(673b) Microscopic Gas Diffusion inside ZIF-11 Crystals Dispersed in Different Polymers to Form Mixed Matrix Membranes

Mixed matrix membranes (MMMs) consisting of zeolitic imidazolate framework (ZIF) filler particles embedded in a polymer matrix have shown great potential for gas separations. These membranes were introduced to combine the gas-separation properties of the filler particle with the ease and scalability of fabrication of pure polymeric films. ZIFs represent a subgroup of metal organic frameworks (MOFs), which exhibit great chemical and thermal stability and functionality.

¹H and ¹³C pulsed field gradient (PFG) NMR was used to study diffusion of ethane and ethylene inside a loosely packed ZIF-11 bed and various ZIF-11-based MMMs at equivalent loading pressures. Diffusion measurements were performed by utilizing a high magnetic field (17.6 T) to obtain sufficiently high signal-to-noise ratios for gas diffusion data and high magnetic field gradients (up to 25 T/m) to enable measurements of diffusivities for the length scales of displacements smaller than the mean size of the ZIF-11 crystals. It was observed that depending on the polymer type, the confinement of the ZIF-11 crystals in the polymer can lead to a reduction in the intra-ZIF diffusivities of the studied gases. This effect was investigated as a function of temperature as well as gas loading inside MMMs. It was tentatively explained by a reduction in the framework flexibility of ZIF-11 crystals due to the crystal confinement in a polymer.