Carbonylation Process of TDI - One-step Method

The synthesis of toluene diisocyanate (TDI) from dinitrotoluene (DNT) and carbon monoxide (CO) can be carried out by either a one-step or a two-step process. This article first introduces the one-step method.

During the 1970s and 1980s, countries such as the United States, Japan, the former Federal Republic of Germany, and Belgium conducted research on the one-step method. The synthesis reaction of TDI from DNT and CO often uses a continuous multi-tank reactor. The reaction generally occurs in the liquid phase. The key to this method is selecting an appropriate catalyst. Catalysts for the reaction include chlorides or cyanide complexes of precious metals such as Pd, Rh, Pt, Ru, Ir, Re, Cs, Ag, and Au. Commonly used are complexes of halogenated metals like palladium chloride, germanium chloride, vanadium chloride, and rhodium chloride. The solvent is o-dichlorobenzene, with a pressure of around 28 MPa, a temperature of 190°C, and a reaction time of 4-5 hours. The conversion rate of DNT is 98%, and the yield of TDI is 46%, with some by-products such as polyisocyanates, 2-nitro-4-isocyanatomethyl toluene, and 4-nitro-2-isocyanatomethyl toluene.

Furthermore, to improve the conversion rate of DNT and the yield of TDI, co-catalysts such as boric acid, pyridine, formic acid, small amounts of water, carbamates, and acyl fluorides can be added.

In addition to the liquid-phase reaction mentioned above, there are reports of gas-phase reactions. For example, silver supported on pumice or manganese supported on alumina can catalyze the reaction of CO and DNT at 195°C, resulting in a TDI yield of 42%. Another example is the use of a PdO-MoO3-Fe2O3 (2-6/1/1) catalyst supported on γ-Al2O3, reacting at 210°C and 30 MPa, with a TDI yield of 32-35%.

An example of a synthesis is as follows: DNT (10%), catalyst (palladium chloride isoquinoline complex, 4% by mass), and co-catalyst molybdenum trichloride (1%) are added to a three-tank continuous reactor with o-dichlorobenzene solvent. CO is introduced, and the reaction proceeds at 200°C and 20 MPa for 1 hour, resulting in a TDI yield of 75.2%, a conversion rate of 99.1%, with the products including 4.7% mono-isocyanates besides TDI.