Carbonylation Synthesis Process of MDI – Carbonylation Process with Carbon Dioxide and Other Methods

Carbonylation Process with Carbon Dioxide

Monsanto Company has published patents for producing TDI and MDI using non-phosgene methods involving amines, carbon dioxide, and dehydrating agents. In this process, carbamates are first formed under near atmospheric and low pressure conditions, which are then dehydrated using phosphorus pentoxide and triethylamine to produce isocyanates.

According to recent reports, the Lanzhou Institute of Chemical Physics of the Chinese Academy of Sciences, Jisu Polyurethane Engineering Technology Center, and Gansu Yinguang Chemical Industry Company have successfully developed a non-phosgene method for producing 1,6-hexamethylene diisocyanate (HDI) and passed small-scale tests. This process involves reacting 1,6-hexamethylene diamine with carbon dioxide to form the corresponding hexamethylene dicarbamate, which is then reacted with suitable dehydrating agents such as phosphorus oxychloride to produce the corresponding isocyanate. This non-phosgene HDI production process achieves high yields and purity. Compared to the current phosgene-based process, this method avoids the hazards of using highly toxic phosgene. By directly using carbon dioxide as the carbonylating agent, it is cost-effective and easy to operate. The main quality indicators of the product meet the standards of similar foreign products.

Other Methods

There are many methods for synthesizing isocyanates, some of which are still used today for the synthesis of mono-isocyanates in fields such as pesticides. Methods such as olefin-isocyanate synthesis, halide-isocyanate synthesis, and chloride-cyanate synthesis are now rarely seen in the synthesis of organic isocyanates with functionalities of two or more and will not be discussed here.

One method involves synthesizing isocyanates using amines and chloroformates as raw materials. The reaction occurs at 40-300°C and 0.02-20 MPa in a solvent, in the presence of a catalyst (activated carbon impregnated with 0.1%-0.5% metal chlorides and inorganic bases or tertiary amines). This method can also produce diisocyanates, including TDI and HDI, with yields reaching 94%-95%.