Experimental Research on NO₂ Viscosity and Absorption for (1-Ethyl-3-methylimidazolium Trifluoroacetate + Triethanolamine) Binary Mixtures

Abstract

The viscosity (9.34–405.92 mPa·s) and absorption capacity (0.4394–1.0562 g·g⁻¹) of (1-ethyl-3-methylidazolium trifluoroacetate + triethanolamine) binary blends atmospheric pressure in the temperature range of 303.15–343.15 K and at different mole fractions of [EMIM] [TFA] have been carried out. The molar fraction of [EMIM] [TFA] dependence of the viscosity and absorption capacity was demonstrated. The addition of a small amount of [EMIM] [TFA] into TEA led to rapidly decreased rates of binary blends’ viscosity and absorption capacity. However, the viscosity and absorption of binary blends did not decrease significantly when [EMIM] [TFA] was increased to a specific value. Compared with the molar fraction of the solution, the temperature had no obvious effect on viscosity and absorption capacity. By modeling and optimizing the ratio of viscosity and absorption capacity of ([EMIM] [TFA] + TEA), it is proven that when the mole fraction of [EMIM] [TFA] is 0.58, ([EMIM] [TFA] + TEA) has the best viscosity and absorption capacity at the same time. In addition, at 303.15 K, ([EMIM] [TFA] + TEA) was absorbed and desorbed six times, the absorption slightly decreased, and the desorption increased.

1. Introduction

Nitrogen oxides (NO_X) are the prominent pollutants in the atmosphere. They are mainly produced by the high-temperature combustion of industrial nitrogenous fuel in which NO_X emission accounts for 50% of total emission when burning petroleum and over 60% (about 7.7 million tons) when burning coal in power plant boilers [1,2,3]. NO_X are the main precursors of acid rain and the direct sources of indoor nitrous acid (HONO) and photochemical air pollution. They greatly increase the risk of lung damage or even lung cancer if exposed to these substances for a long time [4,5,6]. At present, SCR (selective catalytic reduction), SNCR (selective non-catalytic reduction), and NSCR (non-selective catalytic reduction) are the principal force of denitrification techniques. However, these technologies are often accompanied by high technology costs and complex tail gas pollution. In contrast, inexpensive solvents are becoming more attractive. The absorbents (water, acid solution, alkali solution) which target nitrogen dioxide (NO₂) account for the majority of these solvents [7,8]. As a mature and efficient separation system for industrial desulfurization and denitrification, alkanolamines (such as monoethanolamine, diethanolamine, methyldiethanolamine, and triethanolamine) can rapidly transfer acid gas from gas phase to liquid phase and have significant advantages of high absorption and low operation costs [9,10,11]. The mixture of alkanolamines and water is a common form of industrial solvents. The cavities of the water provide larger activity spaces for the molecules of alkanolamines and the two-phase hydrogen bonds significantly prevent the self-association between the molecules [12]. This behavior effectively reduces the viscosities of pure alkanolamines, strengthens the mass transfer, and increases the absorption capacity of gases. However, the degradation and volatilization of alkanolamine solutions—as well as the scaling and corrosion of equipment carried by the water of the compound system—are still unavoidable in the industrial process [13]. The crucial problem is that NO₂ reacts with alkanolamine solutions based on water easily and rapidly which makes the solutions become viscous and foam, at the same time causing the system to generate higher regeneration energy, which challenges the practicability and circulation of the absorption solutions in large absorption equipment [14,15]. Some researchers have carried out a lot of work on the absorption of acid gas by nonaqueous amine solutions [16,17,18]. For example, Kim et al. [17] used alcohol solvent as a substitute for water. Compared with water or short-chain alcohol, the use of (long-chain alcohol + alkanolamines) solution reduces the wastage of high volumes of solvents, but the insufficient absorption capacity in the process still needs to be further studied and solved in the future. Therefore, finding a safe and stable solvent is necessary to enhance the process efficiency further and reduce the environmental impact caused by the volatilization of absorbents.

