Photo-DAC: Light-Driven Ambient-Temperature Direct Air Capture by a Photobase

Direct air capture (DAC) may reduce atmospheric CO₂ concentrations to preindustrial levels, yet the high energies and temperatures involved in current DAC technologies hinder large-scale deployment. In the case of aqueous-based CO₂ absorbents, a large energetic penalty is associated with heating and boiling off water, as required for thermally driven solvent regeneration. This could be avoided via photochemically driven pH swings involving photoacids or photobases, and harnessing abundant and renewable solar energy, though efficient solvent regeneration and recycling in a realistic multicycle DAC process remains challenging. Herein, we report a photochemically driven DAC process (photo-DAC) in which atmospheric CO₂ capture by an aqueous glycylglycine (GlyGly) solution is enabled through a pH swing by a pyridine-substituted diiminoguanidine (PyDIG) photobase. Upon irradiation with UV light, the PyDIG photobase undergoes photoisomerization from the <i>E</i>,<i>E</i> to the <i>Z</i>,<i>Z</i> isomer, corresponding to a p<i>K</i>_a increase of 2.8 units that activates GlyGly for DAC through deprotonation. After the GlyGly/PyDIG solvent is saturated with atmospheric carbon dioxide, leaving it in the dark under ambient conditions leads to the isomerization of PyDIG from the <i>Z</i>,<i>Z</i> back to the <i>E</i>,<i>E</i> isomer, which is accompanied by a pH drop and CO₂ release. To demonstrate the recyclability of the GlyGly/PyDIG solvent, we have completed six consecutive DAC cycles, with a measured average cyclic capacity in the range of 0.21-0.26 mol CO₂ per mol of GlyGly/PyDIG. These results open the prospect for energy-efficient DAC cycles completed entirely at ambient conditions, thereby avoiding the significant energy penalties associated with heating and boiling aqueous solvents.