The Reaction between K2CO3 and Ethylene Glycol in Deep Eutectic Solvents

Understanding intermolecular interactions is important for the design of deep eutectic solvents. Herein, potassium carbonate (K2CO3) and ethylene glycol (EG) are used to form deep eutectic solvents. The interactions between K2CO3 and EG are studied using nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectra. Interestingly, the interaction results indicate that the carbonate anion CO32− can react with EG to form EG-based organic carbonate, which can occur even at room temperature. The possible reaction steps between K2CO3 and EG are presented. As K2CO3 can be prepared from CO2 and KOH, the findings of this work may provide a promising strategy for CO2 capture and conversion.


Introduction
Over the last few decades, deep eutectic solvents (DESs) have been developed and received a lot of attention mainly because of their advantageous characteristics, such as their low volatility, low flammability, simple preparation procedures, and structure tunability [1,2].Generally, DESs are described as a kind of fluid consisting of at least two or more components, while they present lower melting points than each parent component [3,4].To date, DESs can be classified into several types according to their components, and many DESs are prepared by mixing hydrogen bond donors (HBDs) and hydrogen bond acceptors (HBAs) [5][6][7][8].Due to their attractive properties, the applications of DESs have been explored in many fields, including organic reactions, biomass treatment, separations and extractions, electrochemistry and batteries, and gas capture [9][10][11][12].
DESs based on potassium carbonate (K 2 CO 3 ) are also explored mainly because K 2 CO 3 is a common and cheap inorganic base with environmentally benign characteristics [13,14].K 2 CO 3 -based DESs are already used in lignocellulose pretreatment, electrode material separation, supercapacitors, and gas separations [15][16][17][18][19][20][21][22].Among these K 2 CO 3 -based DESs, K 2 CO 3 -EG DESs (EG: ethylene glycol) have received significant attention, as EG is also a cheap and renewable compound.The physiochemical properties of K 2 CO 3 -EG have been thoroughly investigated [14,22], and the interactions between K 2 CO 3 and EG have been studied using theoretical calculations [16,19].The theoretical results suggest that intermolecular hydrogen bonds formed between the -OH hydrogen of EG and the carbonate anion of K 2 CO 3 .
In this work, the interactions between K 2 CO 3 and EG in DESs were further studied using NMR and FTIR spectra.Surprisingly, the NMR results reveal that the CO 2−  3 anion can react with EG to form EG-based carbonate species, which was not disclosed by the previous studies of K 2 CO 3 -EG DESs reported in the literature.The results can be found in the following sections.

Results and Discussion
At first, three DESs K 2 CO 3 :EG (1:6), (1:8), and (1:10) were prepared.The 13 C NMR spectra of these K 2 CO 3 -EG DESs were recorded using DMSO-d 6 as an external solvent.Namely, the interactions between K 2 CO 3 and EG were not disturbed by DMSO-d 6 .As shown in Figure 1 13 CO 3 :EG (1:8) were also investigated.As shown in Figure 2a, the carbon peaks C-b, C-c, and C-d can be observed.The C-d peak in K 2 13 CO 3 :EG (1:8) is significantly enhanced relative to that in K 2 CO 3 :EG (1:8), suggesting that C-d carbon comes from K 2 13 CO 3 .In other words, C-d carbon in K 2 CO 3 -EG DESs is from the CO 2− 3 carbon of K 2 CO 3 .Interestingly, there is another weak peak at 157.4 ppm (Figure 2a), which can be ascribed to the carbonyl carbon of the dianion . This peak is not detected in K 2 CO 3 -EG systems, which may be due to its low concentration in K 2 CO 3 -EG DESs.

