Exploring enthalpies of CO2 and N2 adsorption on Zn- and Co-based zeolitic frameworks at varying temperatures and pressures

Abstract

This study investigates the thermodynamics of CO2 and N2 adsorption on zeolitic imidazolate frameworks (ZIFs), focusing on the effects of metal ions under varying temperatures and pressures. Zn- and Co-based ZIFs were synthesized using 2-methylimidazole and characterized by XRD, FT-IR, Raman spectroscopy, and BET isotherms, confirming high microporosity. Gas adsorption experiments were performed at pressures up to 3000 kPa and temperatures between 268 and 323 K. The CO2 adsorption capacities vary from 274 to 157 cm³/g, while N2 ranged from 67 to 29 cm³/g as temperature increased. The synergistic dynamics of temperature and pressure enhanced CO2 diffusion and induced pore structure changes via a gate-opening mechanism. Isosteric enthalpy (ΔHads) values, determined using Freundlich-Langmuir/Clausius-Clapeyron and virial fits, were -22 ± 5 kJ/mol for CO2 and -15 ± 2 kJ/mol for N2, confirming that physisorption is the dominant mechanism. Gibbs free energy (ΔGads) for CO2 ranged from -12 to -18 kJ/mol, and entropy (ΔSads) from -0.01 to -0.03 kJ/mol/K, suggesting spontaneous and thermodynamically favorable CO2 adsorption, though spontaneity decreased with rising temperature. ZIFs containing Zn exhibited high CO2 adsorption capacity, while those with Co showed better CO2 selectivity over N2, highlighting their potential for efficient CO2 capture under various temperature and pressure conditions.