(School of Chemical Engineering, Tianjin University, Tianjin 300072)
Organic isocyanate is an important compound, which is widely used in polyurethane industry, coating industry, dyes and pesticides and other polymer materials.
rhi 3·3H2O 170 2 62.9 100
Taking the preparation of MDI as an example, the first step is to synthesize DPU (diphenyl urea):
The reaction was catalyzed by diethyl palladium. When aniline was excessive, DPU could obtain higher yield. Under the conditions of120 c, 4 1× 103kPa, the yield of DPU can reach over 90% after 4 hours of reaction.
The second step is the decomposition of DPU by methanol.
The reaction can be carried out without a catalyst. The third step is the thermal decomposition of carbamate, which is similar to the above reaction.
The method has wide sources of raw materials, mild reaction conditions, clear route and few side reactions. However, because the process uses diphenylurea as an intermediate, the application scope of this method is limited, and it can not be applied to diisocyanate with complex structure such as TDI.
South Korea's LG Company completed the synthesis of MDI with formaldehyde as an intermediate. The specific reaction route is as follows:
2
The reaction uses solid acid as catalyst, and the reaction product is benzyl carbamate, which can be decomposed into MDI by heating. The production cost of this method is slightly higher than that of phosgene method.
In a word, among various methods for synthesizing isocyanate by non-phosgene method, the simpler the steps, the more complicated the reaction itself and the more difficult it is to realize. Increasing the reaction route and gradually completing the reaction can greatly improve the feasibility of realizing the reaction. DMC method can be applied to the synthesis of various isocyanates with clear process route, few by-products and mild reaction conditions. Secondly, the methanol produced in the three-step reaction can be recovered and used to produce MDC. Although the production cost is slightly higher than that of phosgene method, it is environmentally friendly and has a good industrial prospect. Moreover, China already has the technology of dimethyl carbonate, and it is expected to realize the environmental protection synthesis of some isocyanate products in the near future, and at the same time develop the production routes of different isocyanate products.
Characteristics of Toluene Diisocyanate (TDI) and Its Harm to Human Body
Toluene diisocyanate (TDI) has two isomers: 2,4-toluene diisocyanate and 2,6-toluene diisocyanate. Toluene diisocyanate (TDI) is a water-white or light yellow liquid with a strong pungent smell, which accumulates and lurks in human body and has a strong irritating effect on skin, eyes and respiratory tract. Inhalation of high concentration toluene diisocyanate vapor can cause bronchitis, bronchopneumonia and pulmonary edema. Liquid contact with skin can cause dermatitis. Liquid contact with eyes can cause serious irritation, which may lead to permanent damage if it is not treated in time. Long-term exposure to toluene diisocyanate can cause chronic bronchitis. People who are allergic to toluene diisocyanate may cause asthma, wheezing, dyspnea and cough.