Finding MOFs for highly selective CO ₂/N ₂ adsorption using materials screening based on efficient assignment of atomic point charges

Electrostatic interactions are a critical factor in the adsorption of quadrupolar species such as CO ₂ and N ₂ in metal-organic frameworks (MOFs) and other nanoporous materials. We show how a version of the semiempirical charge equilibration method suitable for periodic materials can be used to efficiently assign charges and allow molecular simulations for a large number of MOFs. This approach is illustrated by simulating CO ₂ and N ₂ adsorption in ∼500 MOFs; this is the largest set of structures for which this information has been reported to date. For materials predicted by our calculations to have promising adsorption selectivities, we performed more detailed calculations in which accurate quantum chemistry methods were used to assign atomic point charges, and molecular simulations were used to assess molecular diffusivities and binary adsorption isotherms. Our results identify two MOFs, experimentally known to be stable upon solvent removal, that are predicted to show no diffusion limitations for adsorbed molecules and extremely high CO ₂/N ₂ adsorption selectivities for CO ₂ adsorption from dry air and from gas mixtures typical of dry flue gas.