October 10, 2011
Look, Ma—I can make isocyanates without phosgene. Phosgene-free routes to isocyanates have long been a goal of industry. Even though phosgene is extremely toxic, it is a key raw material for making two large-volume materials—polycarbonates and isocyanates. Whereas the chemical industry has developed polycarbonate processes that avoid phosgene, it has not been able to devise a high-yield phosgene-free route to isocyanates. M. Kloetzer, E. Stroefer, and H.-J. Blankertz disclose a method for making aliphatic isocyanates without the need for phosgene.
The invention covers the initial reaction of polyamines with urea to make polyureas, which are then thermally decomposed to the corresponding polyisocyanates. The patent describes the use of this route to make 1,6-hexamethylene diisocyanate (HDI) and isophorone diisocyanate.
To prepare HDI, 1,6-hexamethylenediamine (0.9 kg/h), toluene (8 kg/h), and molten urea (6 kg/h) are pumped to a mixing nozzle and heated to 130 °C. The hot stream is directed through an 8-m-long tubular reactor in the temperature range 170–220 °C under a pressure of 13 bar. Almost 100% of the amine is converted, and the yield of 1,6-hexamethylenediurea is ≈98%.
The diurea is separated from the toluene and fed into a fluidized-bed reactor at 380 °C. Sand is used as the fluidizing medium. The vapors leaving the reactor are flash-cooled with toluene to separate the product HDI from the byproduct ammonia. The yield of HDI is 89%. (BASF AG [Ludwigshafen, Germany]. US Patent 8,026,387, Sept. 27, 2011; Jeffrey S. Plotkin)