Efficient and Reversible Nitric Oxide Absorption by Low-Viscosity

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Efficient and Reversible Nitric Oxide Absorption by Low-Viscosity, Azole-Derived Deep Eutectic Solvents Luhong Zhang, Haopeng Ma, Guangsen Wei, Bin Jiang, Yongli Sun, Xiaowei Tantai,* Zhaohe Huang, and Yang Chen

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School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of China ABSTRACT: Deep eutectic solvents (DESs) have been extensively studied and applied to the absorption of acid gases. However, the absorption of nitric oxide (NO) by DES has rarely been reported. In this work, we found that NO can be absorbed by azole-derived DESs effectively and reversibly. A series of DESs with low viscosity based on tetrazolium (Tetz), triazole, and imidazole as hydrogen bond donors (HBDs) and common ionic liquids as hydrogen bond acceptors (HBAs) were prepared. The absorption results showed that DESs based on tetrabutylphosphine chloride (P4444Cl) as HBA or Tetz as HBD exhibited higher NO absorption. Notably, Tetz/P4444Cl DES exhibited the NO absorption values of 2.10 and 1.01 mol/mol at 101.3 and 12 kPa partial pressures, respectively, and 303.15 K. In addition, the NO absorption capacity decreased continuously with the increasing temperature. The evaporation of Tetz/P4444Cl DES can be ignored during the absorption progress. The results of thermogravimetric analysis measurement and regeneration experiments demonstrated that Tetz/P4444Cl possessed desirable thermostability and reusability. The absorption mechanism of NO by Tetz/P4444Cl was studied by Fourier transform infrared, 1 H NMR, and Gaussian simulations. It was found that there was chemical interaction between NO and the hydrogen-containing nitrogen atom of Tetz.

1. INTRODUCTION Nitrogen oxide (NOx), one of the main harmful substances in air pollution, has caused ecological hazards and environmental pollution, such as ozone hole, acid rain, and photochemical pollution. Combustion of fossil fuels is the main source of NOx, and more than 90% of NOx is nitric oxide (NO) during the coal combustion process under high temperatures.1 At present, different methods have been put forward to absorb NO. In general, according to the difference in their working medium, they can be divided into two types: dry removal (selective catalytic reduction,2 photocatalytic decomposition reduction of NO,3 NO storage and reduction,4 etc.) and wet removal (liquid-phase complexation absorption,5 liquid-phase reduction absorption,6 oxidation absorption,7 etc.) methods. In spite of the comparatively fine denitration efficiency of these means,8 many disadvantages such as the poor recycle performance, secondary pollution, high investment and operating cost, etc. have been criticized.6 In addition, NO not only exists as an important industry material but also plays a significant role in biological processes, such as in nervous system, physiology, and immune system.9 Therefore, with the constant acceleration of industrialization process, developing efficient and environmentally friendly technologies for reversible NO absorption is extremely significant and urgently necessary. Ionic liquids (ILs), low-melting (99.99%) and N 2 (>99.99%) were purchased from Tianjin Shengtang Specialty Gases Co., Ltd. 2.2. Preparation of DESs. The DESs in our present work were synthesized by stirring HBD and HBA at 353.15 K for 2 h until the mixture changed to colorless and transparent. The prepared DESs were dried at 373.15 K under vacuum for 24 h B

DOI: 10.1021/acs.jced.9b00173 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Figure 2. Schematic diagrams of NO absorption at 101.3 kPa and selected temperatures and of an electronic balance. (1) N2 gas cylinder, (2) NO gas cylinder, (3) pressure relief valve, (4) gas rotor flowmeters, (5) gas mixer, (6) mass flowmeter, (7) tee 1, (8) glass gas sampling tube, (9) digital thermostat water bath, (10) tee 2, (11) residual gas absorption bottle (NO is absorbed by H2O2 solution), (12) operation buttons, (13) display screen, (14) level, (15) balance plate, (16) side-door.