In recent years, as one of the hotspots in the field of green chemistry, ionic liquids (ILs) have been proposed as new candidate materials for selective capture of CO₂, SO₂, and H₂S due to their unique physical and chemical properties, such as adjustable structures, low vapor pressure, high thermal stability and chemical stability, high solubility for organic and inorganic compounds, and wide liquid phase range [18,19,20]. ILs have also been studied to absorb NO₂ in recent reports. Kunov Kruse et al. [21] first analyzed the feature spectra of several imidazolyl ILs for capturing NO₂. These ILs can easily overcome the Lewis base catalytic reaction between NO₂ and ILs in an anhydrous state, which prevents the solvent from anion loss and dramatic viscosity changes at high temperatures. Yuan et al. [22] found that NO₂ exists as a dimer among the molecules of [BMIM] [OTF]. Processes similar to molecular-imprinting of NO₂ during absorption–desorption illustrate the adsorption of NO₂ in ILs is controllable and reversible, and the absorption quantity can reach 0.0322 g·g⁻¹ at the normal state. Duan et al. [20] tested the solubility of pure NO₂ in the special amine-functionalized ILs. The best among these ionic liquids was CPL:TBAB (2:1) whose absorption quantity was 0.355 g·g⁻¹, about 10 times of the traditional imidazolyl ILs. It can be concluded that amine-functionalized ILs have similar characteristics to those of organic amine solutions. However, amine-functionalized ILs have fewer basic amine groups than alkanolamines, and their anions easily interact with the tail of cation (—NH₂) through hydrogen bonds, which makes the viscosities of pure amine-functionalized ILs relatively high, further leading to lower gas capture capacity in the industry [23,24]. In this case, ionic liquids used alone are not enough to separate NO₂ maturely and economically.

In recent years, researchers have tried to use binary mixture (ionic liquid + alkanolamine) to absorb acid gas [25,26,27,28,29,30]. It has been found that the mixing of binary systems significantly increases the absorption of acid gas, reduces the possible corrosion to equipment, and reduces the viscosity of the fluid, which is very important. The method can reduce pumping energy consumption and the loss in the flow process and improve equipment durability. However, the absorption of NO₂ by binary mixtures (ILs + alkanolamines) has yet to be studied. According to Wolf et al. [31], based on imidazole ionic liquids in 2012, and Gold et al. [32] on the good trapping effect of triethanolamine on NO2 in aqueous solution. Therefore, we prepared neutral ionic liquid 1-ethyl-3-methylimidazole trifluoroacetate ([EMIM] [TFA]) and mixed it with triethanolamine (TEA) in different proportions, and further studied the relationship between NO₂ absorption and viscosity in the binary system. The experimental temperatures of ([EMIM] [TFA] + TEA) are 303.15 K, 313.15 K, 323.15 k, 333.15 k, and 343.15 k, respectively.

2. Results and Discussion

2.1. Viscosity and Model

The viscosity of TEA and [EMIM] [TFA] in 298.15 K–333.15 K and their comparison with the reported literature values are shown in

Table 1. Comparison of the viscosity between experiment and literature data ^a,b.

T(K)	TEA Viscosity (mPa·s)			[EMIM] [TFA] Viscosity (mPa·s)
	Expt	Lit [39,40]	Lit [34]	Expt	Lit [34,35]
298.15	584.98	606.4	604.8	33.10	32
303.15	405.92	403.0	404.7	26.56	27.38
313.15	207.24	204.1	203.9	17.24	18.25
323.15	108.69	108.50	108.3	13.04	13.72
333.15	60.25	64.6	64.2	10.39	11.09

^a Standard uncertainties u are as follows: u(T) = ±0.1 K, u(P) = ±0.005 MPa, and the relative standard uncertainties u_r in u_r(η) = 0.27%–4.72%. ^b The values in parentheses are the relative uncertainty u_r(η) × 100% of the measured viscosities.

. Figure 1 shows that the viscosity measurement values of TEA are basically consistent with the values of Lopez et al. [33] and Li et al. [34,35], with an average error of 0.7985%. The viscosity values of [EMIM] [TFA] are slightly lower than those of Karim et al. [36] and Hector et al. [37], with an average error of −3.512%. This phenomenon may be due to the extreme sensitivity of [EMIM] [TFA] to chloride ions and trace moisture (>104 ppm) carried in the synthesis process [38].

The viscosity values of ([EMIM] [TFA] + TEA) binary mixtures at different temperatures and molar fractions are shown in

Table 2. Viscosity values of {[EMIM] [TFA](χ₁) + TEA(1 − χ₁)} binary systems (mPa·s) ^a,b.

