Carbon Dioxide Absorption by Polyethylene Glycol Dimethyl Ether Modified by 2-methylimidazole

Abstract

Developing and utilizing capture and storage technologies for CO₂ has become a critical research topic due to the significant greenhouse effect caused by excessive CO₂ emissions. A conventional physical absorption process for CO₂ capture is polyethylene glycol dimethyl ether (NHD); however, its limited application range is caused by its poor absorption of CO₂ at low pressures. In this work, the CO₂ absorption of NHD was enhanced by combining NHD with a novel chemical absorbent 2-methylimidazole (2-mIm)-ethylene glycol (EG) solution to improve CO₂ absorption. Viscosity and CO₂ solubility were examined in various compositions. The CO₂ solubility in the mixed solution was found to be at maximum when the mass fractions of NHD, 2-mIm, and EG were 20%, 40%, and 40%, respectively. In comparison to pure NHD, the solubility of CO₂ in this mixed solution at 30 °C and 0.5 MPa increased by 161.2%, and the desorption heat was less than 30 kJ/mol. The complex solution exhibits high selectivity and favorable regeneration performance in the short term. However, it is more sensitive to moisture content. The results of this study can provide important data to support the construction of new low-energy solvent systems and the development of novel CO₂ capture processes.

1. Introduction

The accelerated pace of global industrialization has triggered a dramatic surge in fossil fuel consumption, leading to continuously rising CO₂ emissions and exacerbated climate challenges [1]. To achieve carbon neutrality, the development of high-efficiency carbon capture and storage (CCS) technologies has become imperative. Among existing methods, physical and chemical absorption techniques dominate industrial applications due to their complementary advantages under specific operational conditions. In addition, researchers are attempting to develop new types of absorbents, such as composite solvents, nanofluids, phase change solvents, and others [2,3].

The physical absorption method uses the difference in solubility of the different gases in a solvent to selectively separate CO₂ [4,5,6,7]. Typical physical absorbers, such as polyethylene glycol dimethyl ether (NHD) [8,9], have the advantages of low regeneration energy, a simple regeneration process, stable solvent, etc., and are especially suitable for low-temperature and high-pressure conditions. In the molecular structure of NHD, the oxygen atom of the ether functional group behaves as a base, so the NHD solution has a strong ability to dissolve and absorb acidic gases (e.g., H₂S, COS, CO₂, etc.) [10]. It is notable that as the temperature decreases, the solubility of NHD for CO₂ increases, but the solubility for H₂ and N₂ decreases. This property enhances CO₂ selectivity under low-temperature conditions (−5 to 0 °C), significantly reducing solvent consumption. However, since NHD follows Henry’s law, it is ineffective for low-pressure CO₂ sources (e.g., flue gas), severely limiting industrial applications. The hybridization of physical and chemical absorbents has emerged as a primary strategy to address the low absorption capacity of physical solvents under reduced pressures.

The chemical absorption method primarily separates CO₂ from other gases by forming intermediate compounds through reversible chemical reactions between CO₂ and chemical absorbents [11,12,13]. The alkanolamine-based method has been extensively studied and applied in recent decades, owing to its high absorption rate under atmospheric pressure, cost-effectiveness, and enhanced chemical absorption performance under high-pressure conditions [14,15,16,17]. However, it also exhibits several common drawbacks of chemical absorbents, such as high CO₂ desorption temperature, substantial regeneration energy consumption, susceptibility to oxidative degradation, and strong corrosiveness. Primary mitigation strategies involve formulating complex solutions with efficient physical absorbents or developing novel chemical absorbents [18,19]. Therefore, combining physical and chemical absorbents to form complex solutions has become a promising strategy to balance absorption performance and energy consumption.

