Preparation background of propylene carbonate
Cyclic carbonates include five-membered ring, six-membered ring, seven-membered ring carbonate and macrocyclic carbonate with seven-membered ring or above, among which propylene carbonate and ethylene carbonate are the most widely used. Ethylene carbonate is an excellent polar high-boiling point solvent and surfactant raw material. It is widely used in plastics, printing and dyeing, polymer synthesis, gas separation and electrochemistry. In South Korea, ethylene carbonate is directly used to remove acidic gases (carbon dioxide, hydrogen sulfide, etc.) from natural gas; in Western Europe and Japan, a large amount of ethylene carbonate is used to replace acrylamide, urea systems and water glass as concrete engineering Pollution-free soil stabilizer.
In recent years, foreign countries have developed processes for synthesizing dimethyl carbonate, ethylene glycol, functional polymers, etc. using ethylene carbonate as raw materials, and used them in polymer modification, demonstrating their broad application in the field of organic synthesis. Purpose: It is a potential basic raw material for organic chemicals. Propylene carbonate is an organic solvent and fine chemical synthesis intermediate with excellent performance. It can be used to absorb carbon dioxide and hydrogen sulfide gas in natural gas and synthetic ammonia feed gas.
In recent years, many domestic urea manufacturers have used propylene carbonate as a solvent for urea decarburization, so the demand for propylene carbonate has increased significantly. Propylene carbonate is used as battery electrolyte and can withstand light, heat and chemical changes under harsh conditions. In addition, it is a good solvent for extracting petroleum components, raw paint, plasticizers and other insoluble substances, and is also a raw material for the production of dimethyl carbonate by transesterification.
Preparation and application of propylene carbonate
Propylene carbonate is an organic solvent with excellent performance, high boiling point and high polarity. It is also an important organic chemical and has been widely used in the fields of organic synthesis, gas separation, battery dielectric and metal extraction. It can be used as a plasticizer, a spinning solvent, a dispersant for water-soluble dyes and pigments, an oily solvent, an extractant for olefins and aromatic hydrocarbons, a decarburization solvent in nitrogen fertilizer production, and used in second-generation lithium-ion batteries. As an electrolyte to protect the graphite anode, if there is research and development of an electrolyte with propylene carbonate as the main solvent, an electrolyte with propylene carbonate as the main solvent involves lithium-ion battery electrolyte.
Contains lithium salt, organic solvent and additives; the organic solvent includes propylene carbonate and chain carbonate; the additives include film-forming additive A and additive B that inhibits the co-intercalation of propylene carbonate; the composition by mass percentage is lithium salt 10 %~18%, propylene carbonate is 25%~60%, chain carbonate is 15%~60%, additive A is 0.5%~5%, and additive B is 0.1%~10%. Sulfite is used as an additive to the lithium-ion battery electrolyte. Since sulfite has a high reduction potential, which is higher than the decomposition potential of propylene carbonate, during the first charging process, sulfite decomposes before propylene carbonate. A stable and dense SEI film is formed on the graphite surface, which effectively inhibits the propylene carbonate solvent from being embedded into the graphite layer along with lithium ions, effectively improving the initial discharge capacity and cycle life of the battery.
In addition, propylene carbonate is also an efficient desulfurization and decarbonization (carbon dioxide) solvent and is used in industrial sectors such as natural gas purification, ammonia feed gas purification and hydrogen production. Another new use of it is for wood bonding. For example, it is mixed with isocyanate in a certain proportion and used as a wood bonding agent polycarbonate. In recent years, propylene carbonate and methanol have been used to transesterify dimethyl carbonate to synthesize dimethyl carbonate. Dimethyl carbonate can be prepared from propylene carbonate. Since dimethyl carbonate has a wide range of uses and is in high demand, this also indirectly This greatly broadens the uses of propylene carbonate.
Preparation of propylene carbonate
Preparation of propylene carbonate 1. Synthesis based on 1,2-propanediol
Because the synthesis technology of 1,2-propanediol is relatively mature and the quality and yield of the product are relatively stable, there are many reports on using propylene glycol as the main raw material to synthesize propylene carbonate.
1) Propylene glycol-phosgene method: The earliest industrial preparation of propylene carbonate was the synthesis reaction of 1,2-propylene glycol and photofluorobenzene boric acid gas,
Because phosgene is a highly toxic substance, it causes serious harm to people and the environment; in addition, the production of hydrochloric acid as a by-product not only reduces the atomic economy of the process, but also increases the process investment cost due to the corrosion of equipment by hydrochloric acid. . Therefore, its use is currently prohibited.
2) Propylene glycol-urea method
The route of synthesizing propylene carbonate by reacting urea with 1,2-propanediol has been widely studied in China. When propylene glycol reacts with urea to synthesize propylene carbonate, the first step is to generate amino carbonate, and the second step is deamination and cyclization of the amino carbonate to generate the target product propylene carbonate, accompanied by the production of ammonia as a by-product. The reaction conditions described in the early reported patent for preparing propylene carbonate using urea and propylene glycol were mild, and the yield of the target product was relatively high. The catalyst introduced is organotin, which has certain toxicity.
