Condensation of aniline with formaldehyde under acidic conditions is a fully mature technical route for the production of MDI abroad, the reactants are neutralized with alkali and then distilled to obtain diphenylmethane diamine (MDA). After dissolving MDA with solvent, it is made into polyphenyl polyisocyanate 4.4-MDI 2.4-MDI 2.2-MDI or mixed PMDI by photogasification reaction, and then distilled and refined to get pure MDI. The reaction between free amine and phosgene is the most important method for MDI, and before the reaction, the amine is dissolved in inert solvent, and then excess phosgene of the same solvent is added continuously at a low temperature to form aminocarbonyl chloride and amine hydrochloride slurry, and the aminocarbonyl chloride and amine hydrochloride slurry are formed. The amine is dissolved in an inert solvent and excess phosgene of the same solvent is added continuously at low temperature to form a slurry of carbamoyl chloride and amine hydrochloride. The reaction takes place in about 20% solution, and to minimize side reactions, the phosgene must be in excess of 50% or more. Phosgene is very toxic to human body and has killing power, for this reason, the central control is very strict, moreover, the phosgene method of production investment, phosgene is inconvenient to transport and storage, the resulting hydrogen chloride corrode the equipment seriously, the production requirements are harsh, the operation of the danger is great, the equipment is not easy to overhaul. Today, people are actively developing alternative methods to phosgene.
The U.S. Monsanto Company has published a patent. The use of amine, carbon dioxide and dehydrating agent and other non-photogas method of production of TDI and MDI. the reaction at nearly atmospheric pressure and lower pressure Mr. Cheng into carbamate, and then use phosphorus pentoxide and triethylamine to do dehydrating agent dehydration to generate isocyanate. In terms of the overall reaction, the future direction is to catalyze the carbonyl synthesis of urethane and then pyrolyze it to MDI, and BASF of Germany is reported to have industrial production plants for the urethane method in Belgium and the U.S. The data reported by Asahi shows that the urethane method is being used to produce urethane at a low pressure. Asahi reported data show that the non-photogas method than the photogas method of production costs reduced by 20%.