Results and Discussion
At first, three DESs K2CO3:EG (1:6), (1:8), and (1:10) were prepared.The 13 C NMR spectra of these K2CO3-EG DESs were recorded using DMSO-d6 as an external solvent.Namely, the interactions between K2CO3 and EG were not disturbed by DMSO-d6.As shown in Figure 1   Furthermore, the 1 H- 13 C HMBC spectra of K2 13 CO3:EG (1:8) were recorded to demonstrate the interactions between K2 13 CO3 and EG.As can be seen in Figure 2b, there is a cross-signal between H-c and C-d, confirming the formation of HO-CH2-CH2-O-13 COO − carbonate.Moreover, the correlations between 13 CO 3 2 carbon and the -OH proton or -

Results and Discussion
At first, three DESs K2CO3:EG (1:6), (1:8), and (1:10) were prepared.The 13 C NMR spectra of these K2CO3-EG DESs were recorded using DMSO-d6 as an external solvent.Namely, the interactions between K2CO3 and EG were not disturbed by DMSO-d6.As shown in Figure 1   Furthermore, the 1 H- 13 C HMBC spectra of K2 13 CO3:EG (1:8) were recorded to demonstrate the interactions between K2 13 CO3 and EG.As can be seen in Figure 2b, there is a cross-signal between H-c and C-d, confirming the formation of HO-CH2-CH2-O-13 COO − carbonate.Moreover, the correlations between 13 CO 3 2 carbon and the -OH proton or - Furthermore, the 1 H-13 C HMBC spectra of K 2 13 CO 3 :EG (1:8) were recorded to demonstrate the interactions between K 2 13 CO 3 and EG.As can be seen in Figure 2b, there is a cross-signal between H-c and C-d, confirming the formation of HO-CH 2 -CH 2 -O-13 COO − carbonate.Moreover, the correlations between 13 CO 2− 3 carbon and the -OH proton or -CH 2 -proton can be found, suggesting that both the -OH proton and -CH 2 -proton in EG can form hydrogen bonds with the O atom of CO 2− 3 in K 2 CO 3 -EG DESs.However, the hydrogen bonds between the -CH 2 -proton and CO 2−  3 in K 2 CO 3 -EG DESs were not revealed in previous reports [16,19].
Figure 3 presents the FTIR spectra of K 2 CO 3 , EG, and K 2 CO 3 -EG used in this work.In comparison with the FTIR spectra of K 2 CO 3 and EG, new bands can be observed at ~1650 cm −1 in the spectra of K 2 CO 3 -EG.The band at ~1650 cm CH2proton can be found, suggesting that both the -OH proton and -CH2-proton in can form hydrogen bonds with the O atom of CO 3 2 in K2CO3-EG DESs.However, hydrogen bonds between the -CH2proton and CO 3 2 in K2CO3-EG DESs were not vealed in previous reports [16,19].
Figure 3   Based on the above spectral results, the reaction between K2CO3 and EG can be e cidated, which may proceed in the following steps.
The overall reaction is shown in Equation (5).
As seen in Figure 2a, the formation of the dianion − OOC-O-CH2-CH2-O-COO − can detected, so the reaction shown in Equation ( 6) can occur in K2CO3-EG DESs.Based on the above spectral results, the reaction between K 2 CO 3 and EG can be elucidated, which may proceed in the following steps.
Molecules 2024, 29, x FOR PEER REVIEW 3 of 6 CH2proton can be found, suggesting that both the -OH proton and -CH2-proton in EG can form hydrogen bonds with the O atom of CO 3 2 in K2CO3-EG DESs.However, the hydrogen bonds between the -CH2proton and CO 3 2 in K2CO3-EG DESs were not revealed in previous reports [16,19].
Figure 3 presents the FTIR spectra of K2CO3, EG, and K2CO3-EG used in this work.In comparison with the FTIR spectra of K2CO3 and EG, new bands can be observed at ~1650 cm −1 in the spectra of K2CO3-EG.The band at ~1650 cm  Based on the above spectral results, the reaction between K2CO3 and EG can be elucidated, which may proceed in the following steps. (1) The overall reaction is shown in Equation ( 5). (5) As seen in Figure 2a, the formation of the dianion − OOC-O-CH2-CH2-O-COO − can be detected, so the reaction shown in Equation ( 6) can occur in K2CO3-EG DESs.
(6) CH2proton can be found, suggesting that both the -OH proton and -CH2-proton in EG can form hydrogen bonds with the O atom of CO 3 2 in K2CO3-EG DESs.However, the hydrogen bonds between the -CH2proton and CO 3 2 in K2CO3-EG DESs were not revealed in previous reports [16,19].
