Diffusion of Light Hydrocarbons in the Flexible MIL-53(Cr) Metal–Organic Framework: A Combination of Quasi-Elastic Neutron Scattering Experiments and Molecular Dynamics Simulations

The dynamics of light hydrocarbons, including ethane, propane, and n-butane, is explored in the highly flexible metal–organic framework MIL-53(Cr) by combining quasi-elastic neutron scattering measurements and molecular dynamics simulations. The loading dependence of the self-diffusivity shows peculiarities, including (i) a relatively rapid decrease of Ds at low loading for ethane and propane and (ii) an unusual increase of Ds for n-butane at high loading, following a decreasing profile up to intermediate loading. These diffusion behaviors are analyzed in light of the structural flexibility of the solids upon alkane adsorption characterized by the neutron measurements. A 1D-type diffusion is evidenced for all alkanes with a jump sequence mainly ruled by the hydroxyl groups present at the surface of the MOF pore wall. This global translational motion is associated with a rotational dynamics that differs according to the nature of the alkane: whereas n-butane follows uniaxial displacements, ethane shows random rotational reorientation.