Investigation of Moisture Swing Adsorbents for Direct Air Capture by Dynamic Breakthrough Studies

Adsorbent materials with humidity-modulated CO₂ sorption capacities are essential for direct air capture (DAC) based on moisture swing adsorption (MSA) processes. These materials have seldom been studied in the context of dynamic breakthrough experiments despite their efficacy in providing valuable equilibrium and kinetics information on the adsorbents and their resemblance to practical processes at large scales. Herein, we performed a series of breakthrough experiments to systematically investigate the DAC properties of the MSA adsorbent IRA-900-C. Prepared from the commercially available anion exchange resin IRA-900 (chloride form), IRA-900-C exhibits a CO₂ capacity of 1.92 mmol g^-1 at 20% RH at 25 °C. The CO₂ uptake capacity in IRA-900-C decreases as the environmental relative humidity (RH) increases at constant temperature. The competitive sorption behavior of CO₂ and H₂O is also revealed by humid CO₂ breakthrough experiments. Breakthrough experiments with different gas velocities and particle sizes of IRA-900-C suggest that the CO₂ adsorption kinetics in IRA-900-C is controlled by internal mass transfer resistances under DAC conditions. A theoretical maximum CO₂ working capacity of 1.27 mmol g^-1 can be achieved with IRA-900-C by swinging the RH from 20 to 50% RH at 25 °C along with constant purge of inert gas, and the feasibility of CO₂ production in a vacuum is experimentally verified. This study highlights the significance of dynamic breakthrough experiments in evaluating the DAC performance of MSA sorbents and providing valuable information for the design and optimization of DAC systems enabled by moisture swing processes.