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Article

Aqueous 2-Ethyl-4-methylimidazole Solution for Efficient CO2 Separation and Purification

by
Xingtian Zhang
1,2,3,
Jun Wu
4,
Xiaoxiao Lu
3,
Yefeng Yang
1,*,
Li Gu
2,* and
Xuebo Cao
3,*
1
School of Materials Science and Engineering, Zhejiang Sci-Tech University, Hangzhou 310018, China
2
School of Materials and Textile Engineering, Jiaxing University, Jiaxing 314001, China
3
College of Biological, Chemical Sciences and Engineering, Jiaxing University, Jiaxing 314001, China
4
College of Advanced Materials Engineering, Jiaxing Nanhu University, Jiaxing 314001, China
*
Authors to whom correspondence should be addressed.
Separations 2023, 10(4), 236; https://doi.org/10.3390/separations10040236
Submission received: 21 February 2023 / Revised: 25 March 2023 / Accepted: 26 March 2023 / Published: 3 April 2023
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Abstract

:
Carbon capture and storage (CCS) technology is considered as one of the most effective short-term solutions in reducing atmospheric CO2 concentrations. A key of CCS technology is to seek the absorbent with low cost, fast absorption rate, and high stability. In this study, we show that 2-ethyl-4-methylimidazole is particularly suitable for efficient CO2 capture. The aqueous solution of 2-ethyl-4-methylimidazole displays a maximum CO2 molar absorption capacity of 1.0 mol∙mol−1 and the absorbed CO2 can be completely released through heating the solution at a relatively low temperature (<100 °C). Stability tests show that the aqueous system is quite stable, with less than 10% loss of the molar absorption capacity after eight absorption–desorption cycles. Time-related in-situ attenuated total reflection infrared absorption spectroscopy and 13C nuclear magnetic resonance spectroscopy studies reveal that the intermediates are HCO3− and H2CO3 in the process of CO2 absorption–desorption. These intermediates are easily decomposed, which are responsible for the low CO2 desorption temperature and high desorption efficiency of the system. Moreover, the aqueous solution of 2-ethyl-4-methylimidazole is able to separate and purify CO2 from flue gas and even ambient air. Consequently, 2-ethyl-4-methylimidazole is a promising low-cost CO2 absorbent for industrial implementation.

Graphical Abstract

1. Introduction

Since the industrial revolution, human-caused CO2 emissions have increased exponentially [1]. Rising concentrations of CO2 in the atmosphere contributes to the greenhouse effect, which makes the global temperature rise continuously [2,3]. The situation is likely to get worse in the future, as an expanding population and industrialization continue to produce CO2 from the burning of fossil fuels. Since the 1970s, this anthropic influence has caused the global mean surface temperature (GMST) to rise at an average rate of 0.2 °C per decade [4]. In order to address this challenge, the Paris Agreement was signed by consensus in 2015, controlling the increase in global average temperature above the pre industrialization level and below 2 °C [5,6]. Carbon capture and storage (CCS) technology is considered as one of the most effective short-term solutions in the fight against global warming and plays a vital role in reducing atmospheric CO2 concentrations [7,8,9]. To keep the targets set by the Paris agreement, about 1 billion tonnes of CO2 must be captured each year and up to 16 billion tonnes stored by 2050 [10]. At the same time, CO2 is considered to be a cheap C1 source, which can be directly used for the subsequent conversion of multi-carbon products after capture, so the early capture is particularly critical [11,12].
CO2 capture is the most economical and feasible method to restrain carbon emissions from industrial point sources [13]. According to the current technical progress, amine chemisorbents, such as ethanolamine (MEA) [14,15], N-methyldiethanolamine (MDEA) [16], and piperazine (PZ) [17], have fast absorption rate, high absorption capacity, and mild operating conditions, and are often used to capture CO2, which is also one of the most mature CO2 separation technologies at present [18,19,20]. Especially, aqueous solutions of MEA have been extensively studied in post-combustion CO2 capture systems, and it is regarded as the benchmark for all CCS processes [14,15,21]. MEA has a strong affinity to CO2, resulting in favorable absorption kinetics. However, this method has three major disadvantages: solvent degradation, equipment corrosion, and regeneration high energy consumption. The solvent regeneration process requires a lot of energy to decompose stable carbamate formed during CO2 capture (temperature is about 120–140 °C), accounting for about 70–80% of the total cost of capture system [22,23,24]. Moreover, the most common impurity in industrial waste gas is O2. In the presence of O2, the absorbent containing amine is rapidly deactivated by the oxidation of amine [25,26,27,28]. There are many obstacles in using amine absorption method to capture CO2 after combustion, which has become an unavoidable challenge for its large-scale commercial application. The affinity between the absorbent and CO2 must be precisely regulated: if the interaction is too strong, the desorption energy will be great, whereas if the interaction is too weak, CO2 will not be completely absorbed [29]. Therefore, it is of great significance to develop MEA alternatives with low energy loss and fast absorption rate.
Imidazole is an interesting class of basic heterocyclic compounds in nature. Its main use is as a precursor material for the synthesis of other more complex compounds [30], and it has not been widely used as an absorbent for CO2 capture except for a few examples. Tomizaki et al. studied the reaction heat and vapor–liquid equilibrium of several imidazoles [31]. Evjen et al. investigated the CO2 absorption capacity of polyalkylated imidazoles and found that these imidazole-based solvents can achieve high CO2 absorption capacity [32]. Shannon et al. found that pure N-alkylimidazole has relatively low viscosity and density compared to its corresponding N-alkylimidazole-based ionic liquid [33]. Chen et al. measured the solubility of CO2 in 5~30 wt% 2-methylimidazole aqueous solution at 293.15–313.15 K [34]. Lin et al. introduced imidazole into polyelectrolyte membrane as an alkaline group and showed a good CO2 selective separation effect [35]. Yan et al. found that the introduction of imidazole improved the adsorption capacity and thermal stability of amine adsorbents [36]. Li et al. demonstrated that 1-methylimidazole is a phase separator that induces phase separation and an absorption promoter that increases the absorption rate [37]. Moreover, imidazole ionic liquids show good CO2 absorption capacity [38]. Cheng et al. studied the CO2 absorption mechanism by deep eutectic solvents based on ethylene glycol and protic ionic liquid ([MEAH][Im]), formed by monoethanolamine with imidazole [39]. Considering that imidazole-based compounds have low saturated vapor pressure and viscosity [40,41], excellent thermal and antioxidant stability [42,43,44], and low reaction heat [31], they should have potential for efficient and low-cost CO2 capture.
In this work, we investigated the CO2 absorption performance of a total of ten low- molecular-weight solid imidazoles. Of them, 2-ethyl-4-methylimidazole was found to be the optimum absorbent for CO2 capture, as it presents the best CO2 absorption performance on the basis of the large absorption capacity, fast absorption rate, relatively low desorption temperature (90 °C), and excellent cycle performance. The mechanism of CO2 absorption–desorption was studied by time-related in-situ attenuated total reflection infrared absorption spectroscopy and 13C nuclear magnetic resonance spectroscopy. Moreover, 2-ethyl-4-methylimidazole is capable of efficiently separating and purifying CO2 from flue gas and ambient air.

