Oxalamide-Functionalized Metal Organic Frameworks for CO₂ Adsorption

Metal-organic frameworks (MOFs) have received great attention in recent years as potential adsorbents for CO₂ capture due to their unique properties. However, the high cost and their tedious synthesis procedures impede their industrial application. A series of new CO₂-philic oxalamide-functionalized MOFs have been solvothermally synthesized: {[Zn₃(μ₈-OATA)_1.5(H₂O)₂(DMF)]·5/2H₂O·5DMF} (Zn-OATA), {[NH₂(CH₃)₂][Cd(μ₄-HOATA)]·H₂O·DMF} (Cd-OATA), and {[Co₂(μ₇-OATA)(H₂O)(DMF)₂]·2H₂O·3DMF} (Co-OATA) (H₄OATA = N,N'-bis(3,5-dicarboxyphenyl)oxalamide). In Zn-OATA, the [Zn₂(CO₂)₄] SBUs are connected by OATA^4- ligands into a 3D framework with 4-connected NbO topology. In Cd-OATA, two anionic frameworks with a dia topology interpenetrated each other to form a porous structure. In Co-OATA, [Co₂(CO₂)₄] units are linked by four OATA^4- to form a 3D framework with binodal 4,4-connected 4²·8⁴ PtS-type topology. Very interestingly, Cu-OATA can be prepared from Zn-OATA by a facile metal ions exchange procedure without damaging the structure while the CO₂ adsorption ability can be largely enhanced when Zn(II) metal ions are exchanged to Cu(II). These new MOFs possess channels decorated by the CO₂-philic oxalamide groups and accessible open metal sites, suitable for highly selective CO₂ adsorption. Cu-OATA exhibits a significant CO₂ adsorption capacity of 25.35 wt % (138.85 cm³/g) at 273 K and 9.84 wt % (50.08 cm³/g) at 298 K under 1 bar with isosteric heat of adsorption (Q_st) of about 25 kJ/mol. Cu-OATA presents a very high selectivity of 5.5 for CO₂/CH₄ and 43.8 for CO₂/N₂ separation at 0.1 bar, 298 K. Cd-OATA exhibits a CO₂ sorption isotherm with hysteresis that can be originated from structural rearrangements. Cd-OATA adsorbs CO₂ up to 11.90 wt % (60.58 cm³/g) at 273 K and 2.26 wt % (11.40 cm³/g) at 298 K under 1 bar. Moreover, these new MOFs exhibit high stability in various organic solvents, water, and acidic or basic media. The present work opens a new opportunity in the development of improved and cost-effective MOF adsorbents for highly efficient CO₂ capture.