Study on CO 2 absorption performance of ternary DES formed based on DEA as promoting factor

: In this study, we used tetraethylammonium chloride ( TEAC ) , diethanolamine ( DEA ) , and N - methyl - diethanolamine ( MDEA ) to prepare ternary DES and binary DES to absorb CO 2 . We found that their formation was due to the hydrogen bond interaction between hydrogen bond acceptor and hydrogen bond donor ( HBD ) . Surprisingly, TEAC/MDEA/DEA can react with CO 2 , but TEAC/MDEA cannot react with CO 2 . Unexpectedly, after adding DEA to TEAC/MDEA, the ternary TEAC/MDEA/DEA DES can react with CO 2 . Nuclear magnetic resonance spectroscopy and Fourier infrared spectroscopy results showed that the accidental CO 2 absorption behavior mainly depended on the HBD DEA, because the imine group in DEA reacted with CO 2 to form carbamate, thereby absorbing CO 2 , while the hydroxyl group on MDEA and the hydroxyl group of DEA did not interact with CO 2 . Through thermal stability analysis, TEAC/MDEA/DEA system with the molar ratio of 1:3:4 is more stable. We further studied the in ﬂ uence of molar ratio, temperature, water content, and other factors on the absorption of CO 2 by ternary DES. In addition, TEAC/MDEA/DEA ( 1:3:4 ) was regenerated at 80°C, and the absorption capacity of DES was almost unchanged after ﬁ ve absorption – desorption cycles.


Introduction
Carbon dioxide (CO 2 ), the most famous greenhouse gas and the main culprit of global warming, mainly comes from the emissions of human activities [1].According to the most recent data reported by the Scripps Institution of Oceanography, on December 8, 2021, atmospheric CO 2 concentration increased dramatically, reaching 415.92 ppm, which is why contemporary research focuses on ways to slow or stop this trend [2].Although many methods have been developed to inhibit carbon emissions to date, the absorption of CO 2 based on an amine aqueous solution is the most used method in the industry, such as monoethanolamine (MEA) and diethanolamine (DEA) [3,4].Unfortunately, this approach has serious inherent flaws: for example, the high volatility of solvents, amine degradation, and intensive regenerative energy [5].Abbott et al. [6] first reported this kind of solvent.They used choline chloride (ChCl) and urea as raw materials to successfully prepare liquid eutectic mixture at room temperature and defined it as deep eutectic solvents (DES).Such as low vapor pressure, wide liquid range, non-flammability, adjustable structure and simple synthesis process have attracted wide attention in many scientific fields.So far, the number of studies on DES has grown rapidly.Most DES reported in the literature is formed through the combination of hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD), due to the formation of hydrogen bond (H bond) [7].At present, due to its unique characteristics, DES has been used in various fields, such as electrochemistry, organic synthesis, gas absorption, functional material synthesis, and extraction separation [8][9][10][11][12].
Sakwattanapong et al. [13] studied the desorption energy consumption of mixed amine MEA + N-methyldiethanolamine (MDEA), DEA + MDEA, and MEA + AMP in a laboratory scale-packed tower, and the results showed that the desorption energy consumption of the three composite absorbents was between the energy consumption of the two amines that constitute the composite absorbents.The energy consumption of desorption decreased with the increase in AMP concentration in MEA + AMP composite absorbent, but the energy consumption remained basically unchanged when the concentration reached 2.7 kmol/m 3 , which indicated that the addition of sterosteric amine could reduce the desorption reaction heat of the composite absorbent.However, the influence of latent heat of water evaporation and sensible heat of absorbent on desorption energy consumption cannot be ignored.
Piperazine (PZ) showed good performance as the activator in the compound absorbent.Kanawade et al. [14] studied the performance of the composite absorbent after adding MEA, DEA, and PZ to the tertiary amine Nethyldiethanolamine, and the results showed that PZ had the best performance as the activator.In a similar manner, MEA is also used as an additive to MDEA aqueous solutions.MEA and MDEA have good characteristics.Compared with a single MEA or MDEA aqueous solution, the amount of CO 2 absorbed in the mixture is reduced, and the oxidation decomposition rate is higher, but it is increased under the condition of low partial pressure of CO 2 [15,16].Choi et al. [17] introduced 1%, 3% and 5 wt% piperazine (PZ), MDEA and hexamethylenediamine (HMDA) into 30 wt% 2-amino-2-methyl-1-propanol (AMP) solution, respectively, and the absorption of CO 2 increased.By adding polyamines (such as TEPA, TETA, DETA, or EDA) into MDEA aqueous solution, Hafizi et al. [18] improved the absorption capacity of CO 2 .The increased absorption capacity is due to the presence of basic nitrogen groups in polyamines.The ternary mixture of MDEA, PZ, and MEA has a higher absorption amount of CO 2 than the binary mixture of MDEA and PZ [19].Therefore, it is expected to mix MDEA with tertiary or secondary amines to improve the absorption amount of CO 2 .
In our previous work, the CO 2 absorption capacity of MEA as HBD was greater than that of MDEA as HBD.Among them, the CO 2 absorption capacity of TEAC/MDEA (1:4) at room temperature is 0.075 g CO 2 /g DES, respectively, indicating that its absorption capacity is not high.This is because MDEA aqueous solution does not undergo carbamate esterification with CO 2 , so the reactivity of MDEA with CO 2 is lower than that of primary and secondary amines.It is hoped that some measures can be taken to further improve the CO 2 absorption capacity of DES with MDEA as HBD.
In this study, an appropriate amount of DEA was added to TEAC/MDEA to further improve the CO 2 absorption capacity of this DES, while maintaining its regenerative capacity.The absorption amount and TGA under several different molar ratios were explored, and the optimal molar ratio was selected.The influence of different temperature and water content on the absorption capacity of CO 2 was explored for the mole ratio of DES, as well as its 1 H nuclear magnetic resonance spectroscopy (NMR), 13 C NMR, and infrared spectra before and after the absorption of CO 2 gas, and its absorption mechanism of CO 2 was investigated.The absorption-desorption experiment was conducted on the DES at the appropriate temperature.After five cycles of absorption-desorption of DES absorb carbon dioxide, to investigate whether DES can still maintain the initial absorption capacity, so as to further explain whether DES can be regenerated and recycled after adding DEA.

