(454b) Anomalous Relationship between Molecular Size and Diffusivity in ZIF-11 By a Combined Application of Diffusion NMR, Volumetric Adsorption Method, and IR Microscopy

Zeolitic imidazolate frameworks (ZIFs), a sub-class of metal organic frameworks (MOFs), have been considered for gas separation due to their great thermal and chemical stability, large porosity, and tunable pore apertures comparable with sizes of gas molecules. ZIFs are known to exhibit significant framework flexibility that allows molecules with sizes larger than the nominal pore aperture size to enter and diffuse inside ZIF crystals. Clearly, framework flexibility can play a large role in diffusion of guest molecules inside ZIF pore systems.

In this work, ¹H and ¹³C pulsed field gradient (PFG) NMR was used to study intracrystalline self-diffusion of ethane and ethylene in batches of ZIF-11 crystals with different mean crystal sizes between around 3 and 100 microns in a broad range of temperatures and sorbate concentrations. The observed agreement between the PFG NMR diffusion data measured with different nuclei types for the same molecules and samples allowed ruling out any measurement artifacts. For ZIF-11 crystals loaded with single sorbates, despite the larger size of ethane compared to ethylene, a larger intracrystalline self-diffusivity was observed for ethane than ethylene at large and intermediate sorbate loadings. It was also observed that replacing a fraction of ethane molecules by ethylene molecules reduces intracrystalline self-diffusivity of ethane. In order to further investigate this anomalous relationship between ethane and ethylene intracrystalline diffusivities in ZIF-11, uptake curves were measured using IR microscopy and volumetric gas adsorption method for single ZIF-11 crystals and thin ZIF-11 crystal beds, respectively. The corrected diffusivities obtained from these complementary measurements under the same or comparable conditions were found to be in agreement with the self-diffusion data from the PFG NMR measurements. The observed anomalous relationship between the diffusivities of ethane and ethylene in ZIF-11 is attributed to a reduction in the framework flexibility of ZIF-11 in the presence of ethylene molecules. This flexibility reduction manifests itself in a decrease in the effective ZIF-11 aperture size.