Challenges in engineering the structure of ionic liquids towards direct air capture of CO2

Abstract

Man-made perturbations over emissions of greenhouse gas (GHG) bring tremendous negative impacts on the survival environment. CO₂ accounts for ~75% of global GHG impacts with others mainly composed of N₂O, CH₄, and small fluorinated gas molecules. Deployment of “negative emission” technologies via direct air capture (DAC) of CO₂ by engineered chemical reactions represents one of the most promising and distinct pathways to limit and alleviate the global warming trend. However, the inherent low concentration of CO₂ during the DAC of CO₂ process (~400 ppm) gives rise to extra requirements and constraints in designing sorbents. The current absorbents capable of achieving DAC of CO₂ via engineered chemical interactions mainly include aqueous hydroxide sorbents (e.g., NaOH, KOH, and Ca(OH)₂)/solid alkali carbonates and porous solid-supported amines. The former has the merit of fast sorption kinetic, but is quite corrosive and energy-intensive for regeneration (T > 250 °C). The latter has moderate desorption temperature (> 120 °C), but its application is limited by the slow sorption kinetic, and chemical and thermal degradation. In addition, maintaining high structural porosity after multiple cycles is required to keep the good performance. Considering these limitations and deficiencies associated with the chemisorption processes in DAC of CO₂, developing new sorbents combining the merits of the liquid and solid systems, that is, good thermal and chemical stability, low volatility, high CO₂ uptake capacity, fast sorption kinetics, and energy-efficient desorption, is highly desired in the practical DAC of CO₂ procedure. Ionic liquids (ILs) have demonstrated good performance in CO₂ capture and separation benefiting from their unique features and molecular structures, including good stability, ultralow volatility, designability, and high CO₂ solubility/selectivity. Particularly, the nonvolatility and structural adjustability of ILs make the energy-efficient CO₂ desorption process feasible. Although the emerging of ILs provides a new and exciting option for CO₂ capture, physisorption ILs cannot still compete with current commercially available solvents (e.g., amines and inorganic base) because of the lower CO₂ absorption capacity, especially for post-combustion flue gas and even DAC with low CO₂ concentration. Therefore, the design and synthesis of functionalized ILs with chemisorption for highly efficient capture of CO₂ becomes the hot topic. Two kinds of task-specific ILs (TSILs) with basic groups have been demonstrated in CO₂ chemisorption, including amino-based ILs (AILs) and superbase-derived task-specific ILs (STSILs). However, most of the studies are still focused on the capture and separation of pure CO₂ or gaseous mixtures with high CO₂ concentrations. To design specific TSILs achieving efficient DAC of CO₂, the following critical aspects and challenges should be addressed by engineering the structure of targeted ILs.

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