Diffusion of CH4 in ZIF-8 Studied by Quasi-Elastic Neutron Scattering

Quasi-elastic neutron scattering (QENS) measurements have been performed to characterize at the molecular level the diffusion of CH4 adsorbed at various concentrations in ZIF-8. This allows to compare for the first time diffusivities obtained in a MOF by microscopic and mesoscopic techniques. The good agreement which is observed between the different methods shows that there are no defects obstructing diffusion, which is often the case in zeolites. Even if the effective size of ZIF-8 windows is larger than the diameter derived from crystallography, the characteristic time for intercage diffusion (similar to ns) is much longer than the time between jumps within a cage (similar to 10 ps), indicating that the organic linkers severely hinder molecule passage.