2. Materials and Methods

2.1. Materials

All the imidazoles (reagent grade) studied in this work were obtained from commercial sources, which were provided by Macklin Reagent (Macklin Biochemical Co., Ltd., Shanghai, China). Ethanolamine was provided by Sinopharm Reagent (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China). Deuterium oxide (99% wt %) used in nuclear magnetic resonance spectroscopy experiments was purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. The purity of CO2, N2, and O2 is 99.99% vol %, provided by Zhejiang Nankai Gas Co., Ltd., Jiaxing, China. The deionized (DI) water was prepared by reverse osmosis ultra-pure water equipment (conductivity ≤ 0.1 µS/cm).

2.2. Absorption Experiment

The absorption experimental setup for CO2 capture is diagrammatically described in Figure S1. The imidazole solid was dissolved in DI water and the aqueous solution is transferred to the bubbling reactor immersed in a water bath to keep the reaction temperature constant CO2 gas bubbles into the reactor, and the unabsorbed CO2 gas is discharged into the tail gas treatment bottle containing sodium hydroxide. The gas flow rate is controlled by the mass flowmeter, and the amount of CO2 absorbed is quantified by gravimetric method [45,46,47]. All absorption experiments were conducted at atmospheric pressure. The temperature was controlled in the range of 25~60 °C. To avoid accidental absorption of CO2 by the solution, all gas pipelines were blown with N2 before the experiment was run. The weight difference of each measurement represents the amount of CO2 absorbed by the solution. As long as the mass does not rise during continuous measurement, the balance of CO2 absorption can be considered to be reached. The cumulative absorption of CO2 can be calculated with Equation (1).
X = ( m t − m 0 ) / 44.01 m l / M m
where X is the CO2 molar absorption capacity (molCO2∙mol−1imidazole), m t the mass of the bubbling reactor after CO2 capture (g), m 0 the mass of the bubbling reactor before CO2 capture (g), m l the mass of the imidazole (g), and M m the molecular mass of the imidazole (g∙mol−1).

2.3. Desorption Experiment

The desorption setup is similar to the absorption reaction setup. As shown in Figure S1b, the gas bubbling tube at the upper part of the reactor is changed into a condensing tube to recover the evaporated liquid returned to the reaction bottle to reduce solution loss. The released gas passes through the cooling device and drying pipe to remove excess water and then enters the gas flow monitor. When the monitored gas flow rate is 0 mL/min, it proves that the CO2 gas is completely released. The tail of the device is a tail gas absorption bottle containing sodium hydroxide solution.

2.4. Flue Gas Capture

For CO2 capture in flue gas, we developed a simplified CO2 capture setup for separating and purifying CO2 from the flue gas. First, ten gas bubbling reactors containing imidazole aqueous solution were connected in series to form the absorption setup (Figure S2a). CO2 (15%) was mixed with N2 (77%) and O2 (8%) to obtain a synthetic flue gas. The synthetic flue gas was continuously feed to the setup and the gas stream at the outlet was monitored through on-line gas chromatograph. In the desorption stage, the solution loaded by CO2 was transferred to a vacuum flask and heated at 90 °C to release the CO2 (Figure S2b).

