Theoretical Optimization of Pore Size and Chemistry in SIFSIX-3-M Hybrid Ultramicroporous Materials

Classical Grand Canonical Monte Carlo (GCMC), classical molecular dynamics (MD), and density functional theory (DFT) have been employed to study the effect of pore-size and pore-chemistry on the adsorption of carbon dioxide in the SIFSIX-3-M family of hybrid ultramicroporous materials (HUMs), where M = Zn 2+, Ni 2+, or Cu 2+ . These HUMs have been shown to exhibit exceptional affinity toward small polarizable molecules such as CO 2, and our simulated isotherms are in good agreement with those obtained experimentally. Isosteric heats of adsorption ( Q st ) calculated using these theoretical methods also follow the experimentally observed trend, decreasing as M varies from Cu to Ni to Zn in SIFSIX-3-M . More specifically, the interaction energy between a CO 2 molecule and HUMs with varying pore-size was calculated using a DFT-D2 level of theory. The strongest interaction energy was calculated for SIFSIX-3-Cu (56.89 kJ mol –1 ) where the pore-size is the smallest. The interaction energy decreased in SIFSIX-3-Ni (52.21 kJ mol –1 ) and SIFSIX-3-Zn (48.46 kJ mol –1 ), each of which exhibiting larger pore dimensions. The increase in the strength of the interaction in SIFSIX-3-Cu is attributed to the shorter distance between the negatively charged equatorial fluorine atoms of the SiF 6 2– pillar and the positively charged carbon atom of CO 2 . Finally, DFT-D2 calculations were used to reconcile the uncertainty in locating the exact crystallographic position of equatorial fluorine atoms in the SIFSIX - 3-M family. The calculated interaction energies agree best with experimental Q st values when equatorial fluorine atoms are at a 45° angle with respect to the crystallographic a- axis. Our theoretical calculations thus create a foundation for first-principles based rational design of SIFSIX-3-M and other HUMs for selective adsorption of carbon dioxide and other relevant sorbates such as acetylene.