χ1	T(K)
	303.15	313.15	323.15	333.15	343.15
0	405.92	207.24	108.69	60.25	36.29
0.1128	320.40	149.99	86.10	52.38	32.87
0.2006	234.79	117.78	65.22	40.93	27.13
0.3337	120.61	69.56	39.08	25.55	18.02
0.4299	67.35	41.65	26.19	17.46	12.69
0.5058	44.99	28.61	19.73	13.72	10.47
0.6051	37.59	23.83	17.74	12.32	9.27
0.6965	37.59	25.49	17.60	12.60	9.64
0.7994	35.58	26.28	17.44	12.36	9.74
0.9001	30.63	19.88	15.61	11.64	9.41
1.0000	26.56	17.24	13.04	10.39	9.34

^a Standard uncertainties u are as follows: u(T) = ±0.1 K, u(P) = ±0.005 MPa, u(χ₁) = ±0.0001, and the relative standard uncertainties u_r(η) = 0.27—4.72%. ^b Values in parentheses are the relative uncertainty u_r(η) × 100% of the measured viscosities.

and Figure 2. It can be seen that the viscosity of binary mixture decreases with the increase in temperature, and the viscosity of the mixture is always between the two-phase pure liquid, that is, η_TEA < η_{([EMIM] [TFA] + TEA)} < η_{[EMIM] [TFA]}. The viscosity of [EMIM] [TFA] decreases only 2.84 times in the temperature T range 303.15 K–353.15 K, while the viscosity of TEA decreases 11.18 times. TEA is more susceptible to temperature variation than [EMIM] [TFA], which can be attributed to the fact that a large number of intermolecular forces in TEA decrease with the increase in temperature [34]. Pseudo-quantitative changes of viscosity η with temperature and composition for the ([EMIM] [TFA] + TEA) binary systems are shown in Figure 3. The viscosity of binary mixtures ([EMIM] [TFA] + TEA) (9.34–405.92 mpa·s) is more susceptible to the influence of the molar fraction rather than temperature, which is consistent with the result of Haghtalab et al. [41]. Viscosity is always negatively correlated with the molar fraction χ₁. The viscosity of the binary system decreases rapidly in the enrichment regions of TEA (0 < χ₁ < 0.5), and the viscosity changes slowly in the enrichment regions of [EMIM] [TFA] (0.5 < χ₁ < 1). The relationship between the viscosity of the binary mixture ([EMIM] [TFA] + TEA) and the molar fraction of [EMIM] [TFA] is well fitted by the sixth order polynomial Equation (1), and the fitting parameters are shown in

Table 3. The fitting parameters (A_i) and the determination coefficients (R₂) of Equation (1) for viscosity of {[EMIM] [TFA] (χ₁) + TEA (1 − χ₁)} binary systems at several temperatures ^a.

T (K)	A0	A1	A2	A3	A4	A5	A6	R2
303.15	405.84	−460.72	−3557.9	9150.8	−5717.9	−1840.4	2047.1	1.0000
313.15	207.20	−715.74	2963.5	−12302	25291	−23213	7786.5	1.0000
323.15	108.72	−162.30	−475.60	1078.0	157.78	−1399.4	705.82	0.9999
343.15	60.272	−13.505	−674.36	1546.2	−1021.8	−125.96	239.58	0.9999
353.15	36.295	−3.7401	−271.20	206.93	807.81	−1308.2	541.51	0.9999

^a Standard uncertainties u are as follows: u(T) = ±0.1 K, u(P) = ±0.005 MPa.

.

H = A₀ + A₁x + A₂x² + A₃x³ + A₄x⁴ + A₅x⁵ + A₆x⁶

(1)

The viscosity deviation values were calculated from the data of viscosity by using the following Equation (2).

$$\Delta \eta =\eta -{\chi }_1{\eta }_1-{\chi }_2{\eta }_2$$

(2)

where η, η₁ and η₂ stand for the viscosity of ([EMIM] [TFA] + TEA), pure [EMIM] [TFA] and pure TEA, respectively. χ₁ and χ₂ stand for the molar fraction of [EMIM] [TFA] and TEA. The calculations are shown in

Table 4. Viscosity deviations of ([EMIM] [TFA] + TEA) (mPa·s) ^a.