2-Methylimidazole (2-mIm) is a hygroscopic white crystalline powder that is readily soluble in water, alcohols, and acetone but is insoluble in benzene. Imidazole derivatives are widely employed for synthesizing ionic liquids, whereby -NH/-NH₂ groups on imidazolium cations chemically capture CO₂. 2-mIm is an important ligand for the synthesis of zeolitic imidazolate frameworks such as ZIF-8 [20]. ZIFs primarily capture CO₂ through physical adsorption, while the 2-mIm solution captures CO₂ mainly through chemical absorption. As a result, 2-mIm exhibits a stronger CO₂ absorption capacity. However, compared to the adsorption of ZIF-8, its desorption heat is higher. Mirroring the structure of imidazolium cations, 2-mIm demonstrates a comparable CO₂ absorption performance with lower viscosity and density [21,22]. Crucially, it exhibits superior thermal/oxidative stability [23] and reduced reaction heat relative to conventional amine solutions [24,25]. In recent years, Liu et al. [26] found that the mixed solution formed by adding a certain amount of 2-mIm to ethylene glycol showed excellent CO₂ absorption performance, with low desorption heat consumption and excellent cyclic regeneration, which has good potential for application. Chen et al. improved the physical absorption process of propylene carbonate (PC) using this mixed solution [27]. Li et al. developed a complex solution by adding an aqueous 2-methylimidazole (2-mIm) solution to N-methylpyrrolidone (NMP) [28]. The complex solution demonstrated significantly high CO₂ selectivity and excellent recyclability. The mixed solution greatly increased the CO₂ solubility performance through chemical absorption while maintaining the benefits of low energy consumption for physical absorbent desorption. Especially at low partial pressures of CO₂, considerable CO₂ absorption can still be achieved due to the presence of chemical absorbents. These works have demonstrated the potential of 2-mIm to improve the conventional physical absorption process.

In this work, a mixed system with 2-mIm as the chemical adsorbent and ethylene glycol as the co-solvent is proposed to optimize the CO₂ absorption rate and capacity of the physical solvent NHD. Through a thorough evaluation of parameters such as CO₂ absorption capacity, desorption energy consumption, viscosity, selectivity, and regeneration, the composition of this mixed solution was optimized. Currently, no literature has reported the use of a complex solution comprising 2-mIm (with ethylene glycol as a cosolvent) and NHD to enhance the absorption performance of NHD under low-pressure conditions. The results of this project can provide important data to support the further construction of new processes for the low-energy consumption of water-based solvent systems and the development of new processes for CO₂ capture.

2. Materials and Methods

2.1. Experimental Materials

The purity and source of the reagents and gases used are listed in

Table 1. Chemicals used in the experiments in this study.

Chemicals	Material Purity	Manufacturer
2-methylimidazole	98%	Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China
Ethylene glycol	Analytical Reagent, AR	Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China
Polyethylene glycol dimethyl ether	Average Mn~250	Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China
Carbon dioxide	>99.99%	Zhongke Gas Co., Ltd., Karamay, China
Nitrogen	>99.99%	Zhongke Gas Co., Ltd., Karamay, China

.

2.2. Experimental Apparatus and Procedure

The CO₂ absorption apparatus is depicted in Figure 1 and is made up of three main parts: a monitoring computer, an absorption cell, and an intermediate cell. The absorption cell has a volume of 115 mL, while the intermediate cell and pipeline have an effective volume of 109 mL. Both the absorption and intermediate cells are placed in a constant-temperature water bath, with the water bath set to the desired experimental temperature. The purpose of setting the intermediate cell is to ensure that the temperature of the gas entering the absorption cell remains consistent. Meanwhile, the amount of gas entering the absorption cell is determined by measuring the pressure change before and after gas intake in the intermediate cell. The intermediate cell contains a piston, and the lower part of the piston is connected to the manual pump. The purpose of the manual pump is to increase the pressure in the middle tank when the gas pressure in the vessel is low. A set of temperature and pressure sensors is installed in both the intermediate cell and absorption cell. The temperature sensor used is a Pt100-type thermocouple, with an accuracy of ±0.1 K. The pressure sensors have a precision of ±10 kPa, and the temperature control accuracy of the constant temperature water bath is ±0.1 K. Data on temperature and pressure are collected by a data acquisition system (DAS).

