Metal–organic frameworks (MOFs) represent the largest known class of porous crystalline materials ever synthesized. Their narrow pore windows and nearly unlimited structural and chemical features have made these materials of significant interest for membrane-based gas separations. In this comprehensive review, we discuss opportunities and challenges related to the formation of pure MOF films and mixed-matrix membranes (MMMs). Common and emerging separation applications are identified, and membrane transport theory for MOFs is described and contextualized relative to the governing principles that describe transport in polymers. Additionally, cross-cutting research opportunities using advanced metrologies and computational techniques are reviewed. To quantify membrane performance, we introduce a simple membrane performance score that has been tabulated for all of the literature data compiled in this review. These data are reported on upper bound plots, revealing classes of MOF materials that consistently demonstrate promising separation performance. Recommendations are provided with the intent of identifying the most promising materials and directions for the field in terms of fundamental science and eventual deployment of MOF materials for commercial membrane-based gas separations.

An optimized acid hydrolysis method was developed to yield carboxylic acid-functionalized PIM-1 (PIM-COOH) with >89% conversion in 48 h using a postpolymerization reaction of PIM-1. Physical characterization of PIM-1 and PIM-COOH revealed that the average size of free volume elements in PIM-COOH decreased relative to that in PIM-1. Compared to PIM-1, PIM-COOH showed a significant increase in CO2- and H2-based selectivities with a corresponding decrease in permeabilities and sorption capacities for all gases considered. The dual-mode sorption model, time-lag method, and sorption–diffusion model were applied to glean molecular-level insights into diffusion and sorption in these polymers. Results indicate that improvements in selectivities for CO2-based gas pairs for PIM-COOH are primarily driven by diffusion selectivity and that PIM-COOH displays transport behavior consistent with the sorption–diffusion model. To better understand performance under more realistic conditions, pure- and mixed-gas permeation values for CO2/CH4 are reported for a 330 day aged PIM-COOH sample.

Gas‐separation polymer membranes display a characteristic permeability–selectivity trade‐off that has limited their industrial use. The most comprehensive approach to improving performance is to devise strategies that simultaneously increase fractional free volume, narrow free volume distribution, and enhance sorption selectivity, but generalizable methods for such approaches are exceedingly rare. Here, we present an in situ crosslinking and solid‐state deprotection method to access previously inaccessible sorption and diffusion characteristics in amine‐functionalized polymers of intrinsic microporosity. Free volume element (FVE) size can be increased while preserving a narrow FVE distribution, enabling below‐upper bound polymers to surpass the H2/N2, H2/CH4, and O2/N2 upper bounds and improving CO2‐based selectivities by 200 %. This approach can transform polymers into chemical analogues with improved performance, thereby overcoming traditional permeability–selectivity trade‐offs.

Partially fluorinated polymers often exhibit exceptional membrane-based separation performance for a variety of gas pairs. While many gas transport studies focus on the incorporation of aliphatic fluorine groups (e.g., −CF3) on the polymer backbone, few studies have systematically investigated structure–property relationships for aromatic fluorine groups. Here, the effect of aliphatic and aromatic fluorine groups on solid-state morphology and gas transport is compared for structural analogues of 6FDA-based polyimides that contain either hydrogen or fluorine functional groups on the diamine monomer. Both fluorinated analogues displayed higher gas diffusivity compared to their hydrocarbon-based counterparts. However, the aromatic fluorinated analogue displayed a larger decrease in diffusivity selectivity due to weakened secondary interchain forces and a larger increase in interchain spacing, suggesting a greater extent of packing disruption resulting from increased steric hindrance associated with aromatic fluorine groups. This study establishes guiding principles for how carbon–fluorine bonds affect macromolecular packing structure and gas separation performance.

Pure-gas transport performance rarely matches mixed-gas performance for industrially relevant membrane applications. While significant effort has focused on studying the adverse effects of plasticization, an additional phenomenon known as competitive...