Direct Air Capture of CO₂ with Aqueous Amino Acids and Solid Bis-iminoguanidines (BIGs)

We report a bench-scale direct air capture (DAC) process comprising CO₂ absorption with aqueous amino acid salts (i.e., potassium glycinate, potassium sarcosinate), followed by room-temperature regeneration of the amino acids by reaction with solid meta-benzene-bis(iminoguanidine) (m-BBIG), resulting in crystallization of the hydrated m-BBIG carbonate salt, (m-BBIGH₂)(CO₃)(H₂O)_n (n = 3-4). The CO₂ is subsequently released by mild heating (60-120 °C) of the carbonate crystals, which regenerates the m-BBIG solid quantitatively. This low-temperature crystallization-based DAC process circumvents the need to heat the aqueous amino acid sorbents, thereby minimizing their loss through thermal and oxidative degradation. The CO₂ cyclic capacity for the sarcosine/m-BBIG system, measured over three consecutive absorption/regeneration cycles, is in the range of 0.12-0.20 mol/mol. The regeneration energy of m-BBIG, comprising the enthalpy of CO₂ and water release, and the sensible heat, is 360 kJ/mol (8.2 GJ/ton CO₂). Alternatively, the aqueous amino acids can be regenerated by boiling under reflux, with measured cyclic capacities of up to 0.64 mol/mol.