Characterization of K-MER and 13X zeolites for humid direct air capture of CO₂ under equilibrium and cycling conditions

The deleterious effects of moisture on uptake capacity and regeneration energy requirements remain a major challenge for zeolite adsorbents in carbon capture applications. This motivates the need to design CO₂-selective and/or more hydrophobic zeolites even at the low Si/Al needed for practical binding site densities, particularly for the dilute CO₂ concentrations (∼400 ppm) involved in direct air capture (DAC) processes. Herein, we present the dry and humid DAC performance of commercial 13X and for the first time, potassium-exchanged merlinoite (K-MER, Si/Al = 3.6). We show that the CO₂ selectivity observed in K-MER for concentrated, humidified CO₂ does not persist at 400 ppm CO₂ and the same humidity (2095 ppm H₂O) at equilibrium. Despite this, we demonstrate that under non-equilibrium cycling conditions using mild regeneration at 100 °C, K-MER retains 73% of its dry uptake capacity over 5 humid DAC cycles, in contrast to 47% measured over 13X. A combination of adsorption isotherm and competitive adsorption/desorption studies in a dynamic column breakthrough (DCB) system, coupled with insights from steady-state and time-resolved diffuse reflectance infrared spectroscopy (DRIFTS), reveals that the more robust humid performance of K-MER compared to 13X can be attributed to more facile regeneration of CO₂ binding sites due to more weakly-bound H₂O and CO₂ (i.e., linear-only) species. These results underscore how the nature and strength of adsorbed species in binary CO₂ and H₂O mixtures are greatly influenced by the combination of framework topology and Si/Al (even for Si/Al < 5), highlighting further design opportunities for humid-tolerant zeolite DAC adsorbents.