Alkaline salts of L-lysine as green solvents for direct air capture: Characterisation and performance analysis

This study investigates the potential of potassium and sodium salts of L-lysine, a basic amino acid, as green solvents for direct air capture (DAC). Such green solvents help to reduce the life-cycle impact of the DAC process, eliminate harmful fugitive emissions, and pave the way for the sustainable development of the latter. This study includes detailed characterisation analysis in terms of functional groups using Fourier Transform Infrared Spectroscopy (FTIR), elemental analysis through Energy Dispersive Infrared Spectroscopy (EDX), and structural studies through X-ray diffraction (XRD) of the alkaline L-lysine salts. FTIR analysis confirmed the formation of sodium and potassium lysinate salts, with characteristic shifts in the O-H and N-H stretching regions. EDX studies revealed that both salts formed stable molecular structures with further oxidation. XRD analysis showed partial crystallinity in both salts in the presence of an amorphous phase, which improved the gas solubility. The salt solutions were tested for carbon dioxide solubility under laboratory conditions at 25 °C and varying pressure (30 – 100 kPa) and flow rates (10 – 30 L/min) with CO₂ loading values as high as 1.0 mol CO₂/mol solvent. The solubility tests signify that the proposed solvent offers comparable performance to other classes of solvents used in DAC, and also experimentally ascertains the DAC potential found in the characterisation results. Overall, these findings suggest that L-lysine-based salts are promising candidates for green DAC applications as they form stable salts with DAC-favouring chemical properties and molecular stability. These results will be helpful in the development of alkaline amino acid salts as potential green solvents for DAC applications on an industrial scale.