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With the development of C1 chemistry, technologies for producing methanol and dimethyl ether (DME) from coal gas, natural gas, and other sources have received increasing attention. These technologies take advantage of China’s abundant coal and natural gas resources, helping to compensate for the shortage of oil resources in the country and addressing the global challenge of balancing energy use with environmental pollution. As DME production technology becomes increasingly mature, DME can serve as a suitable product for the restructuring of the small-scale fertilizer industry; therefore, it is essential to develop downstream products for DME in order to accommodate its potential large-scale production. Furthermore, dimethyl ether is also an important chemical intermediate for fine chemical products. Therefore, the production and application development of DME are of great significance for the development of China’s energy chemicals, fine chemicals, and environmental protection. 1 Properties of dimethyl ether Dimethyl ether, also known as wood ether or methyl ether, is abbreviated as DME; its molecular formula is CH3OCH3. Dimethyl ether (DME) has physical properties very similar to those of liquefied petroleum gas (LPG). It is a colorless gas with a slight ether-like odor; it is non-corrosive and non-carcinogenic. Its vapor pressure at room temperature is approximately 0.5 MPa. It can be liquefied by cooling it to –25°C under normal pressure, or by increasing the pressure to 0.5–0.6 MPa at room temperature. Its boiling point is –24°C, and its freezing point is –140°C. 3,700 mL of dimethyl ether gas can dissolve in 100 mL of water. Dimethyl ether is also readily soluble in various organic solvents such as gasoline, carbon tetrachloride, propane, chlorobenzene, and methyl acetate. Dimethyl ether is difficult to activate at room temperature, but prolonged storage or exposure to direct sunlight can lead to the formation of unstable peroxides, which can explode spontaneously or upon heating. 2 Methods for the preparation of dimethyl ether 2.1 One-step synthesis of DME from syngas The one-step method involves using syngas as a raw material, and directly reacting it over a bifunctional catalyst that facilitates both methanol synthesis and methanol dehydration to produce DME. This method has advantages such as readily available raw materials, a short process flow, small equipment scale, low energy consumption, a high one-pass conversion rate, and immunity to methanol price fluctuations. The companies currently actively developing the one-step syngas process are primarily Danish Topsφe, American Air Products and Chemicals, and Japanese NKK. Although the development is still in the pilot stage, industrial-scale plants will be built in the near future. China’s Shanxi Coal Chemistry Research Institute, Dalian Institute of Chemical Physics, Tsinghua University, East China University of Science and Technology, Zhejiang University, and Southwest Chemical Engineering Design Institute are all conducting research on the one-step synthesis of syngas. Synthetic catalysts have been successfully developed, and industrial-scale tests have been carried out. 2.2 Synthesis of DME from syngas via a two-step process The so-called two-step process involves first synthesizing methanol from syngas, and then producing DME by dehydrating methanol in the presence of a solid acid catalyst. Previously, sulfuric acid was mainly used as a catalyst; nowadays, ZSM-5 molecular sieves made from γ—Al2O3/SiO2 are predominantly used as catalysts. They offer excellent performance and selectivity, enabling the production of high-purity dimethyl ether. However, the one-pass conversion rate in the methanol synthesis process is low, the operating pressure is high, energy consumption is substantial, and costs are high; therefore, this method is rarely used. 2.3 Direct synthesis of DME via CO2 hydrogenation: This method, as a new approach to synthesizing DME, is still in the exploratory stage. CO2 is the most abundant carbon resource on Earth, and the greenhouse effect caused by it has inflicted significant damage on the ecological balance of humanity. Therefore, synthesizing various chemicals from CO2 as a raw material to achieve the recycling of CO2 has attracted the interest of researchers around the world. The production of methanol from CO2 hydrogenation is limited by thermodynamic equilibrium, which has led to interest in the use of CO2 hydrogenation for the production of DME. This can break the thermodynamic equilibrium of CO2 hydrogenation to methanol, thereby increasing the conversion rate of CO2. Currently, scholars at home and abroad have been working on this topic and have achieved certain results. 3 Applications of dimethyl ether Dimethyl ether is a chemical product with a wide range of uses. Its main applications are: (1) as a fuel. Due to its oxygen content, dimethyl ether burns completely without producing carbon deposits or residual substances, and its combustion emissions meet all health standards; hence, it is an ideal clean fuel. At the same time, dimethyl ether can also be blended into liquefied gas, city gas, natural gas, and alcohols to improve the performance of these fuels. The calorific value of dimethyl ether is approximately 64,686 kJ/m3. At low pressures, dimethyl ether is in liquid form, and its properties are similar to those of liquefied gas; it can thus replace liquefied petroleum gas as a clean fuel for domestic use. Furthermore, dimethyl ether can be used directly as a vehicle fuel after being liquefied, which can to some extent overcome the drawbacks of methanol, such as poor starting