Functionalized UNT-14 Metal–Organic Frameworks for Enhanced CO ₂ Adsorption and Separation: Insights from Monte Carlo Simulations and Density Functional Theory

Carbon dioxide (CO 2 ) is a major greenhouse gas responsible for global warming/climate change, whereas methane (CH 4 ) is the primary component of natural gas and serves as a comparatively cleaner energy source than coal and oil. Efficient separation of CO 2 from CH 4 - and N 2 -containing gas mixtures is therefore critical for both environmental mitigation and energy applications. Herein, we investigate the effect of linker functionalization on CO 2 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-NO 2, incorporating cyano (−CN) and nitro (−NO 2 ) groups, respectively, were constructed and systematically analyzed. Grand Canonical Monte Carlo simulations were employed to predict pure-component adsorption isotherms of CO 2, CH 4, and N 2, while density functional theory (DFT) calculations were used to evaluate CO 2 binding energies. Both functionalized frameworks exhibit significantly enhanced CO 2 uptake relative to parent UNT-14, concomitant with higher Henry’s constants ( K H ) and isosteric heats of adsorption at infinite dilution ( Q st0 ). DFT results corroborate these trends, revealing stronger CO 2 ···framework interactions in the functionalized materials. Radial distribution function analysis reveals preferential CO 2 adsorption near the –CN and –NO 2 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 CO 2 /CH 4 and CO 2 /N 2 separation selectivities under ambient conditions. This suggests that linker functionalization is an effective strategy for tuning the adsorption behavior of UNT-14 toward enhanced CO 2 capture and separation, thereby guiding future synthetic efforts.