Direct CO₂ Capture from Air via Crystallization with a Trichelating Iminoguanidine Ligand

Effectively reducing the concentration of CO₂ in ambient air is essential to mitigate global warming. Existing carbon capture and storage technology can only slow down the carbon emissions of large point sources but cannot treat the already accumulated CO₂ in the environment. Herein, we demonstrated a simple direct CO₂ capture method from air via reactive crystallization with a new trichelating iminoguanidine ligand (BTIG). It could strongly bind CO₂ to form insoluble carbonate crystals that could be easily isolated. In the crystal, CO₂ was transformed to CO₃^2– and trapped in a dense hydrogen bonding network in terms of carbonate–water clusters. This capture process was reversible, and the BTIG ligand could be regenerated by heating the BTIG–CO₂ crystal at a mild temperature, which was much lower than the decomposition temperature of CaCO₃ (∼900 °C). Thermodynamic and kinetics analyses indicate that the crystallization process was exothermic with an enthalpy of −292 kJ/mol, and the decomposition energy consumption was 169 kJ per mol CO₂. In addition, BTIG could also be employed for CO₂ capture from flue gas with a capacity of 1.46 mol/mol, which was superior to that of most of the reported sorbents.