Investigating the SO₂ absorption behavior of pyrimidine-based deep eutectic solvents via a dual-site thermodynamic model

Abstract

Deep eutectic solvents (DESs), composed of hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs), are widely used in flue gas desulfurization due to their excellent SO₂ absorption properties. In this work, 14 DESs with pyrimidine derivatives and their isomers (including 2-aminopyrimidine (AmPyr), 2-chloropyrimidine, 2-bromopyrimidine, 4-amino-2-hydroxypyrimidine, 2,4-dihydroxypyrimidine, 4,6-diaminopyrimidene, 2-aminopyrazine, and 3-aminopyridazine, respectively) as HBDs and 1-ethyl-3-methylimidazolium chloride (C₂mimCl) and 1-ethyl-3-methylimidazolium bromide as HBAs have been successfully prepared and used for SO₂ absorption. Among them, C₂mimCl-7 + AmPyr exhibited the highest SO₂ absorption (19.032 mol kg⁻¹, at 298.15 K and 1.0 bar), rapid gas–liquid equilibrium within 40 s, and an exceptional ideal selectivity of 528.7 for SO₂/CO₂. After 30 cycles of absorption–desorption, the SO₂ absorption capacity remained as high as 18.265 mol kg⁻¹. A dual-site reaction equilibrium thermodynamic model (DS-RETM) was established for absorption behavior analysis. Using C₂mimCl-7 + AmPyr as a case study, Henry's constant, equilibrium constants, and other thermodynamic parameters were determined. DS-RETM fitting further enabled visualization of the potential absorption behavior of each DES component, facilitating comparison of HBA and HBD effects on SO₂ absorption. This study offers new insights into the development of high-performance flue gas desulfurization absorbents and introduces a novel model for thermodynamic analysis of SO₂ absorption in DESs.

Keywords: Deep eutectic solvents; SO₂ absorption; Pyrimidine; Thermodynamic; Selectivity.

---

1 Introduction

The various fuels burned in factories during the production process, such as coal, oil, natural gas, etc., will produce a large amount of flue gas.¹ The harmful gases contained therein, especially sulfur dioxide (SO₂), cause serious damage to the ecological environment and pose a significant threat to human health.² In the atmosphere, SO₂ oxidizes into sulfuric acid mist or sulfate aerosols, which are important precursors for environmental acidification. The content of SO₂ exceeding 0.5 ppm has potential impacts on human health, while when its concentration exceeds 1.5 ppm, healthy individuals may suffer from chronic bronchitis, laryngitis, and severe respiratory infections.^3,4 Thus, it is necessary to remove SO₂ before it is released into the atmosphere. Flue gas desulfurization (FGD) is the process of separating SO₂ from flue gas through various technologies and converting it into harmless or usable substances.⁵ According to the state of the absorbent and product during the reaction process, it can be divided into wet, dry, and semi-dry desulfurization. Wet desulfurization technology is relatively mature, efficient, and easy to operate, and is the most widely used desulfurization method, including limestone gypsum method, ammonia absorption method, etc. However, these traditional absorbents have drawbacks such as irreversibility, volatility, and secondary environmental pollution.⁶ Developing efficient, selective, and renewable absorbents is one of the key challenges in FGD technology.

Among all reported efficient absorbents, deep eutectic solvents (DESs) have been regarded as a promising one, because of their high selectivity, remarkable absorption capacity, green characteristics, etc.⁷ DESs were first proposed by Abbott in 2003 and are widely regarded as one of the green solvents.^8,9 They are formed by the interaction between hydrogen bond donors (HBDs) and hydrogen bond acceptors (HBAs) through hydrogen bonds, halogen bonds, or van der Waals forces, and have advantages such as easy synthesis, recyclability, designability, etc.^10–12 Due to the wide variety of HBDs and HBAs, theoretically, there could be at least 10⁶ types of DESs, which can be designed with specific functions based on application requirements.¹³ Since Han et al.¹⁴ first used choline chloride-glycerol deep eutectic solvents to absorb SO₂ in 2013, various DESs have been developed for flue gas desulfurization. Among them, DESs using nitrogen-containing heterocyclic organic compounds as HBDs or HBAs, including imidazole,^15–18 triazole,¹⁹ tetrazole,²⁰ benzimidazole,²¹ pyrazole,²² pyridine,^23–26 etc., have been reported successively.

Pyrimidine, a six-membered nitrogen-containing heterocyclic organic compound, differs from pyridine in that it contains two alternating N atoms and has lower toxicity. Schröder et al.¹ reported that the N atom of the pyrimidine ring can form dipole–dipole interactions with the O atom in S=O, indicating that pyrimidine-based absorbents are excellent candidates for SO₂ absorption. However, the physical interaction formed by electrostatic attraction between polar molecules is relatively weak, which limits the ability of materials to remove SO₂. Notably, DESs prepared from pyridine derivatives containing functional groups such as –NH₂, –OH, and –COOH exhibit excellent SO₂ absorption capacity.^25,26 Thus, to prepare pyrimidine-based DESs with high SO₂ absorption performance, pyrimidine derivatives containing functional groups such as –NH₂, C=O, –Cl, and –Br have been inferred to enhance their interactions with SO₂.

Ulteriorly, to investigate the absorption behavior, the SO₂ absorption process of DESs was analyzed through a thermodynamic model. Zhang et al.²⁷ obtained the physical and chemical absorption enthalpy changes (Δ_rH_m) of acidic protonic ionic liquid (APIL) based DESs through a reaction equilibrium thermodynamic model (RETM). In our previous work,²⁸ we also successfully used a RETM to demonstrate that purine-based DESs exhibit chemical absorption in the low-pressure region and physical absorption in the high-pressure region, and obtained a series of thermodynamic parameters. The absorption behavior of different DESs can be observed through the RETM, but how to gain more comprehensive insight into the absorption capacity of each component still needs further exploration.

In this work, a series of DESs with pyrimidine derivatives (one of 2-aminopyrimidine, 2-chloropyrimidine, 2-bromopyrimidine, 4-amino-2-hydroxypyrimidine, 2,4-dihydroxypyrimidine, and 4,6-diaminopyrimidene) or isomers of 2-aminopyrimidine (2-aminopyrazine or 3-aminopyridazine) as HBDs with 1-ethyl-3-methylimidazolium chloride or 1-ethyl-3-methylimidazolium bromide as HBAs have been successfully prepared. The relationship between temperature and the densities as well as viscosities of the prepared DESs was analyzed, and the thermal decomposition temperature (T_d) was further determined through thermogravimetric analysis. Subsequently, the SO₂ absorption performance of the prepared DESs was measured to investigate the effects of the molar ratio (HBAs : HBDs), HBD type, pressure, and temperature. Moreover, the time-dependent SO₂ absorption, SO₂/CO₂ selectivity and regeneration performance of pyrimidine-based DESs were also explored. Specifically, the physical and chemical absorption behavior of the prepared DESs was investigated using a dual-site reaction equilibrium thermodynamic model (DS-RETM), which can further elucidate the influence of HBAs and HBDs on SO₂ absorption and thermodynamic parameters. Finally, the possible interaction between pyrimidine-based DESs and SO₂ was further elucidated. Our findings offer a novel strategy for the design of high-performance, high-selectivity SO₂ absorbents and introduce a new thermodynamic model for SO₂ absorption in DESs.

