Study on the CO2 absorption performance of deep eutectic solvents formed by superbase DBN and weak acid diethylene glycol

Shihong Guo; Xuzhong Zhou; Chengfang Liu; Shuie Li

doi:10.1515/chem-2025-0146

Article Open Access

Study on the CO₂ absorption performance of deep eutectic solvents formed by superbase DBN and weak acid diethylene glycol

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Published/Copyright: April 29, 2025

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From the journal Open Chemistry Volume 23 Issue 1

Abstract

Deep eutectic solvents (DESs), a new class of green solvents, have attracted considerable attention for CO₂ capture. In this study, we synthesised DESs using a superbase, 1,5-diazabicyclo [4.3.0] non-5-ene (DBN), as the hydrogen bond acceptor and a weak acid, diethylene glycol (DEG), as the hydrogen bond donor in different molar ratios to achieve efficient and reversible CO₂ absorption. Experimental results indicate that DES2 (DBN:DEG = 1:2) exhibits optimal CO₂ absorption performance (a maximum absorption capacity of 0.275 gCO₂/gDES. Deep eutectic solvents at 30°C and flow rate of 30 mL/min). Nuclear magnetic resonance (NMR) spectroscopy reveals that DEG activated by DBN in DESs can react with CO₂ to form carbonates, thereby enabling CO₂ capture. In addition, monoethanolamine was added to the DESs as a promoter, thereby enhancing the CO₂ absorption performance, with a maximum absorption capacity of 0.316 gCO₂/gDES. ¹³C NMR and Fourier transform infrared analyses revealed that the amino and hydroxyl groups in the DESs can simultaneously react with CO₂ to form carbamate and carbonate, achieving synergistic absorption.

Keywords: super-alkaline deep eutectic solvent; carbon dioxide capture; mechanism analysis

1 Introduction

The combustion of fossil fuels generates enormous industrial waste, increasing CO₂ concentration in the atmosphere. CO₂ is one of the major greenhouse gases; it can absorb and emit infrared radiation, warming the atmosphere. This greenhouse effect is the primary driver of global warming and climate change. In addition, atmospheric CO₂ dissolves in oceans, forming carbonic acid, thereby increasing ocean acidity and harms marine ecosystems [1–3]. Therefore, CO₂ emissions have become a global issue. Consequently, various measures, including developing renewable energy, improving energy efficiency, promoting carbon capture and storage technologies, carbon capture and use as well as afforestation, are being implemented globally to reduce CO₂ emissions and mitigate the greenhouse effect [4,5]. Carbon capture technologies can be classified according to the sequence of carbon capture and combustion processes, separation methodologies, and technological advancements. This classification is fundamentally determined by the sequence of carbon capture and combustion processes. They can be categorised into pre-combustion capture, post-combustion capture, and oxy-fuel combustion. Pre-combustion capture and oxy-fuel combustion require the addition of new equipment to process carbon-containing components on an existing industrial base, posing a major challenge to already established industrial lines. In contrast, post-combustion capture only requires CO₂ capture from flue gases after combustion; thus, it is more convenient and suitable for industrialisation [6,7]. Post-combustion capture techniques include chemical absorption, membrane separation, chemical and physical adsorption, ionic liquid (IL) capture, and deep eutectic solvent (DES) capture [8]. In chemical absorption, alkanolamine aqueous solutions are used for industrial-grade CO₂ removal, primarily via the reaction of the amino groups in alkanolamine solutions with CO₂ [9,10]. However, because of the high volatility, strong corrosiveness, and high energy for solvent regeneration during amine scrubbing, considerable efforts are required to develop promising and effective CO₂ absorbents to replace alkanolamines [11,12].

