(612b) Microscopic Diffusion of Mixed and Pure Gases in Mixed Linker ZIF-7-8

Zeolitic imidazolate frameworks (ZIFs), a subset of metal organic frameworks (MOFs), are promising materials for applications in separations and catalysis for their high porosity and the ability to be easily modified using various linkers and metal ions. Recently, there have been many successful attempts to mix different linkers and incorporate them into the same framework. These mixed linker ZIFs have the potential to give tunable material parameters distinct from their parent materials thereby offering a possibility to optimize molecular sieving properties. This work focuses on ZIF-7-8, i.e. a mixed linker ZIF containing ZIF-8 (methylimidazolate, mIm) and ZIF-7 (benzylimidazolate, bIm) linkers. Since the effective pore aperture size accounting for linker flexibility is much smaller in ZIF-7 (~3 Å) than ZIF-8 (~4 Å), lower gas diffusivity values and higher diffusion selectivity are expected in ZIF-7-8 in comparison to ZIF-8.

¹³Carbon, ¹H and ¹²⁹Xe pulsed field gradient (PFG) NMR with high magnetic fields (14 T and 17.6 T) and high field gradients (up to 30 T/m) was used to measure intra-ZIF self-diffusivities (D) of the following mixed and/or pure gases: C₂H₆, C₂H₄, CO₂, CH₄ and Xe. For each studied sorbate the characteristic (mean) self-diffusivities in ZIF-7-8 were found to be smaller than the corresponding self-diffusivities in ZIF-8. Unexpectedly, the PFG NMR signal attenuation curves measured for gas diffusion inside ZIF-7-8 crystals showed deviation from the monoexponential behavior corresponding to normal diffusion with a single diffusivity. This deviation was attributed to an existence of a difference between intracrystalline gas diffusivities in different ZIF-7-8 crystals. Such difference was confirmed by complementary measurements of ethane transport diffusion performed using infrared microscopy (IRM) for different single ZIF-7-8 crystals. Comparison of the IR bands associated with ZIF-8 and ZIF-7 in ZIF-7-8 crystals allowed assigning the observed diffusivity difference to a difference in the fraction of mIm and bIm in different ZIF-7-8 crystals.

The diffusion selectivity (i.e. a ratio of the characteristic self-diffusivities of sorbates x and y in the same material, D_x/D_y) for CO₂/CH₄ mixture was found to be larger in ZIF-7-8 than in ZIF-8. At the same time, the diffusion selectivity for C₂H₄/C₂H₆ was found to be similar in both ZIF-7-8 and ZIF-8. These diffusion selectivity data in combination with the observed dependence of the self-diffusivities in ZIF-7-8 on the sorbate size and loading reveal a prominent role played by the framework flexibility in diffusion inside ZIF-7-8 crystals.