2.6. Thermal Stability and Reusability Experiments of DES. The decomposition temperatures of DES and Tetz were determined by thermogravimetric analysis (TGA, NETZSCH) for obtaining their thermal stabilities. Also, the TGA measured a heating rate of 10 K/min and a purge gas (N2) velocity of 20 mL/min. In the typical NO absorption−desorption cycle experiments, consecutive NO absorption experiments and desorption experiments were carried out five times for researching the reusability of DES. In desorption experiments, N2 with 60 mL/min was bubbled through the NO-saturated DES in a absorption tube, which was partly immersed in an oil bath at 353.15 K. The amount of NO desorption was determined at regular intervals by an electronic balance until the weight remained constant. The weight change of the desorption NO was small after 1 h, and the regenerated DES was used to absorb NO again. 2.7. FTIR and 1H NMR Spectra. The FTIR spectra of DESs were measured by a Nicolet 380 spectrophotometer (Thermo Electron Co.) using a typical thin-film method in the wavenumber range from 500 to 3500 cm−1 under ambient conditions. Moreover, the 1H NMR spectra of DESs were obtained by a 500 MHz Bruker Avance III (Germany).

40Z) that was contacted with the absorbent through a calibrated spring. Since the spring deflection measured by the rotation sensor was proportional to the viscous resistance of the fluid on the spindle, the viscosity of the measured absorbent was directly given on the basis of setting. The viscosities of DESs were measured three times, and the averages were adopted as the final results. 2.4. Experimental Apparatus. An electric-heated thermostatic water bath, a mechanical agitator, and a single-mouth flask were used for DES preparation. The apparatus for NO absorption were made up of two gas cylinders, two gas flowmeters, one electric-heated thermostatic water bath, one absorption tube, and an electronic balance (±0.0001 g). The experimental apparatus diagram is depicted in Figure 2. 2.5. Absorption of NO. In a typical absorption process, a kind of DES (about 1.5 g) as an absorbent in a gas absorption tube was weighed by an electronic balance. Then, the absorption experimental apparatus was loaded. The gas absorption tube was partly immersed in the electric-heated thermostatic water bath so that the DES could keep a stationary temperature. After checking the airtightness of the absorption apparatus, N2 with 60 mL/min was used to drive off the air in the system for 30 min. After the N2 was cut off, the main valve and the pressure relief valve of the NO gas cylinder were sequentially opened and the NO outlet pressure was controlled by the pressure relief valve to 0.1 MPa. NO was bubbled through the absorbent in the gas absorption tube at a rate of 20 mL/min. The weight change of the absorption sample tube was determined every 30 min by an electronic balance, which was used to calculate the NO molar absorption of DES. Before weighing the weight change of the absorbent, NO was first introduced into the residual gas absorption bottle by tee 1. After removing the absorption tube from the system for weighing, the inlet and the outlet of the absorption tube were blocked immediately with rubber stoppers so that the NO gas cannot escape from the DES when weighting. After one weighing was completed, the absorption tube was reintroduced into the absorption apparatus and tee 1 was adjusted to allow NO gas to continue to pass through the DES. When the weight of the DES was kept constant, its NO absorption reached an equilibrium and the absorption process could be ended.

3. RESULTS AND DISCUSSION 3.1. Viscosities of DESs. The viscosities of azole-derived DESs before and after NO absorption were measured at 303.15 K, and the results are listed in Table 3. In general, all of Table 3. Viscosity (η) of Azol-Derived DESs Before (ηb) and After (ηa) Absorption of NO at a Temperature (T) of 303.15 K and a Pressure (p) of 101.3 kPaa DES (molar ratio 1:1)

ηb (mPa s)

ηa (mPa s)

Tetz/P4444Cl Tetz/P4444Br Tetz/N4444Cl Tetz/N4444Br Triz/P4444Cl Imid/P4444Cl

40.22 68.26 87.88 279.15 67.23 52.34

3.15 5.63 9.85 26.32 7.13 5.16

a

Standard uncertainties are u(T) = 0.1 K, u(p) = 0.1 kPa, u(molar ratio) = 0.01, and u(ηb) = u(ηa) = 0.6 mPa s. C