Figure 3   Based on the above spectral results, the reaction between K2CO3 and EG can be elucidated, which may proceed in the following steps. (1) The overall reaction is shown in Equation ( 5).(5) As seen in Figure 2a, the formation of the dianion − OOC-O-CH2-CH2-O-COO − can be detected, so the reaction shown in Equation ( 6) can occur in K2CO3-EG DESs.
(6) CH2proton can be found, suggesting that both the -OH proton and -CH2-proton in EG can form hydrogen bonds with the O atom of CO 3 2 in K2CO3-EG DESs.However, the hydrogen bonds between the -CH2proton and CO 3 2 in K2CO3-EG DESs were not revealed in previous reports [16,19].
Figure 3   Based on the above spectral results, the reaction between K2CO3 and EG can be elucidated, which may proceed in the following steps. (1) The overall reaction is shown in Equation ( 5).CH2proton can be found, suggesting that both the -OH proton and -CH2-proton in EG can form hydrogen bonds with the O atom of CO 3 2 in K2CO3-EG DESs.However, the hydrogen bonds between the -CH2proton and CO 3 2 in K2CO3-EG DESs were not revealed in previous reports [16,19].
Figure 3 presents the FTIR spectra of K2CO3, EG, and K2CO3-EG used in this work.In comparison with the FTIR spectra of K2CO3 and EG, new bands can be observed at ~1650 cm −1 in the spectra of K2CO3-EG.The band at ~1650 cm  Based on the above spectral results, the reaction between K2CO3 and EG can be elucidated, which may proceed in the following steps. (1) The overall reaction is shown in Equation ( 5). (5) As seen in Figure 2a, the formation of the dianion − OOC-O-CH2-CH2-O-COO − can be detected, so the reaction shown in Equation ( 6) can occur in K2CO3-EG DESs.( 6) The overall reaction is shown in Equation (5).
Molecules 2024, 29, x FOR PEER REVIEW 3 of 6 CH2proton can be found, suggesting that both the -OH proton and -CH2-proton in EG can form hydrogen bonds with the O atom of CO 3 2 in K2CO3-EG DESs.However, the hydrogen bonds between the -CH2proton and CO 3 2 in K2CO3-EG DESs were not revealed in previous reports [16,19].
Figure 3   Based on the above spectral results, the reaction between K2CO3 and EG can be elucidated, which may proceed in the following steps. (1) (2) The overall reaction is shown in Equation ( 5).(5) As seen in Figure 2a, the formation of the dianion − OOC-O-CH2-CH2-O-COO − can be detected, so the reaction shown in Equation ( 6) can occur in K2CO3-EG DESs.
(6) (5) As seen in Figure 2a, the formation of the dianion − OOC-O-CH 2 -CH 2 -O-COO − can be detected, so the reaction shown in Equation ( 6) can occur in K 2 CO 3 -EG DESs. (4) The overall reaction is shown in Equation ( 5).(5) As seen in Figure 2a, the formation of the dianion − OOC-O-CH2-CH2-O-COO − can be detected, so the reaction shown in Equation ( 6) can occur in K2CO3-EG DESs.
(6) (6) It is worth noting that the signal of the dianion − OOC-O-CH 2 -CH 2 -O-COO − is much weaker compared to that of HO-CH 2 -CH 2 -O-COO − in Figure 2a, i.e., the main product of the reaction between K 2 CO 3 and EG is HO-CH 2 -CH 2 -O-COO − .Therefore, the main reaction between K 2 CO 3 and EG can be represented by Equation (5).Moreover, the pH values of K 2 CO 3 :EG (1:6) and (1:8) were 12.73 and 13.2 at 30 • C [14], respectively, which implied the formation of the OH − anion in K 2 CO 3 -EG solvents and supported the reaction shown by Equations ( 5) and ( 6).The aforementioned results reveal that the reaction occurs between K 2 CO 3 and EG, forming alcohol-based carbonates, suggesting that the reported theoretical calculations for the interactions between K 2 CO 3 and EG are not accurate [16,19].
All the three DESs K 2 CO 3 :EG (1:6), (1:8), and (1:10) can be formed by heating K 2 CO 3 -EG mixtures at 80 • C and 1.0 atmosphere.It should be noted that K 2 CO 3 :EG (1:10) can also be easily prepared by mixing K 2 CO 3 and EG at room temperature, and the peaks of EG-based carbonate can still be found in the NMR and FTIR spectra of K 2 CO 3 :EG (1:10) prepared at room temperature.In other words, the reaction between K 2 CO 3 and EG could proceed at room temperature.Moreover, as we all know, K 2 CO 3 is a common and cheap inorganic base, which can be produced through the reaction between KOH and CO 2 .Therefore, the findings of our work might be beneficial to developing new pathways for CO 2 capture and conversion.

