Amino-Functionalized Ionic Liquid Phase Change Absorbents for Efficient CO₂ Capture

Abstract

Considering the escalating severity of global climate warming, reducing CO₂ emissions has gained widespread recognition among the international community. Post-combustion capture is one of the most widely adopted technologies in this field. Chemical absorption, though mature, suffers from inherent drawbacks such as high regeneration energy consumption and significant solvent loss. Therefore, novel high-efficiency absorbents are urgently needed. In this study, an amino dual-functionalized ionic liquid [TEPAH]­[ATA] was synthesized via a one-step method and then mixed with NPA and water to construct a ternary phase-change absorption system, [TEPAH]­[ATA]/NPA/H₂O. The CO₂ absorption-desorption performance, phase-change behavior, and reaction mechanism of this system were systematically investigated. Results showed that the CO₂ absorption loading reached 1.79 mol CO₂ mol^–1 IL. Upon saturation, the system spontaneously separated into two liquid phases, where the rich phase accounted for only 28% of the total volume yet contained over 96% of the absorbed CO₂, and its viscosity was 6.59 mPa s. After five absorption-desorption cycles, the regeneration efficiency remained at 88.6%, demonstrating good cyclic stability. This study provides a reference for the application of ionic liquids in CO₂ capture.

1. Introduction

Carbon dioxide emissions from human activities are the primary driver of the rapid increase in atmospheric CO₂ concentrations. These emissions also constitute a major cause of severe environmental issues such as global climate change and glacial melting. Given that fossil fuels are expected to remain central to global economic development as well as modern social operations for the foreseeable future, the comprehensive deployment of CCUS technology is of paramount importance. Currently, CCUS is recognized as an effective pathway for deep CO₂ emission reduction, encompassing three key stages: carbon capture, utilization, and storage. ,

Among existing carbon capture technologies, post-combustion capture is one of the most widely adopted and technologically mature options, particularly suitable for retrofitting existing industrial sources such as coal-fired power plants, cement kilns, and steel mills. Available methods include adsorption, absorption, membrane separation, and cryogenic separation. , Among them, chemical absorption using monoethanolamine (MEA) remains the industrial benchmark due to its superior absorption rate, achieving a loading capacity of 0.5 mol CO₂/mol MEA. However, this method suffers from significant drawbacks, including severe equipment corrosion and high regeneration energy consumption 3.7 GJ/t CO₂, which limit its large-scale deployment. , These limitations have driven ongoing research into novel absorbents that combine high efficiency with long-term stability.

Ionic liquids (ILs) are molten salts composed of anions and cations, with melting points near or at room temperature. , They have emerged as promising candidates for CO₂ capture due to their exceptional chemical stability, structural tunability, and high thermal stability. , Nevertheless, the inherently high viscosity of neat ILswhich may increase several hundredfold upon CO₂ absorptionseverely hinders mass transfer efficiency and poses challenges for industrial implementation. To mitigate this issue, researchers have explored adding cosolvents such as water, alcohols, or ethers to develop hybrid solvent systems. A notable study by Liu et al. demonstrated that using n-propanol (NPA)/ethylene glycol as solvents resulted in viscosities of 3.66 and 7.65 mPa s before and after CO₂ absorption, respectively. These values are significantly lower than those of conventional non-aqueous IL systems.

The structural designability of ionic liquids offers vast potential for functionalized modification. By regulating ionic structures or introducing specific functional groups, researchers can obtain functionalized ionic liquids with tailored properties. Numerous studies confirm that primary and secondary amines, acting as active centers, significantly enhance the CO₂ absorption capacity and selectivity of ionic liquids. Inspired by these findings, researchers have synthesized a series of novel amino-functionalized ionic liquids by grafting amino groups onto ionic liquid structural units through molecular modification. Chen et al. designed and synthesized four amino-functionalized ionic liquids for capturing low-concentration CO₂. All four exhibited excellent CO₂ absorption performance at room temperature, with [TETA]­[HCOO] showing the best performance. Its decarbonization rate reached as high as 98%. The significance of this pioneering achievement is threefold: combining the advantages of chemical absorption with the designability of ionic liquids, providing a model for subsequent research on functionalized ionic liquids for CO₂ capture, and laying a solid theoretical and experimental foundation.

