Evaluating Techniques for Incorporating Metal-Organic Frameworks into Mixed Matrix Membranes for Organic Solvent Nanofiltration

Polymeric membranes have dominated the membrane market because of their low cost, ease of manufacture and mechanical flexibility, the latter of which allows for facile inclusion into industrial membrane modules. However, it is widely known that there is a trade-off between permeability and selectivity for traditional polymeric membranes. Moreover, there is still a need for more chemically and thermally stable membranes for organic solvent nanofiltration (OSN) applications. Inorganic membranes have such enhanced performance characteristics. Membranes based on metal-organic frameworks (MOFs) are particularly interesting because they have tuneable chemistries and well-defined pore sizes, which allow for customizable separations based on molecular sieving and adsorptive affinity. Though inorganic membranes offer significant improvements over polymeric membranes, they are costly to manufacture and difficult to consistently produce without selectivity-reducing defects. Mixed matrix membranes (MMMs) provide a promising alternative to pure polymeric and inorganic membranes. MMMs incorporate inorganic materials into a polymeric matrix. They are capable of achieving some of the superior performance characteristics of inorganic membranes while still maintaining the manufacturability and flexibility of polymeric membranes.

To date, our group has successfully synthesized MMMs based on MOFs for OSN using four distinct techniques. In the first technique, HKUST-1 was synthesized and incorporated into a cross-linked P84 membrane by mixing the MOF in the dope solution prior to phase inversion. While this is the most typical way of forming MMMs, it means that all permeation paths will go through both the polymer and MOF, which limits the influence of the MOF on the membrane performance. To create a potentially continuous MOF flow path, HKUST-1 was incorporated into an existing cross-linked P84 membrane via in situ growth (ISG) [1]. Furthermore, interfacial synthesis was also used to form a thin film of HKUST-1 on a cross-linked P84 support with the goal of enhancing flux relative to that of the ISG membrane [2]. Morphological and chemical composition are explored with FT-IR spectroscopy, SEM and EDX. This data is compared with flux and rejection data, evaluated using acetone-polystyrene solutions, to understand the underlying mechanisms behind membrane performances.

The final method of synthesis involved interfacial polymerization of a polyamide thin-film nanocomposite (TFN) membrane on a cross-linked P84 support using nano-scale ZIF-8, MIL-53(AL), NH2-MIL-53(Al) and MIL-101(Cr) [3]. The MOF’s presence is inferred by a combination of contact angle measurements, FT-IR spectroscopy, SEM, EDX, XPS and TEM. TFN membranes show dramatically increased methanol and tetrahydrofuran permeance when compared to thin film composite controls, without sacrificing rejection.