Quantum chemical screening of eutectic solvent components for insights into CO2 complexation mechanisms†

Abstract

Developing new negative emission technologies (NETs) to capture atmospheric CO₂ is necessary to limit global temperature rise below 1.5 °C by 2050. The technologies, such as direct air capture (DAC), rely on sorption materials to harvest trace amounts of CO₂ from ambient air. Deep eutectic solvents (DESs) and eutectic solvents (ESs), a subset of ionic liquids (ILs), are all promising new CO₂ sorption materials for DAC. However, the experimental design space for different DESs/ESs/ILs is vast, with the exact CO₂ complexation pathways difficult to elucidate; this creates significant limitations in rationally designing new materials with targeted CO₂ sorption energetics. Herein, the CO₂ complexation pathways for a structural library of different DES/ES components are computed using quantum chemical calculations (i.e., density functional theory). For the entire structure library, we report the energies of elementary CO₂ binding and proton transfer reactions as these reactions are fundamental in DAC within DESs and ESs. These elementary reactions are combined to generate CO₂ complexation pathways and calculate their free energies. The different elementary steps and reaction pathways demonstrate the range of CO₂ complexation free energies and the significance between CO₂ binding and proton transfer reactions. We also report the CO₂ complexation free energies with different functional groups around the CO₂ sorption site, supporting the concept of functionalization for tuning CO₂ complexation thermodynamics. Additionally, our findings suggest potential descriptors, such as proton affinity or pK_a, could be useful when identifying candidate species for ESs and predicting/rationalizing product distributions. Our work has implications for experimental synthesis, characterization, and performance evaluation of new DAC sorption materials.