Phase-change absorption is a novel CO₂ capture technology that has received widespread attention in recent years. Its core mechanism lies in the phase separation of the absorbent after CO₂ absorption, forming a CO₂-rich phase and a CO₂-lean phase, with most CO₂ absorption products aggregating in the rich phase. Therefore, only the rich phase requires regeneration during the process, a strategy that significantly reduces regeneration energy consumption. Liquid–liquid phase-change absorbents developed from organic amines can substantially lower energy requirements. Research indicates that the regeneration energy of phase-change absorbents can be substantially reduced compared to the traditional MEA process, demonstrating immense application potential. Zheng et al. innovatively proposed an [IPDAH]­[Im] functionalized ionic liquid, in which CO₂ products enrich into the lower layer and form a white crystalline precipitate after saturated absorption. The volume of this phase accounts for only 60% of the total volume, and its regeneration energy is only 1.23 GJ/t CO₂, far lower than that of MEA. Su et al. combined triethylenetetramine and bromine as an ionic liquid with alcohol and water to develop a liquid–liquid phase-change absorbent. Compared to solid–liquid absorbents, this system achieves phase separation faster and more simply, reaching an absorption loading of 1.33 mol/mol upon saturation. The rich phase absorbs 96.32% of the CO₂ using only 35.3% of the total volume, which greatly reduces regeneration energy consumption. Phase-change absorbents thus exhibit significant application potential in the field of CO₂ capture.

Despite these advances, many existing phase-change absorbents still have notable drawbacks. Conventional amine-based biphasic systems typically suffer from high rich-phase viscosity, often exceeding 20–50 mPa s. This is mainly caused by the formation of extensive hydrogen-bonding networks among carbamate species. The high viscosity severely hinders mass transfer and raises pumping energy consumption.

To address these limitations, we adopt a dual-functionalization strategy. In this design, both cation and anion are engineered to actively contribute to the capture and release of CO₂. The cation [TEPAH]⁺ contains multiple primary and secondary amine groups, providing abundant reactive sites for high CO₂ loading. The anion [ATA]^‑, which bears an azole ring and an amino group, participates not only in CO₂ binding but also in absorption enhancement. More importantly, it promotes carbamate hydrolysis during desorption, thereby reducing the energy barrier for regeneration.

Based on the research background, this paper synthesizes an amino-difunctionalized ionic liquid using tetraethylenepentamine (TEPA) and 3-amino-1,2,4-triazole (ATA) as the primary absorbents. The abundant amino sites in these compounds enhance CO₂ absorption performance. , Water and NPA are employed as a mixed solvent, with NPA also serving to effectively regulate phase separation behavior. A novel liquid–liquid phase-change ionic liquid absorption system, [TEPAH]­[ATA]/NPA/H₂O, is constructed. This system aims to achieve synergistic optimization of three key aspects: high-efficiency CO₂ capture, low viscosity, and low regeneration energy consumption.

2. Experimental Section

2.1. Chemicals

Tetraethylenepentamine (TEPA, 99% purity), 3-amino-1,2,4-triazole (ATA, 99.93% purity), n-propanol (NPA, 99.7% purity), and ethanol (EtOH, analytical grade) were obtained from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Carbon dioxide gas (99.99% purity) was supplied by Guizhou Sanhe Gas Technology Co., Ltd., China. Deionized water was used to prepare all solutions. All chemicals were used as received without further purification.

2.2. Synthesis of [TEPAH]­[ATA]

The amino-functionalized ionic liquid [TEPAH]­[ATA] was prepared by a one-step synthesis. In a typical procedure, 0.0125 mol of TEPA and 0.0125 mol of ATA were dissolved in 30 mL of ethanol/water (4:1 v/v) solution. The mixture was magnetically stirred at room temperature for 12 h, then rotary evaporated at 60 °C under vacuum to remove excess solvent. The product was obtained as a pale yellow viscous liquid, which was the target ionic liquid [TEPAH]­[ATA]. The product was then placed in a volumetric flask and diluted with NPA/water mixtures of varying volume fractions to yield [TEPAH]­[ATA]/NPA/water solutions with a concentration of 0.5 mol/L.

2.3. Experimental Procedure

2.3.1. Absorption Section

The absorbent solution was loaded into a bubbling absorption tube. CO₂ was continuously introduced at a flow rate of 60 mL min^–1, and the water bath was maintained at the designated temperature. The inlet gas flow was regulated by a mass flow controller, while the outlet gas flow rate was measured periodically with a soap film flowmeter. The absorption process was deemed complete when the outlet flow rate matched the inlet flow rate and remained constant for 3 min, indicating saturation of the solution. Upon saturation, the absorbent gradually separated into two phases.

