Characteristic features of molecular transport in MOF ZIF-8 as revealed by IR microimaging

• The methane transport diffusivity strongly increases at surprisingly low loadings.
• This increase disappears in a mixture due to window-blocking effects.
• The transport diffusivity of methanol and ethanol goes through a minimum.
• This odd trend is related to clustering effects by H-bonding.
• Darken relation is validated for correlating transport and self-diffusion.

In this study differences in the diffusion characteristics of several small molecules in MOF ZIF-8 were examined. The diffusion data were obtained using the technique of IR microscopy by investigating individual crystals. Most of the primary data were already published separately but are revisited here, complemented with new data and discussed together as a mini-review on the diffusion characteristics. The range of the reported diffusivities covers six orders of magnitude. This study was only possible owing to the particularly wide range of crystal sizes, from 20 up to 300 μm, available for the diffusion experiments. Different features in the loading dependencies are identified. They reflect a large variety of mechanisms which dominate the mass transfer within the pores, including window blocking effects and molecular clustering. It is demonstrated that the diffusion characteristics may change significantly if a small molecule like methane diffuses in a mixture with other guests. However, this is not a general feature of the system as for larger molecules like ethane and ethene no difference in the mixture was found compared to the single compound. Hence, predictions solely based on single-component data like membrane separation factors may differ notably from the real performance. On the basis of diffusivity measurements with small alcohols the validity of the Darken relation is demonstrated and found to provide a quantitative tool to correlate transport and self-diffusivity in this host system. Besides all details of the underlying transport mechanisms this study provides a broad basis of reference data for the diffusivity in one of the most prominent representatives among the ZIF MOFs.

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