Kinetics and new Brønsted correlations study of CO2 absorption into primary and secondary alkanolamine with and without steric-hindrance

To figure out the steric hindrance and its magnitude effects on CO2 absorption performance and Brønsted correlations of primary and secondary alkanolamines, the pKa values and kinetics data (second order reaction rate, k2) were investigated for nine alkanolamines at 293–313 K using pH meter and stopped-flow technique. The tested amines include monoethanolamine (MEA), diethanolamine (DEA), 2-methyl-ethynolamine (MAE), 2-ethyl-ethynolamine (EAE), 1-amino-2-propanol (1-AP), 3-amino-1-propanol (3-AP), 2-amino-2-methyl-1-propanol (AMP), 2-amino-1-propanol (2-AP) and 3-Amino-1,2-propanediol (3-APD). In addition, the equilibrium solubility of MEA, MAE, EAE, 2-AP and AMP were tested for evaluation of steric hindrance effect on carbamate’s stability. The comprehensive analysis of kinetics, solubility and Brønsted correlations indicated that the steric hindrance caused by alkyl group(s) attached to α-C atom has much significant effect on k2 and solubility than that caused by alkyl group connected to amino group. Moreover, the k2 and pKa values for sterically unhindered and hindered amines were separately fitted at 293–313 K, giving out an AARD of 5.2% and 16.5% between predicted and experimental k2, respectively. Finally, based on molecular structures and experimental results, mechanism of steric hindrance effect on reaction kinetics was proposed.