Subcritical Coadsorption and Metastability of Methane–Carbon Dioxide Mixtures in Metal Organic Frameworks

Metal–organic frameworks (MOFs) are a class of crystalline nanoporous materials with exceptionally high surface areas and tunable pore volumes, making them promising for gas storage, separation, and capture. In this study, we investigate the subcritical coadsorption of methane (CH₄) and carbon dioxide (CO₂) in a representative nanoporous framework, IRMOF-8, using Transition-Matrix Monte Carlo (TMMC) simulations. Systems containing up to 400 CH₄ and 150 CO₂ molecules per unit cell were examined under subcritical conditions to characterize the free-energy landscape and cooperative adsorption behavior. The adsorption of CH₄ exhibits sharp discontinuities in uptake, which shift to lower pressures as the CO₂ concentration increases. Furthermore, the metastable adsorption states previously identified in pure CH₄ systems [1] are progressively destabilized by CO₂ coadsorption. These results reveal how guest–guest interactions modulate the free-energy barriers and equilibrium behavior in multicomponent gas adsorption, providing new insights into the thermodynamics of confined fluid mixtures in nanoporous materials.