Method for converting monoisocyanates to ureas

An MDI and/or polymeric MDI manufacturing process, comprising the steps of:

• a) reacting aniline with formaldehyde to produce a mixture of methylene dianiline (MDA), one or more polymethylene polyanilines having at least three aniline groups (PMDA) and unreacted aniline in a solvent;
• b) distilling aniline from the mixture produced in step a) to produce a process stream containing the MDA, PMDA and residual aniline;
• c) phosgenating the process stream from step b) in a non-polar solvent to form an isocyanate process stream containing the non-polar solvent, MDI, one or more polymethylene polyphenylisocyanates that have at least three phenyl isocyanate groups (PMDI) and phenyl isocyanate;
• d) separating MDI and PMDI from the isocyanate process stream obtained in step c) by distillation to produce a solvent stream containing the non-polar solvent, 0.2 to 10 weight percent phenyl isocyanate based on the weight of the solvent stream and up to 5 weight percent, based on the weight of the solvent stream, of MDI and/or PMDI;
• e) reacting the solvent stream obtained in step d) in the presence of at least 0.025 wt.-% of an organometallic catalyst, based on the weight of the phenyl isocyanate, wherein the organometallic catalyst is selected from the group consisting of bis(cyclopentadienyl) Cr(II), bis(methylcyclopentadienyl) Cr(II) and compounds having at least one metal ion bonded to at least one organic ligand and the metal ion is selected from one or more of Co(II), Mg(II), Y(III), Cr(III) and a lanthanide series metal ion in the 3+ oxidation state, to convert at least a portion of the phenyl isocyanate to 1,3-diphenylurea and optionally aniline, and optionally to thermally deactivate the organometallic catalyst;
• and
• f) recycling 1,3-diphenylurea and optionally non-polar solvent and optionally residues from the thermal deactivation of the catalyst, from step e) directly or indirectly into step d), whereby at least a portion of the 1,3-diphenylurea reacts with at least a portion of the MDI and/or PMDI to form biuret compounds.