Importance of ZIF-90 Lattice Flexibility on Diffusion, Permeation, and Lattice Structure for an adsorbed H₂/CH₄ Gas Mixture: A Re-Examination by Gibbs Ensemble Monte Carlo and Molecular Dynamics Simulations

Abstract

Adsorption and diffusion of the gas mixture H2/CH4 in the metal–organic framework (MOF) of structure type zeolitic imidazolate framework-90 (ZIF-90) are revisited. While the adsorption can successfully be examined in Gibbs ensemble Monte Carlo (GEMC) simulations using the common approximation of a rigid lattice, the dynamics of methane in ZIF-90 is remarkably influenced by the lattice flexibility. Molecular dynamics (MD) simulations not only show a strong influence of the lattice flexibility on the diffusion of methane but even find a slight structural phase transition of the lattice. This structural change appeared at higher temperatures and was not caused by the content of guest molecules like in most former discoveries of gate opening. For prediction of mixed gas ZIF-90 membrane selectivity, the adsorption and diffusion results show that the high CH4 adsorption selectivity is overcompensated by the high H2 mobility. The comparison of our results for the H2/CH4 membrane selectivity with experimental findings from mixed gas permeation through supported ZIF-90 membranes shows better agreement than other simulations that use a rigid lattice for MD. Also, the increase of the membrane selectivity by increased temperature could be found.

A systematic examination of the impacts of MOF flexibility on intracrystalline molecular diffusivities

“…3.3.2 Window exibility. Previous works 8,21,77 have shown that degrees of freedom associated with the MOF structure and their coupling with diffusing molecules can in some cases be vital to accurate description of molecular diffusion. To probe these effects, the time-averaged diameters of the empty and adsorbate-loaded windows for each MOF were calculated.…”

Mixed‐matrix gas separation membranes for sustainable future: A mini review

“…Most of the MOFs seem to host the molecules, larger than the crystallographically analyzed pore aperture, which is the main reason of lattice flexibility in these MOFs. The lattice flexibility has been observed for different MOF structures including but is not limited to ZIF‐7, 72,73 ZIF‐8, 74 ZIF‐90, 75 and MIL‐ 53 76 . Many research group have explored intensively and investigated the phenomenon of lattice flexibility by using different characterization approaches and came to the similar conclusion mentioned above.…”

“…Apart from these, fumed silica, which is non-porous material is used throughout the world in MMM preparation. In terms of fillers choice criteria for MMM preparation, most particles have been chosen based on their gas sorption behavior rather than their transport behavior of gaseous molecules and this principle has been different MOF structures including but is not limited to ZIF-7, 72,73 ZIF-8, 74 ZIF-90, 75 and MIL-53. 76 Many research group have explored intensively and investigated the phenomenon of lattice flexibility by using different characterization approaches and came to the similar conclusion mentioned above.…”

Effect of Si/Al Ratio in the Faujasite Structure on Adsorption of Methane and Nitrogen: A Molecular Dynamics Study

“…The dependence of diffusivity to temperature can be explained by higher thermal energy of particles at higher temperatures in the Arrhenius theory. The diffusion coefficient of gas is also dependent on the thermal velocity of molecules and the size and shape of the cages 43.…”