Chemical names and properties
The chemical names used in this article and their characteristics are shown in Table 1.

Analysis of test instruments
TGA5500 thermogravimetric analyzer of TA instruments, USA was used to determine the thermogravimetric curves of three ternary DES with different molar ratios before and after CO 2 absorption.Trace moisture tester, WS-2A, Shandong Zibo Three Pump Kesen Instrument Co., LTD.

Preparation of ternary deep eutectic solvents
TEAC, MDEA, and DEA were mixed in a closed condition according to the molar ratio of 1:2:4, 1:3:2, and 1:3:4 respectively.The water bath was heated at 70°C and fully stirred until the clarified liquid state was formed.The system was removed from the water bath, cooled, and stood at room temperature for a long time without precipitation and still maintained in the clarified liquid state.Ternary DES.

Performance characterization 2.4.1 Thermogravimetric characterization
The thermogravimetric curves of three ternary DES with different molar ratios after CO 2 absorption were determined using TA instruments TGA5500 thermogravimetric analyzer.and deuterium oxide (D 2 O) were measured before and after the absorption of CO 2 gas by nuclear magnetic resonance spectrometer.

Infrared characterization
Fourier infrared spectrometer was used to measure the infrared spectra of ternary DES before and after CO 2 absorption, and the window material was selected potassium bromide tableting method is adopted.

Desorption of CO 2
The desorption of CO 2 is achieved under the conditions of heat and N 2 purge.The saturated absorbent was added to the desorption bottle and placed in a constant temperature water bath at 80°C.The inlet of the desorption bottle was connected with N 2 .CO 2 release could be monitored by weighing mass changes during desorption.
3 Results and discussion