χ1	303.15 K	313.15 K	323.15 K	333.15 K	343.15 K
0.0000	0.00	0.00	0.00	0.00	0.00
0.1128	−42.73	−35.82	−11.80	−2.25	−0.38
0.2006	−95.03	−51.35	−24.28	−9.32	−3.75
0.3337	−158.72	−74.28	−37.69	−18.06	−9.28
0.4299	−175.48	−83.91	−41.38	−21.36	−12.01
0.5058	−169.05	−82.53	−40.58	−21.31	−12.19
0.6051	−138.78	−68.44	−33.07	−17.76	−10.71
0.6965	−104.11	−49.42	−24.47	−12.92	−7.88
0.7994	−67.08	−29.07	−14.79	−8.03	−5.01
0.9001	−33.83	−16.34	−6.99	−3.73	−2.62
1.0000	0.00	0.00	0.00	0.00	0.00

^a Standard uncertainties u are as follows: u(T) = ±0.1 K, u(P) = ±0.005 MPa, u(χ₁) = ±0.0001.

and the relationship between the molar fraction of [EMIM] [TFA] and viscosity deviations ∆η is shown in Figure 4.

Similar to the behavior of most non-aqueous binary mixtures [41,42,43], the viscosity deviation of ([EMIM] [TFA] + TEA) binary mixtures presents an entirely negative deviation which is different from the ideal situation in the whole range of χ₁ in Figure 4. This behavior can be explained by weakening strong and specific interactions in the blends [44]. At the same time, it was observed that with the increase in [EMIM] [TFA] molar fraction, the viscosity deviation first decreased and then increased. When χ₁ is about 0.4, the absolute viscosity deviation is max, and the maximum value is |∆η_min|. Meanwhile, |∆η_min| decreases from 175.48 mPa·s to 12.19 mPa·s with temperature (303.15 K–43.15 K). Furthermore, the viscosity deviations have also been fitted by a Redlich–Kister [45] type Equation (3), a fourth-order polynomial was found to be optimum for viscosity deviations of binary mixtures ([EMIM] [TFA] + TEA). The standard deviation was calculated by using the following Equation (4). The fitted parameters and standard deviation σ were listed in

Table 5. The fitting parameters (B_P) and the standard deviation (σ) of Equations (3) and (4) for viscosity deviations of {[EMIM] [TFA] (χ₁) + TEA (1 − χ₁)} binary systems at several temperatures ^a.

T (K)	B0	B1	B2	B3	B4	σ
303.15	−677.1505	−346.9174	532.6040	536.1765	−127.0621	0.7056
313.15	−331.3255	−125.2933	399.0103	17.7388	−486.4350	0.2340
323.15	−161.1407	−80.3842	129.4112	89.9606	−44.0274	0.2428
333.15	−84.6904	−32.6001	94.7237	74.1388	−14.9678	0.1724
343.15	−49.0278	−7.6149	74.1177	39.5426	−33.6108	0.0761

^a Standard uncertainties u are as follows: u(T) = ±0.1 K, u(P) = ±0.005 MPa.

.

$$\Delta Q=X_1(1-X_1){{\displaystyle \sum _{P=0}^M{B}_P(1-2X_1)}}^P$$

(3)

$$\sigma ={\left[{\displaystyle \sum _1^{n_{dat}}{(Z_{\exp }-Z_{cat})}^2/n_{dat}}\right]}^{1/2}$$

(4)

where ∆Q represents ∆η is the excess property, χ₁ is the molar fraction of [EMIM] [TFA], B_p is the fitting parameters, M is the degree of the polynomial expansion, which was optimized using the Marquardt algorithm.

where n_dat is the number of experimental data points, Z_exp and Z_cat are the experimental value, calculated by Equation (3), respectively.

2.2. Absorption Capacity

Table 6. The absorption quantities of NO₂ by {[EMIM] [TFA] (χ₁) + TEA (1 − χ₁)} binary systems at 303.15 K to 343.15 K (g _NO2/g_{([EMIM] [TFA] + TEA)}) ^a.

