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 SiF₆ ^2- ("SIFSIX") anions. Grand canonical Monte Carlo (GCMC) 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_st 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_st 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 CO₂ sorption were observed from the GCMC simulations.