(345e) Identifying Best Core MOFs with Open Mg Sites for CO2/N2 Separation Using Computational Tools

Metal-organic frameworks (MOFs) are nanoporous, crystalline structures with favorable properties such as high pore volume, surface area, chemical diversity, flexibility that can be quite beneficial for gas storage, separation/filtering, catalysis applications. Since they are assembled from metal nodes and organic linkers as the building blocks, hypothetically, a limitless number of MOFs can be formed. MOFs with open metal sites are very promising candidates for separations because certain sorbates can have highly favorable interactions with these nodes. Recently, the CoRE (computation-ready experimental) MOF database¹ has been created to enable large-scale computational screening studies. To understand their potential for CO₂/N₂ separation, performing full quantum chemical calculations on each of these MOFs would be computationally prohibitive, but molecular mechanics force fields fail to capture chemisorption interactions.

In this study, the CoRE MOF database is screened for MOFs with open Mg sites that are optimal for CO₂/N₂ separation at flue gas conditions. To this extent, a hierarchical screening approach is employed. First, a geometric approach is used to identify MOFs with open Mg sites. Second, unary adsorption isotherms are determined from a multi-site Langmuir model in which the guest-host interaction energy for the volume element near the open Mg site is computed by periodic density functional theory (DFT) and the remainder of the interactions is calculated by a combination of the TraPPE force field for the sorbate molecules and of the UFF force field for the framework. Third, for MOFs with sufficient CO₂ capacity, the binary CO₂/N₂ adsorption isotherms and selectivities are estimated using the ideal adsorbed solution theory (IAST)². Fourth, the self diffusivities and transport selectivities are estimated. Finally, first principles simulations are performed for the most promising materials.

References

1. Chung, Y. G. et al. Computation-Ready, Experimental Metal–Organic Frameworks: A Tool To Enable High-Throughput Screening of Nanoporous Crystals. Chem. Mater. 26, 6185–6192 (2014).

2. Myers, A. L. & Prausnitz, J. M. Thermodynamics of mixed-gas adsorption. AIChE J. 11, 121–127 (1965).