Acid gas absorption technology is of great importance in
these days for the prevention of global warming and the
resulting worldwide climate change. More efficient
process design and development for the removal of acid
gases has become important, together with the development
of new absorbents as one of urgent areas of research
in addressing global-warming problems. In the present work,
aqueous solutions of 2-amino-2-hydroxymethyl-1,3-propanediol (AHPD), a sterically hindered amine, has
been examined as a potential CO2 absorbent and compared
with the most commonly used absorbent, monoethanolamine
(MEA) solution, through equilibrium solubility measurements
and 13C NMR spectroscopic analyses. The solubilities of CO2
in aqueous 10 mass % AHPD solutions were higher than
those in aqueous 10 mass % MEA solutions above 4 kPa at
298.15 K, but below 4 kPa, the solubility behavior appeared
to be the opposite. The solubility difference between
these two solutions increased with the CO2 partial pressures
above the crossover pressure. Equilibrated CO2−MEA−H2O and CO2−AHPD−H2O solutions at various CO2 partial
pressures ranging from 0.01 to 3000 kPa were analyzed
by 13C NMR spectroscopy to provide a more microscopic
understanding of the reaction mechanisms in the two
solutions. In the CO2−amine−H2O solutions, amine reacted
with CO2 to form mainly the protonated amine (AMH+),
bicarbonate ion (HCO3
-), and carbamate anion (AMCO2
-),
where the quantitative ratio of bicarbonate ion to
carbamate anion strongly influenced the CO2 loading in
the amine solutions. A profusion of bicarbonate ions, but
a very small amount of carbamate anions, was identified in
the CO2−AHPD−H2O solution, whereas a considerable
amount of carbamate anions was formed in the CO2−MEA−H2O solution. AHPD contains more hydroxyl groups than
nonhindered MEA, and hence, the chemical shifts in its 13C
NMR spectra were strongly influenced by the solution
pH values. In contrast, MEA appeared to be insensitive to
pH. The strong interrelations among CO2 solubility, CO2
partial pressure, bulkiness of the amine structure, and pH
identified through the present experimental investigations
can provide basic guidelines for finding new potential organic
absorbents, including specifically designed amine
chemicals.
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