(607d) Resolving Self-Diffusion of Liquids in Particular Local Environments inside ZIF/Polymer Hybrid Membranes By High Field NMR

Mixed matrix membranes (MMMs) are advanced membranes for gas and liquid separations that are formed by embedding porous filler particles, such as zeolitic imidazolate frameworks (ZIFs), into polymer matrices. ZIFs, a sub-class of metal organic frameworks (MOFs), have exceptional molecular sieving properties and great affinity to polymers compared with many other types of porous fillers.

To optimize MMM performance in separations, detailed diffusion studies are needed to characterize their transport properties on different length scales larger and smaller than the filler particles. Such studies were previously reported for gases. However, to our knowledge, no such microscopic studies demonstrating an ability to quantify molecular diffusion inside ZIF or MOF crystals emended in polymers were presented for liquids. Here we report microscopic study of diffusion of methanol, ethanol, p-xylene and o-xylene inside MMMs formed by dispersing ZIF-71 crystals in Torlon polymer (viz. in ZIF-71/Torlon MMMs) as well as in the corresponding beds of loosely packed ZIF-71 crystals. The diffusion studies were carried out under conditions of sorption equilibrium with the saturation vapor pressure of each liquid. Diffusion measurements were performed at various temperatures using ¹³C pulsed field gradient (PFG) NMR at high magnetic fields (14 T and 17.6 T) and high magnetic field gradients (up to 23 T/m). Application of high magnetic field allowed obtaining high signal-to-noise ratios for liquid diffusion data, while high magnetic field gradients enabled measuring intra-ZIF self-diffusivities for displacements smaller than and comparable to the mean size of ZIF-71 crystals. It was observed that the intra-ZIF self-diffusivity of each liquid sorbate in ZIF-71 crystal beds and ZIF-71/Torlon MMMs decreases with increasing diffusion time. This observation is explained by the influence of crystal external surface on the diffusion. Measurements of the diffusion time dependent intra-ZIF self-diffusivities allowed obtaining intra-ZIF self-diffusivities that are not perturbed by crystal external surface. The ratios of the latter diffusivities for different sorbate pairs resulted in the intra-ZIF diffusion selectivities in the studied MMMs. These selectivities will be reported and discussed. To our knowledge, this work represents the first microscopic study of liquid diffusion in MOF/polymer MMMs.