Carbonylation Synthesis Process of MDI: MDA-BTC Carbonylation Process

Bis(trichloromethyl) carbonate (BTC) is a white crystalline substance with a melting point of 81-83°C. It is miscible with ether, tetrahydrofuran, dichloromethane, chloroform, chlorobenzene, toluene, nitrobenzene, and various alcohols, ketones, and esters. Since it is a solid, it facilitates transportation during the production process and ensures accurate stoichiometry during reactions. More importantly, it possesses high chemical stability (thermal decomposition temperature greater than 300°C) and low vapor pressure, making it a safe, stable, and environmentally friendly green chemical raw material that can replace phosgene for carbonylation reactions.

For example, methylene dianiline (MDA) can react with bis(trichloromethyl) carbonate in the presence of dichloromethane solvent and a basic solution to yield 4,4'-methylene diphenyl diisocyanate (MDI). This method involves mild reaction conditions, making it suitable for synthesis at room temperature and atmospheric pressure in a laboratory setting.

Experimental Procedure:

At room temperature, a three-necked flask containing a dichloromethane solution (300 ml) of 4,4'-methylene dianiline (29.7g, 0.15mol) is equipped with two dropping funnels. One funnel contains a dichloromethane solution (200 ml) of BTC (29.6g, 0.1mol), and the other contains an aqueous solution (200 ml) of NaOH (24g, 0.6mol). Both solutions are slowly added dropwise to the vigorously stirred dichloromethane solution of 4,4'-methylene dianiline. The reaction system generates slight heat. The addition speed of the solutions from both funnels is controlled to maintain the pH of the reaction system between 11 and 12. The reaction flask is immersed in a cold water bath to keep the system at room temperature. After the addition is complete, the reaction continues at room temperature for an additional 2 hours.

After the reaction, the aqueous and dichloromethane layers are separated using a separatory funnel. The aqueous layer is extracted with dichloromethane (2x20ml). The organic layers are combined, dried with anhydrous magnesium sulfate, and most of the solvent is removed. The residue is purified by silica gel column chromatography using dichloromethane/light petroleum ether (7/3) as the eluent to obtain a light yellow product (28.45g, yield 87%, melting point 40-42°C).

MDA can be synthesized by conventional methods from aniline and formaldehyde. BTC is produced from dimethyl carbonate and chlorine under ultraviolet light. The key to the synthesis of MDI from MDA and BTC is pH control to achieve the highest yield.