2 Results and discussion

2.1 Characterization of pyrimidine-based DESs

The densities of the as-prepared DESs with C₂mimCl as a HBA were measured at different temperatures, as shown in Fig. 1a and b (Tables S1 and S2). Obviously, as the molar ratio increases from 2 : 1 to 7 : 1, the density of C₂mimCl-n + AmPyr DESs gradually decreases from 1.164 g cm⁻³ to 1.151 g cm⁻³ at 298.15 K. Thus, the increase of C₂mimCl is beneficial to decrease the density of C₂mimCl-n + AmPyr DESs. As for the DESs of C₂mimCl-7 + HBDs, the order of density (arranged by HBDs) at the same temperature is: AmPz < AmPd ≈ AmPyr < ChPyr < DAmPyr < Cyt < Ura < BrPyr. Considering that the molar ratio of C₂mimCl to HBDs remains unchanged, there exists a significant difference in density, indicating that the molecular structure of HBDs will affect the density of DESs significantly.²⁹ The density of DESs was fitted to the temperature (T) (ρ_DESs = α + βT, detailed parameters are provided in Tables S3 and S4). The results show that R₁² and R₂² are 0.999, revealing a linear relationship between densities and temperatures.

Fig. 1 The densities of (a) C₂mimCl-n + AmPyr (n = 2, 3, 4, 5, 6, 7) and (b) C₂mimCl-7 + HBDs; the viscosities of (c) C₂mimCl-n + AmPyr (n = 2, 3, 4, 5, 6, 7) and (d) HBAs-7 + HBDs.

Subsequently, the viscosities of pyrimidine-based DESs were measured at different temperatures, as shown in Fig. 1c and d (Tables S5 and S6). The results in Fig. 1c are similar to the effect of temperature on densities, indicating that increasing C₂mimCl is beneficial for reducing its viscosities, which may be due to the formation of more intramolecular hydrogen bonds.^30,31 In Fig. 1d, the order of viscosities (at 298.15 K) for different C₂mimCl-7 + HBDs DESs is: C₂mimCl-7 + BrPyr < C₂mimCl-7 + ChPyr < C₂mimCl-7 + AmPz < C₂mimCl-7 + Ura < C₂mimCl-7 + AmPd < C₂mimCl-7 + AmPyr < C₂mimCl-7 + DAmPyr < C₂mimCl-7 + Cyt. Owing to the different types, positions, and numbers of functional groups in HBDs, their ability to form intramolecular hydrogen bonds with C₂mimCl differs from one another, demonstrating that the structures of HBAs and HBDs significantly alter the viscosities of DESs.²⁹ In addition, the viscosity of C₂mimCl-7 + AmPyr is higher than that of C₂mimBr-7 + AmPyr, which may be due to the different anions of HBAs, leading to the weaker ability of C₂mimBr and AmPyr to form intermolecular hydrogen bonds.³² The viscosities were fitted to the temperature (T) (VFT equation: η = η₀ exp(t/(T − T₀)),³³ and the detailed parameter data are shown in Tables S7 and 1). The results indicate that R₃² and R₄² are 0.999, suggesting an exponential relationship between viscosities and temperatures.

Table 1 The viscosity fitting results and T_d of C₂mimCl-7 + HBDs

Parameters	HBDs
	Cyt	AmPyr	DAmPyr	AmPd	Ura	AmPz	ChPyr	AmPyr*	BrPyr
AmPyr*: C2mimBr-7 + AmPyr.
η0 × 10^4 (Pa s)	6.591 ± 3.095	5.847 ± 2.319	4.233 ± 1.846	9.167 ± 3.933	13.50 ± 4.797	15.90 ± 3.597	12.20 ± 2.858	14.00 ± 8.586	18.60 ± 4.826
t2 (K)	696.7 ± 83.61	601.2 ± 59.98	800.0 ± 85.88	563.1 ± 66.87	522.0 ± 58.61	448.2 ± 35.34	492.1 ± 41.15	495.7 ± 115.5	396.2 ± 46.08
T0 (K)	208.9 ± 5.231	220.7 ± 3.644	200.0 ± 5.216	219.7 ± 4.482	217.2 ± 4.495	222.0 ± 2.981	214.0 ± 3.567	209.1 ± 10.69	215.4 ± 5.018
R4^2	0.999	0.999	0.999	0.999	0.999	0.999	0.999	0.999	0.999
Td (K)	508	443	503	514	503	519	464	444	393

---

To investigate the thermal stability of pyrimidine-based DESs, HBAs-7 + HBDs were analyzed by thermogravimetry (TG), as shown in Fig. S1 and Table 1. Thermal decomposition temperature (T_d) can be used to judge the absorption temperature of absorbent and desorption temperature during regeneration, which is generally defined as the temperature at which the mass loss exceeds 5%. The maximum T_d value is 519 K (C₂mimCl-7 + AmPz), and the minimum value is 393 K (C₂mimCl-7 + BrPyr), indicating that pyrimidine-based DESs have excellent thermal stability during absorption–desorption processes and have the potential for flue gas desulfurization at high temperature.

2.2 SO₂ absorption of pyrimidine-based DESs

To explore the optimal molar ratio of HBAs to HBDs, the SO₂ absorption capacity of C₂mimCl-n + AmPyr was investigated. Fig. 2a shows the SO₂ absorption of C₂mimCl-n + AmPyr at 298.15 K and 1.0 bar. As the molar ratio of C₂mimCl to AmPyr increases, the SO₂ absorption of C₂mimCl-n + AmPyr increases gradually, reaching a maximum of 19.032 mol kg⁻¹. With the increase of C₂mimCl, the viscosity of C₂mimCl-n + AmPyr decreases, which is more beneficial to mass transfer;³⁴ nevertheless, the charge-transfer interaction between the highly electronegative Cl⁻ in C₂mimCl and the S atoms in SO₂ (Cl⁻⋯SO₂) is beneficial for improving SO₂ absorption.^35,36 According to the above analysis, the optimal molar ratio of C₂mimCl to AmPyr is 7 : 1.

Fig. 2 The SO₂ absorption of (a) C₂mimCl-n + AmPyr and (b) HBAs-7 + HBDs, T = 298.15 K, P = 1.0 bar; (c) the effect of pressure on SO₂ absorption of C₂mimCl-n + AmPyr, T = 298.15 K; (d) the effect of temperature on SO₂ absorption of C₂mimCl-7 + AmPyr; P = 1.0 bar.

