DFT-Assisted Mechanism Evolution of the Carbonylation of Ethylene Glycol to Ethylene Carbonate by Urea over Zn(NCO)2•2NH3 Catalyst

Zn(NCO)(2)center dot 2NH(3) catalyst has been synthesized, characterized by X-ray diffraction and IR spectroscopy, and used as a catalyst for the cyclization of urea and ethylene glycol (EG) to ethylene carbonate (EC) for the first time. A maximum yield of 40% has been obtained with a urea/EG mole ratio of 1:1.5 and a temperature of 150 +/- 2 degrees C. An attempt has been made to predict the mechanism of the reaction with the help of density functional theory calculations. Our calculations suggest the reaction to be a consecutive one. In the first step, urea decomposes to ammonia and isocyanic acid (HNCO). HNCO reacts with EG to produce 2-hydroxyethyl carbamate (2-HEC). At the last step, 2-HEC cyclizes over Zn(NCO)(2)center dot 2NH(3) to EC. Calculations suggest the cyclization of 2-HEC to follow a charge-controlled intramolecular nucleophilic elimination cyclization path.