Metal-organic frameworks (MOFs) are porous, crystalline materials that have exceptional performance for applications in gas storage and adsorption-based separations. Of note, MOFs with open metal sites (i.e., unsaturated metal sites) have stronger adsorption-selectivity than MOFs with fully coordinated metal sites. Using one isostructural family of open metal site MOFs, known as M2(dobdc), where M = Mg, Mn, Co, and Ni and dobdc4- = 1,4-dioxido-2,5-benzenedicarboxylate, we present a method for forming mixed-matrix membranes (MMMs) with polyimides and poly(ether-imide) random copolymers. Co2(dobdc) and Ni2(dobdc) can be synthesized as small nanoparticles (~20 nm in diameter) and easily dispersed in polyimides with loadings up to 33 wt. % to form selective MMMs, and transport properties are presented for these MMMs that surpass the upper bound for ethylene/ethane separation. However, certain isostructural frameworks such as Mg2(dobdc), which is the strongest CO2 adsorbent in this family of MOFs, can only be formed into ~200 nm-sized particles that strongly agglomerate, thereby precluding the formation of selective MMMs with pure polyimides with loadings above approximately 20 wt. %. To overcome this limitation, we present a rubber toughening approach, whereby loadings up 51 wt. % of Mg2(dobdc) can be added to a poly(ether-imide) copolymer. The transport properties in these MMMs are presented for H2, N2, CH4, and CO2. Interestingly, for high ether content, permeabilities of the polymer phase are sufficiently reduced that diffusion information can be extracted from transient permeation experiments, and these results show that improvements in diffusion for CO2, N2, and CH4 in these MMMs are inversely correlated with the adsorption strength of Mg2(dobdc). Additionally, mechanical properties are assessed as a function of MOF and polymer composition to highlight the tradeoff between MOF loading, the ability to form continuous films, and transport performance.

Evaluating an open metal site MOF for mixed-matrix membranes

255th National Meeting (2018)

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