Investigating CO2 Sorption in SIFSIX-3-M (M = Fe, Co, Ni, Cu, Zn) through Computational Studies

A combined Monte Carlo (MC) simulation and periodic density functional theory (DFT) study of CO2 sorption was performed in SIFSIX-3-M (M = Fe, Co, Ni, Cu, Zn), a family of hybrid ultramicroporous materials (HUMs) that consist of M2+ ions coordinated to pyrazine ligands and are pillared with SiF62- ("SIFSIX") anions. Grand canonical Monte Carlo (GCMG) simulations of CO2 sorption in all five SIFSIX-3-M variants produced isotherms that are in good agreement with the corresponding experimental measurements. The theoretical isosteric heats of adsorption (Q(st)) for CO2 as obtained through canonical Monte Carlo (CMC) simulations are also in close agreement with the experimental values. Consistent with experiment, the simulations generated the following trend in the CO2 Q(st): SIFSIX-3-Cu > SIFSIX-3-Ni > SIFSIX-3-Co > SIFSIX-Zn > SIFSIX-3-Fe. The magnitudes of the theoretical Q and relative trend were further supported by periodic DFT calculations of the adsorption energy for CO2 within the respective HUMs. We attribute the observed Q trend in SIFSIX-3-M to their differences in pore size and lattice parameters. Specifically, the sorption energetics decrease with increasing pore size and a/b lattice constant. Simulations of CO2 sorption in SIFSIX-3-Cu resulted in different profiles for the radial distribution function (g(r)) and dipole distribution than within the other analogues due to the smaller pore size and much shorter a/b unit cell lengths of the crystal structure; this is a direct consequence of the Jahn-Teller effect. Although these HUMs are isostructural, notable differences in the classical energy contributions for CO, sorption were observed from the GCMC simulations. Overall, this study demonstrates that the CO2 Q(st) in SIFSIX-3-M can be controlled by the choice of the saturated metal, with values ranging from 42 to 54 kJ mol(-1).