Computational studies of CO₂ sorption and separation in an ultramicroporous metal-organic material

Grand canonical Monte Carlo (GCMC) simulations of CO₂ sorption and separation were performed in [Zn(pyz)₂SiF₆], a metal-organic material (MOM) consisting of a square grid of Zn²⁺ ions coordinated to pyrazine (pyz) linkers and pillars of SiF₆ ^2- ions. This MOM was recently shown to have an unprecedented selectivity for CO₂ over N₂, CH₄, and H ₂ under industrially relevant conditions. The simulated CO ₂ sorption isotherms and calculated isosteric heat of adsorption, Q_st, values were in excellent agreement with the experimental data for all the state points considered. CO₂ saturation in [Zn(pyz) ₂SiF₆] was achieved at near-ambient temperatures and pressures lower than 1.0 atm. Moreover, the sorbed CO₂ molecules were representative of a liquid/fluid under such conditions as confirmed through calculating the isothermal compressibility, β_T, values. The simulated CO₂ uptakes within CO₂/N₂ (10:90), CO₂/CH₄ (50:50), and CO₂/H₂ (30:70) mixture compositions, characteristic of flue gas, biogas, and syngas, respectively, were comparable to those that were produced in the single-component CO₂ sorption simulations. The modeled structure at saturation revealed a loading of 1 CO₂ molecule per unit cell. The favored CO₂ sorption site was identified as the attraction of the carbon atoms of CO₂ molecules to four equatorial fluorine atoms of SiF₆^2- anions simultaneously, resulting in CO₂ molecules localized at the center of the channel. Furthermore, experimental studies have shown that [Zn(pyz)₂SiF₆] sorbed minimal amounts of CO₂ and N₂ at their respective liquid temperatures. Analysis of the crystal structure at 100 K revealed that the unit cell undergoes a slight contraction in all dimensions and contains pyrazine rings that are mildly slanted with an angle of 13.9. Additionally, molecular dynamics (MD) simulations revealed that the sorbate molecules are anchored to the framework at low temperatures, which inhibits diffusion. Thus, it is hypothesized that the sorbed molecules become trapped in the pores and block other sorbate molecules from entering the MOM at low temperatures.