Method for producing mdi, especially 2,4'-mdi

摘要

MDI is prepared by a process comprising the following steps: A) reaction of aniline and formaldehyde in the presence of an acid to give methylene(diphenyldiamines) and polymethylenepolyphenylenepolyamines, the molar ratio of acid to amine being not more than 0.2, B) reaction of the mixture obtained in step A) with phosgene to give methylene(diphenyl diisocyanates) and polymethylenepolyphenylene polyisocyanates, C) separation of the mixture obtained in step B) into monomeric MDI and polymeric MDI and D) separation of the monomeric MDI obtained in step C) by isolating 2,4'-MDI.