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the researched azoles-based DESs exhibited quite low viscosities. Notably, P4444Cl-based DESs exhibited a lower viscosity than that of N4444Cl-based DES. For example, Tetz/ P4444Cl had a viscosity of 40.22 mPa s and Tetz/N4444Cl had a viscosity of 87.88 mPa s. This may be caused by the presence of a nitrogen atom in Tetz/N4444Cl, which led to a more extensive hydrogen bond network than that in Tetz/P4444Cl, resulting in the higher viscosity of Tetz/N4444Cl than that of Tetz/P4444Cl. In addition, compared to the DESs containing bromine salts, the DESs containing chlorine salts as HBA were also less viscous. For instance, the viscosities of Tetz/P4444Cl and Tetz/P4444Br were 40.22 and 68.26 mPa s, respectively. The larger size of the bromide ion in bromine-based DESs was responsible for their greater viscosities. The large ion size and very small void volume of most DESs contributed to their high viscosity.23 Tetz/N4444Br exhibited the highest viscosity of 279.15 mPa s in all of the experimental DESs, perhaps caused by their stronger hydrogen bond network and smaller void volume. Additionally, Tetz/P4444Cl, Triz/P4444Cl, and Imid/ P4444Cl exhibited different viscosities of 40.22, 67.23, and 52.34 mPa s, respectively. This may be caused by the differences in the intermolecular van der Waals forces of DES formed by Tetz, Triz, and Imid. Furthermore, the viscosities of DESs based on three azoles were significantly decreased after NO absorption. For example, Tetz/P4444Cl had a preabsorption viscosity of 40.22 mPa s and a postabsorption viscosity of 3.15 mPa s. A similar phenomenon was found in ILs owning Nazole derivative anions after CO2 absorption.45 This may be explained by the altered absorption system and the altered hydrogen bonding after acid gas absorption. In addition, the viscosity of Tetz/P4444Cl along with the absorption of NO is depicted in Figure 3. The absorption capacity data and

Table 4. NO Mole Absorption Capacity (MAC) Data of Tetz/P4444Cl (Molar Ratio 1:1) and Its Viscosity (η) Data as a Function of Absorption Time (t) at a Temperature (T) of 303.15 K and Pressure (p) of 101.3 kPaa t (h)

MAC (mol/mol)

η (mPa s)

0 1 2 3 4 5 6 7 8 9 10 11

0.000 0.651 0.987 1.234 1.487 1.671 1.849 1.971 2.100 2.100 2.100 2.100

40.22 30.13 22.04 15.05 9.24 6.02 4.03 4.03 3.05 3.05 3.05 3.05

a Standard uncertainties are u(molar ratio) = 0.01, u(T) = 0.1 K, u(p) = 0.1 kPa, u(t) = 0.01 h, u(MAC) = 0.05 mol/mol, and u(η) = 0.6 mPa s.

Figure 4. NO absorption by P4444Cl-based DESs at 343.15 K and 101.3 kPa.

Figure 3. NO absorption capacity of Tetz/P4444Cl and its viscosity with time at 303.15 K and 101.3 kPa.

viscosity data are presented in Table 4. Obviously, the viscosity of Tetz/P4444Cl decreased as the amount of NO absorption increased. Because the viscosity of absorbent has a direct impact on its fluidity and mass transfer, azole-derived DESs with low viscosities are beneficial to NO absorption applications. 3.2. NO Absorption by Different HBD-Based DESs. Because ChCl-based DESs could not be formed in liquid state at room temperature, the prepared P4444Cl-based and ChClbased DESs were employed to absorb NO at 343.15 K and 101.3 kPa, respectively. The absorption results are exhibited in Figures 4 and 5. The same absorption phenomenon that the

Figure 5. NO absorption by ChCl-based DESs at 343.15 K and 101.3 kPa.

absorption rates of DESs containing Tetz were much faster than those of DESs containing Triz and Imid is observed in Figures 4 and 5. It revealed that acidic Tetz-based DESs possess lower viscosities and faster absorption rate. In addition, D



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N4444Cl, Tetz/P4444Br, and Tetz/N4444Br provided evidence that P4444Cl played a vital role in the activation of Tetz on NO absorption. 3.4. Effect of Temperature on the Absorption of NO by DESs. The DESs that were synthesized by Tetz, quaternary phosphonium salts, and quaternary ammonium salts were applied to absorb NO from 303.15 to 343.15 K under 101.3 kPa. Also, the NO absorption results obtained at the corresponding temperature are exhibited in Figure 7, and the