ILs have attracted considerable attention as replacements for alkanolamines in CO₂ absorption. ILs effectively capture CO₂ by adjusting the ionic efficacy of anions and cations. Because of their wide liquid range, flexible DES design, low vapour pressure, broad temperature range, and high thermal stability, ILs can replace alkanolamines in CO₂ capture [13,14,15]. Bates et al. first synthesised a functionalised ILs containing –NH₂ group, i.e., [C₃H₇NH₂-Bmin][BF₄]. At ambient temperature and pressure, the CO₂ absorption rate of [C₃H₇NH₂-Bmim][BF₄] was 7.4 wt%; however, its high viscosity hindered mass transfer [16]. Combining ILs with other organic solvents is considered a feasible approach. Recently, protic ionic liquids (PILs), created by mixing superbases and weak acids, effectively capture CO₂ due to their high CO₂ attraction and low viscosity. Typically, such mixtures are formed by neturalising strong proton acceptors, such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), and weak proton donors, such as pyrrole, imidazole, phenol, and ethylene glycol (EG) [17,18]. Zhang et al. synthesised PIL-based DESs by adding DBN to DESs prepared by mixing imidazole and BmimCl at different molar ratios. They reported that the synergistic action of cations and anions facilitated CO₂ capture, with the protonated superbase playing a pivotal role in the absorption process. Smith et al. designed DES systems by mixing Lewis or Brønsted acids as hydrogen bond donors (HBDs) and bases as hydrogen bond acceptors (HBAs) [19,20]. These systems can include various ions or molecules, and if hydrogen bonding is prominent, mixtures of superbases and weak acids seem more suitable for DESs. DESs exhibit structural parallels to ILs in physicochemical properties, while demonstrating distinctive merits including simple preparative methodologies, non-toxicity, biodegradability, 100% atomic efficiency, and high product purity. Based on these advantages, DESs are potential CO₂-absorbent materials [21–23]. Fu et al. [24] signed and synthesised super basic DES systems – [DBUH][MLU]-EG and [DBUH]₂[MLU]-EG. They investigated the single and dual active centres of these two super basic DES systems for CO₂ capture. Results showed that DBUH⁺ could activate EG to absorb CO₂, indicating CO₂ absorption by DES via amino and hydroxyl functional groups. Ruan et al. [25] designed and synthesised DES systems formed by DBN and Tz. They reported that [2DBN:Tz] had the highest weight capacity of 0.19 gCO₂/gDES at 25°C. The DES systems absorb CO₂ through the synergistic action of DBN molecules and Tz ions activated by DBN.

According to the literature, DESs comprising superbases and weak acids exhibit high CO₂ capture performance. Therefore, we design a DES system for CO₂ capture using a superbase – DBN and a weak acid – diethylene glycol (DEG) – as raw materials. The CO₂ capture capacity of the DESs system was measured under different conditions. Based on absorption experiments and nuclear magnetic resonance (NMR) analysis, the CO₂ absorption mechanism was proposed and its reversible decarbonisation was analysed according to the absorption-DES-desorption cycle performance. In addition, monoethanolamine (MEA) was added to DES as a promoter. Based on absorption experiments, ¹³C NMR data, Fourier transform infrared (FTIR) analysis, and the promotion mechanism was proposed.

2 Experimental sections

2.1 Materials

(Table 1)

Table 1

Materials used in this work

Chemical name	CAS	MW (g mol⁻¹)	Purity (%)	Supplier
DBN	3001-72-7	124.18	98	Macklin
DEG	111-46-6	106.11	98	Macklin
MEA	141-43-5	61.08	98	Macklin
CO₂	124-38-9	44.01	99.99	Guizhou Sanhe Gas Co., Ltd, China
N₂	7729-37-9	28.00	99.999	Guizhou Sanhe Gas Co., Ltd, China

2.2 Analytical testing equipment

Differential scanning calorimeter, Netzsch DSC2 14 from Germany; thermogravimetric (TG) analyser, Netzsch STA 449 F3 from Germany; NMR spectrometer, Bruker Ascend TM 600 MHz from Germany; FTIR spectrometer, Thermo Fisher Scientific Nicolet iS20 from the United States; electronic balance, AR2 24CN, accuracy of ±2 × 10⁻⁴ g, Ohaus Instrument Co., Ltd.