χ1	T (K)
	303.15	313.15	323.15	333.15	343.15
0.0000	1.0562	0.8846	0.7645	0.6506	0.5227
0.2106	0.9465	0.8261	0.7192	0.6159	0.4985
0.4299	0.8570	0.7739	0.6757	0.5820	0.4753
0.5006	0.8119	0.7409	0.6518	0.5683	0.4725
0.6636	0.7687	0.7062	0.6261	0.5440	0.4594
0.8458	0.7139	0.6605	0.5850	0.5163	0.4473
1.0000	0.6795	0.6233	0.5525	0.4933	0.4394

^a Standard uncertainties u are as follows: u(T) = ±0.1 K, u(P) = ±0.005 MPa, u(χ¹) = ±0.0001, u(A_NO2) = ±0.0001 g.

shows the absorption of NO₂ by different molar fractions ([EMIM] [TFA] + TEA) from 303.15 K to 343.15 K. A second-order polynomial Equation (5) was found to satisfactorily correlate the change of absorption with temperature. By fitting these data, it is found that the second-order polynomial Equation (5) has a good correlation with the change of absorption with temperature. The parameters of the equation are shown in

Table 7. The fitting parameters (C_i) and the determination coefficients (R₂) of Equation (5) for absorption quantities of {[EMIM] [TFA] (χ₁) + TEA (1 − χ₁)} binary systems at different mole fractions ^a.

χ1	C0	C1 (10−2)	C2 (10−5)	R2
0.0000	11.9481	−5.622	6.6857	0.9949
0.2106	5.0106	−1.549	0.6857	0.9987
0.4299	0.5809	1.016	−3.05	0.9996
0.5006	0.1247	1.18	−3.1429	0.9992
0.6636	−0.2954	1.352	−3.3	0.9991
0.8458	0.9571	0.449	1.7429	0.9971
1.0000	3.7355	−1.358	1.1571	0.9971

^a Standard uncertainties u are as follows: u(T) = ±0.1 K, u(P) = ±0.005 MPa, u(χ¹) = ±0.0001.

. ([EMIM] [TFA] + TEA) and NO₂ absorption is negatively correlated with temperature, as shown in Figure 5. Even though the viscosity of blends is low at high temperatures, the low physical solubility of ([EMIM] [TFA] + TEA) seems to dominate. The change of absorption with different molar fractions of [EMIM] [TFA] at specific temperatures and at one atmosphere is shown in Figure 6 and Figure 7. With the increase in the TEA component, the absorption of NO₂ increased. It can be considered that the mole fraction of TEA is significant for the interaction between the binary mixture of ([EMIM] [TFA] + TEA) and NO₂. The relationship between the mole fraction of [EMIM] [TFA] and the absorption is well fitted by the fourth-order polynomial Equation (6), and the fitting parameters are shown in

Table 8. The fitting parameters (D_i) and the determination coefficients (R₂) of Equation (6) for absorption quantities of {[EMIM] [TFA] (χ₁) + TEA (1 − χ₁)} binary systems at several temperatures ^a.

T(K)	D0	D1	D2	D3	D4	R2
303.15	1.0560	−0.5343	0.0549	0.2016	−0.0987	0.9953
313.15	0.8844	−0.2580	−0.0812	0.1287	−0.0504	0.9941
323.15	0.7645	−0.2040	−0.0712	0.137	−0.0740	0.9955
333.15	0.6506	−0.1629	−0.0040	0.0139	−0.0043	0.9995
343.15	0.5228	−0.1292	0.0577	−0.0096	−0.0021	0.9969

^a Standard uncertainties u are as follows: u(T) = ±0.1 K, u(P) = ±0.005 MPa.

.

A = C₀ + C₁t + C₂t²

(5)

A = D₀ + D₁x + D₂x² + D₃x³ + D₄x⁴

(6)

With the increase in the molar fraction of TEA in the binary mixture ([EMIM] [TFA] + TEA), the absorption of NO₂ increased. However, at the same time, the viscosity of the binary system increases rapidly, which is easy to make the absorption equipment stagnate and lead to wall adhesion. For example, at 313.15 K, the viscosity of TEA is 207.24 mPa s, which is about 12 times the viscosity of pure ionic liquid under the same conditions. Therefore, the problem of excessive viscosity should be avoided while ensuring excellent absorption. By fitting the relationship between the [EMIM] [TFA] molar fraction, absorption, and viscosity of ([EMIM] [TFA] + TEA) solution, a mathematical model was established with the minimum ratio of viscosity to the absorption as the optimization objective. The solution was optimized by Origin, and the optimal ([EMIM] [TFA] + TEA) molar fraction was carried out under the higher absorption and lower viscosity. The optimization diagram is shown in Figure 8.

