K3PO4 was found to effectively catalyze the carboxylation of amines by CO2, producing corresponding 1,3-disubstituted ureas in high yields and selectivities. X-ray diffraction and FT-IR spectroscopic analysis of the solid mixture, recovered from the carboxylation of n-butylamine, revealed that K3PO4 was completely transformed into KH2PO4 and KHCO3. Mechanistic and computational studies suggested that the high activity of K3PO4 could be attributed to the synergy effect exerted by the co-presence of KHCO3 and KH2PO4, generated from the interaction of K3PO4 with CO2 and water. (C) 2013 Elsevier B.V. All rights reserved.