TDI Carbonylation Synthesis Process - Two-Step Carbon Monoxide Method

In 1977, Japan's Mitsui Toatsu Chemical Co., Ltd. successfully developed a new non-phosgene process for producing TDI. In 1978, in collaboration with American Cyanamid Company, they built a 500-ton/year pilot plant in Oi, Kyushu, Japan to verify the feasibility of this method for industrial production. At that time, there were plans to invest 7-8 billion yen to build a 50,000-ton/year TDI production plant, whereas constructing a plant of the same scale using the phosgene method would require an investment of 20 billion yen. However, due to various factors, the plant construction plan was postponed indefinitely.

The Mitsui Toatsu method uses aromatic nitro compounds and CO as raw materials. In the presence of ethanol, the first step of the reaction produces toluene-2,4-diamino diethyl carbonate [abbreviated as TDC(Et)₂] under high pressure and high temperature. The second step involves the thermal decomposition or catalytic thermal decomposition (which can be referred to as catalytic cracking) of the generated TDC(Et)₂ to produce TDI and ethanol, with the ethanol being recyclable.

The key step is the carbonylation reaction, which uses catalysts similar to those used in one-step carbonylation synthesis, typically noble metal chlorides or their complexes. Japan's Sankyo Toatsu Chemical Co. has published a series of patents for catalysts used in the synthesis of TDC(Et)₂ from aromatic nitro compounds.

To shorten the reaction time and reduce the amount of expensive catalyst used, Mitsui Toatsu Chemical Co. added a small amount of water to the reaction system. However, excess water significantly reduces the yield of TDC(Et)₂. The optimal molar fraction of water to 2,4-DNT is between 10% and 50%, and this amount does not cause corrosion to the reactor. Additionally, the company explored recycling the complex catalyst. About two-thirds of the supported complex catalyst could be recovered, and the amount of FeCl3 and pyridine added during recycling was about 5%-10% of the amount used.

The second step is the thermal decomposition or catalytic cracking of TDC(Et)₂ to produce TDI and ethanol, with ethanol being recyclable.

Thermal Decomposition Method: First, dissolve TDC(Et)₂ in tetrahydrofuran, maintaining its concentration below 0.12g/mL. Then, introduce nitrogen gas and add the solution to high-boiling-point solvent hexadecane. React at 95-100℃ for 1.5 hours, achieving a TDI yield of 95%.

Catalytic Cracking Method: This method is similar to the thermal decomposition method. Use oxides or organic complexes of Mo, V, Fe, and Co as catalysts, with hexadecane as the solvent. Dissolve TDC(Et)₂ in tetrahydrofuran and react in a high-pressure kettle, introducing a certain amount of N2 during the reaction. The addition of tetrahydrofuran and N2 is to prevent TDI and ethanol from re-esterifying into esters in the gas phase. The operation can be carried out under atmospheric or pressurized conditions.

Mitsui Toatsu Chemical Co. uses a reflux cracking method. When using zinc naphthenate as a catalyst, the pressure is 2.7kPa (20mmHg) and the temperature is 250℃, achieving a TDI yield of 91%. With zinc acetate as a catalyst, the TDI yield can reach 94%.

After the decomposition of TDC(Et)₂, the bottom distillate contains 74.9% C10-13 monoalkylbenzene solvent, 0.52% TDI, 2.6% toluene monoisocyanate and mono-carbamate, 1.38% TDC(Et)₂, 2.6% tetrahydrofuran and other by-products. The American Arco Company began researching non-phosgene methods for producing TDI in 1972. Their method involved diluting 50 parts of the bottom distillate with 150 parts of xylene, heating at 290℃ for 2 hours, and recovering all high-boiling-point solvents from the top distillate.