Comparative Adsorption of CO ₂ and Ethane in Natural Zeolites: Insights from Gravimetric Measurements and Modeling Study

The adsorption and storage behavior of fluids in nanoporous materials have been widely explored using synthetic porous media with uniform pore structures; however, the potential of naturally occurring zeolites remains underexamined despite their low cost and abundance. This study investigates the adsorption of carbon dioxide (CO₂) and ethane (C₂H₆) in natural zeolites, clinoptilolite and two chabazite variants to elucidate how temperature, adsorbent, and adsorbate properties influence confined phase behavior. The porous structure and surface chemistry of these zeolites are examined using a variety of characterization methods, including X-ray diffraction (XRD), Fourier-transform infrared (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and low-temperature N₂ sorption isotherms. The adsorption and desorption isotherms were measured over 253 to 283 K using an in-house-built gravimetric nanocondensation apparatus, capable of handling large sample quantities under subambient to near-ambient conditions. All zeolites exhibited stronger adsorption for CO₂ than for C₂H₆, with overall capacity decreasing as temperature increased, consistent with exothermic physisorption. Narrow, temperature-dependent H3 hysteresis loops demonstrated capillary condensation like behavior within slit-like pores. The Aranovich-Donohue-Freundlich (AD-Freundlich) model accurately described all isotherms (<i>R</i>² ≥ 0.994), capturing the distinct adsorption characteristics of both gases across the three zeolites. CO₂ consistently exhibited higher adsorption capacity than C₂H₆, with clinoptilolite showing the strongest overall affinity among the materials. The heterogeneity exponent increased with temperature, indicating more uniform adsorption at higher thermal energy. Isosteric heats derived from the Clausius-Clapeyron relation ranged from 10-21 kJ mol^-1 for CO₂ and 4-7 kJ mol^-1 for C₂H₆, confirming dominant electrostatic interactions for CO₂ and van der Waals-controlled physisorption for ethane. Overall, the results demonstrate consistent structure-temperature relationships and indicate that this study provides valuable insight toward utilizing natural zeolites as low-cost, scalable materials for CO₂ capture and light-hydrocarbon storage.