Impact of water on CO 2 adsorption in LTA zeolites with Na + and Ca2＋: A molecular simulation approach

This work investigates the influence of hydration on CO 2 adsorption in LTA-type zeolites containing Na + and Ca 2＋ charge-compensating cations by means of Grand Canonical Monte Carlo (GCMC) simulations. Anhydrous and hydrated forms of Na-LTA and Ca-LTA were analyzed and compared with the purely siliceous ITQ-29 zeolite. The study focuses on adsorption isotherms, accessible pore volume, water localization, and global average isosteric heats of adsorption. The results show that the presence of water leads to a systematic reduction in CO 2 adsorption capacity for all investigated structures, with more pronounced effects observed in cation-exchanged zeolites. This reduction arises from the preferential occupation of adsorption sites by H 2 O molecules and the consequent decrease in pore volume accessible to CO 2 . Despite the lower uptake, hydrated Na-LTA and Ca-LTA exhibit higher global average isosteric heats of adsorption, which is attributed to the persistence of energetically strong adsorption environments associated with cation-influenced sites. The purely siliceous ITQ-29 framework displays weaker adsorption energies and lower sensitivity to hydration effects, consistent with its hydrophobic character and predominantly dispersive interactions. Overall, the results demonstrate that hydration affects CO 2 adsorption through a combined geometric and energetic mechanism, leading to a decoupling between adsorption capacity and interaction strength. These findings provide important insights into the behavior of zeolitic adsorbents under humid conditions and contribute to the rational design of materials for CO 2 capture applications.