(644b) Quantifying Self-Diffusion of Mixed and Pure Organic Liquids in ZIF-Based Mixed Matrix Membranes By NMR

Mixed matrix membranes (MMMs) are formed by dispersing filler particles, such as zeolitic imidazolate frameworks (ZIFs), in polymer films. ZIFs represent a sub-class of metal organic frameworks (MOFs). In addition to gas separations, ZIFs have shown promising performance in organic liquids separations. The diffusion process through MMMs is rather complex because diffusivities are usually different in different MMM components (i.e. polymer and filler). As a result, microscopic diffusion measurements performed selectively for a particular MMM component are needed in addition to conventional macroscopic measurements through the entire membrane for a better quantification and understanding of the MMM diffusion process. In particular, quantifying diffusion inside a ZIF component of MMMs can shed light on an influence of polymer/ZIF interfaces and possible changes of intra-ZIF transport due to a ZIF confinement in a polymer.

In this work, C-13 pulsed field gradient (PFG) NMR was used to quantify self-diffusion of pure and mixed xylene mixture inside zeolitic imidazolate framework-71 (ZIF-71) crystals dispersed in a Torlon polymer to form a ZIF-71/Torlon MMM. For the studied mixtures, selective C-13 isotopic labelling of each xylene isomer was used to ensure that the self-diffusivity of each isomer in the mixture is measured independently and correctly. The diffusion measurements were performed at different diffusion times. The observed time dependencies of intra-ZIF self-diffusivities were explained by an influence of the external crystal surface on the studied diffusion process. Analysis of these time dependencies measured for each xylene isomer allowed obtaining intra-ZIF self-diffusivities not perturbed by crystal surface effects as well as the corresponding diffusion selectivity for the studied sorbate mixture. These results will be presented and discussed in detail. The reported selectivity demonstrates promising separation behavior of the considered membrane type.