PEP Review 94-1-2
Lucky Nonphosgenation Process: MDI and PMPPI by Carbonylation
Over the past 25 years, SRI has evaluated many alternative phosgenation and nonphosgenation-based process configurations for the manufacture of methylene diphenylene isocyanate (MDI) and the polymethylene polyphenyl isocyanate (PMPPI) polymer homologs. In these configurations pure MDI is distilled from the polyisocyanate polymer mixture that forms after amine intermediate phosgenation, and both coproducts are sold as reaction intermediates for making commercial polyurethane products. The conventional phosgenation-based process appears to be relatively simple. However, there are several inherent disadvantages, the most disagreeable of which is the need to use phosgene as a reactant.
Lucky Limited, a major Korean chemical manufacturer, has been developing an alternative nonphosgenation process on a mini-pilot-plant scale, based on the reductive carbonylation of nitrobenzene in the presence of aniline to form N,N'-diphenyl urea (DPU), which is followed by methanolysis to form methyl phenyl carbamate (MPC). The unreacted aniline is separated from the reaction product and recycled to the carbonylation step. MPC is then condensed with formaldehyde to form methylene diphenyl diurethane (MDU) products. Pure MDU and polymeric MDU are separated by filtration. They are then decomposed in separate parallel operations to form pure MDI solids and polymeric MDI (PMPPI) liquid. By-product methanol recovered from both thermal decomposition units is collected and recycled to methanolysis.