Switchable Aqueous Pentaethylenehexamine System for CO₂ Capture: An Alternative Technology with Industrial Potential

Abstract

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.

Enzyme-Assisted CO2 Absorption in Aqueous Amino Acid Ionic Liquid Amine Blends

“…Increasing the temperature naturally weakens the intermolecular bonds, leading to increased fluidity and decreased density. The higher viscosity and density of the 25% PEHAp solution could be related to intermolecular hydrogen bonding with higher strength between water and PEHA compared to intra- and intermolecular hydrogen bonding in PEHA . The 25% PEHA solution displayed a viscosity of 5.6 mPa·s at 20 °C (data not shown), which can be compared to 4.2 mPa·s for 25% PEHAp.…”

“…The higher viscosity and density of the 25% PEHAp solution could be related to intermolecular hydrogen bonding with higher strength between water and PEHA compared to intra-and intermolecular hydrogen bonding in PEHA. 22 The 25% PEHA solution displayed a viscosity of It can generally be observed that the initial absorption rates for 25% PEHA and 25% MEA are the highest, where PEHA displayed the highest overall absorption rate. In contrast to the other solutions, the 25% PEHA and MEA solutions also maintained a high absorption rate over a larger CO2 load (mol L -1 ) interval, which is consistent with the availability of a high number of strong primary and secondary amines per unit volume ( Figure 4A).…”

“…The initial high desorption rate is most likely related to the amount of bicarbonate which is highest at the maximum load and decomposes more easily to CO2 compared to the decomposition of the carbamates and their respective amines. 22 On a mol L -1 basis, the lean loading for the 25% PEHA, 25% MEA and 25% PEHAp solutions were between at 1.2-1.4 mol L -1 indicating the stability of the carbamates formed with the primary and secondary amines involved ( Figure 5A).…”

“…The results confirm other reports where it was shown that 30% PEHA displayed faster absorption rates and higher loads compared to a 30% MEA solution. 22 Other aqueous polyamine solutions such as tetraethylenepentamine (TEPA) has also shown higher absorption rates compared to MEA 24 The more modest improvements in the absorption rates with further addition of PEHAp may be related to the increasing viscosities with higher amounts of PEHAp and the fact that a larger fraction of the primary and secondary amines must act as proton acceptors themselves during carbamate formation.…”

“…This value is consistent with another study under similar conditions where a load of about 2.46 mol CO2 mol -1 amine for a 30% aqueous PEHA solution was reported. 22 With two primary and four secondary amine sites, PEHA could theoretically be expected to bind 3 mol CO2 mol -1 amine assuming that no other reactions occur. The 25% PEHAp solution displayed a higher capacity (2.88 mol CO2 mol -1 amine) than 25% PEHA (2.46 mol CO2 mol -1 amine) even though one of the nitrogen atom got protonated.…”