Metal-organic frameworks (MOFs) with coordinatively unsaturated metal sites have shown extraordinary performance for industrially relevant adsorption-based separations such as acid gas purification, hydrogen separations, and olefin/paraffin separations. Within this class of materials, one particularly promising candidate is M2(dobdc), where M is a metal cation and dobdc (2,5-dioxido-1,4-benzenedicarboxylate) is an organic linker. Upon synthesis, these MOFs contain solvent molecules bound to the metal center; however, solvent exchange and heating can remove these bound molecules to expose unsaturated metal sites, thereby giving these materials exceptional adsorption selectivities. These strong adsorption sites, coupled with the ability to tune metal cations, organic linkers, and porosity make this family of MOFs of pressing interest for mixed-matrix membrane research.
Herein, we report the synthesis of mixed-matrix membranes formed from isostructural M2(dobdc) MOF nanocrystals, where M = Ni and Mg, dispersed in a series of polyimides and polyimide-based random copolymers. The polyimide considered for this work was synthesized from 2,2’-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and 2,3,5,6-tetramethyl-1,4-phenylenediamine (Durene). Poly(ethylene glycol) (PEG), poly(dimethylsiloxane) (PDMS), and hydroxyl-functional imide segments were copolymerized with 6FDA-Durene to specifically probe polymer/MOF interactions. To formulate structure-property relationships for this class of mixed-matrix membranes, several gas separations, including H2/CH4 and CO2/N2, were considered. In general, for diffusion-based separations, incorporation of flexible segments, such as PEG, into the polymer backbone negatively affects gas transport properties. However, flexible segments can effectively seal defects between the MOF and polymer, thereby permitting higher MOF loading and creating more mechanically robust, ductile mixed-matrix membranes. Transport properties are investigated for each of these systems to elucidate inherent tradeoffs between permeability, polymer composition, MOF loading, and MOF metal sites.