Herein
we report the application of polyamine pentaethylenehexamine
(PEHA, 3,6,9,12-tetraazatetradecane-1,14-diamine) in CO2 absorption with both neat PEHA and aqueous solutions thereof. The
absorption of molecular CO2 in pure PEHA and in PEHA-water
systems resulted in the formation of two chemical species, namely,
PEHA carbamate and bicarbonate. It was observed that, upon formation
of these species, both the CO2 absorption capacity and
CO2 absorption rate were controlled by the amount of water
in the system. During the CO2 absorption, the neat PEHA
and 92 wt % PEHA were capable of forming carbamate species only while
other aqueous analogues with higher dilution allowed for the formation
of both carbamate and bicarbonate species upon exceeding 8 wt % water
in the mixture. The CO2 uptake steadily increased with
an increase in the water concentration in the solvent mixture and
reached the maximum value of 0.25 g of CO2/(g of solvent)
in the case of 56 wt % PEHA in water. However, in the case of more
dilute systems (i.e., <56 wt % PEHA in water), the trend reversed
and the CO2 loading decreased linearly to 0.05 g of CO2/(g of solvent) for 11 wt % PEHA in water. Meanwhile, it usually
took shorter time to achieve the full CO2 absorption capacity
(equilibrium) with increasing water content in all cases. The 13C NMR analysis was used to quantify the relative amount of
PEHA carbamate and bicarbonate, respectively, in reaction mixtures.
The Kamlet–Taft parameters (α, β, and π*) of aqueous solutions for different concentrations of PEHA
were also studied taking advantage of various solvatochromic dyes
and correlated with the CO2 absorption capacity. The thermally
induced switchable nature of CO2-saturated neat and aqueous
PEHA solutions for transformation of ionic PEHA carbamate and bicarbonate
moieties to molecular PEHA is also represented. A comparison between
aqueous PEHA and aqueous monoethanolamine (industrial solvent) for
CO2 capture is reported. Hence, most importantly, a switchable
PEHA system is demonstrated for reversible CO2 absorption
processes.