The Role of K₂CO₃ in the Synthesis of Dimethyl Carbonate from CO₂ and Methanol

Abstract

The synthesis of dimethyl carbonate (DMC) from methanol and CO₂ has also received widespread attention, and K₂CO₃ is usually used as a catalyst in the synthesis of DMC. In this work, the role of K₂CO₃ in synthesizing dimethyl carbonate (DMC) from methanol and CO₂ was revisited. Interestingly, NMR results indicated that K₂CO₃ can react with methanol to form carbonate CH₃OCOO⁻, an essential intermediate in the synthesis of DMC, which can be transformed into DMC in the presence of CH₃I. In other words, K₂CO₃ can act as not only a catalyst but also a reactant to synthesize DMC from methanol and CO₂.

1. Introduction

Global warming is a serious environmental issue primarily due to the dramatic increase in anthropogenic CO₂ emissions, which has triggered disastrous effects on the ecosystems of the Earth. Developing effective methodologies to reduce CO₂ emissions plays a critical role in curbing global warming [1]. Carbon capture and utilization (CCU) technology is recognized as a promising strategy to efficiently control and reduce carbon emissions, in which CO₂ is trapped from carbon sources and then transformed into valuable chemicals [2,3]. To date, many strategies have been developed for transforming CO₂ into high-value chemicals, such as carboxylic acids, carbamates, and organic carbonates [4,5]. The synthesis of dimethyl carbonate (DMC) from methanol and CO₂ has also gained widespread attention due to its simple synthesis procedures and high atom economy [6].

DMC is regarded as a green organic compound due to its low toxicity and good biodegradability [7]. Because of its unique properties, DMC has been utilized in a wide range of fields. It can be used as a gasoline additive and an ideal electrolyte solvent in lithium-ion batteries [8]. Moreover, the applications of DMC can also be found in paints, pesticides, pharmaceutical products, polycarbonate synthesis, and methylation processes [9]. Although the conversion of CO₂ and methanol (CH₃OH) to DMC is a promising and green reaction, this reaction suffers from the inherent drawbacks, including thermodynamic limitations and the activation of the relatively inert C=O bond of CO₂, which results in a low yield of DMC [10,11,12,13]. Therefore, developing efficient catalysts for this reaction is of particular importance. Various catalysts have been designed and used for the synthesis of DMC from CO₂ and CH₃OH, such as alkali metal carbonate salts, organometallic complexes, ionic liquids, metal organic frameworks, nanomaterials based on metal oxides, and heteropoly acid compounds [14,15].

Alkali carbonate salts are cheap catalysts that are usually used in organic synthesis, and the applications of alkali carbonate catalysts in the synthesis of DMC from CO₂ and CH₃OH have also been widely investigated [16]. Among them, K₂CO₃ is found to be the most efficient catalyst [17,18]. It should be noted that CH₃I is used as a promoter during the synthesis of DMC when K₂CO₃ is used as the catalyst. The catalytic mechanism of K₂CO₃ for the formation of DMC is also proposed in the literature [19], which includes the following steps. The acid–base reaction occurs between K₂CO₃ and CH₃OH with the formation of CH₃O⁻ anion and bicarbonate anion $\mathrm{H}{\mathrm{CO}}_3^{-}$, and CH₃O⁻ anion can attack CO₂ to form the carbonate anion CH₃OCOO⁻. The nucleophilic group CH₃OCOO⁻ will react with CH₃I to form DMC and give I⁻ anion. The I⁻ anion interacts with $\mathrm{H}{\mathrm{CO}}_3^{-}$ to form HI, and thus ${\mathrm{CO}}_3^{2-}$ is regenerated. Then, CH₃I is recycled through the reaction between HI and CH₃OH [20].

Herein, the role of K₂CO₃ in the synthesis of DMC was revisited. Surprisingly, the results disclosed that ${\mathrm{CO}}_3^{2-}$ anion can directly react with -OH of CH₃OH to form CH₃OCOO⁻ carbonate in the absence of CO₂. As mentioned above, CH₃OCOO⁻ carbonate is an intermediate in the formation of DMC. In other words, K₂CO₃ is not only a catalyst but also a reagent in the synthesis of DMC, which may be a reason for the deactivation of K₂CO₃ in the formation of DMC from CO₂ and CH₃OH.

2. Materials and Methods

2.1. Materials and Characterizations

K₂¹³CO₃ (99%) and CD₃OD (99.8%) were obtained from Innochem (Beijing, China).

¹H NMR (400 MHz) and ¹³C NMR (100.6 MHz) spectra were obtained on a Bruker spectrometer. The NMR spectra were recorded at 25 °C, and a 5 mm PABBO probe was used. In the ¹H NMR experiment, the relaxation delay was 1.0 s, the acquisition time was 4.09 s, and the number of scans was 16. In the ¹³C NMR experiment, the number of scans was 1024, the relaxation delay was 2.0 s, and the acquisition time was 1.36 s. For the HSQC experiment, the relaxation delay was 2.0 s, and the acquisition time was 0.10 s. For the HMBC experiment, the relaxation delay was 1.5 s, and the acquisition time was 0.38 s.