The experimental procedures are as follows. A mixed solution with a volume of 70 mL was transferred into the absorption cell, while the intermediate cell was filled with gas at a specific pressure. The water bath was set to the desired experimental temperature. The absorption cell was evacuated to remove the air inside. When the temperatures in both the intermediate cell and the absorption cell reached the set point, the pipeline between them was connected, and the gas in the intermediate cell was released into the absorption cell. Once the pressure in the absorption cell reached the target pressure, the inlet valve of the absorption cell was closed. The temperature and pressure before and after releasing the gas in the intermediate cell were recorded. The rotor speed of the absorption cell was set to a constant 300 rpm, and the temperature and pressure variations within the cell were monitored using the online data acquisition system. After 2 h, when the pressure in the absorption cell stabilized and the absorption reached equilibrium, the gas temperature and equilibrium pressure in the absorption cell were recorded. The procedure was repeated to determine a series of temperature and equilibrium pressure points. The solubility of the gas in the absorption cell at equilibrium pressure was then calculated by applying the equation of state. Each group of experiments was repeated at least three times, and the average value was taken as the final data. Relative error was used to represent the testing accuracy. Its definition is (Measured value − Average value)/Measured value × 100%. The relative error of all the data is within 5%.

In the desorption and regeneration experiment, the system after absorption equilibrium was lowered to atmospheric pressure and then connected to a vacuum pump to carry out the vacuum desorption operation. The operation time was 30 min. After desorption, the inlet-absorption equilibrium operation was repeated to calculate the amount of carbon dioxide absorbed by the desorbed solution, which was compared with the fresh solution to determine whether the solution was completely desorbed.

The viscosity of the absorbent was measured using Anton Paar’s Rheolab QC. For this experiment, 35–40 mL of the sample was placed into the sample chamber and the temperature control system was activated, with the temperature set to the target value. After allowing the temperature to stabilize, the rotor was started and its speed was set to 100 rpm. The measurement duration was set to 300 s, with a sampling interval of 10 s, and the measurements were taken for 30 cycles. The average viscosity value was calculated from 30 measurements to ensure the accuracy of the results.

In this experiment, nine types of absorbents with varying component ratios were prepared, and all experimental conditions are summarized in

Table 2. Absorbent composition.

Absorbents	Compositional Ratios (Mass Fraction)
	NHD	2-mIm	EG	H2O
C1	0	0.33	0.67	0
C2	0	0.4	0.6	0
C3	0	0.43	0.57	0
C4	0.17	0.33	0.5	0
C5	0.2	0.4	0.4	0
C6	0.34	0.33	0.33	0
C7	0.4	0.3	0.3	0
C8	0.18	0.36	0.36	0.1
C9	1	0	0	0

.

2.3. Experimental Data Processing

The experiment uses the principle of the conservation of total gas molar quantity to calculate the solubility of CO₂ in the absorbent. For each set of equilibrium experiments, the amount of total gas, n_t, entering the absorption cell from the intermediate cell was calculated using the following equation:

$$n_t=n_0-n_r={\displaystyle \frac{P_b^1\ast V_b}{Z_1\ast R\ast T_1}}-{\displaystyle \frac{P_b^2\ast V_b}{Z_2\ast R\ast T_2}}$$

(1)

where n_t is the difference in the amount of CO₂ substance contained in the intermediate cell before and after venting, representing the total amount of CO₂ substance put into the absorption cell; $n_0$ is the initial amount of gas in the intermediate cell. $n_r$ is the remaining amount of gas in the intermediate cell after injection into the absorption cell. The amount of gas is calculated using the equation of state, combining data on temperature, pressure, and compressibility factor. P_b1, T₁ and P_b2, T₂ are the pressure and temperature before and after venting of the intermediate cell, respectively; Z₁ and Z₂ are the gas compressibility factors corresponding to P_b1 and P_b2, which can be calculated by the Benedict–Webb–Rubin–Storrow (BWRS) equation of state [29]; V_b is the volume of the intermediate cell, and R is the ideal gas constant.