2.3.2. Desorption Section

The CO₂-rich phase was transferred to a bubbling absorption tube, immersed in an oil bath, and magnetically stirred at a controlled temperature to facilitate desorption. The outlet flow rate was also measured periodically using the soap film flowmeter. After desorption, the rich phase was mixed with the lean phase, and the above absorption procedure was repeated to determine the CO₂ loading capacity of the absorbent for the next cycle.

The absorption mechanism of the ionic liquid was investigated by ¹³C NMR spectroscopy, and its thermal stability was evaluated by thermogravimetric analysis.

2.4. Characterization

Viscosity measurements were performed using a digital rotational viscometer (NDJ-1S). A 20 mL aliquot of the sample was placed in the measuring tube and allowed to equilibrate in a thermostated water bath for 5 min to achieve thermal equilibrium. Viscosity was then measured using a suitable rotor and rotational speed. To improve measurement accuracy, the parameters were readjusted, and the measurement was repeated whenever the displayed percentage deviated from 50%. Each sample was analyzed in triplicate, and the average value with a relative deviation of less than ±2% was reported as the final viscosity.

¹³C NMR spectra were recorded on an Agilent DD2-600 MHz spectrometer for samples at various absorption and desorption loadings to investigate the CO₂ reaction mechanism. The carbon resonance peaks were identified and assigned using MestReNova software.

Furthermore, the thermal stability of the ionic liquid was investigated using a Mettler TGA/DSC1 thermogravimetric analyzer and a TA DSC25 differential scanning calorimeter.

2.5. Definition of Parameters

To investigate the effects of temperature and time on the CO₂ absorption characteristics, eqs – were used to estimate the CO₂ absorption rate, capacity, and loading.

The CO₂ absorption rate was determined from the difference between the inlet and outlet flow rates of the bubbling absorption tube, as expressed in the following equation

$$r_{\mathrm{a}}=\frac{Q_{\mathrm{i}\mathrm{n}}-Q_{\mathrm{o}\mathrm{u}\mathrm{t}}}{n_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}\times V_{\mathrm{m}}\times 1000}\left(\frac{P_{\mathrm{a}}{T}_0}{P_0{T}_{\mathrm{a}}}\right)$$ 1

where r_a (mol mol^–1 min^–1) represents the CO₂ absorption rate of the absorbent; Q _in and Q _out (mL min^–1) denote the inlet and outlet gas flow rates of the bubbling tube, respectively; n _total is the mole amount of the ionic liquid; V _m is the solution volume; P ₀ and P _a (kPa) are the standard and actual atmospheric pressures, respectively; T ₀ and T _a (K) are the standard and actual temperatures, respectively.

The outlet flow rate was recorded at 2 min intervals during the first 20 min and at 5 min intervals thereafter. The total CO₂ loading was then obtained by numerical integration of the absorption rate over the entire absorption period.as expressed in the following equation

$$L_{\mathrm{a}}={\int }_0^{t}r\mathrm{d}t$$ 2

where L _a (mol CO₂ mol^–1 ILs) is the CO₂ absorption loading of the solution at saturation; t (min) is the absorption time; and r (mol mol^–1 min^–1) is the CO₂ absorption rate of the absorbent.

The desorption rate and regeneration efficiency of the solution were calculated using eqs and

$$r_{\mathrm{d}}=\frac{Q_{\mathrm{o}\mathrm{u}\mathrm{t}}^{\prime }}{n_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}\times V_{\mathrm{m}}\times 1000}\left(\frac{P_{\mathrm{d}}{T}_0}{P_0{T}_{\mathrm{d}}}\right)$$ 3

$$\eta =\frac{L_n}{L_0}\times 100\%$$ 4

where r _d (mol mol^–1 min^–1) was the desorption rate of the solution; $Q_{\mathrm{o}\mathrm{u}\mathrm{t}}^{\prime }$ (mL min^–1) was the gas flow rate at the outlet of the condenser; η is the regeneration efficiency of the solution; and L ₀ and L _n (mol mol^–1) are the absorption loadings of the fresh solution and the solution after the n _th regeneration cycle, respectively.

3. Results and Discussion

3.1. CO₂ Absorption Performance of [TEPAH]­[ATA]/NPA/H₂O

In the CO₂ absorption process using ionic liquids, the alcohol volume fraction and the absorption temperature both play important roles in the absorption performance. Therefore, the CO₂ absorption capacities were systematically examined at different NPA volume fractions and temperatures, with the results shown in Figure .

