From greenhouse gas to feedstock: formation of ammonium carbamate from CO2 and NH3 in organic solvents and its catalytic conversion into urea under mild conditions

The capture of carbon dioxide by ammonia in both aqueous and non-aqueous solutions was investigated at atmospheric pressure and 273 K under different operating conditions. The CO2 capture is fast and efficient ranging between 78 and 99%, depending on both the NH3 concentration and the solvent nature. The precipitation of solid mixtures of ammonium bicarbonate, ammonium carbonate and ammonium carbamate occurred in ethanol–water solution. Selective precipitation of ammonium carbamate was achieved by reacting gaseous CO2 and NH3 in anhydrous ethanol, 1-propanol or N,N-dimethylformamide (DMF) in a flow reactor that operates in continuous. In the second step of the process, the pure ammonium carbamate is used to produce urea with good yield (up to 54% on carbamate basis) at 393–413 K in the presence of inexpensive Cu(II) and Zn(II) catalysts. The yield of urea depends on several factors including the catalyst, the reaction temperature and the reaction time. Identification and quantification of urea in the reaction mixtures was obtained by analysis of its 13C NMR spectrum. A preliminary mechanistic interpretation of the catalytic reaction is also briefly presented and commented.