Self and Transport Diffusivity of CO2 in the Metal-Organic Framework MIL-47(V) Explored by Quasi-elastic Neutron Scattering Experiments and Molecular Dynamics Simulations

Quasi-elastic neutron scattering measurements are combined with molecular dynamics simulations to determine the self-diffusivity, corrected diffusivity, and transport diffusivity of CO2 in the metal-organic framework MIL-47(V) (MIL = Materials Institut Lavoisier) over a wide range of loading. The force field used for describing the host/guest interactions is first validated on the thermodynamics of the MIL-47(V)/CO2 system, prior to being transferred to the investigations of the dynamics. A decreasing profile is then deduced for D-s and D-o whereas D-t presents a non monotonous evolution with a slight decrease at low loading followed by a sharp increase at higher loading. Such decrease of D, which has never been evidenced in any microporous systems comes from the atypical evolution of the thermodynamic correction factor that reaches values below 1 at low loading. This implies that, due to intermolecular, interactions, the CO2 molecules in MIL-47(V) do not behave like an ideal gas. Further, molecular simulations enabled us to elucidate unambiguously a 30 diffusion mechanism within the pores of MIL-47(V).