1.

CO₂ absorption performance of [TEPAH]­[ATA]/NPA/H₂O solutions. (a,b) Absorption rate and loading of [TEPAH]­[ATA]/NPA/H₂O under different temperatures; (c,d) absorption rate and loading of [TEPAH]­[ATA]/NPA/H₂O at different NPA volume fractions. (C _ILS = 0.5 mol L^–1; Q _CO2 = 60 mL min^–1; V _L = 25 mL; P _absorption = 101.325 kPa; a: φ_NPA = 60%; b: T _absorption = 303.15 K).

To identify the optimal temperature for CO₂ absorption by the [TEPAH]­[ATA]/NPA/H₂O solution, experiments were carried out within the temperature range of 293.15–313.15 K. As depicted in Figure a,b, the absorption rate exhibited an initial increase followed by a decrease as the temperature rose. This phenomenon suggests that elevated temperatures are detrimental to CO₂ capture, mainly due to the reduced solubility of CO₂ in the absorbent, which hinders CO₂ enrichment and leads to a decline in absorption capacity. Given that the optimal performance was achieved at 303.15 K, this temperature was adopted for all subsequent experiments.

After CO₂ absorption, the solution was allowed to stand until saturation, upon which the system underwent liquid–liquid phase separation into two distinct layers, as shown in Figure : the upper phase was colorless and the lower phase pale yellow.

2.

Solution characteristics of the [TEPAH]­[ATA]/NPA/H₂O system before and after CO₂ absorption.

To clarify the roles of NPA and water in the ternary system, we compared the absorption performance of four systems under identical conditions: [TEPAH]­[ATA], [TEPAH]­[ATA]/NPA, [TEPAH]­[ATA]/H₂O and [TEPAH]­[ATA]/NPA/H₂O. As shown in Figure and Table , the [TEPAH]­[ATA] system absorbed CO₂ extremely slowly, with persistent bubble formation that hindered measurement. The [TEPAH]­[ATA]/NPA binary system showed slow absorption and formed a white precipitate on the tube wall, which is unfavorable for liquid-phase operation. The [TEPAH]­[ATA]/H₂O binary system absorbed CO₂ normally but lacked phase separation and showed low capacity.

3.

Absorption states of the absorbent systems: (a) [TEPAH]­[ATA], (b) [TEPAH]­[ATA]/NPA, and (c) [TEPAH]­[ATA]/H₂O.

1. Physical Properties and Absorption Loading of the Absorbent Systems (T = 303.15 K; C _ILS = 0.5 mol L^–1; V _L = 25 mL).

	phase separation		viscosity, mPa s
system composition	before absorption	after absorption	before absorption	after absorption	loading, mol mol–1
[TEPAH][ATA]	N	N	14.62	/	/
[TEPAH][ATA]/H2O	N	N	2.33	5.44	1.07
[TEPAH][ATA]/NPA	N	Y	3.02	/	/
[TEPAH][ATA]/NPA/H2O	N	Y	3.24	6.59	1.79

Only the full ternary system delivered fast absorption, high loading, spontaneous phase separation, and excellent regenerability. These results confirm that NPA and H₂O play distinct but complementary roles: NPA promotes phase separation, while H₂O reduces viscosity and enhances CO₂ solubility. The ternary formulation is therefore essential for achieving the desired overall performance.

Subsequent experimental analysis revealed that the upper and lower layers correspond to an NPA-rich lean phase and an absorbent-rich concentrated phase, respectively. Figure a shows the variation of the rich phase volume fraction with the initial NPA volume fraction. As the NPA content increased, the rich phase volume gradually decreased. However, at excessively high NPA concentrations, the rich phase volume became extremely small and almost negligible, which adversely affected the CO₂ loading. These findings indicate that NPA serves as a phase regulator in the system, and the upper phase is predominantly composed of NPA.

4.

(a) Rich phase volume fraction of the [TEPAH]­[ATA]/NPA/H₂O solution with different NPA volume fractions. (b) Viscosity of the system under different temperatures. (C _ILS = 0.5 mol L^–1; Q _CO2 = 60 mL min^–1; V _L = 25 mL; T _absorption = 303.15 K; P _absorption = 101.325 kPa).

