Desorption Property and Spectral Investigation of Dilute Sulfur Dioxide

Apr 28, 2014 - In a previous work, the binary mixture of ethylene glycol (EG) (1) + N,N-dimethylformamide (DMF) (2) was used as a promising medium for...
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Desorption Property and Spectral Investigation of Dilute Sulfur Dioxide in Ethylene Glycol + N,N-Dimethylformamide System Fei Gao,†,§ Jianbin Zhang,*,† Yanxia Niu,‡ and Xionghui Wei*,‡ †

College of Chemical Engineering, Inner Mongolia University of Technology, Huhhot 010051, China Department of Applied Chemistry, College of Chemistry & Molecular Engineering, Peking University, Beijing 100871, China § Key Laboratory for Green Chemical Technology of the Ministry of Education, School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, China ‡

ABSTRACT: In a previous work, the binary mixture of ethylene glycol (EG) (1) + N,N-dimethylformamide (DMF) (2) was used as a promising medium for the absorption processes of SO2 (J. Chem. Thermodyn. 2013, 62, 8−16). In this work, the desorption property and spectral changes of dilute SO2 in an ethylene glycol (EG) + N,N-dimethylformamide (DMF) binary system were studied to determine the possible intermolecular interaction of SO2 with DMF and DMF + EG. Desorption data showed that SO2 could desorb from the dissolving SO2 mixture, the 98% SO2 could be removed from the dissolving SO2 mixture, and the regenerate solution showed a similar SO2 absorption capacity with fresh solution. For determining the important interaction mechanism, conventional UV, Fourier transform infrared (FTIR), and 1H NMR spectroscopic techniques were used in absorption processes of SO2. Spectral results suggested that EG could interact with DMF by hydrogen bonds in the binary mixture by the -CH2CH2−OH···OCH- bond; furthermore, SO2 could interact with DMF in various binary mixtures by the -HCO···(O)SO bond.

1. INTRODUCTION Sulfur dioxide (SO2), which is emitted from power and industrial plants, is a significant source of atmospheric pollution that threatens the environment and human health.1 Therefore, it is necessary to remove SO2 from flue gases before it is released. Many technologies of removing SO2 have been proposed, of which the conventional procedures, such as limestone scrubbing and amine scrubbing, show some inherent disadvantages, including high capital and operating costs, large water requirement, poor quality of byproduct, and secondary pollution.2,3 Because of the favorable absorption and desorption properties for acid gases, organic solvents have become the subject of increasing interest in recent years.4−6 Wei and Zhang7−17 have paid great attention for several years to the absorption of SO2 in systems containing ethylene glycol (EG) and its similar compounds. EG is a promising medium that may be used in the absorption processes of SO2 because of its strong SO2 absorption and desorption capacities, low vapor pressure, low toxicity, high chemical stability, and low melting point.8 Roizard et al.18,19 reported SO2 solubility in some organic solvents, and it was reported that the SO2 solubility in N,N-dimethylformamide (DMF) was stronger than in EG. However, DMF shows relatively high vapor pressure, which can result in significant vaporization and solvent loss.20 Herein, in our previous study,21 EG was added into DMF to reduce the solvent loss and obtain the efficient desulfurization system. The results suggested the EG (1) + DMF (2) system had strong solubility capacities to dilute SO2, and the solubility of dilute SO2 in the binary mixture was determined in the range from 2.38 to 47.2 mol·m−3 with increasing DMF concentration at T = 308.15 K and p = 122.66 kPa when the SO2 concentration in the gas phase was designed at ySO2 = 5 × 10−4. © 2014 American Chemical Society