2.3 DES preparation

According to the literature [26], DESs was synthesised using the mixed heating method. DBN and DEG were poured into a flask at molar ratios of 1:2, 1:3, and 1:4. The flask was sealed, and the mixture was magnetically stirred in a water bath at 30°C until a uniform transparent liquid was formed. The prepared binary DESs were cooled at room temperature for 24 h. The resulting DESs with molar ratios of 1:1, 1:2, 1:3, and 1:4 are called DES1, DES2, DES3, and DES4, respectively. Using a similar method, MEA was added to the prepared binary DESs with different molar ratios.

2.4 Characterisation

The NMR spectra of the DESs were recorded before and after CO₂ absorption. FTIR spectroscopy was performed in the attenuated total reflection mode to determine the CO₂ absorption mechanism. The thermal stability of the absorbents was determined using TG analysis. The temperature was set from 30 to 500°C at a 10°C/min heating rate under a nitrogen atmosphere.

2.5 CO₂ absorption and DES desorption experiment

According to the literature [27], the CO₂ absorption experiment was performed using a gravimetric method. A DES was poured into a glass container, which was then placed in a water bath, ensuring the DES was below the water surface. Subsequently, CO₂ was introduced into the DES at atmospheric pressure. Using an electronic analytical balance with a precision of 0.005 g, the weight of the glass container was measured every 10 min to obtain the amount of CO₂ captured. Absorption equilibrium was reached when the weight difference between two consecutive measurements was less than 0.005 g. For the CO₂ DES desorption experiment, a CO₂ saturated DES was placed in a water bath and N₂ was introduced at 70°C for DES desorption until the weight returned to the pre-absorption level.

3 Results and discussion

3.1 Effect of DBN:DEG ratio on CO₂ absorption

The absorption curves of DES1(DBN:DEG = 1:1), DES2(DBN:DEG = 1:2), DES3(DBN:DEG = 1:3), and DES4(DBN:DEG = 1:4) were analysed (Figure 1a). At atmospheric pressure, a temperature of 30°C and a duration of 60 min, DES2 exhibited the maximum CO₂ absorption capacity (0.275 gCO₂/gDES; Figure 1a). Next, MEA was added to DES1 as a promoter (Figure 1b). The absorption capacity of DES1 containing MEA increased significantly because of the active absorption sites of MEA. However, increasing the MEA content did not continuously improve the CO₂ absorption capacity because MEA tightened the hydrogen bond network of the DES, thereby increasing steric hindrance and hindering CO₂ binding with the DES. Therefore, there is a balance, and DES containing MEA exhibited the maximum absorption capacity (0.316 gCO₂/gDES) when [DBN][DEG]₂-MEA = 1:2 (hereinafter, DES2-MEA).

Figure 1

Absorption rate curves of different absorbents at 30°C and a flow rate of 30 mL/min.

3.2 Effect of temperature on CO₂ absorption

We investigated the performance of DES2 for CO₂ absorption at atmospheric pressure within 30–60°C. As shown in Figure 2a, as the temperature increased from 30 to 60°C, the CO₂ absorption capacity decreased from 0.275 to 0.164 gCO₂/gDES due to the exothermic nature of CO₂ absorption. An increase in temperature significantly reduced the CO₂ absorption performance, indicating a substantial effect of temperature on CO₂ absorption by DESs. Notably, the absorption rate showed a marked increase with rising temperature, leading to a shorter time to reach absorption equilibrium, as higher temperature favours mass transfer. Given that CO₂ absorption is an exothermic reaction, higher temperatures also facilitate DESs desorption, offering great potential for the regeneration and reusability of absorbents.

Figure 2

Absorption rate curves of DES2 under (a) different temperatures and (b) different flow rates.

