing temperatures, which demonstrates the values of ΔsolGom could be used as a criterion for SO2 absorption capacity in the binary systems of PGME + DMSO. As in the literature,32,38 the absorption process of SO2 in PGME is related to both hydrogen bonding and charge-transfer interaction, and the absorption process of SO2 in DMSO is based on of the O→S dipole−dipole interaction. The entire process indicating the acting force of O→S dipole−dipole interaction was stronger than charge-transfer interaction and hydrogen bonding for SO2. All negative enthalpy change


namic property changes at three temperatures and 123.15 kPa are listed in Table 7. As shown in Table 7, all ΔsolGom values were found to be −13.04 to −20.19 kJ·mol for the systems composed of PGME + DMSO + SO2, which showed that the Gibbs free energy change of dissolving SO2 from gas phase to liquid phase and F



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Table 6. Density, Viscosity and HLC of SO2 Absorption for Different Alcohol and Alcohol Derivatives at T = 298.15 K alcohol/alcohol derivatives

EG

DEG

TEG

PEG300

PGME

ρ/(g·cm−3) η/(mPa·s) HLC (103 × Pa kg mol−1)

1.109833 17.733 24.5318

1.112834 27.534 19.1735

1.119931 37.131 11.9131

1.121830 69.030 8.4430

0.9169 1.56 8.29

Table 7. Henry’s Law Constants (H′), Standard Gibbs Free Energy (ΔsolGom), Dissolution Enthalpy Changes (ΔsolHom), and Dissolution Entropy Changes (ΔsolSom) of Dilute SO2 in Various PGME (1) + DMSO (2) Mixtures at T = 298.15, 303.15, and 308.15 K and p = 123.15 kPa w2

T/K

H′ (103)

ΔsolGom/kJ·mol−1

0% 0% 0% 20% 20% 20% 40% 40% 40% 60% 60% 60% 70% 70% 70% 80% 80% 80% 100% 100% 100%

298.15 303.15 308.15 298.15 303.15 308.15 298.15 303.15 308.15 298.15 303.15 308.15 298.15 303.15 308.15 298.15 303.15 308.15 298.15 303.15 308.15

0.290 0.391 0.501 0.409 0.552 0.731 0.775 0.916 0.962 0.994 1.36 1.47 1.59 2.24 2.55 2.57 2.85 3.15 3.65 4.33 5.12

−20.2 −19.5 −18.8 −19.3 −18.6 −17.9 −17.7 −17.4 −17.2 −17.1 −16.4 −16.2 −16.0 −15.1 −14.8 −14.8 −14.5 −14.2 −13.9 −13.3 −13.0

(ΔsolHom) values indicated that SO2 dissolution in the binary system PGME + DMSO is an exothermic process. In addition, the results of all standard enthalpies of SO2 dissolution indicating the addition of SO2 destroyed original hydrogen bonding of self-association among PGME and DMSO molecules first, and then the new hydrogen bonds were formed among PGME, DMSO, and SO2. All entropy change (ΔsolSom) values were negative, which demonstrated that the entropy chaos degree of solvent molecular decreased when the SO2 molecules in the gas phase dissolved into binary systems PGME + DMSO, namely the interaction between the molecule of SO2 and the solvent lead to molecule ordering and a high ordering degree.27 3.5. Regeneration Property. To illustrate the capture and regeneration properties of the absorbent, the absorption− desorption cycles were repeated five times. The solvent after the absorption of SO2 was used to investigate the regeneration property of binary system for SO2 by heating and bubbling N2. The results shows that 98.5% SO2 molecules can be regenerated from the system of PGME + DMSO (w1 = 50%) during 12 min by heating 333.15 K and bubbling N2 (80 mL/ min). Five times capture and release SO2 cycles for the binary system of PGME (1) + DMSO (2) (w1 = 50%) were performed, and the desorption results are shown in Figure 7. For every cycle, the solvents can be reused without an obvious loss of absorption capacity and the SO2 can be easily stripped out under this operation condition. In addition, the PGME (1) + DMSO (2) binary system presented a serious merit including a low regeneration temperature, higher loading capacity of SO2,