Furthermore, a series of HBAs-7 + HBDs DESs were synthesized, and their SO₂ absorption capacity is shown in Fig. 2b. Besides, Table S8 shows the SO₂ absorption capacity of synthesized DESs compared to other materials, which are superior to many solid absorbents. For DESs with different HBAs, the absorption capacity of C₂mimBr-7 + AmPyr (14.630 mol kg⁻¹) is significantly lower compared to C₂mimCl-7 + AmPyr (19.032 mol kg⁻¹), indicating that anions have a greater influence on SO₂ absorption capacity. In other words, the greater the electronegativity, the higher the absorption capacity of SO₂.^29,32 As isomers of AmPyr, AmPd and AmPz were also used to form DESs with C₂mimCl. The SO₂ absorption capacities of C₂mimCl-7 + AmPz and C₂mimCl-7 + AmPd are 18.017 mol kg⁻¹ and 18.746 mol kg⁻¹, respectively. These values, all lower than that of C₂mimCl-7 + AmPyr (19.032 mol kg⁻¹), demonstrate that the position of the nitrogen atoms within the heterocyclic ring structure of the HBDs also significantly impacts the performance of DESs, and the pyrimidine ring is more conducive to SO₂ absorption.

Hence, to develop pyrimidine-based DESs with superior performance, the SO₂ absorption of DESs using pyrimidine derivatives with different functional groups as HBDs was investigated. Firstly, with BrPyr, ChPyr, and AmPyr as HBDs, the SO₂ absorption capacity is C₂mimCl-7 + BrPyr < C₂mimCl-7 + ChPyr < C₂mimCl-7 + AmPyr. Typically, the higher the viscosity, the smaller the absorption capacity. Here, the main reason that affects the absorption capacity is that AmPyr contains weak basic functional groups (–NH₂), which may have acid–base interaction with SO₂ molecules.³⁷ Then, the SO₂ absorption of DESs with multifunctional pyrimidine derivatives (Ura, Cyt, and DAmPyr) as HBDs was further investigated. The SO₂ absorption of the three DESs is C₂mimCl-7 + Ura (17.791 mol kg⁻¹) < C₂mimCl-7 + Cyt (18.243 mol kg⁻¹) < C₂mimCl-7 + DAmPyr (18.595 mol kg⁻¹). Thus, the pyrimidine-based DESs with optimal SO₂ absorption is C₂mimCl-7 + AmPyr.

It is universally known that pressure plays a key role in affecting the gas absorption capacity of DESs.³⁸ Fig. 2c shows the SO₂ absorption of pyrimidine-based DESs at 298.15 K and 0–1.0 bar. The SO₂ absorption capacity of C₂mimCl-n + AmPyr increases nonlinearly with the increase of pressure. Taking C₂mimCl-7 + AmPyr with the highest absorption capacity as an example, with the pressure increasing from 0.1 bar to 1.0 bar, the absorption capacity of SO₂ increases from 7.911 mol kg⁻¹ to 19.032 mol kg⁻¹. This shows that increasing the pressure can promote the absorption of SO₂. Conversely, reducing the pressure can be considered to desorb SO₂.^39,40 Additionally, the SO₂ absorption increases rapidly between 0 and 0.1 bar, probably owing to the chemical interaction between the absorbent and SO₂ molecules.⁴¹ However, the growth rate of absorption capacity decreases between 0.1 and 1.0 bar, which may be due to the gradual occupation of absorption sites, so rapid absorption has been completed, resulting in the process being dominated by physical absorption. Therefore, it is speculated that the absorption process of SO₂ via pyrimidine-based DESs includes chemical absorption and physical absorption.⁴²

According to the existing literature,^6,22,43–46 temperature is another important factor affecting the absorption capacity of DESs. Hence, C₂mimCl-7 + AmPyr was selected to investigate the SO₂ absorption performance at different temperatures, as shown in Fig. 2d. With the increase of temperature, the SO₂ absorption capacity of C₂mimCl-7 + AmPyr decreases continuously, and the absorption capacities are 22.055, 19.032, 16.972, 14.611 and 11.238 mol kg⁻¹, respectively. This result shows that increasing the temperature is beneficial to the regeneration of absorbent. Furthermore, it should be noted that C₂mimCl-7 + AmPyr still has a high absorption capacity at high temperatures, indicating that pyrimidine-based DESs could be used as candidate absorbents for high-temperature flue gas desulfurization.²⁹

2.3 Analysis of SO₂ absorption behavior for pyrimidine-based DESs

A DS-RETM equation was established for fitting the SO₂ absorption.⁴⁷ It is assumed that the absorption of SO₂ by C₂mimCl-7 + AmPyr DESs involves the following steps:

SO₂ (g) → SO₂ (l) (1)

SO₂ (g) + C₂mimCl (l) → C₂mimCl–SO₂ (l) (2)

SO₂ (g) + AmPyr (l) → AmPyr–SO₂ (l) (3)

where g and l stand for gaseous and liquid states, respectively. C₂mimCl–SO₂ stands for the interaction of C₂mimCl with SO₂, and AmPyr–SO₂ stands for the interaction of AmPyr with SO₂. Eqn (1) denotes the physical absorption process and could be described by the following equation:where P is the pressure (bar) of SO₂ (g); H is the Henry's law constant with the same unit as P; m_phys-SO2 is the physical absorption molality (mol kg⁻¹); γ_SO2 is the activity coefficient; m° is 1.0 mol kg⁻¹. Eqn (2) and (3) denote the chemical absorption process and could be described by following equations:where k_C2mimCl and k_AmPyr stand for the equilibrium constants; γ_{C2mimCl–SO2}, γ_C2mimCl, γ_AmPyr–SO2, and γ_AmPyr are the activity coefficients; m_{C2mimCl–SO2}, m_C2mimCl, m_AmPyr–SO2, and m_AmPyr are the molality (mol kg⁻¹); P° is 1.0 bar; and are constants (mol kg⁻¹) that could be calculated using eqn (9) and (10); M_C2mimCl and M_AmPyr are the molar mass of C₂mimCl and AmPyr, respectively, in g mol⁻¹.

The values of γ_SO2, γ_{C2mimCl–SO2}, γ_C2mimCl, γ_AmPyr–SO2, and γ_AmPyr are all regarded as 1.0.^28,48 Thus, the DS-RETM could be expressed by eqn (11) and (12):

m_t = m_phys-SO2 + m_CmimCl–SO2 + m_AmPyr–SO2 (11)

where m_t (mol kg⁻¹) is the total SO₂ absorption.

The SO₂ absorption of C₂mimCl-7 + AmPyr at different temperatures and pressures was correlated according to eqn (12), and fitted lines are shown in Fig. 3a. Notably, the fitting curve matches well with the experimental data, with R₅² values greater than 0.999, indicating that the established thermodynamic model has extremely high reliability. The corresponding fitting parameters were calculated and revealed in Table 2. The Henry's law constants keep an increasing trend, while the equilibrium constants keep a decreasing trend, which is consistent with the negative dependence of SO₂ absorption on temperature.

Fig. 3 (a) DS-DETM equation fitting of C₂mimCl-7 + AmPyr for SO₂ absorption; the calculated (b) physical and (c) chemical absorption of SO₂ by C₂mimCl-7 + AmPyr.