3.3 Effect of velocity of flow on CO₂ absorption

We investigated the performance of DES2 for CO₂ absorption over 1 h at flow rates ranging from 30 to 70 mL/min (Figure 2b). As the flow rate increases, the performance of DES2 decreases significantly. Specifically, the CO₂ absorption capacity decreases from 0.275 gCO₂/gDES at a flow rate of 30 mL/min to 0.168 gCO₂/gDES at a flow rate of 70 mL/min. The decrease in CO₂ absorption by DES is attributed to the increased flow rate, which reduces the time available for gas molecules to diffuse into the absorbent. Consequently, the CO₂ adsorption capacity decreases as the residence time decreases. This indicates that flow rate significantly influences CO₂ absorption by DESs [28].

3.4 Effect of moisture content on CO₂ absorption

According to the absorption curve in Figure 3, a DES2 with a higher water content exhibits a more pronounced reduction in CO₂ absorption efficiency, reaching saturation earlier. The addition of water to DESs reduces their viscosity, however, disrupting their hydrogen bond network and diminishing their absorption capacity. The reason in this study is that water competes with DEG for the activation sites of DBN, significantly reducing effective binding sites for CO₂ absorption in DESs. Moreover, the reaction product of CO₂ in DESs is aminoformate. Excess water in a DES system reacts with aminoformate to produce carbonate, which decomposes into water and CO₂, thereby reducing the CO₂ absorption capacity. As depicted in Figure 4, the variation in moisture content before and after DES absorption is minimal.

Figure 3

Absorption rate curves of DES2 under different water contents.

Figure 4

Moisture content of DESs before and after CO₂ absorption.

3.5 Cyclic performance of DES

3.5.1 Thermal stability of DES

Figure 5a shows the TG curves of the synthesised DESs system at different molar ratios. The results indicate that DES2 starts to lose weight at 327.13 K and completely decomposes at around 442.73 K, with no significant change in mass observed with increasing temperature. DES2 exhibits a 5% mass loss at a temperature of 365.23 K, demonstrating good thermal stability. Meanwhile, DES3 and DES4 exhibit 5% mass loss at temperatures of 408.78 and 435.66 K, respectively, indicating enhanced stability with higher DEG contents. The main reason for this stability is the formation of a complex hydrogen bond network between HBA and HBD. Thermal stability is a crucial parameter for assessing an absorbent’s performance in practical applications, as higher thermal stability facilitates effective DESs desorption and recycling of the absorbent [22,29].

Figure 5

(a) Thermogravimetric analysis curves of DES2 at different molar ratios; (b) cyclic diagram of DES2 under N₂ purge at 343.15 K.

3.5.2 DES cyclic performance

In industrial applications, the recyclability of decarbonising agents is crucial for their promotion and use. Excellent recyclability not only reduces production costs and emissions but also enhances economic feasibility. We investigated the cyclic absorption properties of DES2. As shown in Figure 5b, after four absorption-DES-desorption cycles, the absorption capacity remained at 0.251 gCO₂/gDES. However, the absorption performance slightly decreased, indicating that some CO₂ was not completely DES desorbed from the system. This is because the carbonates formed by DES2 and CO₂ are relatively stable and do not completely decompose during regeneration, leaving a small amount of CO₂ retained in DES2. Overall, after four cycles, DES2 maintained a stable and considerable absorption capacity, indicating that DESs have good absorption capacity and reusability [30,31].

3.6 Mechanism of CO₂ absorption by DES

To explore the CO₂ absorption mechanism of DES and the promoting effect of MEA on CO₂ absorption, ¹³C NMR spectroscopy was used to characterise DESs before and after CO₂ absorption. Considering DES2 and DES2-MEA as examples, the absorption mechanism of the synthesised DES system was studied (Figure 6b). After CO₂ absorption, new peaks appeared at 63.35 and 156.7 ppm in DES2, attributable to the carbonate formed by the interaction between DEG and CO₂ and the carbomethoxy (RO–COO–) group formed by the reaction of DEG with CO₂, respectively. This indicates that the hydroxyl group of DEG is activated under the influence of DBN, enabling it to absorb CO₂ [32]. As shown in Figure 6a, DES2-MEA exhibits new peaks at 59.85 and 162.14 ppm after CO₂ absorption, attributable to the carbonate formed by the interaction between DEG and CO₂ and the carbamate (–NH–COO–) formed by the interaction between CO₂ and MEA, respectively [33].

