Significance of fluid properties, pore structure, and surface chemistry in CMK series mesoporous carbon materials for N2/CH4/CO2 separation

Abstract

We employed a combination of macroscopic approaches (Ideal Adsorption Solution Theory, IAST, and Two-Dimensional Equation of State, 2D-EOS) and microscopic molecular simulations to explore the adsorption and separation of N2, CH4, and CO2 in mesoporous materials CMK-1, CMK-3, and CMK-5. Our findings reveal that these behaviors are intricately linked to the material's structure, surface properties, and the adsorbed fluid type. For nonpolar N2/CH4 supercritical fluid, both macroscopic theories align remarkably well with molecular simulations. For gas mixtures with a supercritical gas (N2 or CH4) and a subcritical gas (CO2), the impact of material structure on adsorption and separation varies. While both adsorption theories perform well for CMK-3, with excellent agreement to simulations, only IAST accurately predicts CMK-1. This discrepancy indicates that 2D-EOS has a less satisfactory predictive accuracy for CMK-1 compared to CMK-3 due to structural differences. The hexagonal arrangement of nanorods in CMK-3 leads to a more uniform adsorption site, while CMK-1's diverse adsorption sites, including nanorod surfaces and inter-rod grooves, contribute to a more complex adsorption behavior. In CMK-5, a unique jump in the CO2 adsorption selectivity is observed, attributed to a phase transition within the pores. Interestingly, this transition-induced jump defies accurate prediction by common theoretical models, except for IAST, which correctly predicts trends in selectivity and adsorption amounts. This phenomenon is specific to CMK-5's dual-pore structure, where most CO2 adsorption occurs inside the pipes until saturation, followed by adsorption on the external surface. Examining the impact of –COOH and –OH functional groups on adsorption behavior, we find minimal influence on separation in nonpolar N2–CH4 fluid, independent of the material type. However, in CO2-containing systems, the –COOH functional group has a substantial impact, highlighting its significance in influencing adsorption behavior.