2.5. Direct Air Capture

As shown in Figure S3, in order to capture environmental CO2 with a concentration of 400 ppm, imidazole aqueous solution was placed in a beaker and exposed to air. After several hours, the concentrated CO2 was collected by heating the solution and detected by gas chromatography.

2.6. Characterization

The pH of the solution was determined by a ST2100/F pH meter (Ohaus, Pine Brook, MN, USA) equipped with a ST310 pH electrode (Ohaus, Pine Brook, USA). The conductivity of the solution was determined by a DDS-307A conductivity meter (INASE Scientific Instrument Co., Ltd., Shanghai, China). The viscosity of the solution was measured by the NDJ-9S digital display viscometer (Shanghai Gaozhi Precise Instrument Co., Ltd., Shanghai, China). Nuclear magnetic resonance spectroscopy (NMR) measurements performed on the nuclear magnetic resonance spectrometer (Bruker Avance III 400 MHz, Zurich, Switzerland) was used to determine the changes of solvents in the process of CO2 absorption and desorption. The in-situ attenuated total reflection infrared absorption spectrometer (React IR 701 L, Mettler Toledo, Zurich, Switzerland) was used to study the absorption–desorption behavior of CO2. The gas released during desorption was analyzed by gas chromatography (FULI GC9790 plus, Zhejiang Fuli Analytical Instruments Co., Ltd., Wenling, China) equipped with a flame ionization detector (FID) and methanation reactors. The gas chromatograph is calibrated using different concentrations of standard gases. High resolution mass spectrometry (HRMS) was recorded on Thermo Scientific Q Exactive spectrometer (Waltham, MA, USA) equipped with ESI source and data acquisition was performed using MassHunter software (MassHunter 10.0, Agilent, Santa Clara, CA, USA).

