Critical temperature-dependent adsorption selectivity of binary gas mixtures in slit pores: Insights from Gibbs ensemble Monte Carlo simulations

We conducted constant pressure Gibbs ensemble Monte Carlo molecular simulations to explore the adsorption separation of 3 binary gas mixtures: CH₄/CO, C₂F₆/N₂, and SO₂/CO₂ within slit pores. Key findings indicate that CH₄/CO, a mixture of 2 supercritical gases at room temperature, shows modest adsorption selectivity of around 4, even at elevated pressures of 20 MPa. In contrast, the C₂F₆/N₂ mixture, consisting of supercritical N₂ and C₂F₆ near its critical temperature, exhibits significantly higher selectivity, reaching tens to hundreds. The SO₂/CO₂ mixture, with both gases in a subcritical state at room temperature, displays intermediate selectivity between the other 2 systems. Our simulations revealed that the adsorption selectivity for CH₄/CO and C₂F₆/N₂ mixtures displays distinct single- and double-peaked trends with varying pore widths under medium to high pressures, corresponding to monolayer and bilayer adsorption phenomena. The SO₂/CO₂ system, however, presented a more intricate adsorption mechanism, potentially involving 3-layer molecular adsorption within the pores. Expanding our investigation to 276 mixtures, we discovered an important trend: a higher ratio of critical temperatures between mixture components correlates with increased adsorption selectivity and simplified separation processes. Intriguingly, when this ratio approaches unity, separation difficulty escalates. Additionally, we identified a significant linear relationship between adsorption selectivity and the ratio of adsorption heats at low pressures (0.1 MPa) for a pore width of 0.8 nm, underscoring the impact of thermodynamic properties on separation efficacy. These insights are crucial for the development of energy-efficient gas separation materials, which are vital for applications such as natural gas purification and carbon capture and storage, contributing to a sustainable energy future.