Method for manufacture of diphenylmethane diisocyanates

摘要

Manufacture of a diphenylmethane diisocyanate for an N-phenylcarbamate is economically accomplished in high yield with high selectivity by a method which comprises (A) a process of methylenation for the formation of a condensation product containing at lest 80 mol % of a dinuclear diphenylmethane dicarbamate by the steps of (1) causing a methylenating agent to react upon at least 2 moles of an N-phenylcarbamate, based on 1 mole of the methylene group of said methylenating agent, in a liquid phase in the presence of an aqueous inorganic acid solution, (2) separating the resultant reaction mixture into the aqueous inorganic acid solution and an organic phase reaction mixture containing substantially none of said inorganic acid, and (3) subsequently treating said organic phase reaction mixture in the presence of an N-phenylcarbamate and a carboxylic acid having a pKa of not more than 4 in an aqueous solution at a temperature of 25 DEG C. or a solid acid or an acid consisting of said two acids thereby converting a reaction intermediate possessing a methylene-amino bond and contained in said organic phase reaction mixture to a diphenylmethane dicarbamate, and (B) a process of thermal decomposition described herein.