Quantum Chemical Analysis Of CO₂Absorption In Aqueous Amine Solutions

CO2emission reduction has become the hot topic in the world.Coal-fired power plants are the main sources of CO2and it is very necessaryto take measures to capture CO2from the flue gas of power plants. Chemicalabsorption using aqueous amine solutions have been reported to be aneffective industrial method，and it has been widely researched. Quantumchemistry calculation provides an effective method for the screening ofcarbon dioxide absorbents of amine solutions and the development of newabsorbents.A conductor-like screening model for real solvents (COSMO-RS) wasapplied to predict the base strength of different primary amines and secondaryamines. The pKavalues for different amines were calculated by theCOSMO-RS method coupled with the density functional theory (DFT). Thepredicted pKavalues were compared using different DFT levels and basis sets,and the calculated results at the BLYP/TZP level agree with experimentalvalues. The pKavalues of MEA, DEA and AMP at25℃to60℃werecalculated at the BLYP/TZP level, it showed that the rate of amine solutionabsorbing CO2and its pKavalue were positively correlated, proofed with theexisting experimental data. The predicted equilibrium ratio betweencarbamate and bicarbonate anions in CO2-loaded aqueous amine solutions wasused to evaluate the sorbent performance.The two-step reaction mechanism via a zwitterion intermediate is morefavorable for MEA′COO-formation through the activation energy calculations of reaction paths of MEA/CO2system. And MEA′COO-formsmore easily than decomposes reversibly in non-equilibrium states. As a finalproduct at equilibrium, the MEA′COO-product is dominant. Direct formationof HCO-3via the reaction of MEA, CO2and H2O is the most probablemechanism rather than the hydrolysis of MEA′COO-, and the activationenergy of HCO-3formation is fairly high, so the HCO-3product is negligiblyminor.Through the activation energy calculations of reaction paths ofAMP/CO2system, the conclusions are similar to MEA/CO2system’s, butdifferent in details. AMP′COO-forms more easily than decomposesreversibly in non-equilibrium states, but the difference is relatively small,therefore, the AMP′COO-product as a final product at equilibrium is minor.The extent of the positive reaction is greater than the adverse reaction in theprocess of HCO-3formation, so the HCO-3product is numerous for AMP/CO2system.