Controlled Flexibility in Metal–Organic Frameworks for CO2 Capture: A Mini-Review from Selective Triggering to Process-Relevant Breakthrough Performance

Abstract Flexible metal-organic frameworks (MOFs) can translate small differences in gas-framework interactions into large adsorption responses through gate opening, breathing, or local site reconstruction. This review refocuses CO2 capture and separation in flexible MOFs on a single design criterion: useful flexibility must be controlled within a process-relevant response window. In CO2-targeted separations, the flexible response should either be preferentially triggered or stabilized by CO2, or, in inverse-separation systems, still lead to CO2 enrichment through pregate retention, postopening discrimination, and reversible recovery under realistic mixtures. We organize the discussion around three linked levels: mechanistic origins, multiscale regulation, and process-relevant validation. Selective triggering is described as adsorption-stress-assisted barrier crossing, where quadrupolar and polar interactions help CO2 compensate for the energetic cost of deformation. Adaptive site reconstruction is then discussed for breathing frameworks, anion-rich pores, and partitioned adsorption environments that create CO2-favorable local fields. Regulation strategies are further organized into steric confinement, polar-site and anion engineering, defect tuning, particle size control, densification, membrane incorporation, and shaped-body processing. Finally, dynamic breakthrough and humidity tolerance are discussed as immediate process-facing tests, while cyclic regeneration, shaped-body performance, and reversible recovery under realistic mixtures are proposed as necessary validation benchmarks for future evaluations. Flexible MOFs should therefore be evaluated by selective triggering or stabilization, pregate CO2 retention, rapid and reversible transition kinetics, suppressed postopening coentry of competing gases, and reversible recovery under realistic CO2-containing mixtures.