Figure 6

¹³C NMR carbon spectra of different DESs before and after CO₂ absorption.

To elucidate the promoting mechanism of MEA, FTIR analysis was used to study the CO₂ absorption mechanism of DES containing MEA (Figure 7). Comparing the FTIR spectra before and after CO₂ absorption, new peaks appear at 1569.74 and 816.66 cm⁻¹, attributable to the asymmetric stretching and bending vibrations of –COO– in –NHCOO–, respectively. The peak at 2864.7 cm⁻¹ that corresponds to –NH– weakens, indicating carbamate generation. The disappearance of the peak at 3352.2 cm⁻¹ that corresponds to the hydroxyl group after CO₂ absorption suggests CO₂ consumption, while the peak at 1299.82 cm⁻¹ is attributed to the stretching vibration of carbonate, consistent with the previous ¹³C NMR analysis.

Figure 7

Infrared spectra of DES2-MEA before and after CO₂ absorption.

Based on the above analysis, we propose a possible mechanism (Figure 8). The possible absorption mechanism of DESs is illustrated in Scheme 1: DBN deprotonates the hydroxyl group of DEG, leading to the formation of the HO–CH₂–O–CH₂–O⁻ anion, which then combines with CO₂ to form carbonate [32]. In the DES with added MEA, dual absorption of hydroxyl and amino groups occurs, as illustrated in Scheme 2. The activated DEG by DBN reacts with CO₂, and the amino group in MEA reacts with CO₂, achieving cooperative CO₂ capture through amine-hydroxyl dual active sites in the system, thereby enhancing the CO₂ absorption performance. The addition of MEA promotes the CO₂ absorption capacity of DES.

Figure 8

Possible reaction mechanisms of CO₂ with DES2 and DES2-MEA used in this work.

3.7 Comparison of CO₂ absorption capacity with reported absorbents

To further evaluate the CO₂ absorption potential of DBN/DEG, we compared the CO₂ absorption capacity in this study with other absorbents reported in the literature. As shown in Table 2, the DESs employed in this work demonstrate competitive CO₂ absorption capacities compared with most absorbents under relatively mild absorption conditions, indicating significant potential for efficient CO₂ capture.

Table 2

Comparison of CO₂ capacity with other reported absorbents

Absorbents	PCO₂ (bar)	T (^oC)	CO₂ capacity (gCO₂/g solvent)	Reference
DBN:DEG (1:1)	1	30	0.19	This work
DBN:DEG (1:2)	1	30	0.275	This work
[DBN][DEG]₂:MEA (1:2)	1	30	0.316	This work
DBN/EG/MEA (3/1/1)	1	40	0.248	[32]
DBN/EG/MEA (1/1/3)	1	40	0.267	[32]
[DBUH][Car]:EG (1:2)	1	25	1^a	[31]
[EMIM][2-CNpyr]:EG (1:2)	1	25	2.59^a	[30]
K-GLY+MEA	1	25	0.256	[34]
[DBN]:[Tz] (2:1)	1	25	0.19	[25]
ChCl-DEA (1:6)	1	30	0.15	[35]
[DBUH][4-F-PhO] (1:5)	1	25	0.9^a	[23]
DBN-BmimCl-Im (1:1:1)	1	25	1.02^a	[36]
ChCl:MEA (1:2)	1	30	23^b	[37]
ChCl:MDEA (1:2)	10	30	0.0896	[38]
Be-EG-DEA (1:5:8)	1	30	0.207	[39]
Sodium carbonate-based solid sdsorbents	1	0	10.56^c	[40]
The CaO–MgO-based absorber	1	650	0.768	[41]

^aCO₂ absorption capacity (mol CO₂/mol solvent). ^bMass ratio. ^cCO₂ absorption capacity (cm³ CO₂/g absorbent).