Table 2 Calculated Henry's law constants, reaction equilibrium constants, and thermodynamic parameters of SO₂ absorption in C₂mimCl-7 + AmPyr DESs

Parameters	293.15 K	303.15 K	313.15 K	333.15 K
H (bar)	0.10814 ± 0.00279	0.21585 ± 0.00688	0.30983 ± 0.01596	0.66939 ± 0.16117
kC2mimCl	173.03801 ± 72.62040	99.15970 ± 14.25614	36.28764 ± 2.19755	15.10983 ± 0.82593
kAmPyr	1.27796 ± 0.09255	1.11052 ± 0.05036	0.80265 ± 0.04400	0.46775 ± 0.06859
R5^2	0.999	0.999	0.999	0.999
ΔphysHm (kJ mol^−1)	−35.993	−35.993	−35.993	−35.993
ΔphysGm (kJ mol^−1)	−5.421	−3.864	−3.051	−1.112
ΔphysSm (J mol K^−1)	−104.287	−105.983	−105.197	−104.701
ΔchemHm (kJ mol^−1)	−72.131	−72.131	−72.131	−72.131
ΔchemGm (kJ mol^−1)	−13.158	−11.850	−8.779	−5.416
ΔchemSm (J mol K^−1)	−201.169	−198.85	−202.308	−200.254
ΔrHm (kJ mol^−1)	−108.124	−108.124	−108.124	−108.124
ΔrGm (kJ mol^−1)	−18.579	−15.714	−11.829	−6.528
ΔrSm (J mol K^−1)	−305.457	−304.833	−307.505	−304.955

---

Based on the H, k_C2mimCl, and k_AmPyr, the m_phys-SO2, m_{C2mimCl–SO2} and m_AmPyr–SO2 could be obtained. As shown in Fig. 3b and c, both physical absorption and chemical absorption decrease with increasing temperature. The m_phys-SO2 is most significantly controlled by temperature, followed by m_AmPyr–SO2, and m_{C2mimCl–SO2} is least affected, indicating that the interaction between C₂mimCl and SO₂ is stronger than AmPyr, and the physical interaction is the weakest. Besides, the physical absorption capacity increases linearly with increasing pressure, while the chemical absorption capacity increases nonlinearly and gradually approaches equilibrium. Thus, the absorption capacity of DESs for SO₂ gradually increases linearly with the increase of pressure.

According to the relationship of ln H and ln k to 1/T (Fig. 4a), the parameters of enthalpy change (ΔH_m), Gibbs free energy change (ΔG_m), and entropy change (ΔS_m) could be obtained by the following equations:

Δ_physG_m = RT ln H_m (14)

Δ_chemH_m = Δ_C2mimClH_m + Δ_AmPyrH_m (18)

Δ_chemG_m = −RT(ln k_C2mimCl + ln k_AmPyr) (19)

Fig. 4 (a) The fitting results of ln H and ln k to 1/T of C₂mimCl-7 + AmPyr; (b) the calculated SO₂ absorption of C₂mimCl-7 + AmPyr at 298.15 K.

Here, Δ_physH_m, Δ_physG_m, and Δ_physS_m correspond to the physical process; Δ_C2mimClH_m, Δ_AmPyrH_m, Δ_chemH_m, Δ_chemG_m, and Δ_chemS_m correspond to the chemical process (Table 2). As can be seen, the values of ΔH_m, ΔG_m, and ΔS_m are always less than 0, which indicate the absorption process is exothermic, spontaneous, and with increased orderliness.

Besides, the ln H, ln k_C2mimCl, and ln k_AmPyr at 298.15 K can be calculated, and the SO₂ absorption of m_phys, m_C2mimCl, m_AmPyr, and m^f_t could be further obtained. Fig. 4b shows that the calculated SO₂ absorption remains consistent with the experimental values (such as 19.051 mol kg⁻¹ vs. 19.032 mol kg⁻¹ at 1.0 bar), indicating the high reliability of the DS-RETM equation.

To further investigate the absorption behavior of different pyrimidine-based DESs, and in eqn (12) were replaced with the initial molality of the corresponding HBAs and HBDs ( and ), resulting in DS-RETM equations for different HBAs-7 + HBDs DESs. Subsequently, the SO₂ absorption of different pyrimidine-based DESs at 298.15 K was fitted, and the detailed results are shown in Fig. S2 and Table S9. Based on the obtained H, k_HBAs, and k_HBDs, the m_phys, m_HBAs, and m_HBDs of corresponding HBAs-7 + HBDs DESs could be calculated separately, which is of great significance for insight exploration of the absorption behavior of each component in DESs (Fig. S3). Compared with C₂mimBr-7 + AmPyr, the m_HBDs, m_HBAs, and m_phys of C₂mimCl-7 + AmPyr were significantly increased, indicating that Cl⁻ has a key impact on the SO₂ absorption of DESs. Obviously, for DESs using the same HBAs (C₂mimCl-7 + HBDs), there is not much difference in m_HBAs (also known as m_C2mimCl) (Fig. S3a and b), attributed to the same molar mass of C₂mimCl, which forms a strong charge transfer interaction between Cl⁻ and the S atom of SO₂ (Cl⁻⋯SO₂). However, significant differences in m_HBDs and m_phys between C₂mimCl-7 + HBDs DESs could be observed. Notably, the order of m_HBDs is not the same as the SO₂ absorption of their corresponding DESs (Fig. S3c and d). Among them, m_AmPd and m_DAmPyr are both greater than m_AmPyr, but the SO₂ absorption of C₂mimCl-7 + AmPyr is greater than that of C₂mimCl-7 + AmPd and C₂mimCl-7 + DAmPyr. Interestingly, for the physical absorption (m_phys) of C₂mimCl-7 + HBDs (Fig. S3e and f), there is even a phenomenon opposite to the SO₂ absorption of the corresponding DESs. For instance, C₂mimCl-7 + BrPyr has the minimum SO₂ absorption but the maximum m_phys. To sum up, the types and numbers of functional groups in HBDs determine their varying abilities to absorb SO₂, while also affecting the physical absorption properties of the corresponding DESs. Hence, DS-RETM fitting could visualize these potential absorption behaviors, which is more conducive to exploring the physical and chemical absorption processes of the prepared DESs.

2.4 Kinetic analysis of SO₂ absorption for pyrimidine-based DESs

A typical time-dependent SO₂ absorption process involves taking a certain amount of DESs (∼0.10 g) and observing the change in SO₂ absorption capacity over time at an initial pressure of ∼1.0 bar. The time-dependent SO₂ absorption capacity of C₂mimCl-7 + HBDs at 298.15 K is shown in Fig. 5a. Interestingly, the as-prepared DESs can absorb SO₂ rapidly, which suggests that the absorption process is mainly dominated by chemical absorption. To further observe the SO₂ absorption rate, Boltzmann equation (eqn (21)) fitting was performed on the experimental results,³² and the first derivative of the curve is the absorption rate of the as-prepared pyrimidine-based DESs (Fig. 5b). Clearly, C₂mimCl-7 + AmPyr achieved the maximum absorption rate in the shortest possible time. Moreover, the as-prepared DESs can rapidly absorb SO₂ and basically achieve absorption equilibrium within 40 s, which is superior to numerous reported DESs.^35,49–51 This result shows that pyrimidine-based DESs have potential for flue gas desulfurization.where m_SO2 refers to the SO₂ absorption capacity (mol kg⁻¹), t refers to the time of SO₂ absorption (s), and A₁, A₂, t₀, and d_t refer to the empirical constants.

