Process for preparing methylene dianilines

Abstract

An improvement is described in the process of preparing polymethylene polyphenyl polyamines by condensing aniline and formaldehyde in the presence of an acid catalyst in the initial condensation. The improvement enables the amount of acid which has to be neutralized at the end of the reaction to be substantially reduced or eliminated entirely. This is accomplished without sacrifice of the high level of 4,4'-isomer of di(aminophenyl)methane normally present in the diamine content of the polyamines when a mineral acid catalyst is employed in the condensation. The improvement comprises conducting the initial condensation of aniline and formaldehyde in the presence of a strong acid at a level normally employed to obtain high 4,4'-isomer content in the diamine component of the polyamine product, subjecting the reaction mixture, at the stage at which benzylamine formation is substantially complete, to solvent extraction after adjusting the aniline and water content of the reaction mixture to specified levels, recovering the mixture of benzylamine and excess aniline from the solvent extract and subjecting this mixture (optionally after removal of aniline) to rearrangement to the desired polyamine product in the presence of a catalyst which can be a strong acid (at a level substantially below that used in the initial condensation), or a solid catalyst (diatomaceous earth, clay, zeolite). The aqueous layer remaining after the solvent extraction contains aniline hydrochloride and a minor amount of benzylamine hydrochloride and is recovered and used as part of the reactants employed in a subsequent condensation.