After absorption equilibrium is achieved, the amount of CO₂ that is still present in the absorption cell, or n_E, is:

$$n_E={\displaystyle \frac{P_E\ast V_g}{Z_E\ast R\ast T}}$$

(2)

where P_E is the remaining equilibrium pressure, V_g is the remaining gas volume, and Z_E is the compression factor in the absorption cell.

The material amount of CO₂ absorbed by the absorbent(n_a) is:

$$n_a=n_t-n_E.$$

(3)

The solubility S_a of the absorbent for CO₂ is:

$$S_a={\displaystyle \frac{n_a}{V_a}}$$

(4)

where V_a is the volume of the absorbent.

To determine the solubility of N₂ in the absorbent, Equations (1)–(4) can also be used.

The desorption heat of CO₂ from the absorbent, Q, can be calculated using the Clausius–Clapeyron equation (Equation (5)) [30]. The Clausius–Clapeyron equation describes the relationship between the vapor pressure and temperature of a substance. It is a key equation in thermodynamics and is used to understand phase transitions, such as the liquid–vapor phase change.

$$Q=-R{\displaystyle \frac{\ln \left(\frac{P_{E1}}{P_{E2}}\right)}{\frac{1}{T_1}-\frac{1}{T_2}}}$$

(5)

In Equation (5), at a certain CO₂ solubility, P_E1 and P_E2 are the equilibrium pressures at two different temperatures of T₁ and T₂, respectively.

3. Experimental Results and Discussion

3.1. Viscosity of the Mixed Solution

The viscosity variation rule with temperature for mixed solutions with varying compositions is displayed in Figure 2. NHD has low viscosity characteristics, and the viscosity is only 4.13 mPa.s at 30 °C. EG, as the additive, has a viscosity of 14 mPa·s at 30 °C. When 2-mIm–EG is added, the viscosity of the resulting mixed solution increases. As can be seen from the figure, the viscosity of the mixed solution is directly proportional to the 2-mIm content, and the increase in viscosity becomes more obvious when the mass fraction of 2-mIm exceeds 30%. For the 2-mIm–EG mixed solution, when the 2-mIm content reached 43% (mixed solution C3), the viscosity of the mixture solution reached 28.73 mPa·s at 30 °C. When the NHD mass fraction was 20% and both 2-mIm and EG mass fractions were 40%, the viscosity of the mixed solution (C5) was 25.56 mPa·s, and the viscosity rapidly decreased by 36% when the temperature increased by 10 °C. Therefore, the viscosity of the mixed solution can be further reduced by appropriately increasing the absorption temperature.

3.2. CO₂ Solubility in 2-mIm–EG Solution

In this study, the chemical absorbent is a 2-mIm–EG mixed solution, in which 2-mIm is the primary chemical absorbent and ethylene glycol is the co-solvent. Figure 3 shows the CO₂ solubility at 30 °C of the mixed 2-mIm–EG solutions with different compositions, where C2* (40 wt.% 2-mIm–60 wt.% EG) is the literature reference value [25], which is used to check the accuracy of the measured data. The figure shows that the C2 component’s solubility agrees with the data from the literature, demonstrating the accuracy of the measured data.

As shown in Figure 3, the CO₂ solubility gradually increased with increasing pressure, but the rate of increase slowed down. The measured three sets of 2-mIm–EG mixed solutions all contained more than 30 wt.% of 2-mIm, but the change in CO₂ solubility was not significant with the increase in 2-mIm content. This indicated the existence of an optimal concentration range for 2-mIm, which needs to be further determined in conjunction with the results of subsequent experiments. Under low-pressure conditions, the CO₂ solubility of the 2-mIm–EG mixed solution was significantly higher than that of the physical absorber NHD, which also provided better feasibility for the subsequent improvement of NHD. It can also be seen that the CO₂ absorption capacity of ethylene glycol is relatively small and primarily serves as a co-solvent [31].

Figure 4 shows the CO₂ solubility fluctuation rule with pressure for a C2 mixed solution (40 wt.% 2-mIm–60 wt.% EG) at various temperatures. As can be seen from Figure 4, the CO₂ solubility in the C2 mixed solution decreases with increasing temperature. However, when the pressure is high, the mixed solution still retains considerable CO_2, absorbed under high-temperature conditions. This shows that under high-pressure conditions, the mixed solution can be suitably raised in terms of the absorption temperature, which lowers the absorbent’s viscosity and lowers the energy required for the ensuing desorption process.

