Introduction

Ethyl methyl carbonate (EMC) is an environmentally friendly, asymmetrical carbonate ester, which simultaneously contains methyl, ethyl, methoxy, ethoxy, and other reactive groups. EMC combines the advantages of both dimethyl carbonate (DMC) and diethyl carbonate (DEC) such as a low melting point, the fine dissolution compatibility with other carbonate, and the excellent dissolution for the various lithium salts. The growing demand for renewable energy sources emphasizes the need for efficient and sustainable battery technologies. EMC has shown promise as an electrolyte as it could provide improved performance and stability for lithium-ion batteries.This underscores the significance of EMC production in meeting global energy demands, leading to further research in this area.

Properties of ethyl methyl carbonate 

Ethyl methyl carbonate has the chemical formula C₄H₈O₃. EMC is a colorless and transparent liquid at room temperature (Figure 1), with a slight odor, insoluble in water, which can be mixed with any proportion of organic solvents such as alcohols, ketones, and esters. Its boiling point is 107.5℃, melting point is -55℃, and flammable density is 1.01 g/cm3. Ethyl methyl carbonate is non-toxic. It can slowly generate carbon dioxide and ethanol when released into the environment.[1]

Application of ethyl methyl carbonate

The global market size of methyl ethyl carbonate in 2022 is approximately 2.1 billion yuan. China, as the world's largest market, holds approximately 90% of the market share.[2] The structure of EMC has the characteristics of low steric hindrance and asymmetry, making it chemically active and suitable for various reactions. It is often used as an intermediate in organic synthesis. While possessing green and environmentally friendly characteristics, methyl ethyl carbonate can also be used as a raw material for the production of high value-added downstream products, widely used in fields such as automobiles, electronics, construction, packaging, and healthcare. Among them, using EMC as an auxiliary solvent to improve the performance of alkaline metal ion batteries in non-aqueous electrolytes is the most important and meaningful application. Ethyl methyl carbonate as an electrolyte has the following advantages:

(1) Low viscosity and excellent fluidity, which is conducive to the migration of lithium ions;

(2) The electrochemical stability has a wide potential range, which can improve the stability of lithium-ion batteries;

(3) Good thermal stability and wide operating temperature range. EMC can effectively improve the safety performance of lithium batteries and extend their service life;

(4) Stable chemical performance, no chemical reaction with battery current collector and active substances;

(5) Safe, green, low toxicity, biodegradable.

These advantages make EMC an electrolyte suitable for almost all applications. Ethyl methyl carbonate has also become one of the most widely used solvents today. Therefore, it has a very promising market outlook, and its annual demand is constantly increasing. However, the electrolyte solutions currently used in China are mostly imported from Germany or Japan, and how to synthesize EMC more economically and safely is the top priority.

Synthesis methods of ethyl methyl carbonate[3]

Generally, the synthesis methods of EMC could be broadly categorized into three types-phosgenation, oxidative carbonylation, and transesterification-as schematically shown in Figure 2. Phosgenation was the earliest industrial route to producing ethyl methyl carbonate, in which COCl₂ alternately reacted with CH₃OH or CH₃CH₂OH as illustrated in Figure 2(a). Although this method has a high conversion rate, its selectivity is poor. The raw material COCl₂ and intermediate products such as ClOCOCH₃ and ClOCOCH₂CH₃ possess high toxicity, posing significant hazards to both the environment and human health. The HCl produced by the reaction also has a certain degree of corrosiveness to production equipment, which contradicts the green production requirements advocated by China. Therefore, this method is gradually being phased out.

Oxidative carbonylation is typically utilized for the production of symmetric carbonates, offering advantages such as a straightforward process, high atom utilization, and environmental friendliness. However, the utilization of oxidative carbonylation for the production of asymmetric carbonates, such as ethyl methyl carbonate, faces numerous challenges and obstacles. For instance, the synthesis of EMC via oxidative carbonylation involves complex reaction equations, where CH₃OH, CH₃CH₂OH, CO, and O₂ serve as reactants and undergo a chemical reaction under high-temperature and high-pressure conditions in the presence of a catalyst; see the plot in Figure 2(b). From the reaction equations, it is evident that the production conditions for ethyl methyl carbonate via oxidative carbonylation are stringent, requiring high temperatures and pressures, which contribute to the elevated cost. Furthermore, side reactions leading to the formation of DMC or DEC complicate the purification process of ethyl methyl carbonate. Additionally, the residual product of H₂O can compromise the quality of ethyl methyl carbonate, as electrolyte solutions based on ethyl methyl carbonate have stringent requirements for water content. Given these challenges, the oxidative carbonylation route for EMC production has not yet been industrialized.

Transesterification is considered to be the most preferred route for obtaining ethyl methyl carbonate due to its mild reaction conditions, ease of control, and low environmental pollution. In theory, there are two transesterification routes to obtain ethyl methyl carbonate, such as DEC reacting with CH₃OH or DMC reacting with CH₃CH₂OH. In fact, the former route is infrequently reported due to its low reactivity. Therefore, the reaction between DMC and CH₃CH₂OH as shown in Figure 2(c) was usually researched, in which the catalyzer in the reaction system is receiving sufficient focus. The catalysts employed in transesterification routes for the synthesis of EMC can be broadly classified into homogeneous and heterogeneous catalysts. Currently, both types of catalysts are in use, each with its own advantages and disadvantages.

References

[1] Yan BM. Research on EMC catalyst synthesized by DMC and ethanol ester exchange [D]. Fujian Normal University, 2023. (MA Thesis)

[2] Wang J. Study on the characteristics and inhibition of vapor explosion of methyl ethyl carbonate under normal pressure [D]. University of Science and Technology of China, 2024. (MA Thesis)

[3]Huang Y, Luo Y, Wang B, Wang H, Zhang L. Crucial Roles of Ethyl Methyl Carbonate in Lithium-Ion and Dual-Ion Batteries: A Review. Langmuir. 2024;40(22):11353-11370.