On the Impact of Sorbent Mobility on the Sorbed Phase Equilibria and Dynamics: A Study of Methane and Carbon Dioxide within the Zeolite Imidazolate Framework-8

The presented work aims at exploring the influence of the mobility of the sorbent framework on both the equilibrium and the kinetic properties of the sorbed phase by means of molecular dynamics computer experiments under isochoric-isothermal and isobaric-isothermal statistical ensembles for several host model options, combined by Widom averaging along the entire trajectory of the host-guest system toward rigorously obtained sorbate isotherms within a fully flexible lattice. The methodology is adapted to the study of the self-diffusivity and the collective (Maxwell-Stefan and transport) diffusivities of carbon dioxide (CO2) and methane (CH4) within the zeolite imidazolate framework-8 (ZIF-8). The simulation predictions are compared with measurements from pulsed-field gradient nuclear magnetic resonance (PFG NMR), as well as with recently conducted infrared microscopy (IRM) experiments elaborated on the basis of the current modeling in the flexible ZIF-8. The modeling results reveal a significant influence on sorbate transport exerted by the 2-methilimidazolate ligands surrounding the cage-to-cage entrances, whose apertures are commensurate with the guest molecular dimensions. Moreover, calculations of the singlet probability density distribution of the sorbate molecules at selected regions within the imidazolate framework provide a plausible explanation of the transport diffusivity as a function of sorbate occupancy, measured via IRM.