Computational design of functionalized imidazolate linkers of zeolitic imidazolate frameworks for enhanced CO<SUB>2</SUB> adsorption

Zeolitic imidazolate frameworks (ZIFs) represent the class of metal–organic frameworks (MOFs) that possess high porosity, large surface area, exceptional thermal, and chemical stability. Because of these properties, ZIFs are being employed extensively in gas separation and selective CO2 adsorption. We have chosen the structural modification approach to enhance the CO2 binding ability of various imidazolate (Im) linkers of ZIFs by systematically varying the substituents at 2, 4, and 5 positions of Im ring with CH3, Cl, CN, OH, NH2, and NO2 functional groups. Density functional theory (DFT) calculations have been employed to identify and quantify the CO2 binding ability of various adsorption sites present in 137 Im linkers. The study demonstrates that the Im linkers with asymmetrical substitution, viz. NO2/OH, CN/OH, and Cl/OH combinations are highly promising linkers of ZIFs for efficient CO2 adsorption. The QTAIM analysis characterizes these interactions as noncovalent interactions which are stabilized by weak hydrogen bond and van der Waals (vdWs) interactions. Localized molecular orbital energy decomposition analysis (LMO-EDA) performed on substituted Im···CO2 complexes reveals that CO2 binding is governed by a combination of H-bonding, electrostatic, and dispersion interactions. The findings of the study will serve as guide-in principles to synthesize new adsorbents with enhanced and selective CO2 adsorption