Fig. 5 (a) The time-dependent SO₂ absorption and (b) SO₂ absorption rate of pyrimidine-based DESs; T = 298.15 K.

2.5 SO₂/CO₂ absorption selectivity of pyrimidine-based DESs

Owing to the complex composition of actual flue gas, it generally contains 0.04–15% CO₂.⁵² Thus, the designed absorbent for flue gas desulfurization should have high SO₂/CO₂ selectivity. The CO₂ and SO₂ absorption capacities of C₂mimCl-7 + HBDs were compared at 298.15 K and 1.0 bar, as shown in Fig. 6. Obviously, pyrimidine-based DESs show a negative absorption of CO₂. Generally, the selectivity of SO₂/CO₂ keeps the ratio of SO₂ absorption capacity to CO₂ absorption capacity under given conditions (S_DESs = m_SO2/m_CO2).²⁹ The comparison of SO₂ absorption capacity and selectivity with literature data is shown in Table S10. Accordingly, the absorption selectivity of pyrimidine-based DESs is C₂mimCl-7 + ChPyr (451.6), C₂mimCl-7 + Ura (494.2), C₂mimCl-7 + AmPyr (528.7), C₂mimCl-7 + BrPyr (536.5), C₂mimCl-7 + DAmPyr (808.5), C₂mimCl-7 + Cyt (1073.1), respectively. The minimum value of S_DESs has reached up to 451.6, indicating that pyrimidine-based DESs have excellent SO₂/CO₂ selective absorption performance and could be used as a candidate for selective absorption of SO₂ from actual flue gas.

Fig. 6 The gas absorption of C₂mimCl-7 + HBDs; T = 298.15 K, P = 1.0 bar.

2.6 SO₂ absorption mechanism of pyrimidine-based DESs

To explore the mechanism of DES formation and SO₂ absorption, C₂mimCl-7 + AmPyr with the highest absorption capacity was selected for FT-IR and ¹H NMR analysis. In Fig. 7a, the peaks appearing at 3325 and 3161 cm⁻¹ are attributed to the stretching vibration of –N_(g)H₂, the peak around 1645 cm⁻¹ is attributed to the bending vibration of –N_(g)H₂, and two peaks at 1575 and 1556 cm⁻¹ are attributed to the stretching vibration of the pyrimidine ring.⁵³ In DES (C₂mimCl-7 + AmPyr), the peak of –N_(g)H₂ around 1645 cm⁻¹ is red shifted to 1628 cm⁻¹, indicating that AmPyr had hydrogen bond interactions with C₂mimCl (N_(g)–H⋯Cl⁻). Besides, the peak of C₍₂₎–H in C₂mimCl around 3056 cm⁻¹ shifts to 3046 cm⁻¹,⁵¹ which may be due to Cl⁻ forming intramolecular hydrogen bonds (C₍₂₎–H⋯Cl⁻), leading to a decrease in force constant and vibrational energy, and a red shift in functional groups.^23,29,54

Fig. 7 (a) FT-IR spectra of C₂mimCl-7 + AmPyr after SO₂ absorption (DES + SO₂), C₂mimCl-7 + AmPyr after CO₂ absorption (DES + CO₂), C₂mimCl-7 + AmPyr (DES), C₂mimCl and AmPyr; (b) ¹H NMR spectra of DES + SO₂, DES, C₂mimCl and AmPyr.

For the spectrum after SO₂ absorption, four new peaks attributed to SO₂ could be observed. Among them, three peaks are attributed to the bending vibration (529 cm⁻¹), symmetric stretching vibration (1124 cm⁻¹), and asymmetric stretching vibration (1281 cm⁻¹) of S=O, respectively.^6,22,32 Compared with free SO₂, a significant red shift was observed in the stretching vibration of S=O, indicating that Cl⁻ in C₂mimCl may form a charge transfer interaction with S atoms in SO₂ (Cl⁻⋯SO₂).^55,56 Hence, the original hydrogen bonds in DESs get weakened significantly, leading to the peaks of –C₍₈₎H₃, C₍₂₎–H and C₍₄₊₅₎–H shifting from 2976, 3046 and 3140 cm⁻¹ to 2984, 3088 and 3145 cm⁻¹, respectively.^32,49 The fourth peak around 836 cm⁻¹ is attributed to the bending vibration of S–O, indicating that there exists chemical absorption of SO₂.^28,57 Owing to the bending vibration of –N_(g)H₂ shifting and splitting into two peaks around 1693 and 1680 cm⁻¹, it is inferred that the acid–base interactions might be formed between AmPyr and SO₂.²³ Contrary to SO₂ absorption, the FT-IR spectrum after CO₂ absorption showed no significant changes compared to before absorption, indicating that there is almost no interaction between the as-prepared DESs and CO₂, thus exhibiting extremely high SO₂/CO₂ selectivity.

Fig. 7b shows the ¹H NMR spectra of AmPyr, C₂mimCl, C₂mimCl-7 + AmPyr, and C₂mimCl-7 + AmPyr after SO₂ absorption. Notably, the peak of –N_(g)H₂ shifts from 6.59 ppm in AmPyr to 6.66 ppm in DES, and the peak of C₍₂₎–H shifts from 9.70 ppm in C₂mimCl to 9.78 ppm in DES. These down-field shifts are attributed to the formation of N_(g)–H⋯Cl⁻ and C₍₂₎–H⋯Cl⁻,²³ respectively, which would reduce electron cloud density (de-shielding effect).^25,58 Concurrently, the peak of C₍₄₊₅₎–H shifts to down-field, while the peaks of C_(d+f)–H and C_(e)–H shift to up-field. In contrast, the peaks of C₍₂₎–H and –N_(g)H₂ shift to up-field after SO₂ absorption. The former is attributed to the charge transfer between the Cl⁻ and S atoms (Cl⁻⋯SO₂), while the latter could be ascribed to the acid–base interaction between –N_(g)H₂ and SO₂.^25,59 The shift of other C–H signals in C₂mimCl and AmPyr indicates that the original hydrogen bonds in DES have been disrupted by SO₂.

2.7 Regeneration of pyrimidine-based DESs

In order to avoid secondary pollution caused by a large number of waste absorbents, it is usually required that the absorbents have good regeneration performance. As shown in Fig. 8, C₂mimCl-7 + AmPyr maintains the SO₂ absorption capacity of 18.265 mol kg⁻¹ after 30 times of absorption–desorption. This represents excellent regeneration performance compared with the reported DESs for absorbing SO₂.^{16,22,23,38,60} To investigate whether the structure of pyrimidine-based DESs is stable and maintains the original absorption performance, the SO₂ absorption capacity of C₂mimCl-7 + AmPyr was measured every 7 days under the same conditions (Fig. S4). The results showed that C₂mimCl-7 + AmPyr remained liquid after 4 weeks, and maintained excellent SO₂ absorption performance.