3. Results and Discussion

A total of ten low-molecular-weight solid imidazoles (Table S1) were selected as the absorbents for CO2 capture. They are all commercially available and can be used as received. For imidazole-based compounds, there is a lone pair of electrons at the N (3) position of the five membered ring. The carbon atom in CO2 is an electron-deficient center, which can be used as an electrophile to react with nucleophilic reagents. This means a good affinity of CO2 molecule to imidazole-based compounds. Furthermore, the vapor pressure of imidazole solid is extremely low (e.g., 0.008 mmHg for 2-ethyl-4-methylimidazole, Table S1). In contrast, the vapor pressure of MEA, a commonly used CO2 absorbent in industry, is about 0.5 mmHg at 25 °C. A low vapor pressure limits the volitation of the absorbent and hence, is advantageous to reducing the loss of the absorbent in the process of CO2 capture.
To perform the CO2 capture, 1 g of the imidazole solid was dissolved in 14 g of DI water to obtain an aqueous solution. Next, CO2 (purity 99.99%) was continuously input to the solution at a constant flow rate of 90 standard cubic centimeters per minute (SCCM). The weight change of the solution was measured every 2 min (Figure 1a–j). Table S2 shows the summary of CO2 capture capacity of various imidazole aqueous solutions and compares them with the results obtained by Tomizaki et al. [31] and Evjen et al. [32] As shown in Figure S4, the molar CO2 absorption capacity of the imidazoles were positively correlated with pKa. Of the ten imidazole solids (Figure 1k), the aqueous solution of 2-ethyl-4-methylimidazole displayed the shortest saturation time (ca. 8 min) and the maximum molar absorption capacity of 0.88 molCO2∙mol−1imidazole (mol∙mol−1 for clarity). The short saturation absorption time demonstrates that the aqueous solution of 2-ethyl-4-methylimidazole enables a favorable absorption kinetics in CO2 capture. The molar absorption capacity of 0.88 mol/mol indicates that approximately one molecule of 2-ethyl-4-methylimidazole interacts with one molecule of CO2. For the commonly used amine system, it usually needs two molecules of amine to capture of one molecule of CO2 to form carbamate. Consequently, 2-ethyl-4-methylimidazole shows a nearly 1:1 molecular utilization in the CO2 capture besides the favorable absorption kinetics. It should be noted that water in our system is a critical component in the promotion of the capture performance of 2-ethyl-4-methylimidazole. In the control experiments (Figure S5), there is no absorption of CO2 when anhydrous solid of 2-ethyl-4-methylimidazole was used alone. The phenomenon is attributable to the interaction between 2-ethyl-4-methylimidazole and water. There is a lone electron pair on the N (3) of the imidazole ring, and this alkaline site can combine with H+ in H2O to promote the ionization of water [48], thus producing OH− ions to absorb CO2.
The interactions of the aqueous solution of 2-ethyl-4-methylimidazole with nitrogen (N2) and oxygen (O2) were studied too, for the purpose of gaining insight into the selectivity of the system. As seen from Figure S6, the pressure of the flask containing the solution could remain unchanged over one week in the presence of N2 and O2. Under identical condition, the pressure of the flask filled by CO2 was negative after stirring for 3 min, attributable to the absorption of CO2 by the solution. The comparison demonstrates that the aqueous solution of 2-ethyl-4-methylimidazole has a good CO2 selectivity, which means that it is capable of separating CO2 from industrial flue gases and even ambient air, as will be shown later.
Next, the mass ratio of 2-ethyl-4-methylimidazole to water was adjusted to be 0.5:14.5, 2:13, and 3:12 (total mass of the system keeping at 15 g), and the CO2 absorption performance of the system under different mass fraction was studied (Figure 2a). For the solution with a mass ratio of 0.5:14.5, the CO2 saturation absorption time is 6 min. With the increase in the percentage of 2-ethyl-4-methylimidazole in the solution, the saturation absorption time is prolonged, which is around 8, 16, and 26 min corresponding to the system with a mass ratio 1:14, 2:13, and 3:12, respectively. Moreover, it can be seen that a low mass fraction of 2-ethyl-4-methylimidazole is advantageous to achieving a high molar absorption capacity. For the 0.5:14.5 system, the molar absorption capacity can reach 1.01 mol∙mol−1, indicative of a 100% utilization of the 2-ethyl-4-methylimidazole molecule. For the 3:12 system, the molar absorption capacity decreases to 0.69 mol∙mol−1. On the basis of the saturation absorption capacity and the time, an average CO2 absorption rates were calculated to be 0.17, 0.11, 0.05, and 0.03 mol∙mol−1∙min−1 for the system with a mass fraction of 0.5:14.5, 1:14, 2:13, and 3:12, respectively (Figure 2b). Figure S7 shows the CO2 capture capacity of 2-ethyl-4-methylimidazole solutions with different mass fractions. It can be seen that the CO2 capacity of pure water is only 0.024 g, which can be improved by adding 2-ethyl-4-methylimidazole, and it has the CO2 capacity of 0.821 g when the ratio is 3:12. Moreover, one may observe a slight decline of the molar absorption capacity in the 0.5:14.5 and 1:14 systems after the saturation absorption (Figure 2a). It is attributed to the removal of the moisture by the continuous CO2 flow, thus leading to a reduction of the mass of the system. However, in the systems with a mass fraction of 2:13 and 3:12, the loss of the moisture is avoided and the systems are quite stable.
The different CO2 capture performances of the 2-ethyl-4-methylimidazole solutions with a mass fraction of 0.5:14.5, 1:14, 2:13, and 3:12 are related to the viscosity of the system. As shown in Figure S8, the viscosity of the solution is increased with the increase in the proportion of 2-ethyl-4-methylimidazole. High viscosity is disadvantageous to the mass transfer efficiency as well as heat exchange efficiency, thus weakening the interaction of the absorbent with CO2. As a result, the aqueous solution with the lowest proportion of 2-ethyl-4-methylimidazole shows the largest molar absorption capacity and highest rate. On the other hand, a high viscosity prevents the volatilization of the moisture. Consequently, the 2:13 and 3:12 systems could display a better stability. Considering that the solution with the mass fraction of 2:13 has good stability and appropriate absorption capacity, we focus on this system in the subsequent studies.
