Diffusion of molecular hydrogen through porous materials: The importance of framework flexibility

The importance of framework flexibility in facilitating the passage of molecules through confining framework materials is probed via both periodic energy minimizations using dedicated force fields and embedded quantum mechanical/semiempirical cluster calculations. Specifically, molecular hydrogen transport through an all-silica zeolitic structure is investigated. Particular attention is given to the comparison of the two modeling methodologies used and the effect of their corresponding approximations. Regardless of methodological differences, the quantitative and qualitative agreement between the different techniques is surprisingly good, tending to confirm the quality and suitability of each respective method. The choice of rigid framework reference structure is shown in both modeling methodologies to strongly affect the predicted influence of the lattice flexibility on the size of the molecular transport barrier, helping to resolve the differing results of previous studies. In all of our calculations, we find the energetics of molecular transport through a confining porous environment to be strongly dependent on the flexibility of the framework.