3.3. CO₂ Absorption Properties of NHD-2-mIm–EG Mixed Solution

This study examined the impact of compositional changes on CO₂ absorption capacity by comparing the CO₂ dissolution equilibrium curves (Figure 5) of NHD-2-mIm–EG mixed solutions with varying ratios at 30 °C. As shown in Figure 5, the CO₂ solubility curves (C4–C7) of the NHD mixed solutions obtained after adding 2-mIm–EG are almost always above the pure NHD solubility curve (C9), which demonstrates a certain improvement effect. The difference in CO₂ solubility is caused by the difference in composition.

Figure 5 illustrates the impact of different NHD contents on CO₂ solubility. In the C4 and C6 mixed solutions, the 2-mIm content was fixed at 33 wt.%, while the NHD contents were 17 wt.% and 34 wt.%, respectively. As shown in Figure 5, the C6 mixed solution demonstrates a higher CO₂ absorption capacity compared to the C4 mixed solution. When the equilibrium pressure exceeds 0.9 MPa, the CO₂ solubility of the C4 mixed solution is lower than that of pure NHD, whereas the CO₂ solubility of the C6 mixed solution only becomes lower than that of pure NHD at 1.3 MPa. This behavior is attributed to the higher NHD content in the C6 mixed solution, which, as pressure increases, enhances the physical absorption contribution, thereby boosting CO₂ absorption and widening the pressure range for optimal performance of the C6 mixed solution.

Figure 6b shows the impact of varying 2-mIm concentrations on CO₂ solubility. The contents of 2-mIm in C5 and C6 mixed solutions were 40 wt.% and 33 wt.%, respectively. The solubility of CO₂ in NHD follows Henry’s law, with solubility increasing linearly as the pressure increases. In C5 and C6, the absorption involved a mixture of chemical absorption by 2-mIm and NHD, combining the effects of both chemical and physical absorption. As a result, the trends observed for C5, C6, and C9 varied significantly. At low pressures, chemical absorption predominantly contributed, while at high pressures, physical absorption became the main process. As shown in the figure, when the pressure exceeded a certain value, the CO₂ solubility in pure NHD was higher than in the mixed solution. When the absorption equilibrium pressure surpassed 0.5 MPa, the CO₂ solubility in the C5 mixed solution was noticeably greater than that in the C6 mixed solution. This is because the content of 2-mIm in the C5 mixed solution was higher than that of C6 by about 7 wt.%, so the chemical absorption was stronger and increased the chemical absorption of CO₂. Therefore, increasing the addition of 2-mIm can improve NHD absorption performance; however, this is constrained by the viscosity of the mixed solution and the solubility of 2-mIm in ethylene glycol. The mass fraction of 2-mIm in the mixed solution should not be too high, and it is recommended to be controlled within 40 wt.%. Meanwhile, the physical absorbent NHD content should be above 20% to ensure the mixed solution’s pressure applicability range and the CO₂ absorption effect at high pressure. Overall, the best CO₂ absorption effect is shown by the C5 mixed solution (20 wt.% NHD–40 wt.% 2-mIm–40 wt.% EG) in Figure 5. Compared with the reported 2-mIm–NMP–H₂O aqueous solution [30], as shown in Figure 5, the mixed solution C5 in this study exhibited higher CO₂ solubility.

Table 3. Incremental CO₂ solubility (Sr) of the C5 mixed solution compared to pure NHD solvent at different pressures at 30 °C.

