Membrane gas separation technology offers energy-saving and smaller footprints compared to traditional thermally driven gas amine absorption processes. Development of high-performance membrane materials is a key component of membrane technology implementation. However, polymeric membranes, the dominant commercial membrane products for gas separation, are limited by a performance due to trade-off between permeability and selectivity. Although inorganic membranes exhibit much higher gas separation performance, their scalable fabrication remains a challenge. On the other hand, mixed-matrix membranes (MMMs) offer an excellent balance between separation performance and scalability, driven by highly selective and permeable molecular sieve particles dispersed in an appropriate polymer matrix. Such MMMs offer both high separation performance assisted by high performing sieves, and economical fabrication due to reliance on simple extension of processing approaches used for pure polymeric membranes. Purely inorganic zeolite generally shows poor compatibility with organic polymer and often requires additional surface modification to eliminate non-selective interfacial voids. Alternatively, metal-organic frameworks (MOFs) consisting of metal ions/clusters connected by organic linkers are another important class of crystalline molecular sieve porous materials, can be a suitable candidate for MMMs, owing to the diverse functionalities and tunable pore structures.
By matching the transport properties and interphase compatibility between sieve and polymer, we developed a diverse array of MOF MMMs by incorporating different kinds of MOF crystals (e.g., UiO-66, RE-fcu-MOF) in various polymers (e.g., 6FDA-polyimides, PEBAX) for natural gas purification, butane isomers separation and CO2 capture. Transport properties were systematically studied via interpreting the sorption and diffusion behaviors. Theoretical framework was employed to predict permeation properties of pure MOF membrane and MOF MMMs using other polymer matrices. Pure-gas and mixed-gas permeations showed that both permeability and selectivity of the polymeric membranes were highly improved by rationally incorporating MOF crystals. The performance of the resulting MOF MMMs were beyond the upper-bounds for traditional polymeric membranes. We demonstrated that MMM is a promising platform for providing high-performance and scalable gas separation membranes, as well as a powerful tool for studying the transport properties of pure MOF membrane.