(606d) Quantifying Microscopic Diffusion of Gases in Different Local Environments of Hybrid Membranes Formed By Dispersing MOF Crystals in Polymers

Membrane technology provides a high prospect for future development of more efficient and more sustainable gas-separation processes. Mixed matrix membranes (MMMs) consisting of metal organic framework (MOF) fillers embedded in a polymer matrix have shown promising gas separation properties. MOFs possess desirable characteristics, including great thermal and chemical stability, as well as tunable pore sizes, while using polymers improves mechanical properties and scalability of the resulting membranes. Permeance and/or selectivity of MMMs can be enhanced by creating percolation pathways over interconnected MOF crystals inside MMMs by carefully increasing intra-membrane MOF loading. To gain such performance enhancements, a fundamental understanding of microscopic gas transport on different length scales and in different local environments of MMMs is required.

In this work, ¹³C pulsed field gradient nuclear magnetic resonance (PFG NMR) was applied to study microscopic self-diffusion of light gases, including CH₄ and CO₂, in MMMs composed of UiO-66-NH₂ embedded in a 6FDA-Durene polymer matrix. UiO-66-NH₂ was chosen due to its very promising behavior in gas separations. The measurements were performed at high magnetic field of 17.6 or 14 T and at large magnetic field gradient amplitudes up to 17 T/m. PFG NMR diffusion data were obtained for MMMs with different MOF loadings up to around 50 weight percent and at different temperatures. For each studied gas the following two molecular ensembles with distinct diffusivities and fractions were observed: (1) the ensemble corresponding to diffusion mostly inside MOF crystals and through MOF-polymer interphases, and (2) the ensemble corresponding to diffusion mostly inside the polymer phase of the MMMs. The dependencies of the diffusivities and fractions of these ensembles on MOF loading and molecular root mean square displacements will be presented and discussed in detail.