performance and acceleration at low temperatures. When used as a vehicle fuel, dimethyl ether enables exhaust emissions to meet the Euro III standards; when used as a substitute for diesel, its cetane number is 10% higher than that of diesel, resulting in greater engine power and improved performance. (2) Synthesis of low-carbon olefins. Ethylene is one of the most fundamental and important raw materials in the petrochemical industry. To address the issue of its source, research is being actively carried out both domestically and internationally on the production of ethylene, as well as mixed olefins consisting of ethylene, propylene, and butylene, from methanol or dimethyl ether. Our country does not have abundant oil resources, and its petrochemical industry developed relatively late; as a result, ethylene and propylene products still need to be imported to some extent. Therefore, great emphasis is placed on producing ethylene and propylene from coal through methanol and dimethyl ether. At present, this research topic in China is still in the pilot stage. (3) Replacing chlorofluorocarbons as aerosols, refrigerants, and blowing agents: Aerosol sprays primarily use chlorofluorocarbons, and these substances cause severe damage to the atmospheric ozone layer. With the growing environmental awareness around the world, it has become apparent that the production and use of chlorofluorocarbons must be completely halted. Studies have shown that dimethyl ether, as a substitute for chlorofluorocarbons, possesses properties such as non-corrosivity, non-toxicity, and good solubility in water and alcohols. It exhibits excellent performance in aerosol products and is considered an ideal propellant for next-generation aerosols. Furthermore, the easy liquefaction property of dimethyl ether has also attracted the attention of researchers; many studies are being conducted to use dimethyl ether as a substitute for chlorofluorocarbons as a refrigerant, taking advantage of its low pollution level and excellent cooling performance. Dimethyl ether, used as a blowing agent, can make the pore size of products such as foam plastics uniform, while enhancing their flexibility and pressure resistance. http://www.nmtech.com.cn/jishuwang/upload1/0801251548053208.jpg4 Development of DME-F products: China is rich in coal resources, and the conversion of coal into syngas followed by the production of DME has become a topic of interest. As an important chemical raw material, DME can be used to synthesize various chemicals and participate in the synthesis of many other chemicals, as shown in Figure 1 below. Therefore, the development of downstream products for DME is also receiving increasing attention. 4.1 Synthesis of dimethyl sulfate via reaction with SO3 Dimethyl sulfate is an important alkylating agent; common reactions involving it include O-methylation and N-methylation. It is widely used in the field of organic synthesis in industries such as pharmaceuticals, pesticides, fragrances, and dyes. In addition, dimethyl sulfate can also be used as a solvent. The process used in our country to produce dimethyl sulfate involves methanol and sulfuric acid as raw materials, with DME appearing as an intermediate product in the production process. This process not only has a long sequence of steps but also suffers from equipment corrosion issues. In particular, the intermediate methyl bisulfate is more toxic than dimethyl sulfate, resulting in very harsh production conditions. At present, most manufacturers in China still use this process to produce dimethyl sulfate. In the 1970s, Sadilak proposed a method for preparing dimethyl sulfate by reacting DME and SO3. This method avoids the formation of the highly toxic substance methyl hydrogen sulfate; it operates under mild reaction conditions with minimal pollution, and the product selectivity can reach 94%–96%, making it one of the most practical methods for the synthesis of dimethyl sulfate. Compared with other synthesis methods, the DME method features a single reaction product and good selectivity. At present, Wuhan Qingjiang Chemical Co., Ltd. in China produces dimethyl sulfate by first producing dimethyl ether through the etherification of methanol, and then synthesizing it with sulfur trioxide in a methyl ester mother liquor; the pure product is obtained through vacuum distillation, with a purity of over 98.5%. 4.2 Synthesis of alkyl halides Alkyl halides can be synthesized using DME. The Japanese company Soda uses γ—Al2O3 as a catalyst to produce CH3Cl by reacting DME with HCI. Additionally, Germany has also reported that using solid ZnCl2 as a catalyst and a reaction temperature of 129–350°C, DME can react with HCl to produce CH3Cl in high yield and high selectivity. Some also use the reaction of DME with HF to synthesize CH3F. Among alkyl halides, CH3Cl is the most widely used chemical raw material. It is an important methylating agent in organic synthesis, and can be used as a solvent for rubber, resins, and organic compounds, as well as a raw material for polychloromethane. 4.3 Synthesis of N,N-Di**amine N,N-Di**amine is one of the basic raw materials used in the production of basic dyes and alkaline dyes. The reaction of DME with aniline can yield N,N-diamine. Compared to other methods for producing N,N-diamine, such as using aniline and methanol in the presence of a β-zeolite catalyst, the DME-based production method features fewer side reactions, lower reaction temperatures, and higher product selectivity. Therefore, the dimethyl ether method will become one of the most valuable and promising production methods for amine alkylation. 