Viscosity plays a crucial role in the CO₂ absorption process, as it strongly influences the diffusion and mass transfer of CO₂ within the solution, thus affecting both absorption and desorption performance. Figure b presents the viscosities of the [TEPAH]­[ATA]/NPA/H₂O solution measured at different temperatures. The viscosity of each phase in the system was found to decrease sharply with rising temperature.

To demonstrate the role of amino functionalization, we compared the CO₂ absorption performance of [TEPAH]­[ATA] with that of [DETA]­[ATA] under identical conditions. As shown in Figure , the [TEPAH]­[ATA] system exhibited a significantly higher CO₂ loading of 1.79 mol mol^–1, compared with 0.98 mol mol^–1 for the [DETA]­[ATA] system. The absorption rate of [TEPAH]­[ATA] was also higher throughout the entire absorption period. These results confirm that a greater number of amino groups directly contributes to higher CO₂ uptake, as more reactive sites are available for carbamate formation.

5.

Comparison of CO₂ absorption rate and loading between [DETA]­[ATA]/NPA/H₂O and [TEPAH]­[ATA] /NPA/H₂O systems under identical conditions. (C _ILS = 0.5 mol L^–1; Q _CO2 = 60 mL min^–1; V _L = 25 mL; P _absorption = 101.325 kPa; a: φ_NPA = 60%; b: T _absorption = 303.15 K).

Based on a comprehensive consideration of all factors, the optimal absorption performance was obtained at an NPA volume fraction of 60% and a temperature of 303.15 K. At this optimal condition, the absorption loading reached 1.79 mol mol^–1, the rich phase volume fraction was 28%, and the rich phase viscosity was only 6.59 mPa s, which is considerably lower than that of many conventional ionic liquid systems. A comparison of the viscosity and absorption loading between the present work and other reported ionic liquid systems is provided in Table .

2. CO₂ Absorption Performance of Different Absorbent Systems .

absorbents	T, K	loading, mol mol–1	viscosity, mPa s	refs.
[TEPAH][ATA]/NPA/H2O	303.15	1.79	6.59	this work
[DETAH][Tz]/NPA/H2O	303.15	1.713	2.57	Zhan et al.
AMP/MEA/H2O	303.15	0.5	8.75	Fu et al.
[DBAE][Im]	313.15	0.82	82.35	Long et al.
[TEPAH][Im]/DM/H2O	293.15	2.08	/	Chang et al.

^aViscosity values correspond to the rich phase after CO₂ saturation; ″/″ indicates that the CO₂-rich phase was solid, and liquid-phase viscosity could not be measured. For systems without phase separation, the viscosity of the whole homogeneous solution is reported. All loadings are calculated on the same molar basis (mol CO₂/mol absorbent).

The comparison in Table further supports the role of amino functionalization. The [DBAE]­[Im] ionic liquid, which contains only tertiary amine sites, achieves a much lower loading 0.82 mol mol^–1 than our [TEPAH]­[ATA] system. Similarly, the conventional AMP/MEA/H₂O system, which has limited primary/secondary amine functionality, achieves only 0.5 mol mol^–1. These comparisons indicate that the abundant primary and secondary amine groups in [TEPAH]­[ATA] are essential for achieving high CO₂ loading.

3.2. CO₂ Desorption Performance of the [TEPAH]­[ATA]/NPA/H₂O

To evaluate the influence of desorption temperature and duration on regeneration efficiency, the CO₂-saturated solution was subjected to thermal desorption under atmospheric pressure over a temperature range of 373.15–393.15 K. As depicted in Figure a, the regeneration efficiency increased with both rising temperature and extended time; at a fixed time point, a higher desorption temperature yielded greater regeneration efficiency. At 393.15 K and 80 min, the rich phase was regenerated efficiently, with a regeneration efficiency of 94.7%. Extending desorption time from 80 to 100 min at the same temperature did not improve regeneration efficiency. The rich phase was likely already fully desorbed within 80 min. Taking economic and energy consumption into account, the optimal regeneration conditions were established as a desorption temperature of 393.15 K and a duration of 80 min.

6.

Desorption performance of [TEPAH]­[ATA]/NPA/H₂O solutions. (a) Effect of desorption temperature and time on the regeneration efficiency ; (b) Recycle of solvent for CO₂ absorption–desorption. (C _ILS = 0.5 mol L^–1; V _L = 25 mL; P _desorption = 101.325 kPa; φ_NPA = 60%; b: T _desorption = 393.15 K).