Fig. 8 The regeneration performance of C₂mimCl-7 + AmPyr.

3 Conclusions

In this work, 14 different DESs were successfully developed and used for SO₂ absorption. The density and viscosity of the as-prepared DESs show a negative correlation with temperature, but the former is linear and the latter is exponential. As the molar ratio of C₂mimCl to AmPyr increases, the SO₂ absorption capacity of C₂mimCl-n + AmPyr gradually increases, ultimately reaching up to 19.032 mol kg⁻¹ at 298.15 K and 1.0 bar. The thermodynamic analysis of SO₂ absorption by C₂mimCl-7 + AmPyr at different temperatures was conducted using the DS-RETM equation, which not only yielded a series of thermodynamic parameters, but also successfully calculated the SO₂ absorption capacity (19.051 mol kg⁻¹) that is highly consistent with the experimental values at 298.15 K, indicating that this model has extremely high reliability. Subsequently, it was further applied to analyze the absorption behavior of C₂mimCl-7 + HBDs DESs, and the corresponding m_phys, m_C2mimCl, and m_HBDs were obtained, demonstrating the impact of different HBDs on SO₂ absorption. Dynamics analysis shows that the as-prepared DESs could reach gas–liquid equilibrium in 40 s. Besides, the present DESs exhibit remarkable SO₂/CO₂ selectivity, with C₂mimCl-7 + AmPyr achieving a value as high as 528.7. After 30 cycles of absorption–desorption, the SO₂ absorption of the regenerated C₂mimCl-7 + AmPyr still reached 18.265 mol kg⁻¹. Finally, combining thermodynamic analysis, FT-IR, and ¹H NMR results, it is indicated that the SO₂ absorption process involves physical absorption and chemical absorption.

4 Experimental section

Preparation of DESs: C₂mimCl-n + AmPyr DESs (n = 2, 3, 4, 5, 6, and 7) were prepared by combining C₂mimCl and AmPyr at specified molar ratios (2 : 1, 3 : 1, 4 : 1, 5 : 1, 6 : 1, 7 : 1). The mixtures underwent heating at 353.15 K for 180 min with constant stirring. Subsequent cooling to ambient temperature yielded homogeneous liquids (Scheme S2).

C₂mimCl-7 + ChPyr, C₂mimCl-7 + BrPyr, C₂mimCl-7 + Cyt, C₂mimCl-7 + DAmPyr, C₂mimCl-7 + Ura, C₂mimCl-7 + AmPd and C₂mimCl-7 + AmPz were prepared using 2-chloropyrimidine, 2-bromopyrimidine, 4-amino-2-hydroxypyrimidine, 4,6-diaminopyrimidene, 2,4-dihydroxypyrimidine, 3-aminopyridazine and 2-aminopyrazine as HBDs, respectively.

C₂mimBr-7 + AmPyr was prepared using C₂mimBr as a HBA and AmPyr as a HBD.

At a molar ratio of 7 : 1 and 353.15 K, with C₂mimCl as a HBA and TAmPyr as a HBD, DESs could not be formed after vigorous stirring at 800 rpm for 180 min.

For other details not described, please refer to the SI.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5im00317b.

Acknowledgements

All authors appreciate the financial support from the National Science Foundation for Distinguished Young Scholars (No. 22425808), the National Natural Science Foundation of China (No. 22178154, 22578187), the Science Foundation of China University of Petroleum, Beijing (No. 2462022YJRC002, 2462022YJRC003), and the Carbon Neutrality Research Institute Fund (No. CNIF20240103).

