Patent ID: 7495124

Claim:
A process for the production of a fraction of diisocyanates of the diphenylmethane series which contains at least 99 wt. % of 2-nuclear methylenediphenyl diisocyanate (based on the total weight of the fraction), comprising a) reacting aniline and formaldehyde in the presence of an acid catalyst to form diamines and polyamines of the diphenylmethane series containing 2-nuclear methylenediphenyldiamine, b) phosgenating the 2-nuclear methylenediphenyldiamine containing diamines and polyamines of the diphenylmethane series, optionally in the presence of a solvent, to yield a crude diisocyanate and polyisocyanate, c) separating from the crude diisocyanate and polyisocyanate a fraction which containing at least 95 wt. % of 2-nuclear methylenediphenyl diisocyanate having a content of 4,4′-MDI of from 49 to 95.99 wt. %, a content of 2,4′-MDI of from 4 to 45 wt. % and a content of 2,2′-MDI of from 0.01 to 20 wt. %, based on the total weight of the fraction, d) optionally, removing from 10 to 98% of 4,4′-MDI from the fraction obtained in step c), e) separating from 50 to 99.9999% of the 2,2′-MDI from the fraction obtained in step c), thereby forming a fraction containing from 0 to 0.4 wt. % of 2,2′-MDI, from 1 to 95 wt. % of 4,4′-MDI and from 5 to 98.6 wt. % of 2,4′-MDI, based on the total weight of the MDI isomers.