Materials and Characterizations
EG (99.5%) was purchased from J&K Scientific Ltd. (Beijing, K 2 CO 3 and K 2 13 CO 3 were obtained from Innochem (Beijing, China).EG was dried by a 4 Å molecular sieve prior to use, and K 2 CO 3 and K 2 13 CO 3 were dried by a vacuum pump.N 2 (99.999%) was obtained from Beijing ZG Special Gases Sci. and Tech.Co., Ltd.(Beijing, China).
FTIR spectra were recorded on a Nicolet 6700 spectrometer (Waltham, MA, USA) with an attenuated total reflection (ATR) accessory. 1H NMR (400 MHz) and 13 C NMR (100.6 MHz) spectra were obtained on a Bruker spectrometer (Bruker Biospin, Karlsruhe, Germany) and DMSO-d 6 was used as the reference.

Synthesis of DESs
Each K 2 CO 3 -EG DES was prepared by mixing K 2 CO 3 and EG at the desired molar ratio in a round flask (10 mL) under N 2 atmosphere, and the mixture was stirred at 80 • C until a homogenous solution was formed.
For K 2 CO 3 :EG (1:10) DESs, they can also be obtained by stirring K 2 CO 3 and EG at room temperature.

Conclusions
In summary, K 2 CO 3 can react with EG in K 2 CO 3 -EG DESs, resulting in the formation of EG-based organic carbonate HO-CH 2 -CH 2 -O-COO − as the main product, and the reaction can occur at room temperature.The HMBC NMR results disclose that both the -OH and -CH 2 -hydrogens form hydrogen bonds with the O atom of K 2 CO 3 .The transformation of carbon from K 2 CO 3 to EG might bring valuable information for the development of CO 2 capture and conversion technologies.
, there are three new carbon peaks C-b, C-c, and C-d besides the CO 2− 3 carbon and -CH 2 -carbon (C-a) of EG for each K 2 CO 3 -EG used.The three new peaks can be found at 59.6 (C-b), 65.8 (C-c), and 157.7 (C-d) ppm for K 2 CO 3 :EG (1:8).The new peaks C-b and C-c are attributed to the -CH 2 -carbons of HO-CH 2 -CH 2 -O-COO − carbonate, and the peak C-d is attributed to the carbonyl carbon of HO-CH 2 -CH 2 -O-COO − [23-25].The 13 C NMR spectra of K 2 , there are three new carbon peaks C-b, C-c, and C-d besides the CO 3 2 carbon and -CH2-carbon (C-a) of EG for each K2CO3-EG used.The three new peaks can be found at 59.6 (C-b), 65.8 (C-c), and 157.7 (C-d) ppm for K2CO3:EG (1:8).The new peaks C-b and C-c are attributed to the -CH2-carbons of HO-CH2-CH2-O-COO − carbonate, and the peak C-d is attributed to the carbonyl carbon of HO-CH2-CH2-O-COO − [23-25].The 13 C NMR spectra of K2 13 CO3:EG (1:8) were also investigated.As shown in Figure 2a, the carbon peaks C-b, C-c, and C-d can be observed.The C-d peak in K2 13 CO3:EG (1:8) is significantly enhanced relative to that in K2CO3:EG (1:8), suggesting that C-d carbon comes from K2 13 CO3.In other words, C-d carbon in K2CO3-EG DESs is from the CO 3 2 carbon of K2CO3.Interestingly, there is another weak peak at 157.4 ppm (Figure 2a), which can be ascribed to the carbonyl carbon of the dianion − OOC-O-CH2-CH2-O-COO − [26].This peak is not detected in K2CO3-EG systems, which may be due to its low concentration in K2CO3-EG DESs.

