Cooperative CO₂ Absorption Isotherms from a Bifunctional Guanidine and Bifunctional Alcohol

Designing new liquids for CO₂ absorption is a challenge in CO₂ removal. Here, achieving low regeneration energies while keeping high selectivity and large capacity are current challenges. Recent cooperative metal-organic frameworks have shown the potential to address many of these challenges. However, many absorbent systems and designs rely on liquid capture agents. We present herein a liquid absorption system which exhibits cooperative CO₂ absorption isotherms. Upon introduction, CO₂ uptake is initially suppressed, followed by an abrupt increase in absorption. The liquid consists of a bifunctional guanidine and bifunctional alcohol, which, when dissolved in bis(2-methoxyethyl) ether, forms a secondary viscous phase within seconds in response to increases in CO₂. The precipitation of this second viscous phase drives CO₂ absorption from the gas phase. The isotherm of the bifunctional system differs starkly from the analogous monofunctional system, which exhibits limited CO₂ uptake across the same pressure range. In our system, CO₂ absorption is strongly solvent dependent. In DMSO, both systems exhibit hyperbolic isotherms and no precipitation occurs. Subsequent ¹H NMR experiments confirmed the formation of distinct alkylcarbonate species having either one or two molecules of CO₂ bound. The solvent and structure relationships derived from these results can be used to tailor new liquid absorption systems to the conditions of a given CO₂ separation process.