Thermoresponsive CO₂ absorbent for various CO₂ concentrations: tuning the pK _a of ammonium ions for effective carbon capture

Amine absorbents that efficiently absorb and desorb CO₂ in response to small temperature changes are desired for CO₂ separation from concentrated and dilute gases. Thermoresponsive hydrogel films consisting of amine-containing microgel particles (GPs), which capture CO₂ at a low temperature (30 °C) and desorb it upon mild heating (75 °C), are attractive for capturing CO₂ from postcombustion gases containing 10% CO₂ (10 kPa). However, little information has been reported about thermoresponsive GPs for CO₂ separation from gas mixtures with low concentrations of CO₂. Herein, we describe the effect of the pK_a of ammonium ions in GPs on the amount of CO₂ desorption upon heating at 75 °C, which was investigated at various CO₂ concentrations. The efficiency of CO₂ desorption (mol-desorbed CO₂/mol-amine) depends on the pK_a and pK_a shift (ΔpK_a) of the ammonium ions in the range of 30‒75 °C. Computational predictions also indicated that the pK_a values and ΔpK_a are both important for reversible CO₂ absorption. A guideline for designing thermoresponsive amine absorbents for various applications including direct air capture and carbon recycling in closed spaces, such as space stations and submarines, is provided.