References

1. W. Li, J. Li, T. D. Duong, S. A. Sapchenko, X. Han, J. D. Humby, G. F. S. Whitehead, I. J. Victórica-Yrezábal, I. da Silva, P. Manuel, M. D. Frogley, G. Cinque, M. Schröder and S. Yang, Adsorption of sulfur dioxide in Cu(II)-carboxylate framework materials: The role of ligand functionalization and open metal sites, J. Am. Chem. Soc., 2022, 144, 13196–13204.
2. X. Bie, R. Wu, B. Yu, X. Quan, S. Zhang, Q. Li, Y. Zhang and H. Zhou, Deactivation mechanisms of Cu–Zn–Al₂O₃ in CO₂ hydrogenation induced by SO₂ exposure, Ind. Chem. Mater., 2025, 3, 710–722.
3. X. Cao, Y. Liu, Q. Huang, Z. Chen, J. Sun, J. Sun, S. Pang, P. Liu, W. Wang, Y. Zhang and M. Ge, Single droplet tweezer revealing the reaction mechanism of Mn(II)-catalyzed SO₂ oxidation, Environ. Sci. Technol., 2024, 58, 5068–5078.
4. W. Zhao, J. L. Obeso, V. B. López-Cervantes, M. Bahri, E. Sánchez-González, Y. A. Amador-Sánchez, J. Ren, N. D. Browning, R. A. Peralta, G. Barcaro, S. Monti, D. Solis-Ibarra, I. A. Ibarra and D. Zhao, Achieving sub-ppm sensitivity in SO₂ detection with a chemically stable covalent organic framework, Angew. Chem., Int. Ed., 2024, 64, e202415088.
5. W. Gong, P. Gao, Y. Gao, Y. Xie, J. Zhang, X. Tang, K. Wang, X. Wang, X. Han, Z. Chen, J. Dong and Y. Cui, Modulator-directed counterintuitive catenation control for crafting highly porous and robust metal–organic frameworks with record high SO₂ uptake capacity, J. Am. Chem. Soc., 2024, 146, 31807–31815.
6. Q. Zhu, C. Wang, J. Yin, H. Li, W. Jiang, J. Liu, P. Li, Q. Zhang, Z. Chen and W. Zhu, Efficient and remarkable SO₂ capture: A discovery of imidazole-based ternary deep eutectic solvents, J. Mol. Liq., 2021, 330, 115595.
7. Z. Yan, S. Y. Lai, C. L. Ngan, H. Li and A. R. Mohamed, Recent advances in energy-efficient and regenerative SO₂ absorption over deep eutectic solvents, J. Environ. Chem. Eng., 2022, 10, 108967.
8. A. P. Abbott, G. Capper, D. L. Davies, R. K. Rasheed and V. Tambyrajah, Novel solvent properties of choline chloride/urea mixtures, Chem. Commun., 2003, 70–71.
9. Y. Chen and Z. Yu, Low-melting mixture solvents: Extension of deep eutectic solvents and ionic liquids for broadening green solvents and green chemistry, Green Chem. Eng., 2024, 5, 409–417.
10. M. Hu, B. Han, L. Xie, B. Lu, D. Bai, N. Shi, Y. Liao, Y. Wang, L. Liu, S. Wu, R. Lan, X. Lei, C. Shi, D. Huang, Y. Li, L. Lin and J. Zhang, Ultrasonic assisted natural deep eutectic solvents as a green and efficient approach for extraction of hydroxytyrosol from olive leaves, Ind. Chem. Mater., 2024, 2, 309–320.
11. Y. Chen, G. Zhao, J. Dong, J. Wang, D. Dong, Z. Li, M. Zhao, Z. Shi and Z. Niu, Green recovery of all-solid-state sodium-ion batteries/lithium-ion batteries by ionic liquids, deep eutectic solvents and low-melting mixture solvents, Ind. Chem. Mater., 2025, 3, 464–474.
12. E. L. Smith, A. P. Abbott and K. S. Ryder, Deep eutectic solvents (DESs) and their applications, Chem. Rev., 2014, 114, 11060–11082.
13. M. Francisco, A. van den Bruinhorst and M. C. Kroon, Low-transition-temperature mixtures (LTTMs): A new generation of designer solvents, Angew. Chem., Int. Ed., 2013, 52, 3074–3085.
14. D. Yang, M. Hou, H. Ning, J. Zhang, J. Ma, G. Yang and B. Han, Efficient SO₂ absorption by renewable choline chloride–glycerol deep eutectic solvents, Green Chem., 2013, 15, 2261.
15. D. Deng, X. Liu and B. Gao, Physicochemical properties and investigation of azole-based deep eutectic solvents as efficient and reversible SO₂ absorbents, Ind. Eng. Chem. Res., 2017, 56, 13850–13856.
16. K. Zhang, S. Ren, X. Yang, Y. Hou, W. Wu and Y. Bao, Efficient absorption of low-concentration SO₂ in simulated flue gas by functional deep eutectic solvents based on imidazole and its derivatives, Chem. Eng. J., 2017, 327, 128–134.
17. D. Lee, W. Y. Choi, K. Jang, J. Park and Y. Yoo, Functionalized imidazole–alkanolamine deep eutectic solvents with remarkable performance for low-concentration SO₂ absorption, Sep. Purif. Technol., 2023, 307, 122782.
18. X. Li, L. Meng, F. Yang, Z. Yang, J. Li, Y. Chen and X. Ji, Tuning the structure of N-methyldiethanolamine-based deep eutectic solvents for efficient and reversible SO₂ capture, Chem. Commun., 2024, 60, 10560–10563.
19. G. Long, C. Yang, X. Yang, T. Zhao, F. Liu and J. Cao, Bisazole-based deep eutectic solvents for efficient SO₂ absorption and conversion without any additives, ACS Sustainable Chem. Eng., 2020, 8, 2608–2613.
20. L. Zhang, H. Ma, G. Wei, B. Jiang, Y. Sun, X. Tantai, Z. Huang and Y. Chen, Efficient and reversible nitric oxide absorption by low-viscosity, azole-derived deep eutectic solvents, J. Chem. Eng. Data, 2019, 64, 3068–3077.
21. Y. Chen, B. Jiang, H. Dou, L. Zhang, X. Tantai, Y. Sun and H. Zhang, Highly efficient and reversible capture of low partial pressure SO₂ by functional deep eutectic solvents, Energy Fuels, 2018, 32, 10737–10744.
22. P. Zhang, G. Xu, M. Shi, Z. Wang, Z. Tu, X. Hu, X. Zhang and Y. Wu, Unexpectedly efficient absorption of low-concentration SO₂ with phase-transition mechanism using deep eutectic solvent consisting of tetraethylammonium chloride and imidazole, Sep. Purif. Technol., 2022, 286, 120489.
23. Q. Wang, H. Wu, T. Zhang, Y. Fan, W. Zhang and K. He, Efficient absorption of low partial pressure SO₂ by deep eutectic solvents based on pyridine derivatives, Chem. Eng. Res. Des., 2022, 177, 36–44.
24. Z. Wang and D. Yang, SO₂ capture by 2-pyridineethanol through the formation of a zwitterionic liquid, Chem. Commun., 2022, 58, 6212–6214.
25. C. Wang, H. Wu, J. Li, J. Zhang, J. Zhang, J. Ding, H. Li, H. Li and W. Zhu, Surpassingly efficient, selective, and reversible absorption of SO₂ through pyridine based deep eutectic solvents, Chem. Eng. J., 2023, 471, 144394.
26. T. Yang, Y. Wang, Z. Huang, F. Liu, Q. Liao and T. Zhao, Deep eutectic solvents composed of 1-methyl-3-ethylimidazole halides and pyridine derivatives for efficient absorption and conversion of SO₂ into cyclic sulfites under ambient conditions, Sep. Purif. Technol., 2025, 361, 131475.
27. P. Zhang, Z. Tu, X. Zhang, X. Hu and Y. Wu, Acidic protic ionic liquid-based deep eutectic solvents capturing SO₂ with low enthalpy changes, AIChE J., 2023, 69, e18145.
28. Y. Zou, C. Wang, H. Ji, P. Wu, Y. Chao, X. Yu, Z. Yu, H. Liu, Z. Liu and W. Zhu, Rapid, selectivity, and reversibility absorption of SO₂ via purine-based deep eutectic solvents and thermodynamic analysis, Green Chem. Eng., 2026, 7, 180–190.
29. P. Li, X. Wang, T. Zhao, C. Yang, X. Wang and F. Liu, Deep eutectic solvents formed by EmimCl plus lactams: Effective SO₂ capture and conversion into sulphur via DESs-mediated claus process, Chem. Eng. J., 2021, 422, 130033.