The analyses of excess molar volumes for EG (1) + DMF (2)21 suggested that the intermolecular interaction of EG with DMF in the binary mixture (x1 = 0.50, molar fraction of EG) was stronger than all other compositions. Meanwhile, the solubility of dilute SO2 in this binary mixture was 14.9 mol·m−3 at T = 308.15 K and p = 122.66 kPa when the SO2 concentration in the gas phase was designed at ySO2 = 5 × 10−4. On the basis of these data, the binary mixture of EG (1) + DMF (2) (x1 = 0.50) may be used as a promising medium for the absorption processes of SO2, which could lower the vapor pressure of DMF and reduce solvent loss. As a whole, the work includes the following parts: (1) the solubility properties of dilute SO2 in EG (1) + DMF (2) mixtures; (2) the physicochemical properties of EG (1) + DMF (2) mixtures; (3) the desorption capacity of SO2 in EG (1) + DMF (2) mixtures; (4) the mechanism for SO2 absorption in the binary mixtures of EG (1) + DMF (2). The first and second parts were reported in our previous work.21 The present work was mainly focused on showing the desorption results of SO2 in the system of EG (1) + DMF (2) (x1 = 0.50) and investigating the intermolecular interaction of EG with DMF and interaction of SO2 with DMF and/or EG by the conventional UV, FTIR, and 1H NMR spectroscopic techniques.

2. EXPERIMENTAL SECTION The certified gas mixtures of SO2 + N2 (ySO2 = 8 × 10−3) and pure N2 gas (>99.9%) were purchased from the Beijing Gas Received: Revised: Accepted: Published: 7871

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Center, Peking University (China). Analytical grade DMF and EG, which were dried over molecular sieves (type 4A) and degassed ultrasonically before use, were obtained from Beijing Reagent Company (Beijing, China). The purity of DMF and EG was checked by measuring and comparing the density of samples with corresponding literature values at 298.15 K. The density values of DMF and EG are respectively 0.9441 and 1.1102 g·cm−3, which agreed reasonably well with the corresponding literature values of 0.9442 and 1.11009 g· cm−3.22,23 Fourier transform infrared (FTIR) spectra were recorded on a Bruker VECTOR22 spectrometer with 1 cm−1 resolution in the range from 4000 to 400 cm−1. The spectrometer possesses autoalign energy optimization and a dynamically aligned interferometer and is fitted with two constringent ZnS pellets, an OPUS/IR operator, and IR source. A baseline correction was made for the spectra, which were recorded in air, and then, 15 μL of solution was used on the FTIR spectrometer in every one of the measurements, and the thin layer of samples has less than 2 μm (typical thicknesses). UV−vis spectra were recorded on an UNIC 4802 spectrophotometer. 1H NMR spectra were obtained using an AVANCE III Bruker-500 MHz nuclear magnetic resonance spectrometer, and DMSO-d6 was used as a NMR solvent. All spectral experiments were performed at room temperature and atmospheric pressure.

Figure 2. GLE curves for EG (1) + DMF (2) (x1 = 0.50) at T = 308.15 K, p = 122.66 kPa: □, the first absorption; ○, the second absorption; △, the third absorption.

From Figure 1, it was found that SO2 concentration in the binary mixture of EG (1) + DMF (2) (x1 = 0.50) decreases from 15.50 to 0.3813 mol·m−3, and 98% SO2 was removed from the dissolving SO2 mixture. Figure 2 showed that the three groups of GLE data were coincident indicating the regenerate solution had the same absorption capacity of SO2 with fresh solution. The results showed that the binary mixture of EG (1) + DMF (2) (x1 = 0.50) could be circularly used as the solution for the SO2 absorption and desorption processes, and the SO2 absorption capacity does not obviously change. The strong absorption and desorption capacity of the mixture toward SO2 may be related to the intermolecular interaction of DMF with SO2, so conventional UV, FTIR, and 1H NMR spectroscopic techniques were used to explore the interaction among molecules in this work. 3.2. Spectral Properties of DMF + EG. The recorded UV spectra of EG (1) + DMF (2) are shown in Figure 3, and EG was used as the reference solution.

3. RESULTS AND DISCUSSION 3.1. SO2 Desorption Data in EG (1) + DMF (2). Desorption data of SO2 in EG (1) + DMF (2) (x1 = 0.50) with the increasing time are shown in Figure 1.

