Effect of Water and Temperature on Absorption of CO₂ by Amine-Functionalized Anion-Tethered Ionic Liquids

Abstract

Amine-functionalized anion-tethered ionic liquids (ILs) trihexyl(tetradecyl)phosphonium asparaginate [P(66614)][Asn], glutaminate [P(66614)][Gln], lysinate [P(66614)][Lys], methioninate [P(66614)][Met], prolinate [P(66614)][Pro], taurinate [P(66614)][Tau], and threoninate [P(66614)][Thr] were synthesized and investigated as potential absorbents for CO(2) capture from postcombustion flue gas. Their physical properties, including density, viscosity, glass transition temperature, and thermal decomposition temperature were determined. Furthermore, the CO(2) absorption isotherms of [P(66614)][Lys], [P(66614)][Tau], [P(66614)][Pro], and [P(66614)][Met] were measured using a volumetric method, and the results were modeled with two different Langmuir-type absorption models. The most important result of this study is that the viscosity of [P(66614)][Pro] only increased by a factor of 2 when fully complexed with 1 bar of CO(2) at room temperature. This is in stark contrast to the other chemically reacted ILs investigated here and all other amino acid-based ILs reported in the literature, which dramatically increase in viscosity, typically by 2 orders of magnitude, when complexed with CO(2). The unique behavior of [P(66614)][Pro] is likely due to its ring structure, which limits the number and availability of hydrogen atoms that can participate in a hydrogen bonding network. We found that water can be used to further reduce the viscosity of the CO(2)-complexed IL, while only slightly decreasing the CO(2) capacity. Finally, from temperature-dependent isotherms, we estimate a heat of absorption of -63 kJ/mol of CO(2) for the 1:1 reaction of CO(2) with [P(66614)][Pro], when we use the two-reaction model.

Tuning the Carbon Dioxide Absorption in Amino Acid Ionic Liquids

“…To increase their CO 2 absorption capacities, ILs need to be modified in such a way that the chemical absorption of CO 2 becomes accessible. The introduction of amine groups in the cation [18] or anion [19][20][21] is one of the easiest and most straightforward methods among the different strategies developed for the enhancement of CO 2 capacity. [22][23][24][25][26][27][28] Amino acid ionic liquids (AAILs) are particular representatives of amine-functionalized ILs.…”

“…For example, lower temperatures or higher CO 2 partial pressures [21,38,39] shift the equilibrium towards 1:1 capacity. [20,21,38,39] Similarly, the attachment of the amine group to the anion instead of to the cation provides higher capacities. [18][19][20][21] Changing the location of the CO 2 absorption from the bulk to the surface of an IL with an amine-functionalized anion also increases the CO 2 capacity to 1:1 at a moderate CO 2 pressure.…”

Significant Improvements in CO2 Capture by Pyridine‐Containing Anion‐Functionalized Ionic Liquids through Multiple‐Site Cooperative Interactions

“…It was seen (Supporting Information, Figure S4) that CO 2 capacity reduced about 0.1 mol per mol IL using moist CO 2 , where water may be preferentially absorbed, and the absorption rate using moist CO 2 increased significantly because of the decrease in the viscosity during CO 2 absorption. [14] Clearly, these pyridine-containing anion-functionalized ILs exhibit very high capacity for CO 2 capture by functionalized ILs and some other adsorbent materials (Supporting Information, Table S2). Why do these ILs exhibit such high capacities for CO 2 capture?…”

Role of Facilitated Transport Membranes and Composite Membranes for Efficient CO2 Capture – A Review

“…The results showed lower performance by these IL membranes, however, the work conveyed an important concept about IL which have reactive amino groups capable to work as CO 2 carrier. Another study reported that tetrabutylphosphonium glycine ([P(C 4 ) 4 ][Gly]) has a very attractive CO 2 absorption capacity [56,57]. More surprisingly, [P(C 4 ) 4 ][AA]s have high CO 2 uptake capacity in both dry and humid conditions.…”