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

Abstract

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 pKa 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 pKa and pKa shift (ΔpKa) of the ammonium ions in the range of 30‒75 °C. Computational predictions also indicated that the pKa values and ΔpKa 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. Amine-containing microgel particles (GPs), which capture CO2 at a low temperature and desorb it upon a mild heating, are attractive materials for capturing CO2. In this paper, the effect of pKa of ammonium ions in the GPs on the amount of CO2 desorption upon heating was investigated at various CO2 concentrations by experiments and thermodynamic predictions. 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.