Two Closely Related Zn(II)-MOFs for Their Large Difference in CO₂ Uptake Capacities and Selective CO₂ Sorption

Two azo functionalized Zn(II)-based MOFs, {[Zn(SDB)(3,3'-L)_0.5]·xG}, IITKGP-13A, and {[Zn₂(SDB)₂(4,4'-L)]·xG}, IITKGP-13B (IITKGP stands for Indian Institute of Technology Kharagpur), have been constructed through the self-assembly of isomeric N,N'-donor spacers (3,3'-L = 3,3'-azobispyridine and 4,4'-L = 4,4'-azobispyridine) with organic ligand 4,4'-sulfonyldibenzoic acid (SDBH₂) and Zn(NO₃)₂·6H₂O (G represents disordered solvent molecules). Single-crystal X-ray diffraction studies reveal the 2D structure with topology for both MOFs. However, the subtle change in positions of coordinating N atoms of spacers makes IITKGP-13A noninterpenetrated, while IITKGP-13B bears a 2-fold interpenetrated structure. IITKGP-13A exhibits higher uptake of CO₂ over CH₄ and N₂ with high IAST selectivities for mixed CO₂/CH₄ (50:50, biogas) and CO₂/N₂ (15:85, flue gas) gas systems. In contrast, IITKGP-13B takes up very low amount of CO₂ gas (0.4 mmol g^-1) compared to IITKGP-13A (1.65 mmol g^-1) at 295 K. Density functional theory (DFT)-based electronic structure calculations have been performed to explain the origin of the large differences in CO₂ uptake capacity between the two MOFs at the atomistic level. The results show that the value of the change in enthalpy (ΔH) at 298 K temperature and 1 bar pressure for the CO₂ adsorption is more negative in IITKGP-13A as compared to that in IITKGP-13B, thus indicating that CO₂ molecules are more favored to get adsorbed in IITKGP-13A than in IITKGP-13B. The computed values for the Gibbs' free energy change (ΔG) for the CO₂ adsorption are positive for both of the MOFs, but a higher value is observed for the IITKGP-13B. The noncovalent types of interactions are the main contribution toward the attractive energies between the host MOF frameworks and guest CO₂ molecules, which has been studied with the help of energy decomposition analysis (EDA).