Ba-exchanged gismondine for CO2 direct air capture (DAC)

Abstract

The development of efficient solid adsorbents for capturing CO₂ at trace atmospheric concentrations (∼400 ppm) is crucial for direct air capture (DAC). This work investigates the effect of Ba²⁺ exchange in gismondine (GIS) zeolite on CO₂ adsorption under DAC conditions. Ba²⁺ exchange distorts the GIS framework, modifying pore accessibility and generating strong electrostatic adsorption sites within the eight-membered ring channels. As a result, CO₂ adsorption at ultra-low partial pressures is significantly enhanced, with the fully exchanged sample achieving ∼1.15 mmol g⁻¹ at 25 °C and 400 ppm, ranking it among the highest-performing physisorbents for DAC. In situ FTIR spectroscopy combined with multivariate curve resolution (MCR-ALS) shows that Ba²⁺ exchange shifts the CO₂ adsorption from weak, Na⁺-dominated environments to stronger, more uniform Ba²⁺ sites. Two-dimensional infrared inversion spectroscopy (2D-IRIS) reveals a progressive localization and deepening of adsorption free-energy minima with increasing Ba²⁺ content, linking spectroscopic features to adsorption strength. Dynamic breakthrough experiments under dry and humid conditions confirm that the fully Ba-exchanged GIS outperforms the benchmark Na-MOR zeolite in both working capacity and kinetics. These results demonstrate that Ba²⁺ incorporation in small-pore GIS zeolites enables an optimal balance of confinement, adsorption strength, and mass transfer, positioning Ba-GIS as a superior physisorbent for DAC.