ΔsolHom/kJ·mol−1 K−1

ΔsolSom/J·mol−1

−42.2

−73.7

−44.2

−83.5

−13.5

−14.0

−29.9

−43.2

−36.1

−67.8

−17.6

−26.9

−9.44

−44.0

Figure 7. Five consecutive absorption−desorption cycles of SO2 by PGME (1) + DMSO (2) (w1 = 50%). In this representation, absorption (room temperature), regeneration (333.15 K), SO2 uptake denotes one gram of SO2 per gram solvent, and 98.5% SO2 could be separated from the dissolving SO2 solution at 333.15 K and bubbling nitrogen within 12 min.

and reusable solvent, which indicated the PGME (1) + DMSO (2) binary system was a good choice to absorb SO2. 3.6. Spectral Properties. 3.6.1. UV−vis Spectra. The UV− vis spectral changes of PGME + SO2 and PGME + DMSO + SO2 are shown in Figure 8a,b, and the reference solution is PGME. As shown Figure 8a, two absorption bands were observed at nearly 278 and 212 nm, which was due to the n→π* electronic transition of SO2 at longer wavelength and n→σ* for O atom in PGME or π→π* for SO2 at shorter wavelength.31 The G



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Figure 8. (a) UV spectra of SO2 in PGME with different concentrations of SO2. (b) UV spectra of SO2 in PGME + DMSO with different concentrations of SO2.

Figure 9. (a) IR spectra of SO2 in PGME with different concentrations of SO2. (b) IR spectra of SO2 in PGME (1) + DMSO (2) with different concentrations of SO2.

atom in SO2,32 and the absorption intensity increased with the increasing SO2 concentrations. In addition, these results showed that the SO2 molecules could can steady dispersed in the solution, and there are an intermolecular interaction between the system of PGME + DMSO and SO2. 3.6.2. FTIR Spectra. FTIR spectra of PGME + SO2 and PGME + DMSO + SO2 systems (w1 = 50%) were determined and plotted in Figure 9a,b. As shown in the literature,43,44 liquid SO2 has three fundamental vibration frequencies in noncomplexing CCl4, 1361.76, 1151.38, 517.59 cm−1, which is attributed to asymmetrical stretching vibration (vas), symmetrical stretching vibration (vs), and in-plane bending vibration, respectively. From the FTIR spectra of PGME + SO2, the asymmetrical stretching vibrations of SO2 at 1330 cm−1 in PGME was found, while symmetrical stretching vibrations 1199 cm−1 are covered by the stretching vibrations of C−O−C.32 The stretching vibration wavenumbers of C−O−C are constant in PGME before and after SO2 absorption, which indicates that the absorption process of SO2 is the formation of intermolecular hydrogen bond between SO2 and alcohols hydroxyl. The stretching vibration of hydroxyl in PGME shifted to 3386 from 3411 cm−1 after bubbling SO2, which results from the original hydrogen bonding interaction of pure PGME that was gradually destroyed and the new hydrogen bonds between PGME and SO2 that were formed, so that the absorption peaks toward a lower wavenumber. It indicates that the hydrogen interaction was mainly a factor for SO2 absorption process in PGME. The absorption peak at 1110 cm−1 was attributed to in-plane

absorption intensity of the band at nearly 278 nm increases with the increasing SO2 concentration, which may result from the probability of electronic transition of n→π* for SO2 increase when the SO2 concentration increase. The absorption band at (198 to 212) nm was related to the π→π* electron transition of the S atom in SO2 and the n→σ* electron transition of the hydroxyl O atom in PGME.39 The shift from 198 to 212 nm results from the two possible reasons: (i) The effect of O atoms of hydroxyl in PGME on S atom in SO2 decreased with increasing the SO2, which weakened the constraints of the S atoms for the lone pair electrons to make the transition the π→π* electron transition of S atom in SO2 easier;40 (ii) the effect of H atoms of hydroxyl in PGME on S atom in SO2 increased with increasing the SO2, which weakened the bonding of O−H of PGME to result in the n→ σ* electron transition of hydroxyl O atom easier.41 Two characteristic absorption bands are observed in Figure 8b. The shorter band shifts from 212 to 218 nm and the intensity boosted, which results from the n→π* electronic transition of the electronic pair of O atom in DMSO and π→π* electron transition of S atom in SO2.32 The position of shorter band moved to longer wavelength when the SO2 concentrations increase, which result from the decreasing effects of O atoms in PGME or DMSO on S atom in SO2 make the π→π* electron transition of S atom in SO2 change easier;42 also, the increasing conjugated degree in SO2 and DMSO molecules from intermolecular interaction among PGME, DMSO and SO2 lead to electronic transition change easier. The bands at nearly 278 nm was due to the n→π* electronic transition of O H



