CeO₂-catalyzed direct synthesis of dialkylureas from CO₂ and amines

CeO₂ showed higher activity for the direct synthesis of 1,3-dibutylurea (DBU) from CO₂ and n-butylamine than the metal oxides tested. The solvent largely influenced the reaction over CeO₂, and N-methylpyrrolidone (NMP) was preferable among various solvents tested from the viewpoints of activity and selectivity. The catalyst system composed of CeO₂ catalyst and NMP solvent (CeO₂ in NMP) was applicable to the reactions of various amines such as linear primary alkylamines or branched primary alkylamines, although tert-butylamine afforded low conversion. In contrast, secondary amines and aniline provided no yield of the ureas. The combination of 2-cyanopyridine with CeO₂ in NMP (CeO₂ in NMP with 2-cyanopyridine) promoted the transformation of the unreactive amines, showing that tert-butylamine and aniline were converted to the corresponding ureas in 82% and 80% yields, respectively. These yields are much higher than those reported in the previous literatures, indicating that CeO₂ in NMP with 2-cyanopyridine drastically promoted transformation of amines with low reactivity.