Induced Protic Behaviour in Aprotonic Ionic Liquids by Anion Basicity for Efficient Carbon Dioxide Capture

Area of Science:

• Ionic Liquids
• Physical Chemistry
• Materials Science

Background:

• Aprotic ionic liquids (ILs) like tetrabutylphosphonium carboxylates ([P4 4 4 4][CnCOO]) are explored for various applications.
• Understanding their interactions with water is crucial for predicting their behavior and optimizing their use.

Purpose of the Study:

• Investigate the interactions between specific ionic liquids and water.
• Elucidate the mechanisms of ion-pair separation and protic behavior in these systems.
• Explore the implications for carbon dioxide (CO2) absorption.

Main Methods:

• Spectroscopic studies using D2O to probe proton/deuterium exchange.
• Analysis of cation-anion interactions and the role of van der Waals forces.
• Investigation of CO2 absorption mechanisms in neat ILs and IL-water mixtures.

Main Results:

• Water addition causes ion-pair separation in [P4 4 4 4][CnCOO] ILs by localizing around carboxylate groups.
• Protic behavior was observed, with proton/deuterium exchange at the cation's α-1 H, forming an ylide intermediate.
• The [P4 4 4 4][C7COO] system showed enhanced ion re-orientation due to stronger van der Waals forces.
• Neat ILs absorbed CO2 via the ylide intermediate, forming a phosphonium-carboxylate zwitterion.
• IL-water mixtures absorbed CO2 to form hydrogen carbonate.

Conclusions:

• Water significantly influences the structure and reactivity of these ionic liquids.
• The protic nature of the ILs, even without water, facilitates CO2 capture.
• The findings offer insights into designing ILs for gas separation and capture applications.