Reusable Polystyrene-Functionalized Basic Ionic Liquids as Catalysts for Carboxylation of Amines to Disubstituted Ureas

A series of polystyrene (PS)-functionalized basic ionic liquids (BILs) were prepared and used as catalysts for synthesis of disubstituted ureas (DSUs) from amines and carbon dioxide (CO2). The PS-BILs as prepared were characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FT-IR) spectroscopy. For investigating the catalytic activities, all PS-BILs were tested in a model reaction of cyclohexylamine (CHA) and CO2 to synthesize dicyclohexylurea (DCU). Poly-2 having branched [bis-imidazolium]/[bis-bicarbonate] was found to show the highest activity for the DCU formation among all PS-BILs catalysts. For a comparison point of view, ceria (CeO2) as a typical catalyst and original Merrifield's resin (MR) was also applied for the reaction. Reaction conditions were optimized by varying reaction temperature, pressure, reaction time, and amount of catalyst used. Under optimized conditions, reactions of various amines with CO2 to synthesize the corresponding diureas were carried out in the presence of the most active catalyst (poly-2). Furthermore, poly-2 could be easily recovered and reused up to seven consecutive cycles with no significant loss of the catalytic activity.