Adsorption and Diffusion of H2 in the MOF Type Systems MIL-47(V) and MIL-53(Cr): A Combination of Microcalorimetry and QENS Experiments with Molecular Simulations

Quasi-elastic neutron scattering measurements are combined with molecular dynamics simulations to determine the self-diffusivity (Ds) profile of hydrogen in the metal organic framework materials MIL-47(V) and MIL-53(Cr) (MIL, Materials Institut Lavoisier) as a function of loading. Experimentally, a sudden increase in Ds for H-2 at low loading (<= 1 H-2/unit cell) was observed with values at least two orders of magnitude higher than in zeolites. This unusual behavior has been denoted as "super-mobility". Here, two different force fields available in the literature to represent the H-2/H-2 and H-2/MOF framework interactions have been considered to capture such experimental findings via molecular dynamics simulations. We have shown that (i) a similar magnitude of the energetic contribution for the H-2/H-2 and H-2/MOF framework interactions and (ii) a smoothness of the potential energy,surfaces are required in order to match the supermobility of H-2 at low loading. The diffusion mechanism at the microscopic scale was successfully simulated in both materials and described in terms of the chemical features of the MIL framework, i.e., the presence or absence of the mu(2) hydroxyl group. It appears that a one-dimensional (1D) diffusion along the z axis and purely random three-dimensional (3D) diffusion processes are observed for MIL-53(Cr) and MIL-47(V), respectively. The adsorption properties were then simulated using the different force fields initially fitted to the diffusion results and compared to manometry measurements. Finally, the comparison of diffusion and adsorption results for the different force fields leads us to choose the best compromise to describe both dynamic and thermodynamic properties.