Molecular Simulations and Experimental Studies of CO₂, CH₄, and N₂ Adsorption in the UNT-14 Metal-Organic Framework

Selective adsorption of CO₂ over CH₄ and N₂ using porous materials is a promising approach for the capture of CO₂ and upgrading of natural gas. Herein, we present a combined simulation and experimental study of the adsorption uptakes of CO₂, CH₄, and N₂ in UNT-14, a copper-based metal-organic framework. Grand Canonical Monte Carlo (GCMC) simulations were employed to predict the pure component adsorption isotherms at 273 and 298 K using atomic charges calculated via three different charge methods. Among those, the Mulliken charge set agrees best with the experimental adsorption data. UNT-14 exhibits greater affinity for CO₂ as compared to CH₄ and N₂, revealed by higher Henry's constant (K_H) and isosteric heats of adsorption at infinite dilution (Q_st0). Density functional theory (DFT) calculation displays a larger binding energy (BE) value for CO₂ than for CH₄ and N₂. Radial distribution function (RDF) analysis reveals that CO₂ molecules tend to adsorb preferentially on the peripheral benzene rings, whereas CH₄ and N₂ molecules tend to adsorb more preferentially on the central benzene ring of the linker. The ideal adsorbed solution theory (IAST) suggests a favorable adsorption selectivity of UNT-14 for equimolar CO₂/CH₄ and CO₂/N₂ gas mixtures (for both the experimental and simulated data), demonstrating efficient CO₂ capture and natural gas upgrading ability.