Relationship between mixed and pure gas self-diffusion for ethane and ethene in ZIF-8/6FDA-DAM mixed-matrix membrane by pulsed field gradient NMR

Abstract

Pulsed field gradient (PFG) NMR was applied to study self-diffusion of an ethane/ethene mixture and the corresponding pure gases in a mixed-matrix membrane (MMM) formed by dispersing ZIF-8 particles in 6FDA-DAM polymer. In addition to the MMM, diffusion measurements of the pure gases were also carried out for the membrane constituents, i.e. ZIF-8 particle bed and the pure polymer film. PFG NMR studies were performed at a high magnetic field of 17.6T using large magnetic field gradients up to 30T/m. The former allowed achieving sufficiently large signal-to-noise ratios, while the latter enabled diffusivity measurements to be performed for molecular displacements smaller than the size of ZIF-8 particles. As a result, the gas self-diffusivities for the diffusion inside the ZIF-8 particles dispersed in the MMM and for the diffusion inside the surrounding polymer matrix could be obtained separately. For each gas, the diffusivities inside the ZIF-8 and polymer phases of the MMM were found to be smaller than the corresponding diffusivities in the neat ZIF-8 particle bed and pure polymer film. This observation is explained by the reduced framework flexibility of ZIF-8 and the polymer chain rigidification in the MMM. The ethane and ethene diffusivities in the MMM loaded with the ethane/ethene mixture were compared with the corresponding diffusivities in the MMM loaded with a single gas. It was found that the presence of another gas component in the ZIF-8 particles dispersed in the MMM does not change the self-diffusivity inside the particles for both gases. At the same time, the self-diffusivity of the faster-diffusing sorbate (ethene) in the polymer phase of the MMM was reduced by co-adsorption of the slower-diffusing sorbate (ethane) in the MMM. The analytical expression proposed in our previous work for the long-range diffusivity in MMMs was validated for gas mixtures and pure gases based on the experimental data reported in this work.