Carbonylation Synthesis Process of TDI - Two-Step Method with Dimethyl Carbonate

Carbonylation of aromatic or aliphatic amines with dimethyl carbonate (or diethyl carbonate) to obtain carbamates, followed by pyrolysis, is also an important non-phosgene method for synthesizing isocyanates. Enichem has developed a new method for producing toluene diisocyanate (TDI).

DMC reacts with toluene diamine (TDA) to prepare the corresponding ethyl N,N'-diaminobenzoate (or N-amidino-N-alkoxy-toluene, abbreviated as TDU). TDU is then thermally cracked to produce TDI and methanol, which is easily recovered and recycled for the production of DMC.

Example: A mixture of 2,4- and 2,6-TDA in an 80:20 ratio, with an excess of DMC (molar ratio 13.2:1) and a zinc acetate catalyst (mole fraction 2.96%), reacts at 160-175°C and 0.25 MPa. The selectivity yield of TDU is 94%, and the conversion rate of TDA is 99%. Excess DMC is removed by distillation. TDU is finally thermally cracked in a tubular reactor at around 450°C to produce TDI (selectivity yield 93.5%), with a TDU conversion rate of 73.5%. In the presence of a catalyst, the thermal decomposition reaction of ethyl N,N'-diaminobenzoate can occur at 200-250°C to produce TDI.

The dimethyl carbonate method is also suitable for the manufacture of other diisocyanate varieties such as MDI.