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

Abstract

Direct air capture (DAC) technologies that extract carbon dioxide directly from the atmosphere using chemical processes have the potential to achieve zero or negative emissions and restore the atmospheric composition to an optimal level with respect to the CO2 concentration. However, before such technologies can be developed and applied energy-efficiently, we need to gain fundamental understanding of DAC chemistry. Here 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). The crystal structures of the BIGs and their carbonate salts have been analyzed by single-crystal X-ray and neutron diffraction to accurately measure key structural parameters including molecular conformations, hydrogen bonding, and p-stacking. Experimental measurements of key BIG 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. 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.

Direct air capture of CO2viacrystal engineering

“…Nevertheless, the temperatures of regeneration are very similar for GBIG, MGBIG, and DABIG, in the range of 120 to 160 C (under a ow of air). 10 Outstanding challenges and future opportunities With its structural precision and the prospect for tuning the CO 2 binding, transport, and release in crystalline frameworks, crystal engineering of MOFs and HBFs offers unique opportunities for DAC. Table 1 summarizes the key properties for some of these materials, relevant to their DAC performance.…”

“…Indeed, simple thermodynamic considerations define the lower end of the CO 2 binding constant (log K ) to 3.7, corresponding to a free energy of binding of at least −21 kJ mol −1 . 10 Furthermore, considering the entropy of binding is typically negative, the enthalpy of CO 2 binding needs to be significantly exothermic (at least −50 kJ mol −1 ). With regard to selectivity, DAC sorbents need to be selective for CO 2 against water, which is 10–100 times more abundant than CO 2 in the atmosphere and tends to bind relatively strongly to most sorbents.…”

“…9 ). 10 To a large extent, DAC by BIG crystallization is driven by the solubility difference between the neutral BIG ligand and the corresponding BIG carbonate salt. For GBIG, the solubility ratio between the free ligand and its bicarbonate salt is only 1.6, too small to drive DAC.…”

Design, synthesis, and physicochemical study of a biomass-derived CO2 sorbent 2,5-furan-bis(iminoguanidine)

“…reported an innovative iminoguanidine type sorbent, namely 2,6-pyridine-bis(iminoguanidine) (PyBIG) which could capture CO 2 from ambient air, crystallize as an insoluble carbonate and regenerate PyBIG by mild heating with concomitant CO 2 releasing ( Seipp et al., 2017 ; Brethomé et al., 2018 ). Soon after, they disclosed another simple and robust iminoguanidine compound called glyoxal-bis(iminoguanidine) (GBIG), by which the flue gas CO 2 absorption led to the formation of a dehydrated bicarbonate salt ( Williams et al., 2019 ; Garrabrant et al., 2019 ); however, further results proved that the GBIG was ineffective for DAC ( Custelcean et al., 2020 ). In addition, their most recent structure-property relationship study of GBIG, MGBIG (methylglyoxal-bis(iminoguanidine)) and DABIG (diacetyl-bis(iminoguanidine)) revealed that minor modifications in the molecular structures would result in dramatic differences in the crystal structures, aqueous solubilities, conformational flexibilities, as well as free energies for CO 2 absorption ( Custelcean et al., 2020 ).…”

“…Soon after, they disclosed another simple and robust iminoguanidine compound called glyoxal-bis(iminoguanidine) (GBIG), by which the flue gas CO 2 absorption led to the formation of a dehydrated bicarbonate salt ( Williams et al., 2019 ; Garrabrant et al., 2019 ); however, further results proved that the GBIG was ineffective for DAC ( Custelcean et al., 2020 ). In addition, their most recent structure-property relationship study of GBIG, MGBIG (methylglyoxal-bis(iminoguanidine)) and DABIG (diacetyl-bis(iminoguanidine)) revealed that minor modifications in the molecular structures would result in dramatic differences in the crystal structures, aqueous solubilities, conformational flexibilities, as well as free energies for CO 2 absorption ( Custelcean et al., 2020 ). Although great progress has been made, the development of DAC sorbents for CO 2 capture is still in its infancy.…”