Figure 2c shows the CO2 absorption curve of the 2:13 system at different CO2 flow rates. It can be seen that, with the increase in the flow rate, CO2 absorption rate is speed up and the absorption saturation time is shortened. Based on the gas absorption double-film theory, the resistance of the gas absorption process is mainly concentrated in the gas film and liquid film. Increasing the gas flow rate can reduce the thickness of the gas film layer at the interface, thus reducing the gas phase mass transfer resistance and effectively improving the absorption rate. However, the increase in gas flow rate will make the contact time between gas molecules and adsorbent too short, which is not conducive to gas adsorption. Herein, when the gas flow rate increases from 50 SCCM to 90 SCCM, the CO2 absorption capacity slightly decreases from 0.82 mol/mol to 0.80 mol∙mol−1. Therefore, the gas flow rate should be reasonably selected based on absorption time, absorption capacity, and other factors to improve absorption efficiency [49]. In this study, the gas flow rate is controlled at 90 SCCM because of the relatively short saturation absorption time.
Temperature plays a key role in the performance of the absorbent as it affects the evaporation of the absorbent and the solubility of CO2 [50,51]. Figure 2d shows the absorption behavior of the 3:12 system at 25, 30, 40, 50, and 60 °C. The results reveal that the saturation absorption time is inversely proportional to the temperature. At high temperature, the average kinetic energy of gas molecules increases, which accelerates the diffusion of CO2 and improves the interactions of CO2 with the absorbent. But high temperature decreases the solubility of CO2 and facilitates the desorption of CO2 as the desorption reaction is endothermic. This leads to a declined absorption capacity of CO2 at the high temperature (e.g., 0.41 mol∙mol−1 at 60 °C versus 0.76 mol∙mol−1 at 30 °C). Consequently, CO2 capture is performed at 25 °C (room temperature) because this temperature allows a high absorption capacity and does not require additional energy input.
In order to reduce the environmental pollution and investment costs, CO2 absorbent needs to be recycled for many times. For commercial applications, they should be stable in the long adsorption–desorption cycle [52]. Desorption heat is the energy required for the decomposition of substances (such as bicarbonate and carbonate) formed in the reaction process, and is also the main source of energy required for the regeneration of the loaded CO2 solution [53]. Therefore, desorption performance is an important index to evaluate the adsorbent. Figure 2e shows the desorption behavior of the 2:13 system in the temperature range of 50~100 °C. Prior to the desorption experiments, the system was interacted with CO2 at 25 °C to reach the saturation absorption capacity (0.80 mol∙mol−1, indicated by blue column in Figure 2e). When the desorption temperatures of 90 and 100 °C were employed, almost all absorbed CO2 can be released by the system (namely a 100% desorption efficiency). With the decrease in the temperature, the desorption efficiency was declined. At 50 °C, only 31.9% of CO2 was released by the system. This is because the desorption process of CO2 is an endothermic reaction. The higher the temperature, the higher the desorption efficiency and the more CO2 can be released. It is worth noting that the desorption temperature is relatively low herein. To efficiently regenerate CO2 from amine-based systems, a high desorption temperature (>100 °C) is usually involved (Table S3) [51,54,55,56,57,58,59,60,61,62,63]. Figure 2f shows the absorption–desorption cycle results of the 2:13 system. After the eight cycles, the system only loses 5.8% of its initial absorption capacity, suggesting that the aqueous 2-ethyl-4-methylimidazole solution for CO2 capture is cyclable and stable. MEA is known to be widely used in the industry as a CO2 absorbent. For a comparison, we studied the CO2 absorption–desorption performances of MEA under the condition identical to that of 2-ethyl-4-methylimidazole solution. As shown in Figure S9a, the saturation absorption capacity is 0.83 mol∙mol−1, close to the value for 2-ethyl-4-methylimidazole solution. However, the absorption capacity was decreased by 84% in the second cycle (Figure S9b). The control experiment highlights the superiority of 2-ethyl-4-methylimidazole solution for CO2 capture.
To understand the absorption–desorption mechanism of CO2 in the aqueous solution of 2-ethyl-4-methylimidazole, time-related in-situ attenuated total reflection infrared (ATR-IR) absorption spectroscopy and 13C NMR spectroscopy measurements were carried out. The ATR-IR spectrometer equips with a liquid-nitrogen-cooled mercury cadmium telluride (MCT) detector and a plug-in optical fiber probe, which enable to acquire the IR spectrum in real time (Figure 3a,b). In the process of CO2 absorption, three IR bands locating at 1010, 1357, and 2341 cm−1 were developed and their intensities were increased with the prolongation of CO2 absorption time (Figure 3c–e). The band of 1010 cm−1 (Figure 3c) is assigned to the C-OH stretching vibration of the bicarbonate. The band of 1357 cm−1 (Figure 3d) is assigned to the COO− symmetric stretching vibration of the bicarbonate. As for the band of 2341 cm−1 (Figure 3e), it is attributed to the asymmetric stretching vibration of CO2. In situ IR results clearly demonstrate that the aqueous solution of 2-ethyl-4-methylimidazole is capable of capturing CO2. Simultaneously, the IR data indicate that CO2 is transformed to HCO3− in the system. The detailed mechanism of CO2 capture by the system is discussed later. When the system is heated in a constant temperature water bath (90 °C), the three IR bands are weakened and ultimately disappeared (Figure 3f–h), indicating that CO2 could be released thoroughly by the system. According to gas chromatography and mass spectrometry measurements (Figure S10), the released gas is pure CO2. No other gaseous products were detected.
