Influence of 2-ethylimidazole linker-doping in ZIF-8 crystals on intracrystalline self-diffusion of gas molecules by high field diffusion NMR

Abstract Pulsed field gradient (PFG) NMR at high magnetic field is used to study molecular self-diffusion of ethylene, ethane, and propylene in a 2-ethylimidazole (eIm) linker-doped ZIF-8. These eIm-doped materials were synthesized by incorporating increasing fractions of the non-native eIm linker, which substitutes the native 2-methylimidazole linker in the ZIF-8 framework. Self-diffusion of the three hydrocarbon gases was measured in the limits of a high and low loading pressure in six different eIm-doped ZIFs with the eIm linker fractions in the framework between 0 and 22.2%. It was found that the elm linker substitution shows a tendency of increasing gas intracrystalline diffusivities, particularly for larger elm linker fractions and sorbates at low loading pressures. It was also observed that the ethylene/ethane diffusion selectivity, viz. the ratio of the ethylene and ethane intracrystalline diffusivities, does not show any dependence, within uncertainty, on the extent of the linker substitution. Based on these results it was concluded that the elm linker substitution is likely to introduce small changes in the ZIF-8 microstructure which result in slightly larger pore aperture sizes, but smaller framework flexibility in comparison with the parent ZIF-8 material.