Absorption rates of carbon dioxide in amines in hydrophilic and hydrophobic solvents

Abstract

The rate of absorption of CO2 into amines in hydrophilic and hydrophobic solvent mixtures was studied in this work using a stopped-flow apparatus and a stirred-tank reactor. Monoethanolamine (MEA, primary amine) and N-methyl benzylamine (NMBZA, aromatic secondary amine) are chosen for this study and diluted in hydrophobic or hydrophilic solvents at different temperatures (295-343 K) and concentrations (0.01-4 kmol.m(-3)). It was found that the reaction rate between the secondary amine and CO2 in water is faster than the reaction with the linear primary amine (MEA); however, in an ethereal hydrophobic non-aqueous system the NMBZA-CO2 reaction is slower than the reaction of CO2 with MEA. With respect to amine, the reaction orders were found to be 0.9 and 1.2 for hydrophilic systems (MEA + H2O, NMBZA + H2O). For hydrophobic system they were found to be 0.6 and 2.7 (NMBZA + ethereal hydrophobic solvent, NMBZA + MePhOH + ethereal hydrophobic solvent). When amine was mixed with hydrophilic solvent (H2O), the activation energy of amine (NMBZA) to react with CO2 was found to be 27.66 kJ.mol(-1) and decreased to 10.37 kJ.mol(-1) when mixed with hydrophobic solvent (ethereal hydrophobic solvent). The activation energy of hydrophobic solvent was found to increase by adding an alcohol to the solvent (MePhOH + ethereal hydrophobic solvent). Both the zwitterion and termolecular mechanism were applied to the experimental data. It was found that deprotonation of the zwitterion and deprotonation of the loosely bound complex was the rate limiting step for the zwitterion and termolecular mechanism, respectively.

Additionally, activators such as water, piperazine (PZ), and 4-hydroxy piperidinol (HPIP) were added to the ethereal hydrophobic non-aqueous solvent to determine the effect on the reaction rate. It was found that the water-and amine-based activators lead to an increase in the observed reaction rates as the concentration of the activators increased. The reaction of NMBA with CO2 and PZ exhibited a higher reaction rate but the PZ was insoluble in the non-aqueous NMBZA system, whereas HPIP remained soluble and offered a similar reaction rate. The activation energy of ethereal hydrophobic non-aqueous solvent to react with CO2 increased from 27.98 to 32.83 kJ.mol(-1) by adding PZ and 33.51 kJ.mol(-1) by HPIP and remained by adding H2O (26.78 kJ.mol(-1)). From this work, it was found that the non-aqueous solvent plays an important role in the reaction rate of CO2 with amines. Activators and solvents can be selected to increase the reaction rate with CO2.