Beyond Equilibrium: Metal–Organic Frameworks for Molecular Sieving and Kinetic Gas Separation

Metal–organic frameworks (MOFs) are a class of crystalline inorganic–organic hybrid materials that have demonstrated huge potential in gas separation due to their ultrahigh porosity, boundless chemical tunability, as well as surface functionality. Most gas separations realized in MOFs are under an equilibrium state and are dependent on the difference in thermodynamic affinities of gases to MOFs, whereas nonequilibrium separation such as kinetic and molecular sieving separation attracting growing attention in the past decade is achieved based on the difference in the size and diffusivity of gas molecules. In this perspective, we first discuss the pore size, temperature, and pressure effect on gas diffusion as well as nonequilibrium gas separation in MOFs. Second, we introduce current techniques reported to measure intracrystalline gas diffusivity. Third, we review recent progress in MOF-based nonequilibrium N2/O2 separation, CO2 capture, and hydrocarbon separation. In addition, we describe the hydrogen isotope separation based on kinetic quantum sieving in MOFs as a special scenario of kinetic gas separation. Lastly, we summarize general design strategies toward MOF-based nonequilibrium gas separation and propose several directions to advance the study in this exciting area.