1 | INTRODUCTION
Consuming petroleum products, coal and natural gas for human activities releases carbon dioxide (CO2) into the atmosphere which is considered one of the reasons for global warming.1Among the CO2 capture technologies, the aqueous amine solutions have been commonly used as the amine solution can react quickly with CO2, with lower costs than ionic liquids or solid sorbents. Mixtures of aqueous amine solutions are commercially used to capture CO2 in steel production, cement and thermal power plants. The reaction mechanisms between CO2 and amine solutions are different for different types of amines.2 The study of the protonation between CO2 and amines is important to explain the reaction mechanisms.3
The dissociation constants (pKa ) of many alkanolamines were measured by Perrin4 while additional alkanolamines and cyclic amine dissociation constants were reported by Tomizaki et al.5 and Chowdhury et al.6 The relationship between the dissociation constants and the reaction kinetics were reported by Versteeg et al.7 and Sharma.8 Nguyen and Henni9 reported the pKa of four polyamines namely 1,4-Bis(3-aminipropyl) piperazine, 1,3-Bis(aminomethyl) cyclohexane, Tris(2-aminoethyl) amine, and 1-Amino-4-methyl piperazine while Kumar et al.10reported the values for 2-(Butylamino)ethanol, m-Xylylenediamine, 3-Picolylamine, Isopentylamine, and 4-(Aminoethyl)-piperidine. Furthermore, the amines’ chemical and biological behaviors are determined by dissociation constants.9-12
The methods used to measure the dissociation constants include ultraviolet spectro-photometry, conductimetric titration, potentiometric titration and magnetic resonance.13 In these techniques, the potentiometric titration method is commonly used as the method is simple and convenient in the pH range between 2 to 11.13-15 Consequently, this method was used to determine the dissociation constants of Methyldiethanolamine (MDEA), N-(2-aminoethyl)-1,3-propanediamine (n-2AOE13PDA), 2-Methylpentamethylene diamine (2-MPMDA), N, n-dimethyldipropylenetriamine (DMAPAPA), 3,3’-Diamino-n-methyldipropylamine (DAOMDPA), Bis[2-(n,n-dimethylamino)ethyl]ether (2DMAOEE), 2-[2-(Dimethylamino)ethoxy]ethanol (DMAOEOE), 2-(Dibutylamino)ethanol (DBEA) and N-propylethanolamine (PEA) in the temperature range of 298.15 K to 313.15 K with 5 K increment. Measurement for Methyldiethanolamine (MDEA) was done for validation purposes. The compound structures, suppliers, and purities are listed in Table S1 in the Supporting Information section.
As observed in Table S1, the nine amines studied included three triamines, two diamines and four monoamines. The monoamines include three tertiary amines which theoretically have lower heat of reaction with CO2 than primary and secondary amines; however, the reaction rate is slower than with primary and secondary amines. Instead of measuring the kinetic between the amines with CO2, a difficult task, dissociation constants of these amines can be more easily be determined. Furthermore, the thermodynamic properties from the study can be used for solubility modeling. The rest of the amines in the study are polyamines which can absorb more CO2 than monoamines. Therefore, it is important to measure their dissociation constants to ensure the reaction rate between the amines and CO2 is of interest to the industry.