Kinetics and dissociation constants (pKa) of polyamines of importance in post-combustion carbon dioxide (CO2) capture studies

Abstract

Pseudo-first-order overall rate constants for the loss of CO2 via reaction with different types of amines were measured using a stopped-flow technique at 298.15 K. Polyamines and cyclic amines were found to have higher reaction rates than linear primary and secondary amines. Therefore, six aqueous cyclic polyamine solutions were studied at (298.15 to 313.15) K over a concentration range of (20 to 120) mol·m-3 using. The overall reaction orders were calculated using the empirical power law kinetics and were found to be fractional in order, for practically all studied cyclic polyamines. The overall rate constants were fitted with the Crooks-Donnellan termolecular rate expression to determine elementary rate constants. In addition, the dissociation constants (pKa ) were determined using the potentiometric titration method at (298, 303, 313 and 323) K, and predicted using quantum chemistry techniques (IEFPCM continuum solvation model). A trend was found for the variation of the pKa with the addition of different radical groups to the cyclic base molecules. Computational techniques tested for the prediction of pKa involved B3LYP and MP2 levels of electronic structure theory, the addition of an explicit water molecule inside the continuum cavity, and a special scaling of the cavity radii for the ions. The procedure developed in this study reduced the error found in a previous technique for cyclic amines by 62%.