Green Synthesis of N,N′-Dialkylureas from CO2 and Amines Using Metal Salts of Oxalates as Catalysts

N,N′-Dialkylureas were synthesized via the carbonylation of amines by CO2, using, for the first time, metal salts of oxalates as catalysts. Influencing factors, such as the natures of catalyst and amine, the reaction conditions (temperature, time and pressure), the solvent, as well as the dehydrant, were investigated systematically. It was found that, among various catalysts, Y2(C2O4)3 provided the largest yield of N,N′-dialkylurea. The employment of an aprotic solvent, relative to a protic solvent, was much more favorable to the formation of N,N′-dialkylurea. Aliphatic primary amines exhibited a significantly higher reaction activity for the generation of N,N′-dialkylurea than aromatic and secondary amines. The increase in reaction temperature, time, and pressure increased the yield of N,N′-dialkylurea. The employment of 4A zeolite as dehydrant promoted the formation of N,N′-dialkylurea. While Cs2CO3 had been reported to provide an appreciably high yield of N,N′-dialkylurea in the literature, the comparative studies demonstrated that the Y2(C2O4)3 catalyst possessed a better performance than the Cs2CO3 catalyst, with the reactions based on the two catalysts being conducted under the same conditions in our laboratory. A possible mechanism for the formation of N,N′-dialkylurea from the carbonylation reaction of amine and CO2 over the Y2(C2O4)3 catalyst was proposed.