(502c) Non-Phosgene Low Temperature Route for Manufacture of Isocyanates
Isocyanates are commonly used as monomers to make polymers, such as polyurethanes which find significant application in the manufacture of rigid and flexible foams. Polyurethanes are also used in the production of adhesives, elastomers, and coatings. Isocyanates are currently manufactured by the phosgenation route which involves reacting an amine with phosgene. This process requires handling of hazardous chemicals and is also environmentally malignant due to issues related to disposal of the hydrochloric acid, formed as a byproduct.
McGhee et al have previously worked on a route to manufacture isocyanates from primary amine, carbon dioxide, a nitrogenous base and an electrophilic dehydrating agent. However, due to the generation of huge amount of salt of the dehydrating agent and the base and due to the absence of an appropriate recycling strategy this route could not be commercialized.
We present a modification to this route by eliminating the use of the nitrogenous base and hence reducing the salt formed. A recycling strategy is proposed to recycle the chemicals used with the goals of making the process commercially viable. A primary amine in presence of low concentrations of carbon dioxide is converted to their corresponding carbamate salt. In presence of a strong dehydrating agent like trifluoroacetic anhydride, the carbamate is converted to isocyanate and salt of the primary amine and trifluoroacetic acid. In excess of carbon dioxide, the zwitterion intermediates of the primary amines are formed which on reaction with a dehydrating agent forms isocyanate without any salt as byproduct. The theoretical isocyanate to amine ratio is 1:2 with the carbamate route and is 1:1 with the zwitterion route. The solubility of carbon dioxide in solvents increases significantly at lower temperatures and hence the zwitterion route is favored in the presence of carbon dioxide excess. Also, results show that the overall conversion is higher at lower temperatures because of the reaction being exothermic. The effect of different solvents and increasingly low temperatures on the overall yield of isocyanates has been studied with the aim of maximizing the isocyanate content formed via the zwitterion route.
McGhee, W.D., Paster M.D., Riley D.P., Ruettimann K.W., Solodar J.A, Waldman T.E, U.S. Patent # 5,451,697, 1995