Figure 1. SO2 desorption from the system EG (1) + DMF (2) (x1 = 0.50).

The absorption for SO2 in the binary mixture of EG (1) + DMF (2) (x1 = 0.50) was measured at T = 308.15 K and the total pressure of 122.66 kPa with the low partial pressure of SO2, and the mixture dissolving SO2 was treated with gas stripping at T = 308.15 K and atmospheric pressure, and the carrier gas was N2 gas with 500 mL·min−1. After desorption, the gas−liquid equilibrium (GLE) data of SO2 in the regenerated solution was measured to determine the property of dissolving SO2 in the regenerated solution based on the previous work.8 The measurement of desorption and absorption capacities of SO2 in EG (1) + DMF (2) (x1 = 0.50) was repeated three times, and the three groups of GLE data are shown in Figure 2.

Figure 3. UV spectral changes with the increasing EG concentration in the system EG (1) + DMF (2).

From Figure 3, there are a series of absorption bands at nearby 267 nm, which could be due to the n → π* electronic transition of the unshared electronic pair of carbonyl oxygen atom in DMF.24 It is clearly visible that the absorption band blue-shifted from 267 to 264 nm with the increasing EG concentration in the binary mixtures of EG (1) + DMF (2). With increasing EG concentration, hydrogen bonding and 7872

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concentration, which indicated the intermolecular hydrogen bond in EG was broken gradually, and the new hydrogen bond between EG and DMF was formed. In Figure 4, it is also found that the stretching vibrational band of carbonyl in DMF26 shifted to lower frequency from 1674 to 1662 cm−1, and stretching vibrational band of N−CH3 in DMF27 located at 1257 cm−1 did not obviously change with the increasing EG concentration. The fact that the stretching vibrational band of hydroxyl in EG shifted toward lower frequency was due to the vibrational property of hydroxyl in EG; meanwhile, the fact that the stretching vibrational band of carbonyl in DMF shifted to lower frequency was due to the vibrational property of carbonyl in DMF. According the above results, we presented that the possible interactions of EG with DMF resulted from the following: hydrogen bonding and interaction of the carbonyl oxygen atoms in DMF with hydroxyl hydrogen atoms in EG were formed as -CH2CH2−OH···OCH-. 1 H NMR spectral results of EG, DMF, and EG + DMF are shown in Figure 5, panels a, b, and c, respectively. Figure 5a shows that the chemical shifts of hydrogen atoms in −CH2− appear at δ = 3.396, 3.401, 3.397, and 3.407 ppm, and the chemical shifts of hydrogen atom in −OH appear at δ = 4.453, 4.464, and 4.475 ppm in the 1H NMR spectrum of pure EG. Figure 5b shows the chemical shifts of hydrogen atom in -HCO at δ = 7.957 ppm, and the chemical shift of hydrogen atoms in −CH3 appears at δ = 2.735 and 2.895 ppm in the 1H NMR spectrum of pure DMF. However, from Figure 5c, in the EG (1) + DMF (2) (x1 = 0.5), the chemical shifts of hydrogen atoms of −CH2− and −OH in EG are found at δ = 3.397 and

interaction of hydroxyl hydrogen atoms of EG with carbonyl oxygen atom in DMF happened easily so that the n → π* electronic transition of unshared electronic pair of carbonyl oxygen atom in DMF became more difficulty. The present results showed that the hydrogen bonding and interaction of the carbonyl oxygen atom in DMF with hydroxyl hydrogen atoms in EG was formed as −CH2CH2−OH···OCH-. FTIR spectra of DMF + EG are shown in Figure 4.

Figure 4. FTIR spectra of EG (1) + DMF (2): (a) x1 = 1.00; (b) x1 = 0.80; (c) x1 = 0.60; (d) x1 = 0.40; (e) x1 = 0.20; (f) x1 = 0.00.