the saturated NO absorption values of Tetz/P4444Cl, Triz/ P4444Cl, Imid/P4444Cl, Tetz/ChCl, Triz/ChCl, and Imid/ChCl were 1.00, 0.57, 0.34, 0.86, 0.67, 0.47 mol/mol, respectively. In other words, Tetz/P4444Cl or Tetz/ChCl exhibited higher capacity than that of Triz/P4444Cl, Imid/P4444Cl or Triz/ChCl, and Imid/ChCl. Therefore, because of the higher absorption rate and higher saturated absorption capacity of Tetz-based DESs, Tetz had a dominant influence on NO absorption. Besides, as described above, ChCl-based DES could not be formed in liquid state at room temperature and Tetz/P4444Cl possessed a higher NO absorption ability than that of ChClbased DESs; we believed that P4444Cl was more suitable HBA than ChCl for synthesizing DES and absorbing NO at room temperature. Therefore, several quaternary salts and Tetz were chosen for the next research. 3.3. NO Absorption by Different HBA-Based DESs. To investigate the effect of different HBAs with Tetz on NO absorption performance, the NO absorption experiments on different quaternary salt-formed DESs were implemented at 303.15 K and 101.3 kPa, with the outcomes being shown in Figure 6. DESs containing chloride ion exhibited a higher

Figure 7. Effect of temperature (at 101.3 kPa) on NO absorption by DESs.

absorption data are presented in Table 5. Obviously, the NO absorption capacity of all of the DESs decreased continuously with the increasing temperature. For instance, the absorption capacity of Tetz/P4444Cl decreased from 2.10 to 1.00 mol/mol when the absorption temperatures were increased from 303.15 to 343.15 K. Therefore, the lower temperature was beneficial to absorb more amount of NO. Additionally, the results also illustrated the fact that part of the desorption process had taken place at the temperature of 343.15 K. Hence, the NOsaturated DESs could be regenerated by increasing the temperature. 3.5. Effect of NO Partial Pressure on the Absorption Capacity. From the above analysis, the Tetz/P4444Cl displayed a higher NO absorption capacity of 2.10 mol/mol. Therefore, Tetz/P4444Cl was picked out for the next research. In this work, the effect of NO partial pressure on the NO absorption capacity of Tetz/P4444Cl was studied. The experiments were carried out by the partial pressure method that was balanced by N2, and the results are shown in Figure 8. The absorption data at the corresponding NO partial pressure are presented in Table 6. Notably, under low partial pressure ranging from 0 to 12 kPa, the NO absorption capacities increased sharply with the increase of partial pressure, which confirmed the chemical absorption of NO. For example, the NO absorption capacities increased from 0.39 to 1.01 mol/mol as the NO partial pressure increased from 2 to 12 kPa. This result suggested that Tetz/P4444Cl could capture NO efficiently under low NO partial pressure. In addition, it increased linearly when the NO partial pressure increased from 12 to 101 kPa, which was only a physical absorption process. As the NO concentration in flue gas is low, the chemical absorption of NO at low partial pressure is efficient for NO capture. 3.6. Loss of Tetz/P4444Cl DES. Considering that the absorption experiments lasted for 8−10 h and the viscosity of Tetz/P4444Cl decreased as the amount of NO absorption

Figure 6. NO absorption by DESs based on Tetz and different HBAs at 303.15 K and 101.3 kPa.

absorption rate and higher saturated absorption capacity than those of DESs containing bromide ion. For instance, the saturated NO absorption of Tetz/P4444Cl was 2.10 mol/mol in 8 h, whereas it was only 0.48 mol/mol in 4 h by Tetz/P4444Br. A similar phenomenon was also observed between Tetz/ N4444Cl and Tetz/N4444Br. Therefore, the anion of HBA had an important influence on the NO absorption rate and the equilibrious NO absorption. On the one hand, the low viscosities of chloride-based DESs were favorable to accelerate the NO absorption rate. On the other hand, due to the smaller size of the chloride and its stronger interaction force with Tetz than that of the bromide ion, chloride had a more pronounced activation of Tetz on NO absorption capacity than that of the bromide ion. In addition, despite P4444Cl and N4444Cl both contained chloride ions, the absorption capacity of Tetz/ P4444Cl was greater than that of Tetz/N4444Cl. Hence, NO absorption was also affected by the cation of HBA. The larger size of the phosphorus atom in P4444Cl than that of the nitrogen atom in N4444Cl may result in the more free volume of Tetz/P4444Cl, which was beneficial to NO absorption capacity. The smallest viscosity, the best absorption performance, and the highest absorption rate of Tetz/P4444Cl than those of Tetz/ E



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Table 5. Experimental NO Mole Absorption Capacity (MAC) of Absorbents at Temperature (T) and Pressure (p) of 101.3 kPaa absorbent (molar ratio, 1:1)

T (K)

MAC (mol/mol)

absorbent (molar ratio, 1:1)

T (K)

