Dialing in Direct Air Capture of CO2 by Crystal Engineering of Bis-iminoguanidines

We report a structure-properties relationship study of DAC by crystallization of bis-iminoguanidine (BIG) carbonate salts. The study focuses on a series of basic BIG structures including the glyoxal-bis(iminoguanidine) prototype (GBIG) and its simple analogs methylglyoxal-bis(iminoguanidine) (MGBIG) and diacetyl-bis(iminoguanidine) (DABIG). We find that minor structural modifications in the molecular structure of GBIG, such as substituting one or two hydrogen atoms with methyl groups, result in major changes in the crystal structures, induced by the increased conformational flexibility and steric hindrance. As a result, the corresponding aqueous solubilities within the series increase significantly, leading in turn to enhanced DAC performances.