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)(2)SiF6], a metal-organic material (MOM) consisting of a square grid of Zn2+ ions coordinated to pyrazine (pyz) linkers and pillars of SiF62- ions. This MOM was recently shown to have an unprecedented selectivity for CO2 over N-2, CH4, and H-2 under industrially relevant conditions. The simulated CO2 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)(2)SiF6] 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, beta(T), values. The simulated CO2 uptakes within CO2/N-2 (10:90), CO2/CH4 (50:50), and CO2/H-2 (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 SiF62- anions simultaneously, resulting in CO2 molecules localized at the center of the channel. Furthermore, experimental studies have shown that [Zn(pyz)(2)SiF6] sorbed minimal amounts of CO2 and N-2 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 degrees. 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.