Figure 4 shows the 13C NMR spectroscopy study of the absorption and desorption of CO2 in the aqueous solution of 2-ethyl-4-methylimidazole. During the absorption of CO2, bicarbonate ions at 160 ppm were produced (Figure 4a), consistent with the afore-mentioned IR data. Standard 13C NMR spectra of sodium carbonate and sodium bicarbonate were shown in Figure S7 for comparisons. Based on the IR and NMR results, a water-assisted CO2 capture mechanism is proposed and shown in Figure 4b. 2-Ethyl-4-methylimidazole first combines with the hydrogen ion in the solution, inducing the water molecules to ionize continuously. Thus, OH− ions are produced and react with CO2 to yield HCO3−. As the reaction proceeds, the pH value of the solution decreases, the conductivity increases (Figure S12), and the HCO3− signal gradually increases, indicating that bicarbonate ions are accumulated in the process of the absorption of CO2. When the system loaded by CO2 is heated, the signal of the bicarbonate peak at 160 ppm gradually weakens and finally disappears owing to the thermal release of CO2 (Figure 4c). In the process of CO2 desorption, bicarbonate ions act as proton receptors to help the deprotonation of protonated imidazole (Figure 4d). H2O molecules are also underlying proton receptors in the system. However, HCO3− ion is a stronger base than H2O, and protons are preferred to bind with the former to form carbonic acid (H2CO3) rather than H3O+. H2CO3 is an unstable molecule, which is every easy to decompose into CO2 upon heating. Moreover, the mechanism studies clarify the reason why our system is easier to release CO2 in comparison to the amine systems (Table S3) [51,54,55,56,57,58,59,60,61,62,63]. In the amine systems, carbamates are the intermediates in the process of CO2 absorption and desorption. In our system, HCO3− and H2CO3 serve as the intermediates, which are known to decompose at low temperatures. Consequently, the formation of HCO3− and H2CO3 can reduce the heat load in the process of regenerating the 2-ethyl-4-methylimidazole solution. Whereas the decomposition of carbamate intermediates is relatively difficult and hence, a larger heat load is required for the regeneration of the amine systems [59,64,65].
From the perspective of practical implementation, CO2 capture usually deals with CO2 emissions from point sources such as power stations, refineries, cement plants, steel industrial facilities, and so on, where the concentration of CO2 is relatively low (volume fraction 10%~20%). When the synthetic flue gas was passed through the aqueous solution of 2-ethyl-4-methylimidazole at a flow rate of 50 SCCM at room temperature, a saturation absorption of CO2 needs 192 min and the molar absorption capacity is 0.56 mol∙mol−1 (Figure 5a). When the gas flow rate is increased to 90 SCCM, the absorption time is relatively short (124 min) and the molar absorption capacity is 0.54 mol/mol. In comparison to the capture of pure CO2 (Figure 2c), the absorption rate and absorption capacity of 15% CO2 by the 2-ethyl-4-methylimidazole solution are both decreased. The results are understandable because the low CO2 partial pressure in the synthetic flue gas reduces the gas phase driving force and distribution coefficient in the liquid phase, thus leading to a varied performance of the system. Figure 5b shows the cycle capacity of 2-ethyl-4-methylimidazole solution toward the synthetic flue gas at the desorption temperature of 90 °C. It can be seen that under the conditions of low partial pressure of CO2 and the presence of O2, the 2-ethyl-4-methylimidazole solution still display a good stability and cyclability. After eight cycles, the molar absorption capacity is only decreased by 7.1%, slightly higher than the value of 5.8% for pure CO2 feed. The 13C NMR spectrum shows that the bicarbonate ion is generated during CO2 absorption and disappears during desorption (Figure S13a), suggesting a capture mechanism same as that for pure CO2 (Figure 4a). After eight cycles, no other species were detected by 13C NMR spectroscopy except for 2-ethyl-4-methylimidazole in the solution (Figure S13b).
Furthermore, the performance of 2-ethyl-4-methylimidazole solution for CO2 separation from flue gas was studied using the bubble absorption setup (Figure 6a). At the initial stage (0~100 min), CO2 in the flue gas can be captured completely by the setup (Figure 6b). After that, the concentration of CO2 in the offgas is increased gradually and reached a platform at about 500 min (absorption saturation). Through desorption, the purified CO2 can be transfer to the steel cylinder with the assistance of the compressor (Figure 6c,d). By repeating the above process, low-cost CO2 capture and storage can be realized.
In addition to the capture of CO2 from large fixed-point sources, a closed carbon cycle also needs to deal with scattered emissions from small emitters. Compared with point source capture, capturing CO2 from dilution sources presents greater challenges. Herein we studied the direct capture of CO2 from the air by the aqueous solution of 2-ethyl-4-methylimidazole (2 g of 2-ethyl-4-methylimidazole dissolved in 13 g of water). After exposure to air for certain hours, concentrated CO2 was collected by heating the 2-ethyl-4-methylimidazole solution. It can be seen from Figure S14a,b, for an exposure time of 12 h, the concentration of CO2 is about 8000 ppm, 20 folds of that of environmental CO2. With the extension of the exposure time, CO2 can be further concentrated and enriched and its reaches around 40,000 ppm after 72 h, which has increased by 100 folds relative to that of the environmental CO2.