PE (MPa)	SNHD (mol/L)	SC5 (mol/L)	Sr = (SC5 − SNHD)/SNHD × 100%
0.2	0.209	0.546	161.2%
0.5	0.646	1.044	61.61%
1	1.373	1.678	22.22%

shows the percentage increase in CO₂ solubility of the C5 mixed solution (S_C5) compared to the pure NHD (S_NHD) at various pressures, using an absorption temperature of 30 °C as an example. According to the table, the modified NHD with 2-mIm of CO₂ solubility increased significantly under low pressure, increasing by 161.2% at an equilibrium pressure of 0.5 MPa. However, the CO₂ solubility decreased as the absorption equilibrium pressure increased, and when the equilibrium pressure reached 1 MPa, the C5 mixed solution’s CO₂ solubility increased by 22.22% compared to the pure NHD solvent. As a result, this modified NHD solution applies to a wider range of pressures, and the absorption is significantly improved.

3.4. CO₂ Isothermal Absorption Curves and Desorption Heat Consumption of Mixed Solutions

Figure 7 shows the CO₂ solubility curve at various temperatures in the C4 and C5 mixed solution, which is used to examine how easily the mixed solution desorbs. As can be seen from the figure, the CO₂ solubility in the mixed solutions decreased significantly with the increase in temperature. Taking the C5 mixed solution as an example (Figure 7b), the CO₂ solubility at 40 °C drops by 30.03% compared to 30 °C, and at 50 °C, it drops by 40.84% compared to 30 °C when the equilibrium pressure is 0.5 MPa. With the increase in absorption equilibrium pressure, the decrease in CO₂ solubility caused by the increase in temperature decreased slightly. At 1 MPa, the CO₂ solubility decreased by 28.34% at 40 °C compared with 30 °C, and decreased by 37.78% at 50 °C compared with 30 °C. Since CO₂ solubility is low at low pressures, desorption and regeneration of the mixed solution can be achieved by pressure reduction and heating. In the experimental process, it was found that the mixed C5 solution could be fully regenerated by vacuum treatment for half an hour after the full equilibrium of CO₂ absorption at 30 °C, and the desorption speed could be further accelerated by heating.

The energy consumption of desorption after CO₂ absorption by mixed solutions is a key parameter of the absorption process. This study used Equation (5) in conjunction with the CO₂ solubility curves of various mixed solutions at various temperatures to determine the CO₂ desorption heat of various mixed solutions (Figure 8). Figure 8a shows the desorption heat of the C5 mixed solution under different CO₂ solubility. The figure shows that the desorption heat of C5 mixed solution is typically less than 30 kJ/mol, with a desorption heat of 28.8 kJ/mol for a CO₂ solubility of 1 mol/L. This is significantly less than the desorption heat of the widely used alcoholic amine aqueous solution. Due to its relatively weak affinity for CO₂, the reaction rate between 2-mIm and CO₂ is slower than that of other alkanolamine solutions like MEA. However, when compared to MDEA (around 60 kJ/mol) and MEA (around 100 kJ/mol) solutions [32,33]. Meanwhile, the figure indicates that the desorption heat exhibits a trend of gradual decrease as CO₂ solubility increases.

Figure 8b compares the desorption heat of various mixed solutions with pure NHD. It shows that the desorption heat of the C2 mixed solution (40 wt.%) 2-mIm–60 wt.% EG) can reach 38 kJ/mol, while the desorption heat of the C5 mixed solution (20 wt.% NHD–40 wt.% 2-mIm–40 wt.% EG) is 28.8 kJ/mol. For the C4 mixed solution with lesser 2-mIm content, the desorption heat was 22.91 kJ/mol, and the desorption heat of the physical absorber NHD was the lowest at 16 kJ/mol. The addition of 2-mIm makes the heat of CO₂ desorption of NHD increase, but it is still at a low level. Comparing the CO₂ absorption capacity and the desorption heat, the modified hybrid C5 solution meets the requirements of high CO₂ solubility and low energy consumption for desorption.

3.5. Mixed Solution Selectivity

In addition to examining the CO₂ absorption–desorption capacity of the modified NHD solution, this work also investigated its selectivity for N₂ to show high separation performance in capturing CO₂ from the flue gas, which is mainly a mixture of CO₂ and nitrogen.

Table 4. Solubility and selectivity coefficients for N₂ in C5 mixed solutions.

