Switchable aqueous pentaethylenehexamine system for CO2 capture: An alternative technology with industrial potential

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.