From Figure 4, the stretching vibrational band of hydroxyl in EG25 shifts from 3397 to 3362 cm−1 with increasing EG

Figure 5. 1H NMR spectra of EG (a), DMF (b), and EG (1) + DMF (2) mixture (x1 = 0.50) (c). 7873

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4.441 ppm, respectively. Meanwhile, the coupling ways of two kind of hydrogen atoms disappeared, and only two chemical shift peaks are displayed. From Figure 5c, it is also found the chemical shifts of hydrogen atom of -HCO in DMF shifted from 7.957 to 7.954 ppm. The above results were because the interaction of carbonyl oxygen atom in DMF bonding with hydroxyl hydrogen atom in EG decreased the shielding effect of aldehyde group hydrogen atoms by carbonyl in DMF and increased the shielding effect of hydroxyl hydrogen atoms in EG, so these signals of chemical shift moved toward a higher magnetic field. From the 1H NMR spectra, we acquired similar hydrogen bonding and interaction information with the UV and IR spectral results. According to the above UV, IR, and 1H NMR spectral results, it was expected that the hydrogen bonding and interaction of carbonyl oxygen atom in DMF with hydroxyl hydrogen atom in EG was formed as −CH2CH2−OH···O CH-. 3.3. Spectral Properties of DMF + SO2. The recorded UV spectral changes of DMF + SO2 are shown in Figure 6, and DMF was used as the reference solution.

not change in the presence of SO2. The above results showed that carbonyl oxygen atoms in DMF interacted with sulfur atoms, which has an unoccupied orbital in SO2, as the form of -HCO···(O)SO. The new bond made the electron density of carbonyl in DMF lower, so that the vibration of carbonyl changed more easily. 1 H NMR spectral results of DMF + SO2 are shown in Figure 8.

Figure 6. UV spectral changes with increasing SO2 concentration in DMF.

Figure 8. 1H NMR spectra of DMF in the presence of SO2.

From Figure 6, a series of absorption bands were observed at around 273 nm, which were assigned to n → π* electronic transition of oxygen atoms in SO2 (Π43).28 The absorption band red-shifted from 273 to 276 nm, and the intensity of band increased with increasing SO2 concentration. The shift results from the intermolecular interaction of sulfur atoms in SO2 with carbonyl oxygen atoms in DMF. The intermolecular bonding results in that the decreasing effects of sulfur atoms on the oxygen atom of SO2 make n → π* electronic transition of the oxygen atom in SO2 change easier, so red-shift phenomena occurred. The interaction of nitrogen atom in DMF with oxygen atom in SO2 was formed as -HCO···(O)SO. The recorded FTIR spectra of DMF and DMF + SO2 are shown in Figure 7. From Figure 7, two new bands appeared at 1146 and 1319 cm−1 in the presence of SO2. The bands were the stretching vibration (Vs) and asymmetry stretching vibration (Vas) of SO2, respectively.29,30 The results revealed that the absorbed SO2 molecules maintained their intrinsic structure indicating that DMF interacted with SO2. In Figure 7, it was found that the stretching vibrational band of carbonyl in DMF shifted to lower a frequency from 1674 to 1664 cm−1, and the stretching vibrational band of N−CH3 in DMF located at 1257 cm−1 did

From Figure 8, in the presence of SO2, the chemical shifts of hydrogen in -HCO in DMF shifted from 7.957 to 7.952 ppm, and the chemical shift of methyl hydrogen did not change. The result revealed that the carbonyl oxygen atom in DMF interacted with the sulfur atom in SO2, which decreased the shielding effect of the aldehyde group hydrogen atom by carbonyl in DMF, so the signal of chemical shift moved toward a higher magnetic field. According to the above UV, IR, and 1H NMR spectral results, the carbonyl oxygen atom in DMF could interact with the sulfur atom in SO2 in the form of -HCO···(O)SO. 3.4. Spectral Properties of EG + DMF + SO2. The recorded FTIR spectra of EG + DMF in the presence of SO2 are shown in Figure 9. From Figure 9, in the presence of SO2 in EG (1) + DMF (2) (x1 = 0.50), the stretching vibration (Vs) and asymmetry stretching vibration (Vas) of SO2 at 1146 and 1323 cm−1 were observed, which revealed that the absorbed SO2 molecules maintained their intrinsic structure, and the binary mixture of EG (1) + DMF (2) interacted with SO2, so the SO2 could be easily desorbed from the binary mixture. In Figure 9, the stretching vibrational band of hydroxyl located at 3386 cm−1 in the mixture of EG (1) + DMF (2) did not change in the

Figure 7. FTIR spectral changes of DMF in the presence and absence of SO2.

