Proof of concept for direct air capture using encapsulated ionic liquid technology

This study demonstrates the feasibility of encapsulated ionic liquids (ENILs) as efficient sorbents for direct air capture (DAC) of CO 2 . The new aprotic heterocyclic anion ionic liquid (AHA-IL) [P 66614 ][4-BrPyra], initially identified via a priori screening as a promising DAC absorbent, was synthesized and experimentally evaluated, using [P 66614 ][2-CNPyr] as a literature benchmark. The AHA-IL was immobilized in porous sub-micrometric carbon capsules to form ENIL-[P 66614 ][4-BrPyra], creating an efficient gas-liquid contactor designed to overcome mass-transfer limitations under dilute CO 2 while preserving intrinsic AHA-IL absorption capacity. The ENIL material was systematically characterized in terms of microstructure, particle size distribution, thermal stability, and elemental composition. CO 2 sorption isotherms were measured by microbalance and Tristar analyzers in the 500–5000 ppm outdoor-indoor DAC range. Fixed bed tests were carried out to evaluate ENIL performance in DAC process, measuring the breakthrough curves to characterize sorption capacity and kinetics in capture operation. For this purpose, breakthrough curves were successfully modeled using linear driving force (LDF) model in Aspen Adsorption simulator. Close agreement between isotherm and breakthrough capacities demonstrates efficient bed utilization, while ENIL-[P 66614 ][4-BrPyra] delivers markedly higher CO 2 uptake than benchmark [P 66614 ][2-CNPyr] at DAC-relevant concentrations. Obtained overall mass-transfer coefficients ( k MTC ) revealed remarkably high sorption rates even at very low CO 2 partial pressures, indicating a reaction-controlled regime. The kinetic advantage of ENIL-[4-BrPyra] arises from favorable intrinsic IL-CO 2 chemistry combined with the small capsule size, which significantly increases the gas-liquid contact surface. Fixed-bed sorption-desorption cycles further demonstrate effective regeneration and stable performance under DAC operation. Overall, ENIL-[P 66614 ][4-BrPyra] combines high capacity with fast kinetics, making it a strong candidate for energy-efficient DAC technologies.