Computational study of oxygen adsorption in metal-organic frameworks with exposed cation sites: effect of framework metal ions

The current inefficient separation of O-2 from air is an important industrial problem. Metal-organic frameworks containing coordinatively unsaturated metal sites (CUS) have emerged as competitive new adsorbents for such targets. In this study, dispersion-corrected density functional theory calculations were performed to investigate the influence of framework metal ions on the adsorption behavior of O-2 in M-3(BTC)(2)-type materials (M = Cr, Mn, Fe, Co, Ni and Cu; BTC = 1,3,5-benzenetricarboxylate acid). The results show that Ni-3(BTC)(2) can be potentially considered as promising oxygen adsorbent with relatively easier deoxygenation than Cr-3(BTC)(2). The magnitude of charge transfer from the CUS to O-2 molecule was found to have a significant impact on the interaction energies of O-2 with M-3(BTC)(2) except for the Cu version. Furthermore, it was revealed that the origin of the difference in the charge transfer can be attributed to the different electronegativity of the metals.