Persistently high carbon dioxide (CO₂) emissions necessitate creating systems that capture CO₂ from the atmosphere for sequestration. Traditionally, CO₂ has been chemically captured as a carbamate using alkyl amines like monoethanolamine (MEA). Such solutions are then purged of CO₂ by heating to moderate temperatures, known as temperature swing, or by acidifying the solution to favor the breakup of carbamates and dissolution of CO₂, known as pH swing. Both methods use significant energy and generate further waste, motivating the search for a cyclic system that can release CO₂ with a minimal input of additional materials and more efficient use of the injected energy. Metastable photoacids— so-called because they acidify a solution due to a reversible photoisomerization reaction—can achieve a substantial pH swing due to the long lifetime of their photoactivated acidic form. Recently, it was shown that the merocyanine class of metastable photoacids can be used to release CO₂ from aqueous alkaline solutions, but non-aqueous, amine-based solutions were not studied. Here, we show that the merocyanine class of photoacids can release CO₂ from a solution of MEA. In situ detection of CO₂ release upon irradiation by a 405 nm light was recorded by monitoring the CO₂⁺ peak in a residual gas analyzer (RGA) mass spectrometer. Based on the electronic absorption spectra of the photoacid, we propose that the CO₂ capture of MEA and the photocycle of the photoacid form two independent equilibria that are coupled to each other through their shared use of protons. We show that these equilibria will shift by introducing either CO₂ or light to the system and generalize prior studies that only looked at this class of photoacid in aqueous basic environments. This model demonstrates that photoinduced CO₂ capture does not require precise alignment between the effective proton activity of the photoacid and CO₂ sorbent and suggests many paths for optimizing a photo-induced CO₂ capture and release system.

Kana Takematsu

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