H₂O and CO₂ Sorption in Ion-Exchange Sorbents: Distinct Interactions in Amine Versus Quaternary Ammonium Materials

This study investigates the H₂O and CO₂ sorption behavior of two chemically distinct polystyrene-divinylbenzene-based ion exchange sorbents: a primary amine and a permanently charged strong base quaternary ammonium (QA⁺) group with (bi)carbonate counter anions. We compare their distinct interactions with H₂O and CO₂ through simultaneous thermal gravimetric, calorimetric, gas analysis, and molecular modeling approaches to evaluate their performance for dilute CO₂ separations like direct air capture. Thermal and hybrid (heat + low-temperature hydration) desorption experiments demonstrate that the QA⁺-based sorbent binds both water and CO₂ more strongly than the amine counterparts but undergoes degradation at moderate temperatures, limiting its compatibility with thermal swing regeneration. However, a low-temperature moisture-driven regeneration pathway is uniquely effective for the QA⁺-based sorbent. To inform the energetics of a moisture-based CO₂ separation (i.e., a moisture swing), we compare calorimetric water sorption enthalpies to Clausius-Clapeyron-derived total isosteric enthalpies. To our knowledge, this includes the first direct calorimetric measurement of water sorption enthalpy in a QA⁺-based sorbent. Both methods reveal monolayer-multilayer sorption behavior for both sorbents, with the QA⁺-based material having slightly higher water sorption enthalpies at the initially occupied strongest sorption sites. Molecular modeling supports this observation, showing higher water sorption energies and denser charge distributions in the QA⁺-based sorbent at λ_H₂O = 1 mmol/mmol_site. Mixed gas experiments in the QA⁺-based sorbent show that not only does water influence CO₂ binding, but CO₂ influences water uptake through counterion-dependent hydration states, and that moisture swing responsiveness in this material causes hydration-induced CO₂ release and drying-induced CO₂ uptake, an important feature for low-energy CO₂ separation under ambient conditions. Overall, the two classes of sorbents offer distinct pathways for the CO₂ separation.