MAC (mol/mol)

Tetz/P4444Cl = 1:1

303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15

2.100 1.876 1.651 1.471 1.323 1.224 1.108 1.036 1.000 1.46 1.158 0.982 0.821 0.718 0.64 0.561 0.503 0.480

Tetz/P4444Br = 1:1

303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15 303.15 308.15 313.15 318.15 323.15 328.15 333.15 338.15 343.15

0.480 0.364 0.307 0.301 0.297 0.254 0.221 0.221 0.221 0.320 0.304 0.295 0.267 0.252 0.236 0.221 0.221 0.221

Tetz/N4444Cl = 1:1

Tetz/N4444Br = 1:1

a

Standard uncertainties are u(p) = 0.1 kPa, u(molar ratio) = 0.01, u(T) = 0.1 K, and u(MAC) = 0.05 mol/mol.

Table 6. Experimental Values of NO Mole Absorption Capacity (MAC) of Tetz/P4444Cl (Molar Ratio 1:1) as a Function of NO Partial Pressure (p) at a Temperature (T) of 303.15 Ka

Figure 8. Effect of NO partial pressure on the NO capture of Tetz/ P4444Cl DES at 303.15 K.

increased, it was necessary to discuss the evaporation problem of Tetz/P4444Cl DES under the absorption condition. In this work, a condenser tube with 278.15 K was connected vertically above the gas outlet of the absorption tube for absorption experiments and the absorption results were compared with those of the unconnected condenser tube to investigate the effects of the Tetz/P4444Cl DES evaporation on NO absorption. The comparison of NO absorption with or without the condensing tube at 303.15 K and 101.3 kPa is depicted in Figure 9. Obviously, the absorption process with or without the condensing tube were hardly changed, and the absorption results were 2.102 and 2.100 mol/mol, respectively, which was within the error range. Combining the high melting point and the high boiling point of Tetz and P4444Cl in Table 1, we can conclude that the evaporation of Tetz/P4444Cl DES can be ignored during the absorption process. 3.7. Thermal Stability and Reusability of DES. The TGA measurement results of Tetz and Tetz/P4444Cl are exhibited in Figure 10. It was clear that the 5% sample decomposition temperatures of Tetz/P4444Cl and Tetz were

p (kPa)

MAC (mol/mol)

1 2 3 4 5 6 7 8 9 10 11 12 20 30 40 50 60 70 80 90 101.3

0.231 0.393 0.492 0.571 0.662 0.753 0.841 0.890 0.942 0.961 0.992 1.010 1.113 1.256 1.372 1.500 1.653 1.785 1.931 2.014 2.100

a Standard uncertainties are u(molar ratio) = 0.01, u(T) = 0.1 K, u(p) = 0.2 kPa, and u(MAC) = 0.05 mol/mol.

513.15 and 440.15 K, respectively, which were all far beyond the regeneration temperature of 353.15 K. Therefore, Tetz/ P4444Cl met the temperature demands of reversibly absorbing NO. In addition, a two-step weight loss was observed in Tetz/ P4444Cl. The first stage of the descent curve reached at 568.15 K with 19% weight loss, which was equal to the mass fraction of Tetz in the DES, and the second stage was attributed to the thermal decomposition of P4444Cl. F



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desorption cycles. In other words, Tetz/P4444Cl possessed desirable reusability for NO absorption and Tetz/P4444Cl could maintain high NO absorption more than 5 times. Although about 0.72 mol/mol of NO remained after desorption, a desorption rate of nearly 66% was obtained. Thencefrom, Tetz/P4444Cl performed satisfactory desorption capacity and repeatability in terms of NO absorption. In addition, it was obviously that there was chemical NO absorption in Tetz/ P4444Cl that resulted in the unresolved 0.72 mol/mol of NO. 3.8. Mechanism Analysis of NO Absorption. For the purpose of obtaining more message about the NO absorption mechanism of Tetz/P4444Cl, the FTIR and 1H NMR spectra were determined. The infrared diversification before and after NO absorption is displayed in Figure 12. After NO absorption, Figure 9. Comparison of NO absorption with or without condensation tube at 303.15 K and 101.3 kPa.

