Metal-Organic Frameworks with Metal-Catecholates for O₂/N₂ Separation

Oxygen and nitrogen are widely produced feedstocks with diverse fields of applications but are primarily obtained via the energy-intensive cryogenic distillation of air. More energy-efficient processes are desirable, and materials such as zeolites and metal-organic frameworks (MOFs) have been studied for air separation. Inspired by recent theoretical work identifying metal-catecholates for enhancement of O₂ selectivity MOFs, the computation-ready experimental database of MOF structures was screened to identify promising candidates for incorporation of metal-catecholates. On the basis of structural requirements, preliminary Grand-Canonical Monte Carlo simulations, and further constraints to ensure the computational feasibility, over 5000 structures were eliminated and four MOFs (UiO-66(Zr), Ce-UiO-66, MOF-5, and IRMOF-14) were treated with periodic density functional theory (DFT). Metal-catecholates (Mg, Co, Ni, Zn, and Cd) were selected on the basis of cluster DFT calculations and were added to the shortlisted MOFs. Periodic DFT was used to compute O₂ and N₂ binding energies near metal-catecholates. We find that the binding energies are primarily dependent on the metals in the metal-catecholates, all of which bind O₂ quite strongly (80-258 kJ/mol) and have weaker binding for N₂ (3-148 kJ/mol). Of those studied here, Cd-catecholated MOFs are identified as the most promising.