Functionalized UNT-14 Metal-Organic Frameworks for Enhanced CO2 Adsorption and Separation: Insights from Monte Carlo...

Carbon dioxide (CO2) is a major greenhouse gas responsible for global warming/climate change, whereas methane (CH4) is the primary component of natural gas and serves as a comparatively cleaner energy source than coal and oil. Efficient separation of CO2 from CH4- and N2-containing gas mixtures is therefore critical for both environmental mitigation and energy applications. Herein, we investigate the effect of linker functionalization on CO2 adsorption and separation performance in a metal-organic framework we recently developed, UNT-14, using a combined computational approach. Two functionalized analogues, UNT-14-CN and UNT-14-NO2, incorporating cyano (-CN) and nitro (-NO2) groups, respectively, were constructed and systematically analyzed. Grand Canonical Monte Carlo simulations were employed to predict pure-component adsorption isotherms of CO2, CH4, and N2, while density functional theory (DFT) calculations were used to evaluate CO2 binding energies. Both functionalized frameworks exhibit significantly enhanced CO2 uptake relative to parent UNT-14, concomitant with higher Henry's constants (KH) and isosteric heats of adsorption at infinite dilution (Qst0). DFT results corroborate these trends, revealing stronger CO2···framework interactions in the functionalized materials. Radial distribution function analysis reveals preferential CO2 adsorption near the -CN and -NO2 groups in the functionalized structures, in contrast to adsorption near the Cu clusters in parent UNT-14. Ideal adsorbed solution theory calculations further demonstrate improved CO2/CH4 and CO2/N2 separation selectivities under ambient conditions. This suggests that linker functionalization is an effective strategy for tuning the adsorption behavior of UNT-14 toward enhanced CO2 capture and separation, thereby guiding future synthetic efforts.