I’ve been involved in an additional work on UiO-66: The study of the adsorption and diffusion of light hydrocarbons. The publication has just been accepted in J. Phys. Chem. C.
The concentration dependence of the self-diffusivity of short-chain linear alkanes in the narrow windows type MOF UiO-66(Zr) has been studied by means of Quasi-Elastic Neutron Scattering (QENS) measurements combined with Molecular Dynamics (MD) simulations. These computations employ a force field to describe the host/guest interactions which was preliminary validated on the adsorption data obtained for the system of interest via gravimetry and microcalorimetry measurements. The QENS-measured self-diffusivity profile presents a non-monotonic tendency as the alkane loading increases, with the existence of a maximum that depends on the size of the alkane. The comparison with the simulated results obtained using either a flexible or a rigid framework highlights that the consideration of the flexibility is of prime importance when exploring the diffusion of ethane molecules in porous materials. The self-diffusivities subsequently calculated for propane and n-butane corroborate the results obtained for ethane, leading to a similar form for the plots of self-diffusion coefficient vs loading. The global microscopic diffusion mechanism is further shown to involve a combination of intracage motions and jump sequences between the tetrahedral and octahedral cages of the framework. The self-diffusion coefficients which decrease with increasing molecular size, and thus increasing confinement, are further compared to the values previously reported for MOFs with 1D-pore networks.