Figure 12. FTIR spectra of DES before and after absorption of NO.

four new absorption peaks located at 1624, 1410, 1314, and 971 cm−1, respectively, are displayed in Figure 10. Notably, the NO absorption mechanism that NO combined with the nitrogen atom in amines with the structure of −NONO has been proposed in 1960s.46 The new peak that appeared at 162421 cm−1 was attributed to the vibration peak of N−O in NO, which was physically dissolved in DES, whereas peaks at 1410,21 1314,47 and 97146 cm−1 corresponded to NO, N− O, and N−N bonds in the −NONO structure, respectively. The 1H NMR spectra of Tetz and Tetz/P4444Cl DES before and after absorption of NO are exhibited in Figure 13. When Tetz was combined with P4444Cl to form Tetz/P4444Cl DES, the absorption peak of Tetz at 15.8 ppm, which was the absorption peak corresponding to the active hydrogen on the nitrogen atom, disappeared. This phenomenon could be explained by the situation that the active proton of Tetz was dissociated in DES. After the absorption of NO by Tetz/P4444Cl, the new peak at 20.1 ppm appeared, which was attributed to the −NOH bond in NONOH. A similar phenomenon also appeared in SO2 absorption by [TMG][PEH] ionic liquid,48 where a new peak appearing in 1H NMR spectrum was ascribed to S−O−H. Therefore, NO was absorbed by Tetz/P4444Cl DES as a result of the deprotonation of Tetz in the DES system. The absorption mechanism of NO by Tetz/P4444Cl DES was proposed, and its sketch map is shown in Figure 14. Although the molar capacity of NO in the Tetz/P4444Cl is half of that in [P44416][Tetz],21 the mass capacity of NO in Tetz/P4444Cl is 70% of that in [P44416][Tetz]. The higher the molecular weight, the lower the molality for the same amount of

Figure 10. TGA for Tetz and Tetz/P4444Cl DES with a heating rate of 10 K/min and a flow rate of purge gas (N2) of 20 mL/min.

To investigate the reusability of Tetz/P4444Cl, the NO absorption experiments at 303.15 K and 101.3 kPa and the desorption experiments at 353.15 K and 101.3 kPa were carried out five times consecutively. The content of NO (mol/ mol) in Tetz/P4444Cl after absorption and after desorption is displayed in Figure 11. It can be concluded that the absorption amount of NO hardly changed after 5 absorption and

Figure 11. Reusability of Tetz/P4444Cl DES in NO absorption. G



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derived DESs was explained by the lower atomic charge and the easier deprotonation of three azole HBDs. The deprotonation extent of Tetz by Br− and Cl− was optimized, and the results are shown in Figure 16. Because the

Figure 16. (a) Deprotonation of Tetz by Br− and (b) deprotonation of Tetz by Cl−.

electronegativity of chloride ions was greater than that of bromide ions, the distance between Cl− and H (1.940 A) was shorter than that between Br− and H (2.162 A). Moreover, the N−H bond in Tetz was protonated to a greater extent by chloride ion (1.073 A) than that by bromide ion (1.058 A). The experimental results that NO absorption of chlorinederived DESs was greater than that of bromine-derived DESs have been discussed above. Therefore, the mechanism of protonation favoring NO absorption was confirmed again. 3.10. Comparison to Other DESs or ILs. For comparison, the NO absorption capacities of various DESs and functional ILs reported in the literature are listed in Table 7. It can be found that the present Tetz/P4444Cl DES demonstrates a competitive NO absorption capacity than [P66614][Tetz]-functionalized ILs and 1,3-DMTU/P4444Cl (3:1) DES. In addition, the NO absorption capacity of Tetz/ P4444Cl DES is obviously superior to the other DESs, such as 1,3-DMU/P4444Br (3:1) DES and CPL/N4444F (2:1) DES. Moreover, the functionalized ILs suffer from multistep synthesis and high cost; DESs based on amines usually possess relatively higher viscosities. Therefore, the present Tetz/ P4444Cl DES is regarded as a promising NO absorbent as a result of its high absorption capacity, ease of synthesis, and lower viscosity.

Figure 13. 1H NMR (500 MHz, DMSO) spectra of Tetz and DES before and after absorption of NO.

