Highly efficient synthesis of cyclic ureas from CO2 and diamines by a pure CeO2catalyst using a 2-propanol solvent

Abstract

Pure cerium oxide (CeO₂) acts as an effective and reusable heterogeneous catalyst for direct synthesis of cyclic ureas from CO₂ and diamines even at a low CO₂ pressure of 0.3 MPa. 2-Propanol is the most preferable solvent to provide good selectivity. The system composed of a CeO₂ catalyst and a 2-propanol solvent is applied to various diamines to provide the corresponding cyclic ureas in high yields (78–98%), including six-membered-ring ureas that are difficult to be synthesized from CO₂. Based on the kinetic studies on the effect of CO₂ pressure and amine concentration and FTIR studies on adsorption of ethylenediamine and CO₂ onto CeO₂, the following mechanism for the synthesis of cyclic urea is proposed: (1) formation of carbamic acid and carbamate species on CeO₂, (2) decomposition of carbamic acid to a free amino group and CO₂, (3) nucleophilic attack of the amino group on the carbamate on CeO₂ to produce the cyclic urea and (4) desorption of the product and regeneration of CeO₂.