Formation mechanism of ternary DES
Infrared spectroscopy is widely used in the study of the formation mechanism of DES and is a typical method to judge the existence of groups.As we all know, electron effect, hydrogen bonding, and conjugation effect can make infrared absorption peak red shift or blue shift; so infrared spectral analysis is a good method to prove the formation mechanism of DES.At present, it is widely believed that the formation of DES is due to the hydrogen bonding between HBA and HBD; so in this article, FTIR was used to characterize and analyze the structure of several prepared DES, and the results are shown in Figure 1.
As can be seen from Figure 1, it is found that the absorption band peak of DEA at 3309.14 cm −1 shifted blue to 3319.45 cm −1 , the absorption band peak of MDEA at 3354.48 cm −1 shifted red to 3319.45 cm −1 , and the hydroxyl peak of MDEA and DEA widened significantly.The above shows that O-H⋯O, O-H⋯Cl and other hydrogen bonding Study on CO 2 absorption performance of ternary DES  3 structures are formed between TEAC and DEA and MDEA.In addition, the strong peak CH 2 and C-C bonds of DEA redshifted from 1458.09 and 1025.44 cm −1 to 1456.07 and 1021.38 cm −1 , respectively.Similarly, the strong peak CH 2 and C-C bonds of MDEA redshifted from 1456.97 and 880.39 cm −1 to 1456.07 and 875.37 cm −1 , respectively, which also proved that hydrogen bonds were formed between DEA, MDEA, and TEAC. 1 H NMR spectroscopy is often used to characterize hydrogen bond interactions between DES and ILs.It is well known that the strength of hydrogen bonds affects the chemical shifts of different peaks in a substance and can be used to study microstructure and interactions at the molecular level.Therefore, this article uses NMR to characterize and analyze the structure of several DES prepared, and the results are shown in Figure 2.
According to Figure 2, in DEA, 4.86 ppm is the peak of H on hydroxyl in DEA, 3.61 ppm is the peak of H near hydroxy-CH 2 , and 2.70 ppm is the peak of H on -CH 2 near -NH.In MDEA, 5.03 ppm is the peak of H on hydroxyl in MDEA, 3.74 ppm is the peak of H near hydroxy-CH 2 , 2.69 ppm is the peak of H on -CH 2 near nitrogen, and 2.45 ppm is the peak of H on -CH 3 near nitrogen.After the formation of TEAC/MDEA/DEA terra-DES, it can be seen that compared with MDEA and DEA, the chemical shifts of TEAC/MDEA/DEA all shifted, such as the chemical shifts of 4.86, 3.61, and 2.70 ppm in DEA.After the formation of DES, all of them moved to the low fields of 4.83, 3.56, and 2.65 ppm, thus confirming the hydrogen bond interaction between them [20].The infrared and NMR results show that the DES is formed by hydrogen bond interaction.Therefore, DES was successfully prepared.

Formation mechanism of ternary DES
It can be seen from Figure 3 that the absorption band peak of MDEA at 3354.48 cm −1 in DES is redshifted to 3331.95 cm −1 and the hydroxyl peak of MDEA is widened, which indicates that hydrogen bond structures such as O-H⋯O, O-H⋯Cl are formed between TEAC and MDEA; in addition, the strong peak CH 2 bond of MDEA blue shifted from 1456.97 to 1457.83 cm −1 , and the C-C bond redshifted from 880.39 to 878.15 cm −1 , which also proved that the hydrogen bond was formed between MDEA and TEAC.
It can be seen from Figure 4 that in MDEA, 5.04 ppm is the peak of H on the hydroxyl group, 3.74 ppm is the peak of H on the hydroxyl group -CH 2 , 2.69 ppm is the peak of H on -CH 2 near nitrogen, and 2.45 ppm is the peak of H on -CH 3 near nitrogen.After the formation of TEAC/MDEA, it can be seen that compared with MDEA, the chemical shifts of TEAC/MDEA have all moved, such as the chemical shifts of 5.04, 3.74, 2.69, and 2.45 ppm in MDEA, and after the formation of DES, they all moved to the low-field positions of 5.02, 3.56, 2.51, and 2.25 ppm, which can prove that the two chemical shifts are different.

Influence of ternary DES mole ratio on CO 2 absorption capacity
The results are shown in Figure 5.It can be seen from the absorption curve that the absorption of CO 2 is the fastest in the first 10 min and then becomes slower and slower.
When the ratio of DEA in the system is the largest, that is, when the mole ratio is 1:2:4, the absorption capacity is the largest, the CO 2 absorption capacity of the first 10 min is 0.101 g CO 2 /g DES, and the absorption capacity of the system after 80 min is 0.197 g CO 2 /g DES.When the mole ratio of ternary DES is 1:3:2 and 1:3:4, the absorption in the first 10 min is very close, which is 0.089 and 0.085 g CO 2 /g DES, respectively.The absorption capacity after 80 min is 0.197 g CO 2 /g DES.Compared with TEAC/MDEA system, it shows that the addition of DEA can improve the absorption capacity of CO 2 .

