What happens at the microscopic level when CO2 reacts with ammonia or amines in solution or on cryogenic surfaces?

Abstract

The reaction of aqueous ammonia or amines with CO₂ forms the basis of many contemporary methods for the removal of CO₂ from power-plant effluents. However, the reactions involved and products produced under varying conditions of concentration, temperature and solvent are not well understood on a microscopic level. For example, for the reactants CO₂ and NH₃, the amounts of carbamic acid and methyl carbamate produced are complex functions of solvent, temperature and reaction time. We have used computational quantum chemical techniques to evaluate the structures, stabilities and spectral properties of potential reaction products of CO₂ + amines. We have also evaluated the effect of temperature and solvent on the many possible reactions. Our results allow us to identify desirable physical and chemical properties for the amine collector molecules and the solvents used. We establish that all the CO₂ + amine reactions, particularly the acid–base reaction to form carbamate anions and amine cations, are significantly favored by an increase in the dielectric constant of the solvent and by a reduction in temperature. To obtain the necessary difference between high equilibrium constants at the absorber temperature and low equilibrium constants at the stripper (or recovery) temperatures, proper adjustments of both absorber identity and solvent are necessary. Formation of solid CO₂–amine products, particularly carbamic acid dimers, is also considered. Although these compounds are more stable than analogous H₂CO₃ oligomers, their potential for directly sequestering CO₂ is limited by the cost of the amine raw material, suggesting that naturally occurring and/or waste NH-containing compounds will need to be reacted with the CO₂.