CO2-Induced Shifts in the Confined Phase Transformation of CH4 in Metal-Organic Frameworks

Abstract

A sharp increase in gas uptake over a narrow pressure range can occur for fluids confined in nanoporous materials such as metal-organic frameworks (MOFs), reflecting a transformation between distinct adsorption states, often accompanied by the existence of metastable states. Understanding how co-adsorbed species modify this behavior is important for predicting and optimizing gas storage and separation. Here, we investigate the effect of CO2 on the adsorption transformation of confined CH4 in rigid IRMOF-1 and IRMOF-8. Transition-Matrix Monte Carlo (TMMC) simulations were performed for systems containing 0-75 and 0-150 CO2 molecules in IRMOF-1 and IRMOF-8, respectively. While the addition of CO2 decreases the maximum CH4 uptake, consistent with competition for pore space, remarkably, the presence of CO2 significantly shifts the discontinuity in the CH4 adsorption isotherm to substantially lower pressures relative to the pure CH4 system. This promotion of the high-density CH4 phase at lower pressures demonstrates that CO2 substantially alters the thermodynamic landscape governing confined CH4 through changes in both fluid-fluid and fluid-framework interactions. These results establish CO2 as a thermodynamic modifier of confined CH4 behavior and suggest that controlled co-adsorption provides a means of tuning adsorption transformations in nanoporous materials.