Phosgene-Free Route to Isocyanates

This research proposal describes possible phosgene-free routes to produce isocyanate compounds using homogeneous catalysts based on palladium. Di-isocyanates are important building blocks for the manufacture of polyurethanes, thermoplastics and foams. The most important members of the di-isocyanate group are TDI (a mixture of 2,4- and 2,6-toluene diisocyanate) and MDI (bis(4-isocyanatophenyl)methane) and their combined world production volume amounts to about 2 million tonnes/year. They easily react with di- or poly-alcohols to form the corresponding polyurethanes. The manufacture of TDI proceeds via classical process chemistry, i.e. from the dinitro-aromatic compounds via reduction to the corresponding diamine and subsequent reaction with phosgene gives TDI. However, handling of the very reactive and toxic phosgene presents a serious threat to the living environment to the extent that the use of this chemical is severely restricted by environmental regulations and permits in order to prevent disasters, such as occurred in Bhopal (India) in the 1980?s, which killed several thousands of people. A potentially attractive catalytic route to isocyanates proceeds via the direct synthesis of carbamate alkyl esters from aromatic nitro compounds by reductive carbonylation in the presence of simple alkanols. Subsequent pyrolysis of the carbamate ester at elevated temperatures then would give the isocyanate. Further developmental studies at Shell Laboratories and other groups have unfortunately shown that an economically competing process based on the reductive carbonylation of nitro-aromatics, be it highly desirable, still appears an elusive goal. Pyrolysis of carbamate alkyl esters appears to be very cumbersome, while separation of the homogeneous palladium catalyst from the reaction mixture leads to catalyst degradation to the extent that catalyst costs become prohibitive. It is clear that new chemistry has to be developed in order to make a chance of achieving the ultimate goal of both an environmentally and economically sound process to isocyanates. This is exactly which the present proposal for exploratory research aims at. Based on some leads generated from preliminary research it is proposed to develop efficient homogeneous palladium catalyst systems for the reductive carbonylation of nitroaromatics in the presence of weakly nucleophilic reagents such as phenols, to produce aromatic carbamate phenyl esters. It appears that carbamate phenyl esters are much more labile compounds and that pyrolysis to form the isocyanate with the former proceeds under much milder conditions than with the corresponding alkyl carbamate ester and therefore with higher yield. The big challenge will be to develop highly active reductive carbonylation catalyst systems with the weakly nucleophilic alcohols, desired for the lability of the resulting carbamate ester. Carrying out the reductive carbonylation in sufficiently a-polar solvents, in which the carbamate ester will only be sparsely soluble, also potentially offers a principally different means to separate the homogeneous catalyst from the product carbamate, i.e. by crystallisation and thus without exposing the catalyst to high temperatures.