Under the optimized regeneration conditions, the cyclability of the absorbent was further investigated through five consecutive absorption-desorption cycles. As shown in Figure b, the regeneration efficiency remained at 88.6% of its initial value after five cycles, indicating that the absorbent maintains good stability after repeated regeneration. Such stability is beneficial for minimizing solvent loss, making this solution promising for industrial applications.

3.3. Thermal Stability Analysis of the [TEPAH]­[ATA] Ionic Liquid

To systematically evaluate the thermal stability of the [TEPAH]­[ATA] ionic liquid for carbon capture applications, thermogravimetric analysis (TG) and differential scanning calorimetry (DSC) were employed in this study.

The TG results are presented in Figure a. The thermal decomposition of [TEPAH]­[ATA] exhibited a single-stage weight loss profile. At 425.15 K, the sample retained 90% of its initial mass with no significant weight loss, which likely originated from residual moisture. Even at 500 K, the weight loss reached only 50%. In practical carbon capture processes, the absorption temperature typically ranges from 303.15 to 333.15 K, while desorption is conducted at 373.15–393.15 K. The decomposition temperature of [TEPAH]­[ATA] is far above these operating ranges, indicating that the ionic liquid maintains excellent structural integrity and thermal stability without appreciable mass loss during conventional absorption–desorption cycles, thus fully meeting the thermal stability requirements for industrial applications.

7.

Thermal analysis curves of [TEPAH]­[ATA] (a) thermogravimetric curve; (b) differential scanning calorimetry curve.

It is worth noting that the above thermal stability analysis was performed on the neat ionic liquid [TEPAH]­[ATA], whereas the actual working absorbent is a ternary system containing NPA and water. At the desorption temperature (373.15–393.15 K), the water present in the rich phase is volatile, and its evaporation is a normal process feature rather than an indication of poor thermal stability of the absorbent. Nevertheless, the loss of water alters the solvent composition, which may in turn affect the absorption performance. Therefore, in industrial applications, attention should be paid to solvent volatilization losses, and appropriate solvent makeup or recovery strategies should be designed to ensure long-term operational stability and economic viability.

The DSC results are shown in Figure b. In the range of 300–400 K, the normalized heat flow decreased continuously without any endothermic or exothermic peaks, indicating the absence of physical phase transitions such as melting or crystallization, or significant chemical decomposition reactions. Within the core operating temperature window of carbon capture, [TEPAH]­[ATA] displays no abnormal thermal effects, thus avoiding additional energy loss due to phase change or decomposition during absorption–desorption cycles, demonstrating excellent thermal stability and process compatibility.

3.4. Reaction and Phase Transition Mechanism

3.4.1. ¹³C NMR Analysis of Absorption and Desorption Processes

To gain insight into the reaction mechanism of CO₂ absorption by the [TEPAH]­[ATA]/NPA/H₂O biphasic absorbent, ¹³C NMR analysis was carried out on samples taken at various absorption stages, and the obtained spectra are presented in Figure .

8.

¹³C NMR spectra of [TEPAH]­[ATA]/NPA/H₂O. (a) Solutions before and after CO₂ absorption; (b) CO₂ absorption by TEPA before and after the addition of ATA; (c)At different CO₂ absorption loadings; (d) Components of the absorbent and absorption products.

Figure d summarizes the primary chemical species present in the absorbent and the absorption products, namely TEPA, ATA, NPA, carbamates, and carbonates. The ¹³C NMR spectra of the fresh solution and the separated lean and rich phases after CO₂ absorption are shown in Figure (a). In the fresh solution, characteristic peaks of TEPA were identified at 39.87, 48.06, and 50.28 ppm, whereas ATA exhibited peaks at 146.81 and 157.90 ppm. The resonances at 9.28, 24.63, and 62.85 ppm are ascribed to the three carbon atoms of NPA. After CO₂ saturation, the system underwent phase separation into two liquid layers. The rich phase retained all the characteristic peaks of the ionic liquid, and new signals appeared in the 163–165 ppm region, corresponding to the carboxyl carbon (CO) of carbamates. The carbamate peaks at 163–165 ppm are consistent with those reported in the literature. The multiplicity of these peaks suggests the formation of multiple carbamate species, confirming that [TEPAH]­[ATA] offers multiple reactive sites for CO₂ capture. Additionally, the peak at 160.16 ppm is assigned to CO₃ ^2– and HCO₃ ^–. The signals corresponding to CO₃ ^2– and HCO₃ ^– are consistent with those reported by Zhan et al. In addition, they reported that alcohol can react with CO₂ to form alkyl carbonates, and they assigned the signal at 159.17 ppm to CH₃CH₂OCOO^–. Since our system also contains NPA, the signal at 159.27 ppm is similarly assigned to propyl carbonate. In contrast, only trace signals of the ionic liquid and carbamates were observed in the lean phase, confirming that the lean phase consists primarily of NPA, while the absorbed CO₂ is predominantly enriched in the rich phase.

