Restricting Lattice Flexibility in Polycrystalline Metal-Organic Framework Membranes for Carbon Capture

Although polycrystalline metal-organic framework (MOF) membranes offer several advantages over other nanoporous membranes, thus far they have not yielded good CO₂ separation performance, crucial for energy-efficient carbon capture. ZIF-8, one of the most popular MOFs, has a crystallographically determined pore aperture of 0.34 nm, ideal for CO₂ /N₂ and CO₂ /CH₄ separation; however, its flexible lattice restricts the corresponding separation selectivities to below 5. A novel postsynthetic rapid heat treatment (RHT), implemented in a few seconds at 360 °C, which drastically improves the carbon capture performance of the ZIF-8 membranes, is reported. Lattice stiffening is confirmed by the appearance of a temperature-activated transport, attributed to a stronger interaction of gas molecules with the pore aperture, with activation energy increasing with the molecular size (CH₄ > CO₂ > H₂ ). Unprecedented CO₂ /CH₄ , CO₂ /N₂ , and H₂ /CH₄ selectivities exceeding 30, 30, and 175, respectively, and complete blockage of C₃ H₆ , are achieved. Spectroscopic and X-ray diffraction studies confirm that while the coordination environment and crystallinity are unaffected, lattice distortion and strain are incorporated in the ZIF-8 lattice, increasing the lattice stiffness. Overall, RHT treatment is a facile and versatile technique that can vastly improve the gas-separation performance of the MOF membranes.