Membrane separations are a promising alternative to existing energy intensive separations like distillation and absorption, but polymeric materials for these applications are subject to plasticization effects and performance trade-offs. To overcome these challenges, metal–organic frameworks (MOFs) have frequently been pursued in the form of mixed-matrix membranes (MMMs), but it is difficult to access MOF loadings above the percolation limit while avoiding nanoparticle agglomeration and interfacial incompatibility. Recently, an interconnected branched morphology of ZIF-8 was discovered that enables access to MOF percolation at low loadings with enhanced selectivity for H2-based separations when cast in a polyimide matrix. The enhanced selectivity relates to a suppression of the “gate-opening” effect from imide–MOF interactions. In this work, the mechanism of transport in branched materials is further investigated through the synthesis and characterization of a branched ZIF-67 framework. Unlike the zinc-based ZIF-8, ZIF-67 has a cobalt center and distinct surface functionalization, resulting in unique transport performance, despite having an isostructural form. To benchmark findings, ZIF-67 was synthesized in its classic rhombic dodecahedral structure and its novel branched morphology. MMMs were formed from both morphologies of ZIF-67 using a high-performance polyimide (6FDA-DAM) matrix. As expected, pure-gas permeation tests revealed that percolation can be accessed in the branched morphology more easily than the rhombic dodecahedral morphology, but surprisingly, the unique surface functionalization of ZIF-67 results in imide–MOF interactions that differ from those of ZIF-8. In general, branched ZIF-67 had lower selectivities for H2-based separations than branched ZIF-8, likely due to weaker surface interactions between ZIF-67 and the polymer. In depth structural and transport characterization will be presented for light gases (H2, N2, and CH4), along with a focus on methodology for film formation to make uniform and repeatable MMM samples and measurements. Finally, the size cutoff in branched ZIF-67 will be discussed as it relates to the separation of larger gas molecules, such as C2 and C3 olefins and paraffins for enhanced separation performance.