Switching to a solid base catalyst can reduce the toxicity of the process. In the presence of a solid base, such as zinc oxide, the reaction temperature is 100~200°C, nitrogen is introduced, and after a certain period of time, the yield of propylene carbonate calculated as urea can reach 99%. When using complex oxidationWhen using a catalyst, under reduced pressure conditions and a temperature of 150~160°C, the conversion rate of urea is 95%~98%, and the selectivity of propylene carbonate is 90%~98%. The catalyst can be recycled.
MgO calcined from basic magnesium carbonate is used as a catalyst to synthesize propylene carbonate from urea and propylene glycol. At the reaction temperature of 170°C, after 3 hours of reaction, the PC yield is greater than 90%. Inorganic lead and zinc compounds are used as heterogeneous catalysts. At a reaction temperature of 160°C and a reaction time of 6 h, the yield of propylene carbonate is 98% based on urea; the reaction product and the catalyst are easily separated. Using iron and zinc oxide as the catalyst, at a reaction temperature of 170°C, after 2 hours of reaction, the yield of propylene carbonate was 78%. The main active component of its catalyst is ZnO, and ZnO and ZnFe2O4 work together to promote the improvement of catalytic activity. The cost of propylene carbonate synthesized by the propylene glycol-urea method is relatively low and has certain advantages in process raw materials.
3) Propylene glycol-carbon dioxide method
Carbon dioxide is used in this reaction. Carbon dioxide is a greenhouse gas. Due to human activities, the concentration of carbon dioxide on the earth’s surface has increased. Using carbon dioxide as raw material and fixing it into chemicals is a green synthesis idea, and has been reported in practice. Although the carbon dioxide used in most current studies does not come directly from emissions, the ideas are also considered green. The catalyst used in this method is an alkali metal salt or an alkaline earth metal salt, among which potassium carbonate has high catalytic activity. In a homogeneous catalytic system, the yield of propylene carbonate can reach 12.6%.
In order to overcome the difficulties of product separation and catalyst recycling caused by homogeneous catalytic reactions, potassium carbonate was loaded on activated carbon to perform heterogeneous catalytic reactions. The results showed that the selectivity of the product was improved. The solvent acetonitrile is used in the synthesis process of this method, which reduces the greenness of the process. Organic compounds of tin, such as Bu2SnO or Bu2 Sn(OMe)2, can also catalyze the reaction of 1,2-propanediol and carbon dioxide under supercritical conditions to generate propylene carbonate.
The addition of co-solvent or the presence of dehydrating agent is beneficial to the production of products and the improvement of yield. Water is generated during the reaction between 1,2-propanediol and carbon dioxide, which reduces the atom utilization rate of the reaction process and the product will be hydrolyzed, so the yield of the product will be inhibited by water. This is a big problem to be solved in the process of industrialization of this process.
4) Exchange method of propylene glycol and ester
The preparation of propylene carbonate can be completed by transesterification reaction between 1,2-propanediol and diethyl carbonate or dimethyl carbonate.
Using alkali metal or alkaline earth metal as catalyst, reacting at 144°C and normal pressure for 12 hours, the yield of propylene carbonate was 88%. If dibutyltin dilaurate and a trace amount of strong base are used as the catalyst for the transesterification reaction, xylene reflux is used to control the reaction temperature, and the by-product ethanol is continuously fractionated, the operating steps can be reduced. However, since the raw materials used in this method are relatively expensive and the catalyst organotin is relatively toxic, it is not an ideal green process.
Preparation of propylene carbonate 2. Synthesis based on propylene oxide
The reaction of addition cyclization of propylene oxide and carbon dioxide to produce propylene carbonate is an exothermic and volume shrinking reaction. Therefore, low temperature and high pressure conditions are conducive to the reaction. Since it is an addition reaction, the atom economy of the process can theoretically reach 100, but the actual situation is related to the catalytic system used.
Catalytic systems mainly include homogeneous catalytic systems and heterogeneous catalytic systems. In a homogeneous catalytic system, the complex catalyst can catalyze the reaction of propylene oxide and carbon dioxide to produce propylene carbonate. The disadvantage is that the catalyst concentration used is relatively high and the reaction yield is relatively low. Quaternary ammonium salt, quaternary phosphonium salt and alkali metal salt catalysts have high catalytic activity for the addition reaction of propylene oxide and carbon dioxide, and the conversion rate is relatively high.
A type of homogeneous metal ion complex catalyst, code-named MC-3, catalyzes the reaction of propylene oxide and carbon dioxide at a reaction temperature of 135°C and a pressure of 3 MPa. The yield of propylene carbonate is greater than 94%. In addition, alkali metal salt catalysts can also catalyze the synthesis reaction of propylene carbonate with the help of macrocyclic crown ethers and the like. Due to the relatively strong toxicity of macrocyclic crown ethers, the practical value of this synthetic method is reduced.
