(351aa) Self-Diffusion of Small Alcohols inside ZIF/Polymer Hybrid Membranes By Pulsed Field Gradient (PFG) NMR

Mixed matrix membranes (MMMs) formed by dispersing zeolitic imidazolate frameworks (ZIFs) crystals in a polymer have emerged as promising materials for gas and liquid separations. ZIFs represent a class of metal organic frameworks (MOFs) with zeolite-like structural properties. ZIF/polymer MMMs benefit from molecular sieving properties of ZIFs and advantages associated with a fabrication of pure polymeric membranes.

¹³C pulsed field gradient (PFG) NMR was used to study self-diffusion of methanol and ethanol inside loosely packed beds of ZIF-71 crystals and ZIF-71 / Torlon MMM under conditions of sorption equilibrium with the saturation vapor pressure of each alcohol. Diffusion measurements were performed at 253 K by utilizing a high magnetic field (14 T) and high magnetic field gradients (up to 18 T/m) to obtain intra-ZIF self-diffusivities for the length scales of displacements smaller than and comparable to the average size of the ZIF-71 crystals. To confirm an absence of any measurement artifacts, additional measurements were performed at 17.6 T. It was observed that the intra-ZIF self-diffusivities of the same liquid in ZIF-71 crystal beds and ZIF-71 / Torlon MMM were the same, within the experimental error, for the smallest root mean square displacements (root MSDs) corresponding to no or little influence of the crystal external surface on the diffusion process. For larger root MSDs, the intra-ZIF self-diffusivities in ZIF-71 / Torlon MMM were observed to decrease with increasing root MSD values. This observation is tentatively explained by the presence of diffusion barriers on the interfaces separating individual crystals in ZIF-71 interconnected clusters, which are believed to form in the studied MMM. The diffusion data for liquid sorbates is discussed in the context of the results previously measured by PFG NMR for gas diffusion in a similar ZIF / Torlon MMM.