Induced selective $N_2$ binding under electric field in MOF-74

Abstract

Dinitrogen is the most abundant species in the air and an inert gas molecule making its separation difficult in other gas mixtures. In this theoretical study, selective binding of dinitrogen to the coordinatively unsaturated metal site in M-MOF-74 (M=Mg, V, Cr, Mn, Fe, Co, Ni, Cu) under external electric field is investigated. Simulation results suggest that external electric field enhances the $π^*$ back-bonding between the transition metal and dinitrogen molecule while weakening σ bond between the metal and other small gas molecules such as $CH_4$ and $CO_2$. In particular, Co-MOF-74 and Fe-MOF-74 show the highest increase in dinitrogen binding energy. As a result, the binding energy of dinitrogen is more than twice that of $CH_4$ and $CO_2$. Our work demonstrates that asymmetric effect that the electric field casts on different gas molecules can serve as another dimension of design that can be exploited in small gas molecule separations via metal-organic frameworks.