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4. CONCLUSION In this work, it was confirmed that the binary mixture of EG (1) + DMF (2) (x1 = 0.5) could be used as the effective desulfurization solution, and the corresponding desorption efficiency of SO2 in the mixture was at 98%. Meanwhile, the regenerate solution showed the same absorption capacity of SO2 with fresh solution. On the basis of the various results, we propose that the carbonyl oxygen atom in DMF could bond with the hydroxyl hydrogen atom in EG and form a hydrogen bond in the binary mixture of EG (1) + DMF (2) in the form of -CH2CH2−OH···OCH-. In addition, in the absorption processes of SO2 in pure DMF or the binary mixture of EG (1) + DMF (2) (x1 = 0.5), spectral analyses suggested that the carbonyl oxygen atom of DMF interacted with the sulfur atom in SO2 in the form of -HCO···(O)SO.

Figure 9. FTIR spectral changes of EG (1) + DMF (2) mixture (x1 = 0.50) in the presence and absence of SO2.

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presence of SO2, which indicated that the addition of SO2 did not affect the original hydrogen bonding and interaction among EG and DMF molecules. From Figure 9, it was found that the stretching vibrational band of carbonyl in the mixture shifted to a lower frequency from 1666 to 1664 cm−1 and the stretching vibrational band of N−CH3 in DMF located at 1257 cm−1 did not change in the presence of SO2. The above results showed that carbonyl oxygen atoms in the binary mixture of EG (1) + DMF (2) interacted with sulfur atoms in SO2, and the original hydrogen-bonding interaction among EG (1) + DMF (2) molecules was not affected. The new bond made the electron density of carbonyl lower, so the vibrations of carbonyl changed more easily. The 1H NMR spectral result of EG + DMF + SO2 is shown in Figure 10.

AUTHOR INFORMATION

Corresponding Authors

*(X.H.W.) Tel.: +86-10-62751529. Fax: +86-10-62751529. Email: [email protected]. *(J.B.Z.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21166017), the Research Fund for the Doctoral Program of Higher Education of China (20111514120002), the Natural Science Foundation of Inner Mongolia Autonomous Region (2011BS0601), Program for New Century Excellent Talents in University (NCET-121017), the Inner Mongolia Science and Technology Key Projects, the Program for Grassland Excellent Talents of Inner Mongolia Autonomous Region, Program for Young Talents of Science and Technology in Universities of Inner Mongolia Autonomous Region (NJYT-12-B13), the Inner Mongolia Talented People Development Fund,the “western light” visiting scholar plan, and Yongfeng Boyuan Industry Co., Ltd. (Jiangxi Province, China).

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Figure 10. 1H NMR spectra of EG (1) + DMF (2) mixture (x1 = 0.50) in the presence of SO2.

From Figure 10, in the presence of SO2 in EG (1) + DMF (2) (x1 = 0.50), the chemical shifts of hydrogen of -HCO in DMF shifted from 7.954 to 7.950 ppm. The phenomenon was related to the interaction of carbonyl oxygen atoms in the mixture with sulfur atoms in SO2 decreasing the shielding effect of aldehyde group hydrogen atoms by carbonyl in DMF, so the signal of chemical shift moved toward a higher magnetic field. According to the above IR and 1H NMR spectral results, carbonyl oxygen atoms in the binary mixture of EG + DMF could interact weakly with sulfur atoms in SO2 in the form of -HCO···(O)SO, and the original hydrogen bond among EG + DMF molecules was not broken. Of course, additional results require further investigation. 7875

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