Taking the result at 303.15 K as an example, the results show that the function has a minimum value of 44.80 when the molar fraction of [EMIM] [TFA] is 0.58, the viscosity is 35.51 mPa·s, and the adsorption capacity is 0.7927 g·g⁻¹, at this time ([EMIM] [TFA] + TEA) has a lower viscosity and a higher adsorption capacity. With the increase in molar fraction (χ₁), the ratio of viscosity to absorption almost reached equilibrium. In order to verify the absorption effect of the experimental data ([EMIM] [TFA] + TEA) on NO₂, we compared it with the ratio of viscosity value and absorption quality of NO₂ absorbents prepared in the existing literature (at the same temperature and pressure). MDEA (101.00 mPa·s/0.021 g·g⁻¹ = 4809.00) [10]; CPL-TBAB (6502.38 mPa·s/0.355 g·g⁻¹ = 18,316.56) [20,21]; [BMIM] [OTF] (132.70 mPa·s/0.032 g·g⁻¹ = 4146.88) [22,46]. The absorption capacity of NO₂ by the mixture of ionic liquid created in this experiment is significantly better than those of other absorbents, and the negative effect of water was eliminated.

2.3. Theoretical Analysis

Spectroscopic studies show that there is a reversible equilibrium between NO₂ and N₂O₄, (the partial pressures of N₂O₄ at different temperatures are shown in the Supplementary Data [47,48]), and NO⁺ and NO₃⁻ are easily produced by autoionization in liquids or organic phases [49]. The molecular adduct [(Don)_n·NO⁺]NO₃⁻ will be formed reversibly When N₂O₄ is absorbed by the organic solvent (Don) [49]. If n > 2, the molecular adduct is more stable. Based on the experimental phenomena and theoretical analysis, we thought that the reaction of NO₂ in pure TEA is similar to that introduced by Addison, and the reaction product is a stable ionic substance [{(HOC₂H4)₃N}₂·NO⁺]NO₃⁻, as shown in Figure 9. We found that the system’s viscosity tends to decrease when adding ionic liquid [EMIM] [TFA]. The viscosity of the binary mixture drops sharply when χ₁ = 0–0.5, which is similar to the mixing of H₂O and TEA. It can be attributed to the fact that [EMIM] [TFA] provides a large number of cavities for TEA, which hinders the self-association of TEA molecules. However, [EMIM] [TFA] has little effect on the binary system except for the absorption decrease in the range of χ₁ = 0–1. Compared with the characteristic spectrum of pure ionic liquid, there are no new peaks and relative peak displacements in the characteristic spectrum of the regenerated [EMIM] [TFA] (Figure 10). Therefore, it is considered that [EMIM] [TFA] only has physical absorption in ([EMIM] [TFA] + TEA) and does not change the properties of the system.

2.4. Reusability of ([EMIM] [TFA] + TEA)

The recovery rate and reuse effect of ([EMIM] [TFA] + TEA) can be determined by absorption and desorption experiments. Therefore, the purpose of the technical efficiency and economic feasibility can be realized. In this work, the reusability of ([EMIM] [TFA] + TEA) was studied. Usually, saturated absorption amount was observed in 60 min at 303.15 K and 1.01 × 10⁵ Pa, with a stream of 30 mL·min⁻¹ pure NO₂, and the captured NO₂ can be removed in 40 min at 70 ℃ under vacuum (4.24 kPa). When the molar fraction of [EMIM] [TFA] was 0.58, a recycle of six times NO₂ absorption and desorption of ([EMIM] [TFA] + TEA) at 303.15 K was observed (Figure 11). The quantitatively recovered ([EMIM] [TFA] + TEA) was directly used in the following absorption process without further treatment. The results showed that the absorption was highly reversible, and the absorption amount remains basically unchanged after six consecutive absorption/desorption cycles. The slight decrease in absorption may be due to the loss of TEA and the increase in ILs mole fraction., but the advantage of this situation was that the desorption quantities of NO₂ were increased.