To examine the synergistic effect of the [TEPAH]⁺ cation and [ATA]^‑ anion in the absorption process, a control experiment was designed in which CO₂ absorption was carried out using an aqueous solution of pure TEPA, while ATA was added to the experimental group prior to absorption. After absorption was completed and the solution turned homogeneous, the samples were characterized by NMR, with the spectra presented in Figure b. The addition of ATA resulted in a marked increase in the signals at 160.16 ppm, which are assigned to CO₃ ^2– and HCO₃ ^–, demonstrating that ATA facilitates the conversion of TEPACOO^– into carbonates. As depicted in Figure c, the carbamate signal intensity rose progressively with increasing CO₂ loading, indicating the gradual transformation of the absorbent.

3.4.2. Absorption and Desorption Mechanism

Based on the experimental results and ¹³C NMR analysis above, the CO₂ absorption mechanism and phase separation behavior of the [TEPAH]­[ATA]/NPA/H₂O system are proposed as follows. The absorbent contains a dual-functionalized ionic liquid, [TEPAH]­[ATA], which possesses multiple primary and secondary amine active sites, each with a high capacity for CO₂ uptake. During absorption, these sites interact with CO₂ via a zwitterionic mechanism to form carbamates, as shown in eqs and that follow.

$$\mathrm{I}\mathrm{L}\mathrm{s}-\mathrm{N}\mathrm{H}+{\mathrm{C}\mathrm{O}}_2\to {\mathrm{I}\mathrm{L}\mathrm{s}-\mathrm{N}\mathrm{H}}^{+}{\mathrm{C}\mathrm{O}\mathrm{O}}^{-}$$ 5

$${\mathrm{I}\mathrm{L}\mathrm{s}-\mathrm{N}\mathrm{H}}^{+}{\mathrm{C}\mathrm{O}\mathrm{O}}^{-}+\mathrm{I}\mathrm{L}\mathrm{s}-\mathrm{N}\mathrm{H}\to {\mathrm{I}\mathrm{L}\mathrm{s}-\mathrm{N}\mathrm{H}}_2^{+}+{\mathrm{I}\mathrm{L}\mathrm{s}-\mathrm{N}\mathrm{C}\mathrm{O}\mathrm{O}}^{-}$$ 6

As the absorption proceeds, some of the carbamate and CO₂ undergo hydrolysis to form CO₃ ^2– and HCO₃ ^–, with the corresponding reactions presented in eqs and

$${\mathrm{I}\mathrm{L}\mathrm{s}-\mathrm{N}\mathrm{C}\mathrm{O}\mathrm{O}}^{-}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{I}\mathrm{L}\mathrm{s}-\mathrm{N}\mathrm{H}+{\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}$$ 7

$${\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}+{\mathrm{H}}^{+}$$ 8

The appearance of propyl carbonate peaks in the ¹³C NMR spectrum (Figure ) confirms that NPA reacts to form propyl carbonate, thereby facilitating CO₂ hydrolysis and contributing to the high regeneration efficiency of the absorption system, as presented in eq

$${\mathrm{I}\mathrm{L}\mathrm{s}-\mathrm{N}\mathrm{C}\mathrm{O}\mathrm{O}}^{-}+{\mathrm{C}}_3{\mathrm{H}}_7\mathrm{O}\mathrm{H}\to \mathrm{I}\mathrm{L}\mathrm{s}-\mathrm{N}\mathrm{H}+{\mathrm{C}}_3{\mathrm{H}}_7{\mathrm{O}\mathrm{C}\mathrm{O}\mathrm{O}}^{-}$$ 9

