High-Performance CO₂-Selective Hybrid Membranes by Exploiting MOF-Breathing Effects

Conventional CO₂ separation in the petrochemical industry via cryogenic distillation or amine-based absorber-stripper units is energy-intensive and environmentally unfriendly. Membrane-based gas separation technology, in contrast, has contributed significantly to the development of energy-efficient systems for processes such as natural gas purification. The implementation of commercial polymeric membranes in gas separation processes is restricted by their permeability-selectivity trade-off and by their insufficient thermal and chemical stability. Herein, we present the fabrication of a Matrimid-based membrane loaded with a breathing metal-organic framework (MOF) (NH₂-MIL-53(Al)) which is capable of separating binary CO₂/CH₄ gas mixtures with high selectivities without sacrificing much of its CO₂ permeabilities. NH₂-MIL-53(Al) crystals were embedded in a polyimide (PI) matrix, and the mixed-matrix membranes (MMMs) were treated at elevated temperatures (up to 350 °C) in air to trigger PI cross-linking and to create PI-MOF bonds at the interface to effectively seal the grain boundary. Most importantly, the MOF transitions from its narrow-pore form to its large-pore form during this treatment, which allows the PI chains to partly penetrate the pores and cross-link with the amino functions at the pore mouth of the NH₂-MIL-53(Al) and stabilizes the open-pore form of NH₂-MIL-53(Al). This cross-linked MMM, with MOF pore entrances was made more selective by the anchored PI-chains and achieves outstanding CO₂/CH₄ selectivities. This approach provides significant advancement toward the design of selective MMMs with enhanced thermal and chemical stabilities which could also be applicable for other potential applications, such as separation of hydrocarbons (olefin/paraffin or isomers), pervaporation, and solvent-resistant nanofiltration.