The pKa values of amine based solvents for CO2 capture and its temperature dependence-An analysis by density functional theory

A computational chemistry study has been employed to calculate the pKa values of alkylamines (primary, secondary and tertiary) used in the removal of carbon dioxide. A DFT-B3LYP level of theory with 6-311++G (d, p) basis set is used for the computational analysis. The pKa values of amines are of great importance as it has a major role in carbon dioxide capturing mechanism. The temperature dependence of pKa values has also been evaluated and it has been observed that the pKa values of alkylamines shows an inverse relation with the temperature. The Secondary and tertiary amines show maximum and minimum pKa values respectively. The gas phase proton affinity (PA) and gas the phase basicity (GB), which is difficult to measure experimentally, have been computed easily and accurately by this method. It is observed that the PA and GB values increase from primary to tertiary alkylamines. The linear relationship between the pKa values and the free energy of protonation has been computationally verified; the results are in good agreement with the experimental results and the method is simpler and less expensive. The work can be extended to study the carbamate stability and carbon dioxide capture mechanism and there by screening the candidates for the Carbon dioxide Capture and Storage.