3. Materials and Methods

3.1. Materials

1-Methylimidazole (≥98%, Shijiazhuang Sydano Fine Chemical Co. Ltd, Shijiazhuang, China); silver trifluoroacetate (≥99%, Tianjin Hines Biochemical Technology Co. LTD, Tianjin, China); bromoethane (≥99%, Tianjin Damao chemical reagent plant, Tianjin, China); acetonitrile (≥99%, Tianjin Bodi Chemical Co. LTD, Tianjin, China); triethanolamine (≥99%, Tianjin Hedong District Hongyan reagent plant, Tianjin, China); NO₂ (≥99.5%); NaOH (≥96%, Xinkou Industrial Park, Xiqing Development Zone, Tianjin, China). All drugs are commercially available and have no further purification (See

Table 9. Sample purity description.

Chemical Names	CAS	Level	Purity c	Final Water Mass Fraction	Source
TEA	102-71-6	Analytical reagent	≥0.99	0.0001 a	Tianjin Hedong District Hongyan reagent plant
[EMIM] [TFA]	174899-65-1	-	≥0.99	0.00167 b	Homemade
1-Methylimidazole	616-47-7	Analytical reagent	≥0.98		Shijiazhuang Sydano Fine Chemical Co. Ltd
Silver trifluoroacetate	2966-50-9	Analytical reagent	≥0.99		Tianjin Hines Biochemical Technology Co. Ltd
Bromoethane	74-96-4	Analytical reagent	≥0.99		Tianjin Damao chemical reagent plant
NO2	10102-44-0	Analytical reagent	>0.9999		Qingdao Antaike Gas Co. Ltd
Glycerol	56-81-5	Analytical reagent	≥0.99	0.0005 b	Tianjin Yongda Chemical Reagent Co. Ltd
Water	7732-18-5	-			UPH-1V-10T, Chengdu ultra pure water technology Co. Ltd

^a Water content of TEA is stated by the merchant. ^b water contents of [EMIM] [TFA] and glycerol were measured by Karl Fischer titration. ^c Purity of the homemade [EMIM] [TFA] was determined by NMR spectroscopy, and the purity of other chemicals are stated by the merchants.

for details of samples). [EMIM] [TFA] was prepared similarly to the literature method [50,51]. 1-methylimidazole (30 mL) and ethyl bromide (90 mL) were fully mixed in a 250 mL round bottom flask and heated under reflux for 8 h to obtain an oily liquid. Then, the liquid was cooled to room temperature and a white solid was obtained. Then, the white solid was dissolved using acetonitrile and filtered, the ethyl acetate was added to the filtrate to obtain a white solid. The above operations were repeated, and the recrystallized product was dried under vacuum for 36 h to obtain 1-ethyl-3-methylimidazolium bromide (EMIMBr). Then, 19.1 g of EMIMBr and 22.4 g of silver trifluoroacetate were weighed out and added to 200 mL of acetonitrile. Finally, the silver bromide precipitate was filtered and the acetonitrile was removed by rotary evaporation. The final product [EMIM] [TFA] was obtained after vacuum drying at 60 ℃. The reaction process is shown in Figure 12. The structure spectrum of [EMIM] [TFA] is as follows. IR film, cm⁻¹: C=N (3374.54 cm⁻¹); -CH₂-(2941.08 cm⁻¹); C=O (1683.35 cm⁻¹); C=C(1572.82 cm⁻¹); -CH₂-(1456.16 cm⁻¹); C-F(1065.90 cm⁻¹). ¹H NMR(500 MHz, DMSO-d6, δ, ppm): 1.364 ppm (t, 3H, -(CH₂)-CH₃); 3.757 ppm(s, 3H, CH₃); 4.081 ppm(q, 2H, CH₂); 7.285 ppm(s, 1H, -CH=); 7.362 ppm(s, 1H, -CH=); 8.582 ppm(s, 1H, -CH=). The results for the FT-IR (S1) and NMR (S2) spectra are also provided in the Supplementary Data to confirm compound identity.

3.2. Instruments and Procedures

The binary mixture of ([EMIM] [TFA] + TEA) was prepared using a precise electronic balance (±10⁻⁴ g, Shanghai Precision and Scientific Sky Beautiful Instrument Co. Ltd, Shanghai, China). The viscometer (Zhejiang Jiaojiang Glass Instrument Factory, Taizhou, China) was calibrated in a super constant temperature bath (Shanghai Pingxuan Scientific Instrument Co. LTD, Shanghai, China, ±10⁻² K) with glycerol, glycol, and water. The viscosity was detected by the Pinkevitch method (according to GB/T 10247-2008), and the viscosity meter was calibrated by glycerol [39,40]. H-NMR spectra were collected on a Bruker AVANCE II 400 M spectrometer using d6-DMSO as the solvent and TMS as the internal standard. FTIR spectra were collected on a Bruker Tencer 27 spectrometer.

