Urea production process: propylene carbonate/dimethyl carbonate route

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Author: Unknown. Journal: Unknown. Date: Unknown. BTW: This article was reproduced from the website “Chemical Engineers”; I thought it was good, so I shared it. It may not fully comply with the publication guidelines, so please understand! This process is carried out in two steps: urea reacts with propylene glycol to produce propylene carbonate, which is then reacted with methanol to produce dimethyl carbonate. The ester exchange of propylene carbonate with methanol to produce dimethyl carbonate is a relatively mature process, in which propylene carbonate is typically synthesized through the cycloaddition of carbon dioxide and propylene oxide. Catalysts include non-metallic catalysts such as ammonium salts, amines, and phosphines, as well as metallic catalysts such as nickel, zinc, and aluminum. The catalyst has low activity, so the reaction must take place at higher temperatures and pressures. In recent years, ionic liquid catalysts have been proposed, but they have low catalytic activity and their preparation is complicated. This reaction requires the addition of various organic solvents as a catalyst system, and the reaction conditions are stringent (reaction temperature of 170 – 210°C, pressure of 6.0 – 12.0 MPa). The one-pass conversion rate of propylene oxide is below 80%, and the yield of the product is low (typically 83%–92%). During the reaction, the catalyst, organic solvents, and product mix together; therefore, a complex separation process is necessary to obtain pure propylene carbonate. Moreover, the equipment required for this process is expensive, resulting in high production costs. The key to this route is the catalyst for the first step of the reaction between urea and propylene glycol (or ethylene glycol), as well as the optimization of the two-step process. According to the patents currently available on the preparation of acrylonitrile carbonate using urea and propylene glycol, it is believed that this reaction can take place with or without a catalyst, and with or without a solvent; it can also be carried out under pressure or reduced pressure, under mild reaction conditions ； Polar aprotic solvents are recommended, with dimethylformamide and diethylformamide being the preferred choices. Under catalyst-free conditions, with 60 g of urea and 102 g of 1,2-propanediol used, the reaction was carried out at 170°C for 3 hours; the selectivity for propylene carbonate was 84%, while the conversion rate of 1,2-propanediol was 43%. Organotin compounds, such as dibutyltin dilaurate, are considered to be effective catalysts for this reaction. Existing patents disclose that propylene glycol or ethylene glycol is added to the reactor in a molar ratio of 100–1 to urea, while a solid alkali catalyst is added in a molar ratio of 0.001–10 to urea. The reaction is carried out at a temperature of 100–200 °C, under a vacuum pressure of 6.67–70 KPa or in an atmosphere of nitrogen, with a reaction time of 0.5–20 hours. The reaction activity of various catalysts was investigated. Using Ca(OH)2 as a catalyst, the conversion rate of propylene glycol was 49.67%, while the yield of propylene carbonate, expressed in terms of urea, was 99.25%. With zinc oxide as the catalyst, at a reaction temperature of 105°C, a nitrogen flow rate of 10 ml/min, and a reaction time of 20 hours, the conversion rate of propylene glycol was 65.28%, and the yield of propylene carbonate, expressed in terms of urea, was 99.55%. During the reaction between urea and propylene glycol, a white solid may precipitate on the condenser. When heated, this substance releases a pungent ammonia smell; it is likely that under the reaction conditions, carbon dioxide produced by the decomposition of a small amount of urea combines with the ammonia generated in the reaction to form ammonium carbonate. Improvements have been made to the commonly used reaction apparatus to prevent this phenomenon from occurring. The formation of ammonium carbonate increases the difficulty and cost of ammonia recovery, which is unfavorable to the technical economics of this route. The key to solving this problem is to reduce the decomposition of urea under reaction conditions; a certain catalyst can also be added. There is extensive research on the reaction of ethylene carbonate (or propylene carbonate) with methanol to produce dimethyl carbonate; the catalysts used are mainly substances with weak acidity, such as sodium methoxide, anion exchange resins, and various metal chlorides. The reaction temperature and pressure are generally maintained at 150–200°C and 1–2 Mpa, respectively. The transesterification reaction is reversible, and reactive distillation can be employed to enhance the reaction process. Jiang Qi et al. studied various inorganic basic catalysts, including IIA-group metal oxides, main-group metal hydroxides, and IA-group metal carbonates. The activity of the catalyst is closely related to its basicity; the stronger the basicity, the higher the catalyst’s activity. In carbonate catalysts, the introduction of crystalline water causes a sharp decline in the catalyst’s activity. This is because propylene carbonate is prone to hydrolysis in alkaline environments, and the introduction of crystalline water accelerates this hydrolysis reaction, thereby affecting the yield of dimethyl carbonate. Wei Tong et al. studied the effect of the basicity of the catalyst on the reaction of propylene carbonate with methanol to produce dimethyl carbonate, concluding that the stronger the basicity of the catalyst, the higher its activity, whereas the selectivity for dimethyl carbonate decreases. It is generally believed that the role of basic catalysts is to activate methanol by adsorbing H+, thereby producing MeOd-. The greater the negative charge carried by MeOd-, the easier it is for MeOd- to attack the carbonyl group in propylene carbonate, resulting in the removal of a molecule of propylene glycol and the formation of dimethyl carbonate.