Figure 1 .
Figure 1.The 13 C NMR spectra of K2CO3-EG DESs.Letters a-d are labels of carbons of EG and EGbased carbonate.

Figure 1 .
Figure 1.The 13 C NMR spectra of K 2 CO 3 -EG DESs.Letters a-d are labels of carbons of EG and EG-based carbonate.
, there are three new carbon peaks C-b, C-c, and C-d besides the CO 3 2 carbon and -CH2-carbon (C-a) of EG for each K2CO3-EG used.The three new peaks can be found at 59.6 (C-b), 65.8 (C-c), and 157.7 (C-d) ppm for K2CO3:EG (1:8).The new peaks C-b and C-c are attributed to the -CH2carbons of HO-CH2-CH2-O-COO − carbonate, and the peak C-d is attributed to the carbonyl carbon of HO-CH2-CH2-O-COO − [23-25].The 13 C NMR spectra of K2 13 CO3:EG (1:8) were also investigated.As shown in Figure 2a, the carbon peaks C-b, C-c, and C-d can be observed.The C-d peak in K2 13 CO3:EG (1:8) is significantly enhanced relative to that in K2CO3:EG (1:8), suggesting that C-d carbon comes from K2 13 CO3.In other words, C-d carbon in K2CO3-EG DESs is from the CO 3 2 carbon of K2CO3.Interestingly, there is another weak peak at 157.4 ppm (Figure 2a), which can be ascribed to the carbonyl carbon of the dianion − OOC-O-CH2-CH2-O-COO − [26].This peak is not detected in K2CO3-EG systems, which may be due to its low concentration in K2CO3-EG DESs.

Figure 1 .
Figure 1.The 13 C NMR spectra of K2CO3-EG DESs.Letters a-d are labels of carbons of EG and EGbased carbonate.

Figure 2 .
Figure 2. The 13 C NMR (a) and 1 H-13 C HMBC (b) spectra of K 2 13 CO 3 :EG (1:8).Letters a-d are labels of carbons or hydrogens of EG and EG-based carbonate.

( 1 )
Molecules 2024, 29, x FOR PEER REVIEW 3 of 6 presents the FTIR spectra of K2CO3, EG, and K2CO3-EG used in this work.In comparison with the FTIR spectra of K2CO3 and EG, new bands can be observed at ~1650 cm −1 in the spectra of K2CO3-EG.The band at ~1650 cm −1 is ascribed to the C=O asymmetrical stretching mode of R-OCOO − [27,28].The FTIR results again suggest the formation of EG-based carbonate.The new bands for K2CO3:EG (1:6), (1:8), and (1:10) are at 1653, 1651, and 1650 cm −1 , respectively.

( 2 )
Molecules 2024, 29, x FOR PEER REVIEW 3 of 6 presents the FTIR spectra of K2CO3, EG, and K2CO3-EG used in this work.In comparison with the FTIR spectra of K2CO3 and EG, new bands can be observed at ~1650 cm −1 in the spectra of K2CO3-EG.The band at ~1650 cm −1 is ascribed to the C=O asymmetrical stretching mode of R-OCOO − [27,28].The FTIR results again suggest the formation of EG-based carbonate.The new bands for K2CO3:EG (1:6), (1:8), and (1:10) are at 1653, 1651, and 1650 cm −1 , respectively.
presents the FTIR spectra of K2CO3, EG, and K2CO3-EG used in this work.In comparison with the FTIR spectra of K2CO3 and EG, new bands can be observed at ~1650 cm −1 in the spectra of K2CO3-EG.The band at ~1650 cm −1 is ascribed to the C=O asymmetrical stretching mode of R-OCOO − [27,28].The FTIR results again suggest the formation of EG-based carbonate.The new bands for K2CO3:EG (1:6), (1:8), and (1:10) are at 1653, 1651, and 1650 cm −1 , respectively.