Dialing in Direct Air Capture of CO₂ by Crystal Engineering of Bisiminoguanidines

Direct air capture (DAC) technologies that extract carbon dioxide from the atmosphere via chemical processes have the potential to restore the atmospheric CO₂ concentration to an optimal level. This study elucidates structure-property relationships in DAC by crystallization of bis(iminoguanidine) (BIG) carbonate salts. Their crystal structures are analyzed by X-ray and neutron diffraction to accurately measure key structural parameters including molecular conformations, hydrogen bonding, and π-stacking. Experimental measurements of key properties, such as aqueous solubilities and regeneration energies and temperatures, are complemented by first-principles calculations of lattice and hydration free energies, as well as free energies of reactions with CO₂, and BIG regenerations. Minor structural modifications in the molecular structure of the BIGs are found to result in major changes in the crystal structures and the aqueous solubilities within the series, leading to enhanced DAC.