The absorption device of NO₂ is shown in Figure 13. The device mainly includes one homemade NO₂ generator, an MFC (Tianjin Jisite Instrument Co. Ltd, Tianjin, China, with an accuracy of ±1% F.S.), two sealed flasks, one three-necked flask, one constant temperature reaction bath (DF-101SZ Gongyi Yuhua Instrument Co. Ltd, Gongyi, China, Temperature control accuracy is ±0.1 K), and one temperature sensor.

The three-necked flask was immersed into the constant temperature water bath of a constant temperature magnetic stirrer. NO₂ was continuously passed into the vessel after its flow rate v_NO2 (v_NO2 = 30 mL·min⁻¹) was controlled by MFC to be constant. At the end of the evacuation, quantitative and different proportions of ([EMIM] [TFA] + TEA) solvent were injected into the three-necked flask, and the molar fraction of [EMIM] [TFA] in the mixture was defined as χ₁ = n_{[EMIM] [TFA]}/(n_{[EMIM] [TFA]} + n_TEA). The mass of absorbent ([EMIM] [TFA] + TEA) absorbing NO₂ gas and three-necked flask were weighed by an electronic balance at a constant time interval (t = 10 min). Repeat weighing was carried out until the range of mass variation was less than 1%, considered a reaction balance. The NO₂ load was defined as A_NO2= mNO₂/(m_{[EMIM] [TFA]} + m_TEA). The unabsorbed residual exhaust was introduced into the sodium hydroxide aqueous solution for recovery. The whole system operated at a constant atmospheric pressure of 0.1 ± 0.005 MPa.

4. Conclusions

In summary, the viscosity of binary mixtures of 1-ethyl-3-methylimidazolium trifluoroacetate ([EMIM] [TFA]) + triethanolamine (TEA) was determined, and their application in NO₂ absorption was studied at 303.15–343.15 K and 1.01 × 10⁵ Pa. At the same time, the ionic liquid [EMIM] [TFA] was characterized by IR and ¹H NMR spectra. The results showed that the viscosity values of ([EMIM] [TFA] + TEA) are between 9.34–405.92 mPa·s at the measured temperature and the viscosity values decrease with an increase in temperature. At the same temperature, the viscosity values decrease with an increase in [EMIM] [TFA] molar fraction, and the relationship between viscosity and [EMIM] [TFA] molar fraction was in accordance with the sixth-order polynomial equation. The viscosity deviation of ([EMIM] [TFA] + TEA) was negative in the range of measured temperature and mole fraction, and the viscosity deviation of ([EMIM] [TFA] + TEA) binary mixtures decreased with an increase in temperature at the same molar fraction. The fourth-order Redlich–Kister equation well fitted the relationship between viscosity deviation and molar fraction. Moreover, the NO₂ absorption quantities of ([EMIM] [TFA] + TEA) binary mixtures decrease with an increase in temperature. At the same temperature, the NO₂ absorption quantities of ([EMIM] [TFA] + TEA) binary mixtures increased with an increase in the ratio of triethanolamine (TEA) in ([EMIM] [TFA] + TEA) binary mixtures, and the relationship between absorption quantities and molar fraction was in accordance with the fourth-order polynomial equation. The experimental results of cyclic absorption showed that the absorption capacity of NO₂ was almost unchanged through repeated recycling. The results of FT-IR spectra and known literature indicated that the absorption of NO₂ by ([EMIM] [TFA] + TEA) was a physical and chemical absorption process. Furthermore, the established mathematical model and the optimization calculation by Origin software showed that when the molar fraction of [EMIM] [TFA] was 0.58 at 303.15 K, the corresponding viscosity was 35.51 mPa·s and the absorption amount was 0.7927 g·g⁻¹. The ([EMIM] [TFA] + TEA) shows great potential as an alternative absorbent in industrial denitrification due to its sound absorption and low viscosity.