Kinetic research on ion exchange regeneration of quaternary ammonium-based CO₂ sorbent for direct air capture

To satisfy the Paris Agreement's 1.5 °C goals, direct air capture (DAC) has become an increasingly crucial technology. However, the ultralow partial pressure of CO₂ in ambient air contributes to a low desorption rate and high regeneration energy consumption. In this study, we proposed a fast regeneration method for CO₂ sorbents based on an ion exchange process. Utilizing an alkaline solution rich in hydroxide (OH^–) ions, the quaternary ammonium (QA)-based ion exchange resin can be changed from carbonate (CO₃^2–) to OH^– form, which directly captures CO₂ from ambient air. The results showed that the working capacity of the sorbents in the OH^– form was 1.85 mmol⋅g⁻¹, which was double that of the CO₃^2– form (0.88 mmol⋅g⁻¹). The kinetics showed that the ratio of OH^–/HCO₃^– at inlet, concentration, and external diffusion influence the desorption rate of CO₃^2– because of the ion exchange reaction. With a high ratio of OH^–/HCO₃^– at inlet, 1 M alkaline solution, and 3 mL⋅s⁻¹ cyclic flow rate, the desorption time of the sorbents was approximately 20 min. Finally, because the energy consumption of the entire process comprises mainly electric energy required to regenerate the alkaline solution after ion exchange, it is expected to establish an electrochemical DAC system to utilize renewable energy efficiently.