4. Conclusions

In conclusion, a series of low molecular weight imidazole solids were demonstrated as the absorbent for CO2 capture. Among them, the aqueous solution of 2-ethyl-4-methylimidazole is found to show excellent CO2 capture performance. The solution displays a maximum molar absorption capacity of 1.0 mol∙mol−1 at the mass ratio 0.5 g 2-ethyl-4-methylimidazole to 14.5 g water, demonstrating that one molecule of 2-ethyl-4-methylimidazole can capture one molecule of CO2, and all absorbed CO2 is able to be released upon heating the 2-ethyl-4-methylimidazole solution at 90 °C. After eight absorption–desorption cycles, the reduced molar absorption capacity of the system is less than 10%, suggesting a good stability. Mechanism studies reveal that HCO3− and H2CO3 serve as the intermediates in the process of CO2 absorption and desorption. Moreover, the aqueous solution of 2-ethyl-4-methylimidazole is capable of efficiently separating and purifying CO2 from flue gas and even ambient air. Therefore, aqueous solution of 2-ethyl-4-methylimidazole is a promising low-cost CO2 absorbent for practical implementation.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/separations10040236/s1. Figure S1: Schematic of experimental setups. (a) CO2 absorption setup. (b) CO2 desorption setup. Figure S2: (a) Schematic of the bubbling absorption setup, which consists of 10 bubbling reactors connected in series. (b) Schematic of desorption and compression setup. Figure S3: Schematic of imidazole aqueous solution direct air capture and heat concentration of CO2. Figure S4: Relationship between molar absorption of CO2 and pKa. Figure S5: Study on CO2 absorption capacity of 2-ethyl-4-methylimidazole by using anhydrous solid and aqueous solution. (a) Photographs showing the absorption of CO2 by pure 2-ethyl-4-methylimidazole solid. (b) Photographs showing the absorption of CO2 by 2-ethyl-4-methylimidazole aqueous solution. Figure S6: Study on selective gas absorption. (a) Photograph of reaction experiment setup. (b–d) Photographs showing that the reading of the pressure gauge pointer changes under different gas feeds. (b) N2, (c) O2, (d) CO2. Figure S7: CO2 absorption capacity of 2-ethyl-4-methylimidazole solution with different mass fractions. Figure S8: (a) Photograph of 2-ethyl-4-methylimidazole solution with different mass fractions. (b) Viscosities of various 2-ethyl-4-methylimidazole solutions. Figure S9: CO2 absorption–desorption performance of ethanolamine. (a) Absorption capacity curve of ethanolamine with time (2 g MEA + 13 mL H2O). (b) After desorption at 90 °C, the secondary reabsorption rate of MEA significantly decreases. Figure S10: (a) Gas chromatography measurements of heat released gas. (b) Mass spectrum of heat released gas. Figure S11: (a) 13C NMR spectrum of NaHCO3 aqueous solution. (b) 13C NMR spectrum of NaCO3 aqueous solution. Figure S12: Changes in pH value and conductivity with time during CO2 absorption by 2-ethyl-4-methylimidazole aqueous solution. (a) pH change. (b) Conductivity change. Figure S13: (a) 13C NMR spectra of 2-ethyl-4-methylimidazole during absorption–desorption of CO2 from the synthetic flue gas. (b) 13C NMR spectrum of 2-ethyl-4-methylimidazole after 8 cycles. Figure S14: The capture of ambient air by 2-ethyl-4-methylimidazole. (a) Curve of CO2 enrichment concentration with capture time. (b) Corresponds to the change of peak area detected by gas chromatography. Table S1: Basic properties of ten solid imidazoles. Table S2: Summary of CO2 capture capacity of various imidazole aqueous solutions. Table S3: Comparison of 2-ethyl-4-methylimidazole and amine absorbents.

Author Contributions

Conceptualization, methodology, X.Z. and X.C.; validation, J.W. and X.L.; formal analysis, X.L.; investigation, J.W.; data curation, X.Z.; writing—original draft preparation, X.Z.; writing—review and editing, Y.Y. and X.C.; visualization, X.Z. and L.G.; supervision, Y.Y. and L.G.; project administration, X.C.; funding acquisition, X.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the National Natural Science Foundation of China (Nos. 22275074).

Data Availability Statement

Not applicable.

Acknowledgments

The authors thank the support of the National Natural Science Foundation of China.

Conflicts of Interest

The authors declare no conflict of interest.

