The Reaction between K₂CO₃ and Ethylene Glycol in Deep Eutectic Solvents

Abstract

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 ${\mathrm{CO}}_3^{2-}$ 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.

1. 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 (K₂CO₃) are also explored mainly because K₂CO₃ is a common and cheap inorganic base with environmentally benign characteristics [13,14]. K₂CO₃-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 K₂CO₃-based DESs, K₂CO₃-EG DESs (EG: ethylene glycol) have received significant attention, as EG is also a cheap and renewable compound. The physiochemical properties of K₂CO₃-EG have been thoroughly investigated [14,22], and the interactions between K₂CO₃ 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 K₂CO₃.

In this work, the interactions between K₂CO₃ and EG in DESs were further studied using NMR and FTIR spectra. Surprisingly, the NMR results reveal that the ${\mathrm{CO}}_3^{2-}$ anion can react with EG to form EG-based carbonate species, which was not disclosed by the previous studies of K₂CO₃-EG DESs reported in the literature. The results can be found in the following sections.

2. Results and Discussion

At first, three DESs K₂CO₃:EG (1:6), (1:8), and (1:10) were prepared. The ¹³C 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, there are three new carbon peaks C-b, C-c, and C-d besides the ${\mathrm{CO}}_3^{2-}$ 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⁻ [23,24,25]. The ¹³C NMR spectra of K₂¹³CO₃: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 K₂¹³CO₃:EG (1:8) is significantly enhanced relative to that in K₂CO₃:EG (1:8), suggesting that C-d carbon comes from K₂¹³CO₃. In other words, C-d carbon in K₂CO₃-EG DESs is from the ${\mathrm{CO}}_3^{2-}$ 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. The ¹³C NMR spectra of K₂CO₃-EG DESs. Letters a–d are labels of carbons of EG and EG-based carbonate.

Figure 2. The ¹³C NMR (a) and ¹H-¹³C HMBC (b) spectra of K₂¹³CO₃:EG (1:8). Letters a–d are labels of carbons or hydrogens of EG and EG-based carbonate.

Furthermore, the ¹H-¹³C HMBC spectra of K₂¹³CO₃:EG (1:8) were recorded to demonstrate the interactions between K₂¹³CO₃ 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-¹³COO⁻ carbonate. Moreover, the correlations between ¹³${\mathrm{CO}}_3^{2-}$ carbon and the -OH proton or -CH₂- proton can be found, suggesting that both the -OH proton and -CH₂- proton in EG can form hydrogen bonds with the O atom of ${\mathrm{CO}}_3^{2-}$ in K₂CO₃-EG DESs. However, the hydrogen bonds between the -CH₂- proton and ${\mathrm{CO}}_3^{2-}$ 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⁻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.

Figure 3. The FTIR spectra of K₂CO₃, EG, and K₂CO₃-EG DESs.

Based on the above spectral results, the reaction between K₂CO₃ and EG can be elucidated, which may proceed in the following steps.

(1)

(2)

(3)

(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)

It is worth noting that the signal of the dianion ⁻OOC-O-CH₂-CH₂-O-COO⁻ is much weaker compared to that of HO-CH₂-CH₂-O-COO⁻ in Figure 2a, i.e., the main product of the reaction between K₂CO₃ and EG is HO-CH₂-CH₂-O-COO⁻. Therefore, the main reaction between K₂CO₃ and EG can be represented by Equation (5). Moreover, the pH values of K₂CO₃: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 K₂CO₃-EG solvents and supported the reaction shown by Equations (5) and (6). The aforementioned results reveal that the reaction occurs between K₂CO₃ and EG, forming alcohol-based carbonates, suggesting that the reported theoretical calculations for the interactions between K₂CO₃ and EG are not accurate [16,19].

All the three DESs K₂CO₃:EG (1:6), (1:8), and (1:10) can be formed by heating K₂CO₃-EG mixtures at 80 °C and 1.0 atmosphere. It should be noted that K₂CO₃:EG (1:10) can also be easily prepared by mixing K₂CO₃ and EG at room temperature, and the peaks of EG-based carbonate can still be found in the NMR and FTIR spectra of K₂CO₃:EG (1:10) prepared at room temperature. In other words, the reaction between K₂CO₃ and EG could proceed at room temperature. Moreover, as we all know, K₂CO₃ is a common and cheap inorganic base, which can be produced through the reaction between KOH and CO₂. Therefore, the findings of our work might be beneficial to developing new pathways for CO₂ capture and conversion.

3. Materials and Methods

3.1. Materials and Characterizations

EG (99.5%) was purchased from J&K Scientific Ltd. (Beijing, China). K₂CO₃ and K₂¹³CO₃ were obtained from Innochem (Beijing, China). EG was dried by a 4 Å molecular sieve prior to use, and K₂CO₃ and K₂¹³CO₃ were dried by a vacuum pump. N₂ (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 ¹³C NMR (100.6 MHz) spectra were obtained on a Bruker spectrometer (Bruker Biospin, Karlsruhe, Germany) and DMSO-d₆ was used as the reference.

3.2. Synthesis of DESs

Each K₂CO₃-EG DES was prepared by mixing K₂CO₃ and EG at the desired molar ratio in a round flask (10 mL) under N₂ atmosphere, and the mixture was stirred at 80 °C until a homogenous solution was formed.

For K₂CO₃:EG (1:10) DESs, they can also be obtained by stirring K₂CO₃ and EG at room temperature.

3.3. NMR and FTIR Data

NMR data:

K₂CO₃:EG (1:6): ¹³C NMR (100.6 MHz, DMSO-d₆, ppm) δ = 59.4, 61.9, 65.7, 157.4, 167.4.

K₂CO₃:EG (1:8): ¹³C NMR (100.6 MHz, DMSO-d₆, ppm) δ = 59.6, 62.1, 65.8, 157.7, 167.4.

K₂CO₃:EG (1:10): ¹³C NMR (100.6 MHz, DMSO-d₆, ppm) δ = 59.6, 62.8, 65.8, 157.7, 167.4.

FTIR data:

K₂CO₃:EG (1:6): 3188, 2915, 2855, 1653, 1324, 1085, 1041, 880, 861, 516 cm⁻¹.

K₂CO₃:EG (1:8): 3256, 2918, 2861, 1651, 1328, 1084, 1039, 880, 860, 515 cm⁻¹.

K₂CO₃:EG (1:10): 3273, 2923, 2864, 1650, 1329, 1083, 1037, 880, 860, 514 cm⁻¹.

4. Conclusions

In summary, K₂CO₃ can react with EG in K₂CO₃-EG DESs, resulting in the formation of EG-based organic carbonate HO-CH₂-CH₂-O-COO⁻ as the main product, and the reaction can occur at room temperature. The HMBC NMR results disclose that both the -OH and -CH₂- hydrogens form hydrogen bonds with the O atom of K₂CO₃. The transformation of carbon from K₂CO₃ to EG might bring valuable information for the development of CO₂ capture and conversion technologies.