30. X. Y. Luo, X. Fan, G. L. Shi, H. R. Li and C. M. Wang, Decreasing the viscosity in CO₂ capture by amino-functionalized ionic liquids through the formation of intramolecular hydrogen bond, J. Phys. Chem. B, 2016, 120, 2807–2813.
31. M. Pan, Y. Zhao, X. Zeng and J. Zou, Efficient absorption of CO₂ by introduction of intramolecular hydrogen bonding in chiral amino acid ionic liquids, Energy Fuels, 2018, 32, 6130–6135.
32. K. Sheng, D. Li and Y. Kang, Unexpectedly promoted SO₂ capture in novel ionic liquid-based eutectic solvents: The synergistic interactions, J. Mol. Liq., 2021, 337, 116432.
33. Y. Cao, J. Zhang, Y. Ma, W. Wu, K. Huang and L. Jiang, Designing low-viscosity deep eutectic solvents with multiple weak-acidic groups for ammonia separation, ACS Sustainable Chem. Eng., 2021, 9, 7352–7360.
34. G. Long, C. Yang, X. Yang, T. Zhao and M. Xu, Deep eutectic solvents consisting of 1-ethyl-3-methylimidazolium chloride and glycerol derivatives for highly efficient and reversible SO₂ capture, J. Mol. Liq., 2020, 302, 112538.
35. B. Jiang, H. Zhang, L. Zhang, N. Zhang, Z. Huang, Y. Chen, Y. Sun and X. Tantai, Novel deep eutectic solvents for highly efficient and reversible absorption of SO₂ by preorganization strategy, ACS Sustainable Chem. Eng., 2019, 7, 8347–8357.
36. X. Wu, R. Guan, W. Zheng and K. Huang, New deep eutectic solvents formed by 1-ethyl-3-methylimidazolium chloride and dicyandiamide: Physiochemical properties and SO₂ absorption performance, J. Taiwan Inst. Chem. Eng., 2021, 119, 45–51.
37. D. Deng, G. Han and Y. Jiang, Investigation of a deep eutectic solvent formed by levulinic acid with quaternary ammonium salt as an efficient SO₂ absorbent, New J. Chem., 2015, 39, 8158–8164.
38. C. Wang, G. Cui, X. Luo, Y. Xu, H. Li and S. Dai, Highly efficient and reversible SO₂ capture by tunable azole-based ionic liquids through multiple-site chemical absorption, J. Am. Chem. Soc., 2011, 133, 11916–11919.
39. Y. Jiang, X. Liu and D. Deng, Solubility and thermodynamic properties of SO₂ in three low volatile urea derivatives, J. Chem. Thermodyn., 2016, 101, 12–18.
40. D. Yang, Y. Han, H. Qi, Y. Wang and S. Dai, Efficient absorption of SO₂ by EmimCl-EG deep eutectic solvents, ACS Sustainable Chem. Eng., 2017, 5, 6382–6386.
41. K. Zhang, S. Ren, Y. Hou and W. Wu, Efficient absorption of SO₂ with low-partial pressures by environmentally benign functional deep eutectic solvents, J. Hazard. Mater., 2017, 324, 457–463.
42. G. Cui, J. Liu, S. Lyu, H. Wang, Z. Li and J. Wang, Efficient and reversible SO₂ absorption by environmentally friendly task-specific deep eutectic solvents of PPZBr + Gly, ACS Sustainable Chem. Eng., 2019, 7, 14236–14246.
43. Z.-M. Li, W.-Q. Gong, J.-F. Li, S.-X. Zhu, D.-J. Tao and Y. Zhou, Efficient and selective absorption of SO₂ by low-viscosity matrine-based deep eutectic solvents, J. Mol. Liq., 2022, 367, 120521.
44. Y. Zhao, J. Dou, R. Dai, H. Bai, S. Khoshk Rish, X. Xiao and J. Yu, Superefficient absorption of low-concentration SO₂ in flue gas using new ternary imidazole-based deep eutectic solvents: Mechanism, thermodynamics, and process analysis, Energy Fuels, 2022, 36, 8351–8359.
45. Y. Zhao, J. Dou, H. Li, R. Dai, H. Bai, S. Khoshk Rish, X. Chen, X. Xiao and J. Yu, Low-cost Na₂S-EG-MTPB deep eutectic solvents absorb SO₂ effectively at a high temperature in flue gas, Sep. Purif. Technol., 2022, 303, 122283.
46. T. Zhou, Y. Zhao, X. Xiao, Y. Liu, H. Bai, X. Chen, J. Dou and J. Yu, Effective absorption mechanism of SO₂ and NO₂ in the flue gas by ammonium-bromide-based deep eutectic solvents, ACS Omega, 2022, 7, 29171–29180.
47. W. J. Jiang, F. Y. Zhong, L. S. Zhou, H. L. Peng, J. P. Fan and K. Huang, Chemical dual-site capture of NH₃ by unprecedentedly low-viscosity deep eutectic solvents, Chem. Commun., 2020, 56, 2399–2402.
48. P. Liu, K. Cai, X. Zhang, X. Wang, M. Xu, F. Liu and T. Zhao, Rich ether-based protic ionic liquids with low viscosity for selective absorption of SO₂ through multisite interaction, Ind. Eng. Chem. Res., 2022, 61, 5971–5983.
49. D. Yang, S. Zhang, D. E. Jiang and S. Dai, SO₂ absorption in EmimCl-TEG deep eutectic solvents, Phys. Chem. Chem. Phys., 2018, 20, 15168–15173.
50. T. Zhao, J. Liang, Y. Zhang, Y. Wu and X. Hu, Unexpectedly efficient SO₂ capture and conversion to sulfur in novel imidazole-based deep eutectic solvents, Chem. Commun., 2018, 54, 8964–8967.
51. M. Lv, D. Yang and J. Chen, Deep eutectic solvents consisting of 1-ethyl-3-methylimidazolium chloride and biobased 2-pyrrolidone for reversible SO₂ capture, ChemistrySelect, 2020, 5, 7142–7147.
52. H. Wang, P. Wu, C. Li, J. Zhang and R. Deng, Reversible and efficient absorption of SO₂ with natural amino acid aqueous solutions: Performance and mechanism, ACS Sustainable Chem. Eng., 2022, 10, 4451–4461.
53. S. Akyüz and T. Akyüz, FT-IR spectroscopic investigations of surface and intercalated 2-aminopyrimidine adsorbed on sepiolite and montmorillonite from Anatolia, J. Mol. Struct., 2003, 651–653, 205–210.
54. C. Wang, Q. Bi, Y. Huo, Z. Zhang, D. Tao, Y. Shen, Q. Zhu, Z. Chen, H. Li and W. Zhu, Investigation of amine-based ternary deep eutectic solvents for efficient, rapid, and reversible SO₂ absorption, Energy Fuels, 2021, 35, 20406–20410.
55. X. Yang, Y. Zhang, F. Liu, P. Chen, T. Zhao and Y. Wu, Deep eutectic solvents consisting of EmimCl and amides: Highly efficient SO₂ absorption and conversion, Sep. Purif. Technol., 2020, 250, 117273.
56. K.-Y. Lee, C.-S. Kim, H.-G. Kim, M.-S. Cheong, D. K. Mukherjee and K.-D. Jung, Effects of halide anions to absorb SO₂ in ionic liquids, Bull. Korean Chem. Soc., 2010, 31, 1937–1940.
57. Z. L. Li, L. S. Zhou, Y. H. Wei, H. L. Peng and K. Huang, Highly efficient, reversible, and selective absorption of SO₂ in 1-ethyl-3-methylimidazolium chloride plus imidazole deep eutectic solvents, Ind. Eng. Chem. Res., 2020, 59, 13696–13705.
58. W. S. Ham, H. Choi, J. Zhang, D. Kim and S. Chang, C₂-selective, functional-group-divergent amination of pyrimidines by enthalpy-controlled nucleophilic functionalization, J. Am. Chem. Soc., 2022, 144, 2885–2892.
59. C. Wang, J. Zhang, Y. Zou, J. Ding, H. Li, M. Zhang, H. Li and W. Zhu, Effects of attractive electrostatic interactions on sulfur dioxide capture by functionalized deep eutectic solvents with abundant negative sites, Chem. Eng. J., 2025, 519, 165030.
60. S. Sun, Y. Niu, Q. Xu, Z. Sun and X. Wei, Efficient SO₂ absorptions by four kinds of deep eutectic solvents based on choline chloride, Ind. Eng. Chem. Res., 2015, 54, 8019–8024.

---