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Separations 10 00236 g001 550
Figure 1. Studies of CO2 capture over ten different imidazole solids. (a) Imidazole, (b) 2-methylimidazole, (c) 4-methylimidazole, (d) 2-ethylimidazole, (e) 2-propylimidazole, (f) 2-isopropylimidazole, (g) 2-buthylimidazole, (h) 1,2-dimethylimidazole, (i) 2,4-dimethylimidazole, and (j) 2-ethyl-4-methylimidazole. (k) Comparisons of CO2 absorption capacity and absorption time of the ten imidazole solids.
Figure 1. Studies of CO2 capture over ten different imidazole solids. (a) Imidazole, (b) 2-methylimidazole, (c) 4-methylimidazole, (d) 2-ethylimidazole, (e) 2-propylimidazole, (f) 2-isopropylimidazole, (g) 2-buthylimidazole, (h) 1,2-dimethylimidazole, (i) 2,4-dimethylimidazole, and (j) 2-ethyl-4-methylimidazole. (k) Comparisons of CO2 absorption capacity and absorption time of the ten imidazole solids.
Separations 10 00236 g001
Separations 10 00236 g002 550
Figure 2. CO2 absorption–desorption performance of 2-ethyl-4-methylimidazole. (a) CO2 absorption curves of the 2-ethyl-4-methylimidazole solution with a mass fraction of 0.5:14.5, 1:14, 2:13, and 3:12 (CO2 flow rate 90 SCCM). (b) Average molar absorption rates of the four 2-ethyl-4-methylimidazole solutions. (c) CO2 absorption curves of the 2:13 system at different gas flow rates. (d) CO2 absorption curves of the 2:13 system at different temperatures. (e) CO2 desorption capacity of the 2:13 system at different temperatures. (f) Cycle stability of the 2:13 system (desorption temperature 90 °C).
Figure 2. CO2 absorption–desorption performance of 2-ethyl-4-methylimidazole. (a) CO2 absorption curves of the 2-ethyl-4-methylimidazole solution with a mass fraction of 0.5:14.5, 1:14, 2:13, and 3:12 (CO2 flow rate 90 SCCM). (b) Average molar absorption rates of the four 2-ethyl-4-methylimidazole solutions. (c) CO2 absorption curves of the 2:13 system at different gas flow rates. (d) CO2 absorption curves of the 2:13 system at different temperatures. (e) CO2 desorption capacity of the 2:13 system at different temperatures. (f) Cycle stability of the 2:13 system (desorption temperature 90 °C).
Separations 10 00236 g002
Separations 10 00236 g003 550
Figure 3. In situ ATR-IR spectra of CO2 absorption–desorption process. (a) Schematic of in situ ATR-IR CO2 absorption setup. (b) Schematic of in situ ATR-IR CO2 desorption setup. (c–e) In situ ATR-IR absorption CO2 spectra of 2-ethyl-4-methylimidazole. (f–h) In situ ATR-IR desorption CO2 spectra of 2-ethyl-4-methylimidazole.
Figure 3. In situ ATR-IR spectra of CO2 absorption–desorption process. (a) Schematic of in situ ATR-IR CO2 absorption setup. (b) Schematic of in situ ATR-IR CO2 desorption setup. (c–e) In situ ATR-IR absorption CO2 spectra of 2-ethyl-4-methylimidazole. (f–h) In situ ATR-IR desorption CO2 spectra of 2-ethyl-4-methylimidazole.
Separations 10 00236 g003
Separations 10 00236 g004 550
Figure 4. Mechanism of absorption–desorption of CO2. (a) Changes in 13C NMR spectra of 2-ethyl-4-methylimidazole during CO2 absorption. (b) Mechanism of CO2 absorption by 2-ethyl-4-methylimidazole. (c) Changes in 13C NMR spectra of 2-ethyl-4-methylimidazole during CO2 desorption. (d) Mechanism of CO2 desorption by 2-ethyl-4-methylimidazole.
Figure 4. Mechanism of absorption–desorption of CO2. (a) Changes in 13C NMR spectra of 2-ethyl-4-methylimidazole during CO2 absorption. (b) Mechanism of CO2 absorption by 2-ethyl-4-methylimidazole. (c) Changes in 13C NMR spectra of 2-ethyl-4-methylimidazole during CO2 desorption. (d) Mechanism of CO2 desorption by 2-ethyl-4-methylimidazole.
Separations 10 00236 g004
Separations 10 00236 g005 550
Figure 5. The capture of CO2 in flue gas by 2-ethyl-4-methylimidazole solution. (a) CO2 absorption curves of the 2-ethyl-4-methylimidazole solution at different gas flow rates. (b) Cycle stability of the 2-ethyl-4-methylimidazole solution (desorption temperature 90 °C).
Figure 5. The capture of CO2 in flue gas by 2-ethyl-4-methylimidazole solution. (a) CO2 absorption curves of the 2-ethyl-4-methylimidazole solution at different gas flow rates. (b) Cycle stability of the 2-ethyl-4-methylimidazole solution (desorption temperature 90 °C).
Separations 10 00236 g005
Separations 10 00236 g006 550
Figure 6. 2-Ethyl-4-methylimidazole solution for separation and purification of CO2 from flue gas. (a) Photograph of bubbling absorption setup. (b) Absorption curve of CO2 capture from flue gas by bubble absorption setup (gas flow rate: 90 SCCM). (c) Photograph of desorption setup. (d) Photographs showing that the released CO2 can be compressed by an air compressor and stored in a steel cylinder.
Figure 6. 2-Ethyl-4-methylimidazole solution for separation and purification of CO2 from flue gas. (a) Photograph of bubbling absorption setup. (b) Absorption curve of CO2 capture from flue gas by bubble absorption setup (gas flow rate: 90 SCCM). (c) Photograph of desorption setup. (d) Photographs showing that the released CO2 can be compressed by an air compressor and stored in a steel cylinder.
Separations 10 00236 g006
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MDPI and ACS Style

Zhang, X.; Wu, J.; Lu, X.; Yang, Y.; Gu, L.; Cao, X. Aqueous 2-Ethyl-4-methylimidazole Solution for Efficient CO2 Separation and Purification. Separations 2023, 10, 236. https://doi.org/10.3390/separations10040236

AMA Style

Zhang X, Wu J, Lu X, Yang Y, Gu L, Cao X. Aqueous 2-Ethyl-4-methylimidazole Solution for Efficient CO2 Separation and Purification. Separations. 2023; 10(4):236. https://doi.org/10.3390/separations10040236

Chicago/Turabian Style

Zhang, Xingtian, Jun Wu, Xiaoxiao Lu, Yefeng Yang, Li Gu, and Xuebo Cao. 2023. "Aqueous 2-Ethyl-4-methylimidazole Solution for Efficient CO2 Separation and Purification" Separations 10, no. 4: 236. https://doi.org/10.3390/separations10040236

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