Self-diffusion of mixed xylene isomers in ZIF-71 crystals dispersed in a polymer to form a hybrid membrane

Abstract

13 C pulsed field gradient (PFG) NMR was used to quantify self-diffusion of a p -xylene/ o -xylene mixture inside zeolitic imidazolate framework-71 (ZIF-71) crystals dispersed in a Torlon polymer to form a ZIF-71/Torlon mixed-matrix membrane (MMM). The corresponding reference PFG NMR measurements with a bed of a loosely packed bed of ZIF-71 crystals were also performed. This study is motivated by a demonstrated potential of ZIF-based MMMs for liquid separations. Selective 13 C 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 at 296 K. The observed dependencies of intra-ZIF self-diffusivities in ZIF-71/Torlon MMMs and the corresponding ZIF-71 crystal beds on diffusion time were explained by an influence of the external crystal surface on the studied diffusion process. Analysis of the time dependencies of the self-diffusivities 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. The reported selectivity of around 4 demonstrates promising separation behavior of the considered membrane type. • First measurements of liquid mixtures in ZIF crystals located in a mixed-matrix membrane (MMM). • Intra-ZIF diffusivities of mixtures are quantified as a function of diffusion time by PFG NMR. • Diffusion selectivity is reported separately for a ZIF component of the MMM. • The reported diffusion selectivity shows promising potential of the studied MMM.