How is TDI synthesized?

Toluene diisocyanate (abbreviated as TDI) generally exists in two isomeric forms: 2,4-toluene diisocyanate and 2,6-toluene diisocyanate. Depending on the synthesis route or raw material specifications, three different isomeric products can be produced: pure 2,4-toluene diisocyanate, commercially known as TDI-100; a product containing 80% 2,4-toluene diisocyanate and 20% 2,6-toluene diisocyanate, commercially known as TDI-80; and a product containing 65% 2,4-toluene diisocyanate and 35% 2,6-toluene diisocyanate, commercially known as TDI-65.

Toluene diisocyanate is synthesized from toluene diamine and phosgene. Toluene diamine is synthesized by hydrogen reduction of dinitrotoluene, which is obtained from the nitration of toluene.

(1) Synthesis of Dinitrotoluene

There are two methods for synthesizing dinitrotoluene (DNT): the two-step nitration method and the one-step nitration method. The first method is described as follows:

Using toluene as the raw material and a mixed acid composed of 25%-30% nitric acid and 55%-58% sulfuric acid, nitration is carried out at 35-45°C to obtain a mixture of nitrotoluene, containing 35%-40% para-nitrotoluene, 55%-60% ortho-nitrotoluene, and 2%-5% meta-nitrotoluene. The mixture can be separated to obtain three relatively pure nitrotoluene isomers.

For the production of TDI-80, the nitrotoluene mixture is further nitrated with a mixed acid containing 60% nitric acid and 30% sulfuric acid without separation to produce dinitrotoluene with isomer ratios of 80% 2,4-DNT and 20% 2,6-DNT. The crude DNT is then washed sequentially in three washing towers with water (deionized), alkali (ammonia water), and water to remove heavy metal impurities that could affect the activity and lifespan of the hydrogenation catalyst in the reduction stage.

For the production of TDI-65, the ortho-nitrotoluene separated from the mixture is nitrated with a mixed acid containing 60% nitric acid and 30% sulfuric acid at 60-65°C to produce a mixture containing 65% 2,4-DNT and 35% 2,6-DNT.

For the production of TDI-100, the para-nitrotoluene separated from the mixture is nitrated under the same conditions to produce 100% 2,4-DNT.

The one-step nitration method for toluene involves using a mixed acid composed of 64.0% sulfuric acid, 27.2% nitric acid, and 8.8% water. Nitration is carried out at 60-65°C to obtain a dinitrotoluene mixture containing 80% 2,4-DNT and 20% 2,6-DNT.

(2) Reduction Reaction

The DNT is dissolved in methanol and 2% (by mass) Raney nickel (Rancy Ni) catalyst is added. This mixture is continuously fed into the reaction tower under a hydrogen pressure of 15-20 MPa at 100°C. Part of the reaction product is recycled, while the rest is distilled after removing the catalyst to obtain toluene diamine (TDA). The catalyst consumption is below 0.3%. If the TDA contains 2,3- and 3,4-isomers, it directly affects the yield of the phosgenation stage because these isomers form resinous materials during phosgenation, reducing the overall yield of TDI. Therefore, chemical treatment or distillation is required to remove these isomers.

When using a vanadium-carbon catalyst, the reduction can be carried out without a solvent, or using water or alcohol. The hydrogenation reduction conditions are a pressure of 1.0 MPa and a temperature of 10-140°C, which are relatively mild. Using Pd or Pt/C systems as hydrogenation catalysts, the conditions are even milder: a temperature of 100°C and a pressure of 1.0-2.0 MPa, with a catalyst particle size of 40 μm and a usage amount only one-180,000th of the DNT, indicating very high catalytic activity. Depending on the hydrogenation catalyst used, the process conditions vary, with a general trend towards atmospheric pressure, low temperature, and high-efficiency catalysis. Iron powder reduction can also be used to produce toluene diamine.

(3) Phosgenation Reaction

Diamines react with phosgene to synthesize diisocyanate compounds using kettle, tower, pressurized, or one-step high-temperature processes. Phosgene is synthesized from chlorine and carbon monoxide. The solvent used in the phosgenation process depends on the product. Industrially, monochlorobenzene or ortho-dichlorobenzene, which are inert chlorinated aromatic hydrocarbons, are used. Monochlorobenzene has a lower boiling point, resulting in less heat loss during distillation. Ortho-dichlorobenzene has a higher boiling point, leading to higher heat consumption during distillation but allows faster reactions at higher temperatures.

First, molten toluene diamine is dissolved in chlorobenzene and reacts with phosgene at 35-45°C for the low-temperature reaction, followed by a high-temperature reaction below 130°C. After the reaction, hydrogen chloride is purged with nitrogen to obtain the TDI product.

To improve product yield and reduce side reactions, a salting method can be used. This involves reacting the diamine compound with dry HCl gas before phosgenation to form a slurry containing about 75% diamine hydrochloride, which is then phosgenated. This increases the yield of isocyanate to 97%. Phosgene can be used in gaseous or liquid form, with gaseous phosgene being safer and liquid phosgene having higher purity. The amount of phosgene used is 2-3 times the theoretical amount required for TDA. Excess phosgene is beneficial for the reaction but increases the workload of phosgene recovery.

There are strict requirements for the specifications of raw materials used in the production of toluene diisocyanate. Impurities such as sulfides and heavy metals affect catalyst activity, and moisture affects phosgene quality and corrodes equipment, thus further affecting the yield.