Thermal stability analysis of ternary DES before and after CO 2 absorption
To select the ternary DES system with the optimal molar ratio, the TGA5500 thermogravimetric analyzer was used in this section to conduct thermogravimetric analysis of the system before and after CO 2 absorption.The three   systems set the same heating procedure: the heating rate is 10°C/min and the temperature range is 30-500°C.For DES with a mole of 1:2:4, as shown in Figure 6a, it can be seen from the thermogravimetric curve that the system begins to lose weight at about 50°C, indicating that the system is prone to thermal degradation.By comparing the curves before and after absorption, it can be seen that the system after absorbing CO 2 is more stable, indicating that the system has poor thermal stability at this molar ratio and cannot be recycled.For the system with a molar ratio of 1:3:2, as shown in Figure 6b, comparing the weight loss curves before and after absorption, it can be seen that the system before CO 2 absorption is more stable before 121°C.At 70°C, the difference in the percentage of weight loss before and after absorption is about 4.8%, and at 100°C, the difference is about 5.8%, indicating that CO 2 can be desorbed by heating after the absorption of CO 2 gas by the system, so as to recycle the system.For the system with a molar ratio of 1:3:4, as shown in Figure 6c, the system after absorbing CO 2 is more stable before 104°C.
At 70°C, the weight loss percentage difference before and after absorption is about 6%, and at 100°C, the weight loss percentage difference before and after absorption is about 7.5%, indicating that the system can also be recycled.
According to the CO 2 absorption capacity and thermogravimetric analysis of ternary DES with three molar ratios, it can be concluded that the optimal molar ratio of TEAC/MDEA/DEA system is 1:3:4.At this molar ratio, DES not only has a good absorption capacity but also can be recycled.Therefore, the TEAC/MDEA/DEA system under 1:3:4 molar ratio is mainly analyzed in the following.

NMR characterization of ternary DES before and after CO 2 absorption
To explore the mechanism of CO 2 absorption in TEAC/ MDEA/DEA ternary DES system with the molar ratio of   C NMR Figure 8 shows that after CO 2 absorption, a new peak appears at 161 ppm, corresponding to the carbon absorption peak on carbamate, while the other peaks are not shifted.Based on the analysis of 1 H NMR and 13 C NMR, the CO 2 absorption mode of TEAC/MDEA/DEA system is mainly the reaction of -NHgroup on DEA with CO 2 to produce carbamate.

Infrared characterization of ternary DES before and after CO 2 absorption
To further investigate the mechanism of CO 2 absorption of ternary DES with molar ratio of 1:3:4, infrared spectra    temperature, at the same time, the viscosity of ternary DES decreases and the mass transfer resistance decreases, making it easier for CO 2 molecules to escape from ternary DES.Therefore, the temperature increase is not conducive to the absorption of CO 2 by ternary DES.
Figure 10b shows the influence of TEAC/MDEA (1:3) on the CO 2 absorption capacity.It can be seen from the figure that, at 30°C, its maximum absorption capacity is 0.075 g CO 2 /g DES, indicating that its absorption capacity is very low.When the promoting factor DEA was added, the absorption capacity was significantly increased.

Influence of ternary DES water content on CO 2 absorption capacity
Water with a mass percentage of 10 and 20 wt% was added to TEAC/MDEA/DEA (1:3:4) system to prepare water-containing Ternary DES and let them absorb CO 2 at 30°C.The absorption capacity was compared to investigate the influence of water content on the absorption capacity of Ternary DES, as shown in Figure 11.
According to the absorption curve, the higher the water content of the Ternary DES, the more obvious the reduction of the CO 2 absorption effect of DES.This is because the addition of water to TEAC/MDEA/DEA reduces the viscosity of TEAC/MDEA/DEA, thus destroying the hydrogen bond network and leading to the reduction of the absorption capacity of DES.In addition, water will compete with CO 2 as the active site of DEA; so the presence of water can greatly inhibit the absorption of CO 2 by DES containing secondary amine, which is similar to the study of Shukla and Mikkola.In addition, the reaction product of DES and CO 2 in this study is carbamate, and excessive water in the system will react with carbamate to form carbonic acid, which will decompose into water and CO 2 [21], resulting in the decline in the CO 2 absorption ability of DES.The more hydrogen on the amine, the more favorable it is to absorb CO 2 by forming hydrogen bond.Compared with DES reported in the literature, the synthesized ternary DES has greater advantages in the synthesis process and raw material cost, which is worthy of further study.

