Single-File Diffusion of Neo-Pentane Confined in the MIL-47(V) Metal-Organic Framework

Single-file diffusion of neo-pentane in the channel-like MIL-47(V) was recently revealed by quasi-elastic neutron scattering experiments. The origin of such unprecedented dynamic behavior in the field of metal organic frameworks (MOFs) has not been elucidated at the atomistic level yet. Here, molecular dynamics simulations are performed to first confirm this single-file diffusion and further unveil the molecular insights into this abnormal diffusion process. Whatever the loading, a subdiffusive regime is highlighted, in good agreement with the single-file diffusion experimentally detected. We show that this subdiffusive regime is uncorrelated with the flexibility of the MIL-47(V) and comes mostly from the pore dimension of the MOF that hinders the crossing of molecules along the channel. Neither translational jumps nor correlation dynamics of the guest molecules present in the same channel and in the neighbor channels were observed. Moreover, the rotational dynamics was also carefully explored, and a relatively homogeneous rotational motion was evidenced along the three directions. While the translational diffusion coefficient decreases when the loading increases, the rotational diffusion coefficient remains constant, which corresponds to a clear deviation to the Stokes-Einstein relation.