2.2. Synthesis of K₂¹³CO₃-CD₃OD Mixture

K₂¹³CO₃ was added into CD₃OD at room temperature, and the mass fraction of K₂¹³CO₃ was 3 wt%. The mixture was stirred at room temperature for 24 h.

2.3. NMR Data

K₂¹³CO₃-CD₃OD: ¹³C NMR (100.6 MHz, CD₃OD-d₄) δ = 161.3, 170.0 ppm.

3. Results and Discussion

NMR spectra were used to investigate interactions between K₂CO₃ and CH₃OH. Due to the low solubility of K₂CO₃ in CH₃OH, the NMR spectra of K₂¹³CO₃ in methanol-d₄ were recorded to obtain the strong signal of carbonate anion in ¹³C NMR spectra. The mixture of K₂¹³CO₃-CD₃OD (3 wt%) was stirred 24 h at room temperature. After stirring, solid particles of K₂CO₃ can still be observed at the bottom of the mixture, suggesting K₂CO₃ was not completely dissolved in methanol, and the methanol phase became a suspension. The suspension was used to obtain NMR spectra.

As can be seen in Figure 1a, the carbon signals of ${\mathrm{CO}}_3^{2-}$ and CD₃OD appear at 170.0 [21] and 49.0 ppm, respectively. Besides these two peaks, a new peak was found at 161.3 ppm, which can be attributed to the carbonyl carbon of methanol-based carbonate [22,23].

The ¹H NMR spectra of CD₃OD with and without K₂¹³CO₃ were also investigated and shown in Figure 1b. After mixing CD₃OD with K₂¹³CO₃, the residual -OH peak and CHD₂- peak in deuterated methanol shifted downfield from 4.84 and 3.31 ppm to 5.03 and 3.34 ppm, respectively. Interestingly, a new peak can be observed at 3.38 ppm after the addition of K₂¹³CO₃ into CD₃OD. This peak can be attributed to the signal of CH₃- group of CH₃OH, which is supported by the HSQC spectra (Figure 1c). The appearance of CH₃- signal revealed that ${\mathrm{CO}}_3^{2-}$ can act as a catalyst for the hydrogen–deuterium exchange reaction between -OH and the deuterated methyl group [24,25]. Moreover, the hydrogen signals of CH₃OCOO⁻ and CHD₂OCOO⁻ were detected using ¹H-¹³C HMBC spectra (Figure 2). As can be seen in Figure 2, hydrogen signals of CH₃OCOO⁻ and CHD₂OCOO⁻ were correlated with the carbonyl carbon of methanol-based carbonate, suggesting again the carbonate carbon from K₂CO₃ was attached to the O atom of methanol.

Based on the above NMR results, the possible reaction steps between CH₃OH and ${\mathrm{CO}}_3^{2-}$ are shown as follows.

(1)

(2)

(3)

(4)

Using Equations (2) and (3), the following Equation (5) can be obtained.

(5)

The overall reaction between CH₃OH and ${\mathrm{CO}}_3^{2-}$ can be presented by Equation (6) below.

(6)

As revealed by Equation (6), ${\mathrm{CO}}_3^{2-}$ can react with CH₃OH to form CH₃OCOO⁻ carbonate, which can react with CH₃I to form DMC. Therefore, it is reasonable to conclude that K₂CO₃ also acts as a reactant in the synthesis of DMC.

Based on the above results, the possible reaction mechanism for the synthesis of DMC from CO₂ and methanol with K₂CO₃ as a reactant is shown in Scheme 1. When CO₂ is introduced into the reaction system, CO₂ can react with OH⁻ to form $\mathrm{H}{\mathrm{CO}}_3^{-}$. The formed $\mathrm{H}{\mathrm{CO}}_3^{-}$ can react with CH₃OH to form CH₃OCOO⁻ (Equation (5)). In other words, both CO₂ and ${\mathrm{CO}}_3^{2-}$ can be transformed to CH₃OCOO⁻, and then DMC is formed after the reaction between CH₃OCOO⁻ and CH₃I. In addition, it is worth noting that the role of K₂CO₃ as a reactant may be a reason for the deactivation of K₂CO₃ during the synthesis of DMC [20], which is detrimental to the stability and reusability of K₂CO₃. As reported by Fujita and co-authors, the amounts of K₂CO₃ recycled after the synthesis of DMC were not high [20]. The effect of reaction parameters, such as temperature, pressure, and concentration, on the DMC yield had been studied by Hou and co-authors [26].

4. Conclusions

In summary, the reaction between K₂CO₃ and CH₃OH was revealed. K₂CO₃ could react with CH₃OH to form carbonate CH₃OCOO⁻, which was an important intermediate and can react with CH₃I to form DMC. The results demonstrated that K₂CO₃ can function as not only a catalyst but also a reactant during the synthesis of DMC from CO₂ and CH₃OH. The insights of this work may be useful to understand the role of K₂CO₃ in the DMC synthesis and offer valuable information for the design of efficient catalysts for synthesizing DMC.