The absorption of CO₂ in aqueous NH₃ solutions occurs with high efficiency and loading capacity at room temperature and atmospheric pressure producing the ammonium salts of bicarbonate (HCO₃ ⁻), carbonate (CO₃ ²⁻), and carbamate (NH₂CO₂ ⁻) anions. ¹³C NMR spectroscopy at room temperature has been proven to be a simple and reliable method to investigate the speciation in solution of these three ionic species. Fast equilibration of HCO₃ ⁻/CO₃ ²⁻ anions results in a single NMR peak whose chemical shift depends on the relative concentration of the two species. A method has been developed to correlate the chemical shift of this carbon resonance to the ratio of the two anionic species. Integration of the carbamate carbon peak provided the relative amount of this species with respect to HCO₃ ⁻/CO₃ ²⁻ pair. No other species was detected in solution by ¹³C NMR, and no solid compounds separated out under our experimental conditions. Finally, the relative amount of HCO₃ ⁻, CO₃ ²⁻, and NH₂CO₂ ⁻ in solution have been correlated to the molar ratio between free ammonia in solution and absorbed CO₂.

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.