Figure 14. Mechanism of NO absorption by Tetz/P4444Cl.

absorbed NO. The molecular−NONO interaction force in Tetz/P4444Cl is likely to be less than the anion−NONO interaction force in [P44416][Tetz], leading to the reduced NO absorption capacity in Tetz/P4444Cl DES. In addition, the larger absorbent−NONO interaction force is not conducive to NO desorption and the higher NO desorption rate has been implemented in this work than [P44416][Tetz]. 3.9. Charge Distribution of HBDs and the Deprotonation of Tetz. To further explain experimental phenomena and the NO absorption mechanism, the density functional theory (DFT) calculations at the B3LYP/6-311G++ (d,p) level was conducted, and the results of Mulliken population atomic charge of Tetz, Triz, and Imid are shown in Figure 15. It was clear that the N1 atom charges on the three HBDs were quite different. The Mulliken population atomic charges on the N1 atom were −0.370, −0.361, and −0.547, respectively. According to the results of discussion above, the NO absorption by Tetz-derived and Triz-derived DESs is higher than that by Imid-derived DESs, the absorption mechanism that lower electron cloud density of N in N−H bond was favorable to the deprotonation of N−H and that it improved the NO absorption performance was proved. Although the electron density of N1 in Triz was lower than the electron density of N1 in Tetz, the electron cloud density of N2 in Triz was greater than the electron cloud density of N2 in Tetz. Therefore, the higher NO absorption capacity of three azole-

4. CONCLUSIONS The three azoles (Tetz, Triz, and Imid) were chosen as suitable HBDs and their charge distributions were calculated. The azole-based DESs were prepared, and the viscosity measurements demonstrated that the viscosities of azolebased DESs were significantly low and their viscosities were further reduced as NO absorption. The absorption results showed that the DESs based on P4444Cl as HBA or Tetz as HBD exhibited higher NO absorption capacity. Notably, Tetz/ P4444Cl DES exhibited the NO absorption values of 2.10 and 1.01 mol/mol at 101.3 and 12 kPa partial pressures and 303.15

Figure 15. Charge distribution of (a) Tetz, (b) Triz, and (c) Imid. H



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Table 7. NO Mole Absorption Capacity (MAC) of Different DESs and ILs at Pressure (p) of 101.3 kPaa MAC absorbents (molar ratio)

Tabs (K)

mol NO/mol abs

g NO/g abs

Tetz/P4444Cl (1:1) Tetz/N4444Cl (1:1) Tetz/P4444Br (1:1) Tetz/N4444Br (1:1) Triz/P4444Cl (1:1) Imid/P4444Cl (1:1) Tetz/ChCl Triz/ChCl Imid/ChCl [P66614][Tetz] 1,3-DMTU/P4444Cl (3:1) 1,3-DMTU/P4444Cl (2:1) 1,3-DMTU/P4444Cl (1:1) 1,3-DMTU/P4444Br (1:1) 1,3-DMTU/N4444Cl (1:1) 1,3-DMTU/N4444Br (1:1) 1,3-DMU/P4444Br (3:1) 1,3-DMU/P4444Br (2:1) 1,3-DMU/P4444Br (1:1) CPL/N4444F (2:1) CPL/N4444Cl(2:1) CPL/N4444Br (2:1)

303.15 303.15 303.15 303.15 303.15 303.15 343.15 343.15 343.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 303.15 338.15 343.15 343.15

2.10 1.46 0.48 0.32 0.71 0.16 0.86 0.67 0.47 4.52 4.25 3.18 2.13 1.13 2.05 1.00 1.17 0.92 0.66 0.16 0.11 0.09

0.173 0.126 0.035 0.024 0.059 0.013 0.123 0.096 0.068 0.246 0.210 0.190 0.160 0.076 0.161 0.070 0.058 0.054 0.046 0.010 0.007 0.005

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K. Moreover, the loss of Tetz/P4444Cl DES could be ignored during the absorption process. In addition, the TGA examination and regeneration experiments demonstrated that Tetz/P4444Cl performed satisfactory thermal stability and reusability. Additionally, the NO absorption mechanism by Tetz/P4444Cl DES was studied using 1H NMR and FTIR. The results illustrated that there was chemical interaction between NO and the hydrogen-containing nitrogen atom of Tetz. The DFT calculation further proved that the greater extent of protonation was favorable to the higher NO absorption. In conclusion, an easily synthesized and lower-viscosity benign solvent Tetz/P4444Cl DES was proposed and its properties were beneficial for NO absorption.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +86 22 27400199. ORCID

Luhong Zhang: 0000-0002-7073-4793 Xiaowei Tantai: 0000-0003-2925-0444 Notes

The authors declare no competing financial interest.

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Efficient and Reversible Nitric Oxide Absorption by Low-Viscosity

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