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.