Comparing the mechanism and energetics of CO2 sorption in the SIFSIX series

Abstract

Simulations of CO₂ sorption were performed in five members of the previously discovered SIFSIX series: SIFSIX-1-Cu, SIFSIX-2-Cu, SIFSIX-2-Cu-i, SIFSIX-3-Zn, and SIFSIX-3-Cu. These metal–organic materials (MOMs) consist of metal ions that are coordinated to linear ligands and are pillared with SiF₆²⁻ (“SIFSIX”) ions. These MOMs mostly differ in the ligand that is used to synthesize the material, although interpenetration and substitution of the metal ion is also responsible for the formation of some members. The pore size in this series ranges from 3.54 to 13.05 Å. The simulated CO₂ sorption isotherms and isosteric heat of adsorption (Q_st) values for all five MOMs are in good agreement with the corresponding experimental data. Consistent with experimental measurements, our simulations demonstrate that the Q_st for CO₂ increases as the pore size decreases in this series. It was found that SIFSIX-1-Cu and SIFSIX-2-Cu-i exhibit favorable MOM–sorbate and sorbate–sorbate interactions upon CO₂ sorption, which explains why the CO₂Q_st increases as a function of loading in these variants. Further, SIFSIX-3-Zn and SIFSIX-3-Cu display nearly constant Q_st values for all loadings considered because only one type of sorption site is present in these MOMs. Notable differences in the repulsion/dispersion and electrostatic contributions for CO₂ sorption were observed in all five SIFSIX MOMs, with no particular trend upon variation of the pore size. SIFSIX-2-Cu-i exhibits the highest contributions from repulsion/dispersion interactions, while SIFSIX-3-Cu displays the highest percentage from electrostatic interactions within the series. Polarization interactions are negligible for all five members. Overall, our simulations reveal that these five SIFSIX MOMs display different CO₂ sorption mechanisms and energetics. This led to differences in the sorbate induced dipole distribution and the radial distribution functions (g(r)) about the Si atoms for all MOMs.