The chemical tunability and well-defined pore structures of metal-organic frameworks (MOFs) make these materials ideal candidates for membrane-based separations. However, MOFs by themselves are often brittle and crystalline making it difficult to form these materials into membrane films. One promising solution to overcoming these challenges is to combine MOF nanoparticles with polymers to form composite systems known as mixed-matrix membranes (MMMs). Unfortunately, these MMMs frequently form interfacial defects, which deteriorate property sets and preclude the use of transport models to determine intrinsic permeabilities in the MOF phase. To overcome these interfacial challenges, this study investigates the surface modification of UiO-66-NH2 by covalently tethering a rigid polyimide oligomer, 6FDA-Durene, to the surface of the MOF. Unlike more flexible polymers, 6FDA-Durene does not block pore access, and when the coated MOF is dispersed in bulk 6FDA-Durene, the interfacial chemistries are identical, enabling the formation of defect-free MMMs up to 40 wt % loading. A series of MMMs with various weight loadings were formed and tested for H2, N2, O2, CH4, and CO2 permeabilities. Interestingly, both permeabilities and selectivities increased concomitantly for several gas pairs with increased MOF loadings. This consistent improvement in property sets permitted the application of the Maxwell Model to predict the intrinsic permeabilities of gases in UiO-66-NH2 for a variety of separations. For CO2/CH4 and H2/CH4 separations, the predicted pure-MOF permeabilities and selectivities were far beyond those accessible to pure polymers, indicating the value of using UiO-66-NH2 for certain membrane-based separations. Additionally, MMM exposure to high-pressure CO2 was investigated, and increased MOF loading improved the stability of the MMMs to CO2. The mechanism behind this induced stability is related to the imide layer being covalently linked to the surface of the UiO-66-NH2 particles, which hinders polymer chain mobility.

Creating an ideal interface to form defect-free mixed-matrix membranes with UiO-66-NH2

2019 Spring National Meeting

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