Manufacture of isocyanates without the use of phosgene remains a priority in the polyurethanes industry. One route that has received some attention is the production of carbamates from the precursor amines by reaction with dimethyl carbonate. The carbamate can then be converted to the corresponding isocyanate by thermolysis or potentially by catalytic cracking. We studied the kinetics of the carbamate forming reactions, catalyzed by zinc carboxylates. Simple zinc carboxylate salts, like zinc acetate showed a significant induction period and first-order influence on the rate of carbamate formation. In contrast, zinc 2-ethylhexanoate, which exists as a tetrahedral Zn4O(L)6 species, shows no induction period but the reaction rate is ½ order in the amount of catalyst precursor added. We’ve proposed a reaction network in which the active catalytic species exists in equilibrium with the tetrameric complex. This enables us to explain the different apparent reaction orders and the very different induction periods observed with zinc acetate versus zinc 2-ethylhexanoate. Interestingly, the model correctly predicts the surprising result of using a mixed catalyst system.

Kinetics of zinc carboxylate catalyzed production of carbamates as non-phosgene isocyanate intermediates

2019 Spring National Meeting

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