Tailored carbon dioxide capacity in carboxylate-based ionic liquids

Abstract

We have used a library of thermally stable tetraalkylphosphonium carboxylate ionic liquids that were easily prepared from available carboxylic acids. Depending on the pK_a in water of the precursor acids, the resulting ionic liquids either dissolve or reversibly chemically absorb CO₂, with some exhibiting notable gas capacities, reaching a CO₂ mole fraction of 0.2 at 1 bar and 343 K. While equilibrium constants and ionic liquid capacities generally correlate with the pK_a of the acids, certain exceptions underscore the influence of liquid structure and physical properties of the ionic liquids, elucidated through molecular dynamics simulations and density functional theory calculations. Unlike the trends observed in other CO₂-absorbing ILs, phosphonium carboxylates do not experience increased viscosity upon gas absorption; instead, enhanced diffusivities are observed, facilitating efficient gas–liquid transfer.