Ultramicroporous flexible metal-organic frameworks: synthetic strategies, structural characteristics, adsorption and separation applications

<jats:p>Efficient gas separation and purification are essential for the sustainable advancement of modern industry, yet they demand highly selective porous materials with exceptional adsorption capacity. Ultramicroporous flexible metal-organic frameworks (UF-MOFs), combining the benefits of subnanometer pore dimensions (&lt;7 Å) and structural dynamism, have emerged as a transformative solution. Nevertheless, the integration of rigid ultramicroporous frameworks and large-pore flexible MOFs, both of which have been independently researched, remains a critically underexplored frontier. When integrated into UF-MOFs, the tiny pores in ultramicroporous frameworks provide the necessary spatial confinement for molecular size-sieving while the framework offers the required flexibility for adaptive responses to specific guests, effectively overcoming the inherent “trade-off” in conventional rigid adsorbents. This review provides a survey of UF-MOF synthetic strategies spanning from solvothermal to room-temperature assembly, and summarizes their representative guest- and temperature-dependent separation behaviors. Furthermore, representative application benchmarks of UF-MOFs, including light hydrocarbon separation, industrial carbon capture, and noble gas recovery, are examined in depth. Finally, the challenges and prospects associated with the large-scale industrial synthesis and commercial applications of UF-MOFs are presented. This review aims to provide a clear trajectory for the future design and development of UF-MOFs toward practical application.</jats:p>