PE (MPa)	SN2 (mol/L)	SCO2 (mol/L)	K = SCO2/SN2
0.294	0.0107	0.712	66.54
0.471	0.0191	1.00	52.36
0.707	0.0298	1.34	44.97
1.087	0.0485	1.76	36.29
1.228	0.0571	1.89	33.10

shows the variation of solubility and selectivity of CO₂ and N₂ with pressure at 30 °C for the preferred C5 mixed solution (20 wt.% NHD–40 wt.% 2-mIm–40 wt.% EG).

Table 4 shows that the C5 mixed solution’s CO₂ selectivity (K) ranges from 33 to 66 across the range of measured pressures and progressively drops as the equilibrium pressure rises. This is because the increase in pressure leads to an increase in the solubility of the gas, so the selectivity of the mixed solution for CO₂ deteriorates, but it is still at a high level. In general, the selectivity of the C5 mixture at low pressure is satisfactory, and the selection of this solvent needs to be considered for the appropriate pressure range.

3.6. Water Resistance and the Recyclability of a Mixed Solution

To investigate the influence of raw gas with water on the absorption effect of the mixed solution, this article takes the C5 mixed solution as the basis and adds 10 wt.% of water to obtain the water-containing mixed solution C8 for CO₂ solubility determination. The CO₂ solubility of the C8 mixed solution at different temperatures is shown in Figure 9a.

As can be seen in Figure 9a, the CO₂ solubility was significantly increased when 10 wt.% of water was added to the C5 mixed solution. The CO₂ solubility of the C8 mixed solution at 30 °C and 0.5 MPa is 1.486 mol/L, which is 1.42 times higher than that of the C5 mixed solution. This is because CO₂ reacts with 2-mIm through a novel reaction mechanism when water is present, which is comparable to the aqueous solution of alcoholamine. As proposed by Liu et al. [26], the reversible reaction between 2-mIm, CO₂, and H₂O leads to the formation of bicarbonate (Equation (6)). As a result, the amount of CO₂ that is absorbed increases. In contrast to the presence of water, a specific 2-methylimidazole alkyl carbonate salt is formed in the presence of CO₂ (Equation (7)), glycol, and 2-methylimidazole, with CO₂ acting as a “switch” to trigger this transformation.

(6)

(7)

Additionally, the presence of water results in the formation of strong chemical bonds between CO₂ and 2-mIm, increasing the energy required for the mixed solution’s desorption. It was calculated that the desorption heat of the C8 mixed solution increased to 65.7 kJ/mol when 10 wt.% of water was added, which was 2.28 times higher than the desorption heat of the C5 mixed solution. The carbon dioxide absorption increased when the mixed solution was exposed to water, but the desorption energy consumption was more sensitive to the presence of water. This also provides an idea for future research, which is to suitably add water to the mixed solution containing 2-mIm and then modify the water content to increase the CO₂ absorption capacity and regulate the mixed solution’s desorption energy consumption at an appropriate level.

Figure 9b illustrates that the mixed solution C5 performs well in both CO₂ desorption and regeneration. After the CO₂ has been fully absorbed, it can be fully regenerated by vacuuming and depressurizing for half an hour. The regenerated solution still maintains a relatively stable amount of CO₂ that is absorbed after several absorption–regeneration experiments at 30 °C.

4. Conclusions

In this work, 2-mIm was used as a chemical absorption body to be compounded with NHD to form a mixed solution to improve its CO₂ absorption performance. The presence of 2-mIm was found to decrease the fluidity of the system and increase the viscosity of the mixed solution; therefore, its content must be kept within a reasonable range. Studies in the CO₂ absorption capacity of mixed solutions with varying compositions revealed that the 20 wt.% NHD–40 wt.% 2-mIm–40 wt.% EG mixed solution exhibited optimal absorption under the tested conditions, with preliminary regeneration stability, selectivity, and a desorption heat of less than 30 kJ/mol. When the raw materials were combined with water, free water existed in the mixture, which greatly enhanced the absorption of CO₂. However, this resulted in a significant rise in the energy required for the desorption of the mixture.