4.4 Reaction with CO: DME reacts with CO via carbonylation to produce methyl acetate and acetic anhydride, which are converted into acetic acid upon hydrolysis. Their reaction equations are as follows: CH3OCH3 + CO → CH3COOCH3 (1) CH3COOCH3 + H2O → CH3COOH + CH3OH (2) CH3OCH3 + 2CO → (CH3CO)2O (3) In these reactions, rhodium carbonyl-iodide is commonly used as a catalyst. Although rhodium catalysts have high activity, their high cost limits their development to some extent. As can be seen from Equation (7), the synthesis of acetic anhydride from DME is an atom-economical reaction, as no water is generated during the process, making it more advantageous than the direct synthesis of acetic anhydride from methanol. Acetic acid and acetic anhydride are important basic organic chemical raw materials that are widely used in the pharmaceutical industry, synthetic fibers, light industry, textiles, leather processing, pesticides, **, rubber and metal processing, food production, and the synthesis of fine organic chemicals. Currently, the demand for acetic acid and acetic anhydride in our country is on the rise. Therefore, there are good prospects for vigorously developing its synthetic products. 4.5 Synthesis of dimethyl carbonate Dimethyl carbonate is an important raw material and intermediate in organic synthesis, and is regarded as a green chemical product of the 21st century. It contains carbonyl groups, —COOCH3 groups, and —CH3 groups, and possesses high reactivity; it is an excellent solvent that can also be used in the synthesis of polycarbonates, as well as as intermediates in the pharmaceutical and pesticide industries. It is insoluble in water, can be mixed with alcohols and esters, is non-corrosive, is flammable, and has very broad market prospects. Currently, the synthesis of dimethyl carbonate mostly employs the methanol carbonylation method, using halides of copper and palladium as catalysts. The formation of water in the reaction reduces the equilibrium yield, causes severe corrosion to the equipment, and makes product separation difficult. Many researchers are seeking various solutions. For example, dimethyl carbonate is produced by oxidative carbonylation using DME as a raw material. Its reaction equation is as follows: CH3OCH3 + CO + 1/2 O2 → (CH3O)2CO (4) 4.6 Synthesis of ethylene Ethylene, as one of the most fundamental and important chemical raw materials, is an important indicator for measuring the production capacity of an industry. Currently, many researchers are actively working on the synthesis of ethylene from methanol and dimethyl ether. Our country is short of oil; therefore, how to produce ethylene and propylene from syngas has increasingly become a focus of attention in our country. The Dalian Institute of Chemical Physics, Chinese Academy of Sciences, has achieved good results in the research on producing ethylene from syngas via dimethyl ether. Its reaction equation is: CH3OCH3=CH2CH2 + H2O (5) 4.7 Synthesis of dimethyl sulfide Dimethyl sulfide is a raw material for the production of dimethyl sulfoxide; it is also used as an intermediate in the manufacture of pesticides and dyes, as well as a flavoring agent in sauces. Additionally, it serves as a solvent in organic synthesis, polymerization reactions, and cyanation reactions, and as a spinning solvent for various synthetic fibers. It can also be used as an industrial cleaning agent, a coating stripping agent, a solvent for hydraulic oil, an anti-corrosive agent for batteries at low temperatures, and a drug penetration agent. It can be synthesized from DME and H2S, or prepared by reacting DME with CS2. Both reactions use γ—Al2O3 as a catalyst. Both methods achieve high selectivity in the production of dimethyl sulfide, with the product purity reaching over 90%. 4.8 Other reactions: As a coupling agent, DME can react with silicon at high temperatures in sealed systems to form organosilicon compounds. Aluminum nitride-alumina-silica ceramic materials with high strength and corrosion resistance are obtained by reacting with soluble silanes, aluminum trichloride, and ammonia. DME reacts with O2 and NH3 to produce hydrocyanic acid; when mixed with air, it reacts to form formaldehyde and other compounds. It also reacts with ethylene oxide to yield a mixture of dimethyl ethylene glycol, dimethyl diethylene glycol, dimethyl triethylene glycol, and dimethyl tetraethylene glycol. Under the action of the solid catalyst SnO/MgO, DME can be oxidized and dimerized to produce dimethoxyethane. It reacts with ammonia on modified acidic molecular sieves to produce dimethylamine. It reacts with P2O5 to produce alkyl polyphosphates. 5 Conclusion DME is readily available and has a wide range of applications; it holds great potential as a fuel and for the production of olefins, and has thus become a focus of research in the fields of energy, environment, and chemical engineering. China is rich in coal resources, and natural gas chemistry already has a certain foundation in some areas; there is an ample supply of raw materials for DME production. This advantage can be utilized to vigorously develop DME and its downstream products. From a long-term perspective, one-step production of DME has high economic value. Direct production of DME from CO2 hydrogenation holds great theoretical and strategic significance, as it allows for the full utilization of CO2 as a carbon resource while also addressing its pollution issues. Vigorously promoting and utilizing DME, this new type of clean energy, not only helps to optimize China’s energy structure and reduce its dependence on imported oil, a strategic resource, but also holds great significance for the coordinated development of resource utilization and environmental protection in the country.