Ethene/ethane mixture diffusion in the MOF sieve ZIF-8 studied by MAS PFG NMR diffusometry

The adsorption and diffusion of ethene/ethane mixtures is explored by ¹H and ¹³C MAS NMR spectroscopy and by the combination of PFG NMR with magic-angle spinning (MAS PFG NMR). Some indication for a preferential adsorption of the molecules close to the methyl-groups of the imidazole-rings was found, however no evidence for structural changes upon adsorption of an ethene/ethane mixture.MAS PFG NMR allows an individual but simultaneous observation of both molecules in adsorbed state and as well as in the gas phase. The intracrystalline MAS PFG NMR diffusivities are in consistent agreement with our previous data obtained by IR microscopy. Our consideration includes the loading dependence and the correlation of self-diffusion with transport diffusion by accounting for the influence of the thermodynamic factor.The diffusion selectivity is determined to D_ethene:D_ethane = 5.5 at a loading of four molecules per cavity. The higher mobility of ethene can be rationalized by its smaller size compared to ethane. This conclusion is validated by the measured activation energies for diffusion which are considerably higher for ethane. On the other hand, differences in the guest-host interaction between the saturated and non-saturated molecules can be excluded as possible reason for the different diffusivities.