Regeneration of ternary DES
The reusable performance of the absorbent has an important impact on the operating cost of the absorption process and can reduce the generation of pollution.Based on this, the recycling performance of TEAC/MDEA/DEA (1:3:4) system was studied in this work, and the results are shown in Figure 12.
As can be seen from the figure, after DES has carried out five cycles of absorption-desorption, the lowest CO 2 absorption capacity of DES can still reach 0.175 g CO 2 /g DES, which has no significant change compared with the CO 2 absorption capacity of the primary absorption process (0.191 g CO 2 /g DES), indicating that the absorbent has a good cycle.After the recycling of DES, the captured CO 2 was not completely released, and its CO 2 absorption decreased slightly.This is because the carbamate generated by the chemical reaction between DES and CO 2 was relatively stable, which was not easy to be completely decomposed into CO 2 and DES during the regeneration; so a small amount of CO 2 remained in DES, leading to the reduction of CO 2 absorption.

Conclusion
In this work, we report the effective CO 2 absorption of ternary DES based on DEA as a promoter.The CO 2 absorption capacity of DES can be obtained by weighing method, and DEA has great influence on CO 2 absorption behavior.By adjusting the ratio of MDEA and DEA, different absorption capacities can be achieved.The absorption amount of ternary DES CO 2 of TEAC/MDEA/DEA with the molar ratio of 1:3:4 is 0.191 g CO 2 /g DES.CO 2 can chemically react with imines on DEA to produce carbamate.And the absorbed CO 2 can be resolved even at 80°C.We believe that this work reveals a promising DES design strategy for carbon capture efficiency.

Figure 1 :
Figure 1: Infrared spectra of ternary DES and its components.

Figure 3 :
Figure 3: Infrared spectra of TEAC/MDEA DES and its components.

Figure 5 :
Figure 5: CO 2 absorption of ternary DES with different molar ratios.

Figure 6 :
Figure 6: (a-c) The TG of ternary DES with different molar ratios before and after CO 2 absorption.

1: 3
:4, 1 H NMR and13 C NMR were determined before and after absorption using DMSO as deuterium reagent.1 H NMR, as shown in Figures7 and 8, after absorbing CO 2 , shifts from 4.91 to 5.29 ppm, and a new peak appears at 3.46 ppm, which represents the chemical reaction between imino (-NH-) on DEA and CO 2 to generate carbamate.

Figure 7 :
Figure 7: 1 H NMR spectra of DES before and after CO 2 absorption.

Figure 8 :
Figure 8: 13 C NMR spectra of DES before and after CO 2 absorption.

Figure 11 :
Figure 11: Effects of different TEAC/MDEA/DEA water contents on CO 2 absorption capacity.

Table 1 :
Chemical names and properties Guizhou Sanhe Gas Co., Ltd.,China

Table 2 :
Comparison between CO 2 absorption capacity of ternary DES and that of DES in literatureStudy on CO 2 absorption performance of ternary DES  93.9Comparison of CO 2 absorption by DESTo evaluate the CO 2 absorption performance of the DES studied in this study, the absorption capacity of the DES studied was compared with that of other DES reported in the literature, and the results are listed in Table2.As shown in Table2, the absorption capacity of CO 2 in this experiment is compared with the data in literature.The research shows that the prepared Ternary DES has a good absorption capacity of CO 2 .The absorption capacity of the prepared and synthesized Ternary DES in this study is within the range of 0.169-0.197g CO 2 /g DES.The absorption capacity of CO 2 in Ternary DES is higher than that of most common DES, e.g., [TETA]Cl-thymol(1:3), [P 2222 ][Triz]-EG(1:2), and [HDBU][Triz]-EG(7:3)].However, it is lower than [MEA][Cl]-EDA(1:3) DES, because EDA contains two NH 2 , while DEA contains only one imine group.