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bending vibration of alcoholic hydroxyl in PGME and shifted to 1108 cm−1, which is further evidence that there are only hydrogen bonds between hydroxyl hydrogen in PGME and O atom in SO2. In addition, the interaction between SO2 and DMSO was similar to that reported in the literature,45 which was attributed to the analogous structure of SO, and the reason for existing a good affinity between DMSO and SO2 may be the analogous structure of SO. As shown in Figure 9b, the asymmetry stretching vibration (vas) peaks of SO2 appeared at 1330 cm−1 and no new absorption peaks were observed after SO2, which indicated the captured SO2 molecules maintained their intrinsic structure, which was due to interaction among PGME, DMSO, and SO2.42 Meanwhile, the stretching vibrational peak of hydroxyls in PGME shifted to 3425 from 3427 cm−1 after absorbing SO2. The phenomenon was related to the fact that SO2 destroyed the original hydrogen bonding interaction of PGME···PGME and PGME···DMSO, and the new hydrogen bonding between hydroxyl H atom in PGME and O atom in SO2 or DMSO was formed. The fact suggested that the absorption of SO2 in PGME + DMSO mixture was physical dissolution, and the SO2 could be desorbed from the PGME (1) + DMSO (2) + SO2 (3) mixture by changing operation condition. 3.6.3. 1H NMR Spectra. 1H NMR experiments were conducted to investigate the absorption mechanism of SO2 in binary systems of PGME + DMSO. The 1H NMR spectral results of pure PGME and the systems of PGME + DMSO in the presence and absence of SO2 are shown Figure 10 a,b, respectively, and the pure DMSO absorption of SO2 could be referenced to external literature.46 From Figure 10a, two bands of the chemical shift of H atom in hydroxyl at d = 4.44 ppm and d = 4.43 ppm were moved upfield to about d = 4.09 ppm after addition of SO2, which meant that an interaction of H atom in hydroxyl of PGME with O atom in DMSO or SO2. The chemical shift of H atom in −OCH3 appeared at d = 1.11 and no obvious shift was observed, which indicates the ether bond (COC) is not involved in hydrogen bonding between SO2 and PGME. In Figure 10b, compared before and after SO2 absorption, no significant chemical shift was found, indicating that there was no the formation of new compounds. It also indicating that the affinity interaction between DMSO and SO2 plays a vital role in SO2 absorption processes and it happened before the hydrogen bonding of PGME and SO2. Additionally, the above results demonstrated the intermolecular hydrogen bonding association among PGME, DMSO, and SO2 molecules was formed.

Figure 10. (a) 1H NMR spectra and chemical shifts changes of PGME before and after SO2 absorption (DMSO as an external reference). (b) 1 H NMR spectra and chemical shifts changes of PGME (1) + DMSO (2) before and after SO2 absorption (CDCl3 as an external reference).

SO2 cycles experiments showed that the binary of systems for absorption SO2 is reversible and has a good regeneration performance. All spectral results indicated that the intermolecular hydrogen bonding interaction among PGME, DMSO, and SO2 was formed.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-471-6575722. Fax: +86-471-6575722. E-mail: [email protected] (J.B.Z.). ORCID

Jianbin Zhang: 0000-0001-8271-8865

4. CONCLUSION The solubility of SO2 decreased with the increasing temperatures by determining the solubility for the PGME (1) + DMSO (2) binary system at three temperatures. Solubility of SO2 in alcohol, alcohol derivatives, and alcohol + DMSO systems were compared from the values of Henry’s law constants. Standard Gibbs free Energy (ΔsolGom), dissolution enthalpy changes (ΔsolHom), and dissolution entropy changes (ΔsolSom) of dilute SO2 in various PGME (1) + DMSO (2) mixtures were obtained by the GLE data and Henry’s law constants, which demonstrate the dissolving SO2 from gas phase to liquid phase and it is a spontaneous process, an exothermic process and the interaction between the molecule of SO2 and the solvent lead to molecule ordering and a high ordering degree, respectively. Five times capture and release

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Inner Mongolia Science and Technology Key Projects, the Program for Grassland Excellent Talents of Inner Mongolia Autonomous Region, Program for New Century Excellent Talents in University (NCET-121017), and training plan of academic backbone in youth of Inner Mongolia University of Technology.

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Solubility Properties and Absorption Mechanism Investigation of Dilute

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