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

Adsorption and diffusion of the gas mixture H-2/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 H-2 mobility. The comparison of our results for the H-2/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.