4 Conclusions

In this study, a DES system comprising DBN and DEG as the HBA and HBD, respectively, was DES designed and synthesised. The CO₂ absorption capacity of DESs was measured using a gravimetric method. The effects of adjusting the HBD and HBA molar ratio, water content, temperature, and flow rate on CO₂ absorption performance were investigated. Optimal absorption conditions were determined; a molar ratio of 1:2, temperature of 30°C, and flow rate of 30 mL/min achieved the maximum CO₂ absorption capacity (0.275 gCO₂/gDES). The addition of water inhibited CO₂ absorption. The reusability of DESs was also studied. After four absorption-DES-desorption cycles, DESs maintained a stable and considerable absorption capacity. Proton NMR spectroscopy indicated that DBN in DESs activated the hydroxyl groups of DEG, enabling them to react with CO₂ to form carbonate and consequently absorb CO₂. Furthermore, MEA was added to DESs as a promoter, achieving an optimal absorption capacity of 0.316 gCO₂/gDES. According to NMR and FTIR analyses, activated DEG and MEA in DESs could simultaneously react with CO₂ to form carbamate and carbonate. The addition of MEA increased the CO₂ absorption capacity. Therefore, DESs can be considered promising absorbents. Moreover, the addition of the promoter MEA a new approach for DES designing efficient DESs for CO₂ capture.

Acknowledgments

This work was financially supported by Science and Technology Department of Guizhou Province in China (project number: K22-0110-034).

Funding information: This work was financially supported by Science and Technology Department of Guizhou Province in China (project number: K22-0110-034).
Author contributions: Shuie Li: methodology; Xuzhong Zhou: investigation; Chengfang Liu: conceptualisation and methodology; Shihong Guo: data curation, software, writing – original and draft, writing – review. All the authors approved the final manuscript version submitted.
Conflict of interest: The authors state no conflict of interest.
Informed consent: Not applicable.
Ethical approval: The conducted research is not related to either human or animal use.
Supporting information: ¹H NMR hydrogen spectra of DBN, DEG, and DBN-DEG(PDF).
Data availability statement: The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

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Received: 2024-09-04

Revised: 2025-02-14

Accepted: 2025-02-18

Published Online: 2025-04-29

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/chem-2025-0146

Keywords for this article

super-alkaline deep eutectic solvent; carbon dioxide capture; mechanism analysis

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BY 4.0

Study on the CO2 absorption performance of deep eutectic solvents formed by superbase DBN and weak acid diethylene glycol

Article

Abstract

1 Introduction

2 Experimental sections

2.1 Materials

2.2 Analytical testing equipment

2.3 DES preparation

2.4 Characterisation

2.5 CO2 absorption and DES desorption experiment

3 Results and discussion

3.1 Effect of DBN:DEG ratio on CO2 absorption

3.2 Effect of temperature on CO2 absorption

3.3 Effect of velocity of flow on CO2 absorption

3.4 Effect of moisture content on CO2 absorption

3.5 Cyclic performance of DES

3.5.1 Thermal stability of DES

3.5.2 DES cyclic performance

3.6 Mechanism of CO2 absorption by DES

3.7 Comparison of CO2 absorption capacity with reported absorbents

4 Conclusions

Acknowledgments

References

Supplementary Material

Articles in the same Issue

Articles in the same Issue

Articles in the same Issue

Study on the CO₂ absorption performance of deep eutectic solvents formed by superbase DBN and weak acid diethylene glycol

2.5 CO₂ absorption and DES desorption experiment

3.1 Effect of DBN:DEG ratio on CO₂ absorption

3.2 Effect of temperature on CO₂ absorption

3.3 Effect of velocity of flow on CO₂ absorption

3.4 Effect of moisture content on CO₂ absorption

3.6 Mechanism of CO₂ absorption by DES

3.7 Comparison of CO₂ absorption capacity with reported absorbents