Upon CO₂ saturation, the absorbent system separates into two distinct liquid phases. The upper phase is predominantly composed of NPA, whereas the absorption products are enriched in the lower phase. This phase separation arises from the incompatibility between highly polar absorption products, such as carbamates, and the relatively low-polarity NPA, which disrupts the original solution homogeneity and promotes spontaneous segregation into a product-enriched polar phase and a solvent-enriched low-polarity phase. In this context, NPA serves not only as a phase separation promoter but also as a modulator of the phase separation behavior through its partitioning between the two phases. Following phase separation, regeneration is achieved by thermal desorption of the rich phase alone, with the corresponding desorption reactions shown in eqs –

$${\mathrm{I}\mathrm{L}\mathrm{s}-\mathrm{N}\mathrm{C}\mathrm{O}\mathrm{O}}^{-}+{\mathrm{H}}^{+}\to \mathrm{I}\mathrm{L}\mathrm{s}-\mathrm{N}\mathrm{H}+{\mathrm{C}\mathrm{O}}_2$$ 10

$${\mathrm{H}\mathrm{C}\mathrm{O}}_3^{-}+{\mathrm{H}}^{+}\to {\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$$ 11

$${\mathrm{C}}_3{\mathrm{H}}_7{\mathrm{O}\mathrm{C}\mathrm{O}\mathrm{O}}^{-}+{\mathrm{I}\mathrm{L}\mathrm{s}-\mathrm{N}\mathrm{H}}_2^{+}\to {\mathrm{C}}_3{\mathrm{H}}_7\mathrm{O}\mathrm{H}+\mathrm{I}\mathrm{L}\mathrm{s}-\mathrm{N}\mathrm{H}+{\mathrm{C}\mathrm{O}}_2$$ 12

Based on the above analysis, the proposed reaction mechanism for CO₂ absorption by the [TEPAH]­[ATA]/NPA/H₂O system is schematically illustrated in Figure . During the absorption process, the primary (−NH₂) and secondary (−NH−) amine groups in [TEPAH]­[ATA] first undergo nucleophilic addition with CO₂ via a zwitterionic mechanism, forming a zwitterionic intermediate (ILs⁺–NH⁺COO^–). This intermediate is then rapidly deprotonated by another basic molecule (free amine or water), yielding stable carbamates (ILs–NCOO^–). Subsequently, some carbamates undergo transesterification with n-propanol to form propyl carbonate (C₃H₇OCOO^–), while the rest hydrolyze to HCO₃ ^– and CO₃ ^2–. The highly polar products (carbamates, propyl carbonate, and carbonates) are enriched in the lower aqueous phase, and the less polar n-propanol migrates to the upper organic phase, according to the “like dissolves like” principle, resulting in liquid–liquid phase separation.

9.

Reaction mechanism of CO₂ in [TEPAH]­[ATA]/NPA/H₂O solution.

In this process, n-propanol not only acts as a phase-separation promoter but also reacts with carbamates to form propyl carbonate, which has lower thermal stability. This significantly reduces the activation energy barrier for desorption, thereby facilitating CO₂ release. During the desorption stage, the protonated ionic liquid (ILs–NH₂ ⁺) continuously reacts with carbamates and propyl carbonate, shifting the chemical equilibrium toward CO₂ release and ultimately achieving absorbent regeneration and recyclability. This synergistic mechanism endows the system with both high absorption capacity and low regeneration energy consumption.

4. Conclusions

Overall, we synthesized a new ionic liquid [TEPAH]­[ATA] and used it to build a high-performance [TEPAH]­[ATA]/NPA/H₂O ternary absorbent. Experimental results demonstrated that at an NPA volume fraction of 60% and a temperature of 303.15 K, the system attained an optimal CO₂ loading of 1.79 mol CO₂ mol^–1 ILs. Notably, the system exhibited pronounced phase separation, with the CO₂-rich phase comprising only 28% of the total volume yet concentrating over 96% of the absorbed CO₂. This significant enrichment of CO₂ in a reduced phase volume greatly lowers the thermal energy demand for regeneration. Moreover, the saturated rich phase displayed a low viscosity of 6.59 mPa s, effectively addressing the high mass-transfer resistance typically associated with conventional ionic liquids. Collectively, this work establishes a solid theoretical and experimental basis for the development of energy-saving CO₂ absorbents, with promising implications for the industrial-scale application of CCUS technology.

Jindong Yang: Methodology, Investigation, Data curation, Writingoriginal draft. Mingkun Wu: Methodology, Supervision. Zhenghai Yang: Methodology. Mengkui Tian: Formal analysis, Funding acquisition, Resources, Supervision.

The authors declare no competing financial interest.