Liquid-Phase CO₂ Capture by a Nonaqueous Cooperative Absorption Mechanism

Anthropogenic carbon dioxide emissions remain a critical driver of climate change, necessitating the development of efficient carbon capture technologies. While aqueous amines are widely used, they suffer from high regeneration energies and solvent degradation. Here, we report a nonaqueous, cooperative CO₂ absorption system using 1-methylpiperazine (MPZ) dissolved in a family of aromatic additives. Among these, the formulation with 2'-hydroxyacetophenone (2'HAP) exhibited the best overall performance, with a stepped sorption isotherm indicating cooperative absorption and a CO₂:MPZ stoichiometry of up to 0.93 at moderate pressures (∼163 kPa) without the addition of water. While other structurally related additives, including acetophenone (AP), 2'-methoxyacetophenone (2'MAP), and 1,4-diisopropylbenzene (DIPB), also enabled high uptake capacities (absorbed CO₂:MPZ stoichiometries up to ∼0.85), the stepped behavior enables enhanced working capacity with reduced temperature differentials, offering potential energy savings in temperature-swing absorption processes. Measurements of performance with different additives including acetophenone isomers and phenolic analogs revealed that CO₂ uptake and isotherm shape are governed by solvent acidity, carbonyl presence, and the ability to stabilize carbamic acid intermediates, perhaps through hydrophobic interactions. Equilibrium network modeling supported the proposed mechanism, illustrating how additive-amine interactions shift key equilibria to enable stepped isotherm behavior. Control experiments with monoethanolamine (MEA) and morpholine (MP) underscored the importance of the diamine nature of MPZ in facilitating cooperative uptake. Breakthrough experiments confirmed the system's robust performance across varying CO₂ concentrations (4.5-25%). This work provides mechanistic insights into additive-amine cooperativity and highlights MPZ-based nonaqueous systems as promising candidates for energy-efficient CO₂ capture.